Stratospheric aerosol injection, or SAI, is the most discussed form of solar geoengineering. The idea is to mimic the cooling effect of large volcanic eruptions by deliberately injecting sulphur dioxide into the stratosphere, where it converts into tiny, long-lived sulphate aerosol particles. These particles scatter incoming sunlight back to space, offsetting some of the warming caused by greenhouse gases. The precedent is real: the 1991 eruption of Mount Pinatubo in the Philippines injected roughly 15 to 20 teragrams of sulphur dioxide into the stratosphere and temporarily lowered global mean surface temperatures by more than one degree Celsius. But deliberate SAI remains deeply controversial, with potential side effects including shifts in regional precipitation, altered temperature gradients between hemispheres and between poles and equator, and delayed recovery of the stratospheric ozone layer. A sudden halt to a deployment could even trigger a catastrophic rapid warming known as the termination shock.

The governance problem is what makes detection so urgent. No global framework currently exists to regulate SAI, despite repeated calls from the United Nations Environment Programme, the European Parliament, and national scientific academies. According to studies under the EU-funded Co-Create project, small-scale outdoor SAI experiments could be carried out today with existing technology at a cost of between 10 and 100 million US dollars, using stratospheric balloons to release anywhere from a few kilograms to a few hundreds of tonnes of sulphur dioxide. Such near-term and mid-term experiments are widely seen as a likely entry point to larger, uncoordinated deployments. A typical moderate volcanic eruption injects a few teragrams of sulphur dioxide, tens of thousands to billions of times more than these exploratory experiments, which is precisely why existing satellites, designed to monitor volcanic plumes, would miss them entirely.

The study tested whether a future satellite instrument could close this surveillance gap. The researchers focused on the Changing-Atmosphere Infra-Red Tomography explorer, or CAIRT, a mission concept that was one of four candidates down-selected in the European Space Agency’s Earth Explorer 11 call. CAIRT would have been the first limb sounder in space to use imaging Fourier-transform thermal infrared technology, observing the limb of the atmosphere at tangent heights from 5 to 115 kilometres with a spectral resolution of 0.25 wavenumbers or better across the mid-infrared range from 718 to 2200 wavenumbers. This design would deliver three-dimensional profiles of trace gases and aerosols with vertical resolution between 1 and 3 kilometres and horizontal resolution between 100 and 300 kilometres, along with temperature profiles resolved to 2 kilometres or finer. Although CAIRT was ultimately not selected for implementation, its scientific goals were recognised as timely, and the concept remains unique in its potential to observe the middle atmosphere.

To evaluate the concept rigorously, the team built what they call a pseudo-reality from state-of-the-art climate model simulations. They used the Community Earth System Model version 2 with the Whole Atmosphere Community Climate Model version 6, run under the Geoengineering Model Intercomparison Project within CMIP6. Specifically, they took the Geo SSP5-34-OS 2.0 scenario, in which sulphur dioxide is injected at four latitudes between roughly 20 and 25 kilometres altitude to hold global warming to 2 degrees Celsius. In the first simulated deployment year, 2034, the injection rates are deliberately tiny: about 0.035 teragrams per year at 15 degrees north, equivalent to roughly 100 tonnes per day, and about 0.005 teragrams per year at 15 degrees south, around 15 tonnes per day. These amounts are characteristic of the near-term experiments that could realistically be performed unilaterally.

From this pseudo-reality, the researchers generated realistic synthetic observations, or pseudo-observations, of what CAIRT would actually measure, using a dedicated Fast Level 2 Simulator that propagates the instrument’s expected random and systematic errors and spatial smoothing characteristics, derived from the mission’s Phase 0 and Phase A feasibility studies. The technique exploits the distinctive rotational-vibrational absorption and emission bands of sulphur dioxide near 8.7 and 7.3 micrometres, and the spectral signatures of sulphate aerosol droplets peaking around 12.5 and 8.3 micrometres, which arise from the undissociated sulphuric acid in the highly acidic stratospheric droplets. Typical total uncertainties for individual retrievals were 0.15 to 0.25 parts per billion in molar fraction for sulphur dioxide and 1.0 to 1.5 parts per trillion in mass fraction for sulphate aerosol, remarkably close to background stratospheric levels.

The results were striking. In the simulated February 2034 scene, CAIRT-like pseudo-observations clearly detected both injection sites, pinpointed their altitude at 25 to 30 kilometres, and quantified the intercepted sulphur dioxide mass at 3.1 tonnes against a true value of 3.4 tonnes at the northern site, and 0.4 tonnes against 0.5 tonnes at the southern site, biases of roughly 10 and 25 percent. Even the weaker Southern Hemisphere injection, cumulatively only a few hundred tonnes by the end of February, stood well above the detection limit. The plume’s descent, rapid zonal dispersion, and poleward transport were all faithfully captured. In practical terms, this means a CAIRT-like instrument could detect and quantitatively characterise a unilateral geoengineering test within hours to a few days of the first injection, something no current satellite can do.

The technique also proved capable of tracking the aftermath. Over the simulated year 2034, the pseudo-observations followed the conversion of sulphur dioxide into sulphate aerosol as the new layer formed at the injection longitudes, spread zonally to encircle the globe, and then dispersed toward higher latitudes in both hemispheres. Differences between the synthetic observations and the underlying model truth generally did not exceed 10 percent within the plume. The instrument even reproduced the self-lofting of the aerosol plume, a subtle process in which the particles absorb solar radiation, heat the surrounding air diabatically, and lift the layer to higher altitudes over time, a phenomenon recently observed after volcanic eruptions such as Raikoke in 2019.

By contrast, the study’s audit of existing capabilities reads as a catalogue of gaps. Solar occultation instruments like SAGE III/ISS achieve excellent precision but have sparse coverage and would only distinguish SAI perturbations at injection rates of 1 to 2 teragrams of sulphur per year, far above exploratory levels. Limb-scattering instruments such as OMPS-LP have worse signal-to-noise ratios, and both operate in the visible range, so they cannot chemically identify sulphate aerosol. Space LiDARs like EarthCARE’s ATLID detect weak aerosol layers but cannot determine composition, while nadir infrared sounders like IASI can identify sulphate but with little or no vertical resolution. The infrared occultation instrument ACE-FTS is expected to be discontinued within a few years, and several other middle-atmosphere missions are nearing retirement, threatening what researchers have called a data desert in stratospheric composition monitoring.

The message of the study is therefore twofold. First, the world is currently blind to the earliest and most plausible forms of geoengineering, the small tests that could precede larger unilateral deployments. Second, high-spectral-resolution mid-infrared limb emission sounding offers a proven-in-simulation path to closing that gap, with benefits extending well beyond geoengineering oversight to wildfire and space-debris aerosol sources, gravity wave characterisation, and understanding of stratospheric circulation in a changing climate. Whether a CAIRT-like mission flies in the coming decades may determine not just what science can see, but whether the international community can verify, and thus govern, one of the most consequential climate interventions humanity might ever attempt.

Subject of Research: Satellite detection of stratospheric aerosol injection geoengineering using mid-infrared limb emission sounding

Article Title: The enhanced capabilities of mid-infrared limb emission sounding to observe stratospheric aerosol injection geoengineering

Article References: Sellitto, P., Kosary, M., Höpfner, M., Funke, B., Hoffmann, A., Ungermann, J., Errera, Q., Tilmes, S., & Sinnhuber, B.-M. (2026). The enhanced capabilities of mid-infrared limb emission sounding to observe stratospheric aerosol injection geoengineering. Atmospheric Measurement Techniques, 19(19), 6379-6393. https://doi.org/10.5194/amt-19-6379-2026

Image Credits: AI Generated

DOI: 10.5194/amt-19-6379-2026

Keywords: stratospheric aerosol injection, geoengineering, CAIRT, mid-infrared limb sounding, sulphur dioxide, sulphate aerosols, satellite remote sensing, solar radiation management, ESA Earth Explorer, climate monitoring, stratosphere, unilateral deployment

The study, led by Sean M. Davis of the NOAA Chemical Sciences Laboratory in Boulder, Colorado, brought together an international team of more than forty researchers to tackle a deceptively simple question. Over the tropics, the belt between 30 degrees south and 30 degrees north that covers half the planet’s surface and is home to roughly 60 percent of humanity, how much has ozone actually changed since the year 2000, and do the world’s chemistry-climate models agree with what the satellites have seen? The answer, it turns out, depends heavily on which dataset you trust, and that dependence has consequences for how confidently we can forecast ozone’s trajectory through the rest of this century.

To disentangle the picture, the team divided the atmosphere into four vertical layers: the troposphere, which holds about 13 percent of the total ozone column; the lower stratosphere, contributing roughly 28 percent; the middle stratosphere, the largest share at about 41 percent; and the upper stratosphere, which supplies the remaining 18 percent. This layering matters because different physical processes dominate each region. In the lower stratosphere, ozone is governed mainly by transport, as the greenhouse-gas-driven acceleration of the Brewer-Dobson circulation pushes air upward through the tropical tropopause and thins the ozone layer there. In the upper stratosphere, chemistry rules: declining chlorine from the Montreal Protocol and cooling temperatures from rising carbon dioxide both push ozone upward, producing robust recovery. The troposphere adds its own complexity, because ozone there is manufactured from precursor emissions such as nitrogen oxides, which are rising across the developing tropics even as they fall over the midlatitudes of the Northern Hemisphere.

The statistical engine of the study was a dynamical linear model, a flexible regression technique that separates the slow background trend from the noise of natural variability, including the El Niño Southern Oscillation, the quasi-biennial oscillation of equatorial winds, the 11-year solar cycle, and stratospheric aerosol from volcanic eruptions. Applying this method to more than a dozen merged satellite datasets and reanalyses, and to two generations of coordinated chemistry-climate model simulations known as CCMI-1 and CCMI-2022, the researchers computed probability distributions of ozone change over the 2000 to 2021 period and measured how much those distributions overlapped.

The headline numbers look encouraging at first glance. Both model generations simulate an increase in tropical total column ozone since 2000, and the newer CCMI-2022 ensemble, with a mean change of +2.5 Dobson units, agrees slightly better with the observational estimate of +3.3 Dobson units than the older CCMI-1 ensemble does at +1.6 Dobson units. But that apparent agreement conceals systematic differences lurking beneath the surface. In the troposphere, the newer models show a substantial increase of about +1.5 Dobson units since 2000, driven largely by stronger rises in precursor emissions, whereas the older generation shows almost no change at +0.3 Dobson units. That single difference explains most of the gap between the two model generations in total column ozone.

More troubling still is the behavior of the observations themselves. In the upper stratosphere, observational estimates of the 2000 to 2021 change range from +0.6 Dobson units in the GOZCARDS dataset to +2.6 Dobson units in the SAGE-OSIRIS-SAGE composite, with as little as 2 percent overlap between the two distributions. In the middle stratosphere, the spread is equally stark, with one dataset showing essentially no change and another showing an increase of 2.5 Dobson units. Even datasets built largely from the same Aura Microwave Limb Sounder instrument, such as SWOOSH and GOZCARDS, diverge, apparently because of differences in how the pre-2004 data from earlier instruments were processed and merged. In the troposphere, the disagreement is arguably the worst of all: one composite shows zero change while another shows an increase of 2.4 Dobson units, with only 1 percent overlap between them.

Why does this matter so much? Because the models exhibit a tantalizing property that, in principle, could turn observations into a predictive tool. The researchers found that across the older CCMI-1 ensemble, models with faster ozone recovery during the first two decades of the twenty-first century also tend to project stronger ozone growth through the rest of the century, a correlation that holds in every layer of the stratosphere and is especially strong in the middle stratosphere. This kind of relationship, known as an emergent constraint, allows scientists to combine a measured present-day trend with the model-derived correlation to narrow the range of possible futures. Before the satellites can play that role, however, two conditions must hold: the correlation must be robust across model generations, and the observational trend estimates must be precise enough to be useful.

Both conditions fail. The emergent relationship that is so clear in the older model ensemble largely disappears in the newer CCMI-2022 simulations, surviving only as a borderline correlation in the middle stratosphere. The reasons remain uncertain, but the authors point to differences in greenhouse gas forcing between the two experimental designs, improved but changed representations of the quasi-biennial oscillation, and documented differences in stratospheric transport between model generations as plausible culprits. Meanwhile, when the researchers propagated the spread of observational trends through the model-derived relationships, the resulting uncertainty in the observationally informed end-of-century ozone projection was as large as, or larger than, the spread of the raw model projections themselves. Depending on which satellite record is treated as truth, one could conclude that the models either systematically overestimate or systematically underestimate future tropical ozone recovery.

Not everything in the analysis is mired in ambiguity. The lower stratosphere, the region where earlier studies first flagged an unexpected decline, shows the most consistent story. Nearly all observational datasets and both model ensembles agree that tropical lower stratospheric ozone has decreased since 2000, with multi-model means of −0.6 Dobson units for CCMI-1 and −1.1 Dobson units for CCMI-2022, and the observations trending somewhat more negative on average than the models. This decline is broadly consistent with the expected greenhouse-gas-driven speeding up of the shallow branch of the Brewer-Dobson circulation, although questions remain about the roles of tropopause height changes and very short-lived halogen compounds. Looking further ahead, the models robustly project continued lower stratospheric ozone decline through 2100, averaging about −5 Dobson units, even as the upper stratosphere gains roughly +6 Dobson units from chemical recovery.

The ultimate message of the study is a call to action for the observational community. The spread among existing satellite trend estimates, particularly in the troposphere and the middle and upper stratosphere, is wide enough to swamp the signal scientists are trying to measure, and reconciling those differences, possibly by hunting for instrumental drifts in the source records, is now a critical priority. Stable, trend-quality measurements from current and future satellite platforms, combined with a better understanding of why successive model generations behave differently, will be essential before observations can serve as the yardstick against which projections of the ozone layer’s future are judged. Until then, the recovery of the ozone layer over the tropics, the region where billions of people and much of Earth’s biodiversity depend on it, remains a story whose next chapter cannot yet be read with confidence.

Subject of Research: Uncertainties in observed tropical stratospheric and tropospheric ozone trends and their implications for constraining model projections of future ozone change

Article Title: Uncertainties in recent tropical stratospheric and tropospheric ozone changes limit the use of observational constraints for assessing future ozone change

Article References: Uncertainties in recent tropical stratospheric and tropospheric ozone changes limit the use of observational constraints for assessing future ozone change. (n.d.). https://doi.org/10.5194/acp-26-13817-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13817-2026

Keywords: ozone layer, tropical ozone, stratosphere, troposphere, satellite observations, chemistry-climate models, Montreal Protocol, Brewer-Dobson circulation, ozone recovery, emergent constraint, CCMI, Dobson units

The biographical frame is striking in itself. Born in Amsterdam on 3 December 1933, Crutzen began his career as a bridge-building engineer before pivoting to meteorology in Stockholm, where he completed his doctorate and, in 1973, the Filosofie doktor, roughly equivalent to a habilitation. After stints at NOAA and NCAR in Boulder, Colorado, where he became a director in 1977, he moved in 1980 to the Max Planck Institute for Chemistry in Mainz as director of the Air Chemistry Department. He remained a Dutch citizen throughout his life, married Terttu Soininen in 1958, and raised two daughters. He died in Mainz on 28 January 2021, leaving behind what his colleagues describe as a lineage of scientific children, grandchildren and even great-grandchildren trained under his mentorship.

His scientific range was extraordinary. In 1970 he first described the nitrogen-oxide-driven ozone-loss cycle in the stratosphere, and he warned that a fleet of supersonic aircraft could damage the ozone shield through their NOx emissions. He traced how nitrous oxide from the troposphere ascends into the stratosphere to form reactive nitrogen, and he was among the first to work out the photochemical mechanisms that create ozone in the lower atmosphere. Later he contributed key ideas on the chemistry of the Antarctic ozone hole, showed how biomass burning and aerosol particles reshape tropospheric chemistry, and, with John Birks, calculated that smoke from a nuclear war could plunge the planet into what became known as nuclear winter. In 2006 an essay of his reignited the geoengineering debate, and in 2000 he co-founded the journal Atmospheric Chemistry and Physics, a pioneer of open access and transparent peer review.

The anecdotes collected by Müller and his co-authors reveal how these landmark results emerged from an intensely human style of working. Lennart Bengtsson recalls a phone call in the summer of 1995 in which Crutzen asked, with characteristic indirectness, whether Bengtsson could arrange an invitation to that year’s Nobel Prize ceremonies in Stockholm. Bengtsson, who knew things he was not permitted to reveal, could only say he would work on it and was confident matters would resolve themselves. They did, of course, and by December Crutzen was in Stockholm sharing the chemistry prize with Mario Molina and F. Sherwood Rowland.

Other memories underline Crutzen’s instinctive kindness. John Birks met him on a flight from Illinois to Washington, discovered mid-journey that his checked bag would not follow him to the destination, and scrambled to recheck it during a stopover. As the cabin door was about to close with Birks still off the plane, Crutzen persuaded the flight attendants to hold the aircraft. Birks later followed Crutzen from the University of Colorado to Mainz for a sabbatical, and the two eventually reunited to study the atmospheric effects of asteroid impacts. John Burrows, who first encountered Crutzen at a NATO winter school in the Italian Dolomites in 1977, remembers the senior scientist delivering the second half of a lecture series on a broken leg sustained skiing down the Marmolada in bad weather, filling young researchers with enthusiasm for stratospheric and tropospheric ozone chemistry.

Burrows, who joined Crutzen’s new department in Mainz in late 1981, describes a science manager who led by example and was, in his words, a why-not person rather than a why person, a calculated risk-taker who listened carefully to the ideas of others. It was over a coffee-time discussion in 1983 that Burrows suggested the differential optical absorption spectroscopy technique, developed by Dieter Perner and Ulrich Platt, could be adapted for space-borne spectrometers. Crutzen backed the idea, and although the first proposal to ESA failed, it paved the way for the SCIAMACHY and GOME satellite instruments that transformed atmospheric remote sensing. Crutzen advised Burrows to keep space experiments well focused and to manage expectations realistically, and he liked to call SCIAMACHY sky magic. He also served as a government adviser on the German Bundestag’s Enquete Commission on protecting the Earth’s atmosphere from 1987 to 1991, work that helped shape national and European support for ozone research at a time when West Germany alone accounted for more than ten percent of global consumption of long-lived ozone-depleting substances.

Not every memory is flattering, and that is part of the charm. Rich Stolarski of NASA Goddard recalls borrowing the code of Crutzen’s two-dimensional atmospheric model in the 1980s, only to find it littered with cryptic comments such as a note that there seemed to be too much diffusion in the upper stratosphere, followed by an abrupt halving of the eddy diffusion coefficient. Stolarski eventually gave up on the punch cards, concluding that while many colleagues wrote better code, Crutzen was an artist who wrote programs to test ideas; for him the idea, not the code, was the goal. Stolarski also remembers a 1970 AGU meeting where the then-unknown Crutzen projected a slide of chemical equations that came up out of focus, cheerfully explaining that he merely wanted to impress the audience with how many reactions his model contained.

The chemistry itself produced memorable exchanges. After Stolarski and Ralph Cicerone presented work on stratospheric chlorine at a 1973 meeting in Kyoto, Crutzen sat down with Stolarski and walked through a series of equations suggesting chlorine might actually increase ozone. He later admitted he spent several months trying to prove it, an effort Stolarski now regards as thoroughness rather than waste, since it verified that no simple mistake had been made. Decades later, Monte Carlo uncertainty calculations showed that chlorine-driven ozone increase was theoretically conceivable only if the rate of the OH plus HO2 reaction were vanishingly small, which laboratory measurements subsequently ruled out. The episode captures Crutzen’s method: test every idea, including your own doubts, against the chemistry.

Life in Mainz had its frictions. Ruprecht Jaenicke recounts that the Max Planck Society hesitated over Crutzen’s appointment, perhaps because of his unusual career path, and that the University of Mainz long refused to recognize his Swedish Filosofie doktor as habilitation-equivalent, addressing him as Dr. rather than Professor even after he had held professorships in Chicago and San Diego. Only shortly before the Nobel Prize was he made an Honorarprofessor. Meanwhile the university chemists of the era dismissed atmospheric trace constituents as scientifically negligible, so Crutzen relied on the outstanding atmospheric chemist Peter Warneck to supervise doctoral students. Rolf Müller adds lighter touches: colleagues piled important papers on Crutzen’s chair during his travels, only for him to sweep the entire heap aside and sit down to discuss science; and when pressed for model plots for a 1992 paper on Antarctic ozone chemistry, he replied with self-irony that he already knew the outcome of the simulations but needed the plots for the paper.

Perhaps the most vivid scene comes from Jülich in February 1996. Müller had invited his former boss to give a seminar, only for the date to coincide with Fettdonnerstag, the carnival day when costumed revelers traditionally cut off men’s ties. The institute’s directors hesitated to welcome a brand-new Nobel Laureate amid the festivities, but rescheduling was impossible given Crutzen’s packed calendar. The lecture hall filled, partly with people in costume, to hear him speak on halogen chemistry in the troposphere. A few minutes in, the doors swung open and a band of witches advanced with an enormous pair of scissors, only to find that Crutzen was not wearing a tie. He resumed his lecture after about five minutes with the words that he hoped he could now focus on his talk again. That blend of scientific intensity and unflappable good humor, his colleagues conclude, was inseparable from the achievements: a modest man, always impatient for new results, who kept one piece of work with him even on family holidays, and who, when a university president credited the Nobel Prize to his institution, quietly observed that what counts, in any case, is the future.

Subject of Research: Personal recollections of Nobel laureate atmospheric chemist Paul J. Crutzen by his friends and colleagues

Article Title: Paul J. Crutzen – interactions with friends and colleagues

Article References: Müller, R., Bengtsson, L., Birks, J., Burrows, J., Jaenicke, R., & Stolarski, R. S. (2025). Paul J. Crutzen – interactions with friends and colleagues. History of Geo- and Space Sciences, 16(2), 31-40. https://doi.org/10.5194/hgss-16-31-2025

Image Credits: AI Generated

DOI: 10.5194/hgss-16-31-2025

Keywords: Paul Crutzen, atmospheric chemistry, ozone, Nobel Prize, Anthropocene, nuclear winter, stratosphere, SCIAMACHY, Max Planck Institute for Chemistry, biography, history of science, geoengineering

The team deployed AirCore samplers during three Northern Hemisphere summer campaigns at sites spanning a wide latitudinal range: Trainou in France at roughly 48 degrees north, and the polar-latitude stations of Kiruna in Sweden and Sodankylä in Finland, both near 67 degrees north. An AirCore is essentially a long coil of stainless-steel tubing, internally coated with a passivating layer to prevent the tube from chemically altering the air it captures. Launched beneath a stratospheric balloon, the device fills passively during descent as the ambient pressure gradient pushes air through the tube. Because radiosondes record pressure and temperature throughout the flight, analysts can calculate exactly how many moles of air entered the tube during any interval, allowing each aliquot of gas to be mapped back to the altitude at which it was sampled. The result is a continuous vertical profile of atmospheric composition, offering far finer vertical resolution than satellite remote sensing can typically achieve.

Once recovered, the samples were analyzed immediately on a dual-laser Quantum Cascade Laser Spectrometer, a mid-infrared absorption instrument capable of measuring carbonyl sulfide, methane, nitrous oxide, carbon dioxide, carbon monoxide and ozone simultaneously from a single air sample. This combination proved crucial, because the entire lifetime analysis hinges on comparing the stratospheric behavior of carbonyl sulfide and methane against nitrous oxide, a long-lived tracer of tropospheric origin whose own stratospheric lifetime is comparatively well constrained at around 116 plus or minus 9 years. The researchers also drew on version 5.3 data from the Atmospheric Chemistry Experiment Fourier Transform Spectrometer, a satellite instrument that retrieves vertical profiles using solar occultation, selecting summer observations between 45 and 49 degrees north for comparison with Trainou and between 65 and 69 degrees north for the polar sites. Obtaining consistent answers from two such different observational methodologies, the authors argue, provides strong evidence for the reliability of both.

The analytical framework rests on tracer-tracer correlation theory first formalized by Plumb and Ko in 1992. In the lower stratosphere, long-lived gases destroyed by similar chemistry display tight, approximately linear relationships with one another when their mole fractions are plotted against each other. The slope of this relationship, combined with the ratio of the gases’ tropospheric burdens and the known lifetime of the reference gas, yields the stratospheric lifetime of the target species. The team applied this classic method alongside a more comprehensive refinement by Volk and colleagues from 1997, which accounts for non-diffusive mixing between the tropics and mid-latitudes by extrapolating the correlation slope to the extratropical tropopause and using globally integrated atmospheric burdens rather than simple tropospheric averages. For the burden estimates, the authors followed the approach of Andrews and colleagues, averaging twelve-month running means of surface observations from Mauna Loa and American Samoa, delayed by two months to reflect transport to the stratosphere.

Careful data curation proved essential. Just above the thermal tropopause, the correlations between carbonyl sulfide and nitrous oxide, and between methane and nitrous oxide, bent away from linearity, likely reflecting the mixing of seasonally varying tropospheric air into the lowermost stratosphere. Including this region would have biased the regressions, so the team restricted their analysis to nitrous oxide mole fractions below roughly 295 to 300 parts per billion, depending on the campaign year. Curvatures at higher altitudes, where different stratospheric chemistry alters the source-sink balance differently for each gas, further limited the usable range to altitudes up to about 20 to 22 kilometers. One flight, designated KRN-a, was flagged as an outlier for carbonyl sulfide after showing anomalously low mole fractions between 17 and 20 kilometers, possibly due to instrument instability or contamination, while another flight had to be excluded entirely because data selection left only a single usable point.

The headline results are strikingly consistent. Using the Plumb and Ko method, the stratospheric lifetime of carbonyl sulfide came out between 69 and 90 years, with an overall average of 76 plus or minus 6 years, corresponding to a stratospheric sink of roughly 37 plus or minus 3 gigagrams of sulfur per year. The Volk method produced an overlapping range of 71 to 83 years and a sink of 35 plus or minus 3 gigagrams of sulfur per year. For methane, the Plumb and Ko approach yielded lifetimes between 153 and 166 years, while the Volk method gave 149 to 162 years, translating into stratospheric sinks of approximately 24 to 27 teragrams of carbon per year. These figures sit comfortably within the ranges reported by earlier studies, which span 39 to 76 years for carbonyl sulfide and 152 to 160 years for methane, and they agree closely with independent estimates derived from ACE-FTS satellite observations, which suggested 75 to 76 years for carbonyl sulfide and 146 to 172 years for methane.

Perhaps the most consequential finding is what the data do not show: no significant long-term trend in the stratospheric removal of either gas, despite clear trends in their tropospheric abundances. Nitrous oxide and methane rose steadily across the sampling years, from 332.0 to 336.8 parts per billion and from 1844.6 to 1899.2 parts per billion respectively between 2019 and 2023, while carbonyl sulfide declined from 490 to 479 parts per billion between 2019 and 2021 before recovering to 487 parts per billion by 2023. This decline in the carbonyl sulfide burden, confirmed by other recent observational records between 2016 and 2020, contrasts with the continued growth of the other two gases and slightly reduces the estimated carbonyl sulfide sink relative to some earlier studies. The authors also note that lifetime estimates differed subtly with latitude, with carbonyl sulfide lifetimes running slightly longer at polar latitudes and methane lifetimes slightly shorter, though in neither case did the differences reach statistical significance.

The study also exposed genuine atmospheric variability that remains poorly understood. Tracer-tracer correlations varied from flight to flight, and even within the same campaign, pointing to day-to-day changes in the composition of the lower stratosphere. To investigate whether these differences reflected sampling location rather than measurement error, the team computed equivalent latitudes from ERA5 potential vorticity fields, a coordinate that captures the transport history of air parcels more faithfully than geographic latitude. The analysis revealed that most sampled air had mid-latitude origins, even over the polar sites, and that the balloon profiles captured localized, latitudinally dependent air masses that zonally averaged satellite data cannot fully resolve. This may partly explain why methane lifetime estimates from the Volk method diverged somewhat between the two observational platforms, though the discrepancies were not statistically significant.

Beyond the specific numbers, the work carries broader implications for how the atmospheric science community constrains trace-gas budgets. The authors found that when they recalculated the lifetimes reported by previous studies using standardized tropospheric burdens, the revised estimates shifted but never significantly departed from the original values, underscoring how sensitive these calculations are to methodological choices about burden definitions, regression altitude ranges and reference tracers. Earlier investigations used everything from cryogenic whole-air samplers to balloon-borne spectrometers and ground-based Fourier transform instruments, and some sampled only the lowest few kilometers of the stratosphere, where tropospheric mixing can corrupt the correlations. Given these inconsistencies, the team advocates for a more standardized methodology across the field. What is already clear, however, is that a modest coil of steel tubing dangling beneath a balloon, paired with laser spectroscopy on the ground, can now deliver stratospheric lifetime estimates rivaling those from orbiting satellites, offering an affordable and repeatable tool for monitoring how the upper atmosphere processes the gases that shape our climate.

Subject of Research: Observational estimates of stratospheric lifetimes and sinks of carbonyl sulfide and methane from balloon-borne AirCore vertical profiles

Article Title: Consistent estimates of carbonyl sulfide and methane stratospheric lifetimes retrieved from AirCore profiles at different latitudes

Article References: Zanchetta, A., van Heuven, S., Kivi, R., Ramonet, M., Engel, A., Krol, M., & Chen, H. (2026). Consistent estimates of carbonyl sulfide and methane stratospheric lifetimes retrieved from AirCore profiles at different latitudes. Atmospheric Chemistry and Physics, 26(19), 13909-13927. https://doi.org/10.5194/acp-26-13909-2026

Image Credits: AI Generated

DOI: 10.5194/acp-26-13909-2026

Keywords: carbonyl sulfide, methane, stratosphere, AirCore, atmospheric lifetime, nitrous oxide, tracer-tracer correlations, ACE-FTS, balloon sounding, greenhouse gases, stratospheric sink, atmospheric chemistry

The research, led by Martin Andrews of the Met Office Hadley Centre in the United Kingdom, brought together twelve climate models from eleven modelling centres around the world under the umbrella of the Quasi-Biennial Oscillation initiative, known as QBOi, part of the World Climate Research Programme’s activities on atmospheric processes. The team ran three coordinated experiments covering the period 1979 to 2020. In the first, the models generated their own QBOs freely, as they normally would. In the second, the tropical stratospheric winds were nudged, or gently relaxed, towards the winds observed in the ERA5 reanalysis dataset, effectively forcing every model to carry the real QBO. In the third, the QBO was suppressed altogether by relaxing the winds towards a climatological average, erasing the oscillation entirely. Comparing the three allowed the researchers to isolate, with unusual clarity, what a realistic QBO does and does not deliver in terms of remote atmospheric influence.

The headline target was the Holton-Tan Effect, the classic teleconnection first described in 1980, in which the phase of the QBO modulates the strength of the Northern Hemisphere winter polar vortex, the swirling westerly jet known as the Polar Night Jet that circles the Arctic stratosphere. Observations show that during easterly QBO winters the polar jet weakens, because the zero-wind line separating tropical and extratropical flow shifts poleward, channelling large-scale planetary waves into the vortex where they deposit easterly momentum and slow it down. During westerly QBO winters the waves spread more freely and the vortex strengthens. The observed signal is dramatic: in January the difference between easterly and westerly composites reaches about 20 metres per second in the reanalysis data.

What the models produced was far more modest. In the free-running experiment, the ensemble mean captured a statistically significant but weak Holton-Tan-like response, peaking at only about 2 metres per second, roughly a tenth of the observed magnitude, and it entirely missed the reversal of the signal seen in observations in March. When the models were nudged towards the observed QBO, the response roughly doubled, peaking near 5 metres per second in December and January, and the number of models showing a negative polar jet response during easterly phases rose from five out of eleven to ten out of twelve. Yet even this improved response amounted to only about 20 per cent of the observed signal. Statistical sub-sampling showed that the nudged and free-running distributions of polar jet responses were not significantly different from each other, suggesting that the improvement came mainly from correcting the too-weak amplitude of the simulated QBOs rather than from strengthening the underlying teleconnection mechanism itself.

The study also probed how well the QBO phase influences sudden stratospheric warmings, the dramatic events in which the polar vortex breaks down and stratospheric temperatures can leap by tens of degrees within days. Observations indicate roughly 0.9 warmings per easterly-QBO winter compared with about 0.5 per westerly-QBO winter, and warmings tend to arrive about a month earlier during easterly phases. The models captured only a small and marginally significant increase in warming frequency during easterly phases, and even then only when the analysis was restricted to models with a significant Holton-Tan Effect. More strikingly, neither the free-running nor the nudged experiments reproduced the observed separation in warming timing between the two QBO phases, nor the observed tendency for final warmings of spring to arrive earlier after westerly-QBO winters. The team notes that models with weaker climatological polar vortices produce more warmings overall, a bias that may be muddying the picture.

One genuinely encouraging result concerned predictability. When the nudged ensemble was compared directly with the observed January polar jet over the 42-year period, the correlation was a highly significant 0.61, implying that up to 30 to 40 per cent of the observed year-to-year variability of the January vortex may be attributable to the equatorial stratosphere, including the QBO. In the experiment with the QBO erased, the correlation collapsed to an insignificant 0.06, despite identical sea surface temperatures, sea ice and atmospheric forcings. This is a powerful demonstration that a forecast system carrying the true phase and amplitude of the QBO holds real predictive information about the winter polar vortex, information that vanishes when the oscillation is removed. The authors suggest that the combination of reasonable skill with a too-weak predicted signal may reflect the well-known signal-to-noise paradox that afflicts seasonal-to-decadal forecasting.

The connection to surface weather proved harder to pin down. The North Atlantic Oscillation, the seesaw of pressure between the Azores and Iceland that steers winter storms across Europe, is only weakly correlated with the QBO even in observations, with correlations of about 0.2 to 0.3 in January. In the model ensembles the correlation was smaller still, generally below 0.1 and statistically insignificant, accounting for less than one per cent of the NAO’s variability. Even when the analysis was restricted to the subset of models with a significant Holton-Tan Effect, only a weak positive January correlation emerged, and its significance vanished when an alternative two-station NAO index was used. Because the downward pathway from a strong or weak polar vortex to the tropospheric circulation is itself well captured by the models, the failure appears to lie in the weak QBO-to-vortex link upstream, which starves the surface response of amplitude.

In the North Pacific, the picture was more nuanced. Observations show that the QBO phase shifts the subtropical jet, with easterly-QBO winters producing a weakening of the jet around 30 degrees north and a strengthening near 50 degrees north, a signature of a slight northward displacement. The nudged ensemble captured a significant wind anomaly on the equatorward flank of the jet, something the free-running ensemble failed to produce, and it also reproduced observed patterns of tropical precipitation anomalies over the Pacific that may act as a tropospheric bridge for QBO influences. However, the model response poleward of 35 degrees north actually degraded with nudging, and the entire latitudinal pattern appeared shifted about five degrees too far north, a systematic bias the authors could not fully explain. The confounding influence of the El Nino-Southern Oscillation loomed large here: filtering to only ENSO-neutral winters changed both the sign and the spatial structure of the wind and precipitation signals, underscoring how entangled these tropical drivers are.

The overall message is a careful one. Bias-correcting the QBO in climate models demonstrably helps: it strengthens the polar jet response, improves the representation of tropical precipitation and subtropical wind anomalies, and unlocks genuine predictive skill for the January vortex. But a realistic QBO alone is not sufficient. The models’ internal machinery, the way planetary waves propagate, break and deposit momentum in response to the equatorial winds, remains too feeble, and key observed signatures such as the timing of stratospheric warmings and the March reversal of the Holton-Tan signal remain out of reach. Whether the remaining gaps stem from biases in the extratropical stratosphere or from a fundamentally incomplete representation of the coupling mechanisms is, the authors conclude, a central question for the next generation of experiments. For now, the QBO’s whisper to the mid-latitudes is real, measurable, and only partially heard by the models that try to simulate it.

Subject of Research: The influence of the equatorial Quasi-Biennial Oscillation on extratropical stratospheric and tropospheric circulation in nudged multi-model climate experiments

Article Title: Extratropical teleconnections in a multi-model ensemble nudged towards the observed QBO

Article References: Andrews, M. B., Butchart, N., Anstey, J. A., Bednarz, E., Elsbury, D., García-Franco, J. L., Kumar, V., Palmeiro, F. M., Trencham, N. E., Yoshida, K., Chai, Z., Hong, D.-C., Huang, K., Jaison, A. M., Kawatani, Y., Knight, J. R., Lin, P., Lott, F., Lu, Y., … Xie, J. (2026). Extratropical teleconnections in a multi-model ensemble nudged towards the observed QBO. Weather and Climate Dynamics, 7(3), 1797-1820. https://doi.org/10.5194/wcd-7-1797-2026

Image Credits: AI Generated

DOI: 10.5194/wcd-7-1797-2026

Keywords: Quasi-Biennial Oscillation, stratosphere, polar vortex, Holton-Tan Effect, sudden stratospheric warming, North Atlantic Oscillation, subtropical jet, climate models, teleconnections, QBOi, nudging, ERA5 reanalysis

Short-lived halogens, or SLH, are a deceptively simple family of compounds with outsized consequences. The group includes organic very short-lived substances such as bromoform, dibromomethane and methyl iodide, which are emitted largely by biologically active oceans as by-products of algae and phytoplankton metabolism, as well as inorganic halogen families — collectively labelled Cly, Bry and Iy — that cycle rapidly between reactive and reservoir forms. Unlike the infamous chlorofluorocarbons that linger for decades and only break down in the stratosphere, these short-lived species are destroyed quickly, releasing chlorine, bromine and iodine atoms low in the atmosphere where they catalytically attack ozone, alter the abundance of the hydroxyl radical that acts as the atmosphere’s detergent, and reshape the partitioning of nitrogen oxides that govern smog formation.

The new release is technically ambitious. CESM2-SLH embeds offline emission inventories for nine oceanic halocarbons derived from SeaWiFS chlorophyll-a climatology, anthropogenic emissions of dichloromethane and perchloroethylene based on recent inventories, and — crucially — online, interactive sources of inorganic halogens. These online sources include ozone-driven oxidation of iodide at the sea surface, which releases hypoiodous acid and molecular iodine and accounts for roughly three-quarters of all atmospheric iodine, and sea-salt aerosol dehalogenation, in which oxidised halogen reservoirs collide with airborne sea salt and liberate molecular chlorine, bromine and mixed halogens. The team had to retune accommodation coefficients — reducing them by factors of three to eight — because CESM2 represents sea-salt aerosol with a different modal scheme than its predecessor, producing up to ten times larger aerosol surfaces in the boundary layer.

Chemistry itself was expanded dramatically. The mechanism adds twelve chlorine, nine bromine and nineteen iodine species participating in more than two hundred new reactions spanning photolysis, gas-phase kinetics, heterogeneous recycling on aerosols, ice crystals and cloud droplets, and stratospheric activation pathways. Rate constants were updated to the latest Jet Propulsion Laboratory and IUPAC recommendations, and the photolysis of higher-order iodine oxides — species whose inclusion proved essential to reproduce upper-tropospheric iodine observations — is now handled explicitly. The computational price is a twenty to twenty-five percent increase in model runtime, a modest cost for what the chemistry delivers.

The headline result is striking: including short-lived halogens cuts global surface ozone by twenty-one to twenty-eight percent and tropospheric ozone by seventeen to twenty-two percent, with stratospheric columns dropping by up to three percent. Hydroxyl radical abundances fall by two to nine percent and nitrogen dioxide by one to ten percent, depending on configuration and resolution. Because ozone is both a pollutant and a greenhouse gas, and because OH controls the lifetime of methane, these numbers ripple through every projection of future air quality and climate forcing made with the model. The authors emphasise that halogen-driven ozone destruction in the troposphere reaches roughly 660 teragrams per year, dominated — somewhat surprisingly to non-specialists — by iodine chemistry, which alone accounts for up to thirty percent of ozone loss in the tropical free troposphere.

Validation against observations is encouraging. Simulated vertical profiles of organic source gases in the tropical tropopause layer fall within the ranges assessed by the most recent World Meteorological Organization ozone assessment, and the model reproduces the split between source gas injection and product gas injection that determines how efficiently halogens reach the stratosphere. Bromine from short-lived sources adds about five parts per trillion to the stratospheric loading — a twenty-five percent enhancement over long-lived sources — while iodine reaches the stratosphere almost entirely in inorganic form at roughly 0.7 parts per trillion. Comparisons with ship-based campaigns measuring iodine monoxide across the Pacific and Atlantic, and with coastal bromine observations at Cape Verde, show the model capturing magnitudes and day-to-day variability, if not always exact timing.

Perhaps the most consequential finding concerns the Antarctic ozone hole. When short-lived bromine and iodine chemistry is included, springtime ozone destruction over the polar vortex deepens by more than twenty Dobson units — about fourteen percent — in October, confirming earlier CESM1 results with the new model generation. Because the radiative balance of the climate system is exquisitely sensitive to ozone changes in the lowermost stratosphere, this region matters far beyond its thin vertical extent. Previous work by the same groups has shown that these halogens exert an overall cooling influence on climate through coupled effects on ozone, methane, aerosols and stratospheric water vapour, meaning that models lacking this chemistry may systematically misjudge both air quality trajectories and radiative forcing.

The release is deliberately user-friendly. Six new model compsets span nudged and free-running configurations at coarse and fine horizontal resolutions, from 1.9 by 2.5 degrees down to 0.9 by 1.25 degrees, and from low-top configurations reaching about forty kilometres to whole-atmosphere WACCM setups extending into the lower thermosphere. A new namelist section exposes scaling factors that let users adjust sea-salt dehalogenation and washout efficiencies when switching resolutions or meteorological datasets, and a technical appendix walks newcomers through spin-up requirements, emission specifiers, deposition lists and cross-section files. The authors recommend that new users start with the coarse, nudged configuration, where transport is consistent across experiments and chemical signals can be isolated cleanly from dynamical noise.

Not every development from the CESM1 era made the cut. Emission-driven methane simulations, continental inorganic halogen emissions from coal burning, polar sea-ice halogen sources, iodine release from dust, and the expanded sulphur chemistry involving methanethiol and hydroperoxymethyl thioformate are all absent from this first release, flagged for future work. The authors are candid that these omissions mean the simulated halogen influence is, in most respects, a lower limit. Even so, the message to the community is unambiguous: any serious simulation of past, present or future atmospheric composition — from urban smog to the ozone hole — should now include the reactive halogens that the ocean, sea salt and human industry continuously pump into the air. With CESM2-SLH freely available, there is little excuse left to leave them out.

Subject of Research: Implementation of short-lived halogen sources and chemistry in the Community Earth System Model v2

Article Title: Short-lived halogen sources and chemistry in the Community Earth System Model v2 (CESM2-SLH)

Article References: Fernandez, R. P., Cuevas, C. A., Villamayor, J., Feinberg, A., Kinnison, D. E., Vitt, F., Bossolasco, A., Barrera, J. A., Reynoso, A., Tomazzeli, O. G., Li, Q., & Saiz-Lopez, A. (2026). Short-lived halogen sources and chemistry in the Community Earth System Model v2 (CESM2-SLH). Geoscientific Model Development, 19(19), 9325-9376. https://doi.org/10.5194/gmd-19-9325-2026

Image Credits: AI Generated

DOI: 10.5194/gmd-19-9325-2026

Keywords: short-lived halogens, CESM2, atmospheric chemistry, ozone depletion, iodine emissions, bromine, chlorine, sea-salt aerosol, stratosphere, tropospheric ozone, hydroxyl radical, climate modelling

The project, named STRAT-NAV, forms part of ARIA’s Enduring Atmospheric Platforms Programme, a £70 million, three-and-a-half-year initiative led by Programme Director Rico Chandra. The programme has funded 18 research and development teams, all working on the same fundamental challenge: how to keep aircraft operating reliably and cost-effectively in the stratosphere for extended periods. The ambition behind the programme is considerable. Platforms capable of lingering aloft for weeks or months could provide the high-performance communications infrastructure needed for truly global connectivity, support the next generation of AI and digital services, and reduce dependence on conventional satellites, which are expensive to launch and impossible to repair once in orbit.

The aircraft at the centre of this vision are known as High-Altitude Pseudo-Satellites, or HAPS. These long-endurance vehicles fly in the stratosphere, a layer of the atmosphere that is relatively calm, above most weather, and far below orbital space. From this vantage point, a HAPS platform can deliver remote-sensing observations or communications services over a wide area, behaving in many respects like a satellite but with the crucial advantage of being recoverable, upgradable and re-deployable. Most HAPS designs are solar-powered ultralight gliders, exquisitely engineered to be as light as possible. That lightness, however, comes at a cost: the onboard battery capacity is limited, and the aircraft face acute power challenges, particularly during the long winter nights experienced at UK latitudes, when solar energy falls to its lowest levels.

This is where the Bath team sees an opportunity hiding in plain sight. Atmospheric gravity waves are created when air is forced upwards — by mountain ranges or by deep convection — and then falls back down under gravity in a wave-like motion, transferring energy through the atmosphere much as ripples transfer energy across the surface of a pond. The vertical air motions associated with these waves can be strong enough for a suitably equipped aircraft to exploit. Just as eagles ride thermals to gain altitude without flapping their wings, STRAT-NAV aims to guide HAPS platforms to soar on atmospheric gravity waves. According to the project team, the energy savings from this ‘gravity wave assist’ would be equivalent to carrying at least a 20 to 40 per cent bigger battery — at no additional cost, weight or complexity to the aircraft itself.

Dr Neil Hindley, a research fellow in the Centre for Climate Adaptation and Environment Research and the project’s lead, described the award as an exciting opportunity to translate decades of atmospheric science into a practical tool. In his words, the project could transform high-altitude aviation and unlock the stratosphere as an operating environment. He noted that atmospheric gravity waves have been studied by scientists for decades, but that until recently the community lacked the sub-kilometre-scale forecasting capability needed to predict them accurately enough to use operationally. If HAPS aircraft can be helped to navigate and exploit these naturally occurring energy sources, he argued, it could fundamentally change what is possible for long-endurance flights over the UK and beyond.

The choice of the United Kingdom as a proving ground is not accidental. The country sits beneath the undulating path of the jet stream, a fast-moving ribbon of air whose meanders are a prolific generator of gravity waves. Add the rugged mountain terrain of Wales and the Pennines, which forces air upward as weather systems pass over, and the stratospheric polar vortex that forms over the pole each winter, and the UK becomes a global hotspot for gravity wave activity. Hindley described exploiting stratospheric gravity waves as a uniquely advantageous strategy for the UK because of this geography. Crucially, the timing works in the aircraft’s favour: while available solar energy falls to a minimum during winter, gravity wave activity over the UK reaches its maximum at exactly the moment it is most needed. This seasonal complementarity between sunlight and wave energy is what could allow HAPS to operate year-round, realising the vision of an operational fleet providing services that until now have only been possible from orbit.

The technical challenge lies in the fact that gravity waves, despite containing abundant energy, are notoriously difficult to forecast and exploit. They are small in horizontal scale compared with the synoptic weather systems that drive them, often just a few kilometres across, which places them below the resolution of standard numerical weather prediction models. A HAPS aircraft therefore needs reliable, high-accuracy forecasts and intelligent routing systems to find and use these rising currents safely and efficiently. STRAT-NAV will tackle this with AI-enhanced, ultra-high-resolution, real-time atmospheric modelling, validated against observations, to forecast the gravity wave field around a HAPS platform up to 72 hours in advance. On top of the forecast layer, the project will develop platform-specific intelligent routing algorithms that maximise the energy harvested from gravity waves through soaring while maintaining the communications coverage that these aircraft exist to provide.

The programme is structured to move from simulation to the real sky. Work begins in September 2026 with model development, followed by observational campaigns in late 2026 and 2027 designed to validate the gravity-wave forecasts against direct measurements of the atmosphere. Only once the forecasts have been proven will the project progress to flight trials with HAPS industry partners later in the programme. This staged approach reflects a broader lesson from aviation history: exploiting natural atmospheric energy is not a new idea — glider pilots have ridden mountain waves and thermals for a century — but doing so autonomously, at 20 kilometres altitude, guided by machine-learned forecasts, is something genuinely novel.

One of the most commercially significant aspects of the design is its platform-agnostic nature. The gravity wave assist capability is intended to work with any HAPS platform, whether a lightweight fixed-wing aircraft or even a rotary-wing vehicle, and requires no modifications to the aircraft and no expensive ground infrastructure. That could lower the barrier to adoption across an industry in which every gram of payload and every watt-hour of battery counts. For operators, the prospect of effectively free energy harvested from the sky — savings equivalent to a battery a fifth to two-fifths larger — could be the difference between a seasonal demonstrator and a viable year-round service.

If STRAT-NAV succeeds, the implications extend well beyond the UK. A demonstrated ability to forecast and exploit stratospheric gravity waves would give long-endurance aircraft a renewable energy source that no battery chemistry or solar array can match in cost, and it would arrive precisely when solar power is weakest at high latitudes. The wider Enduring Atmospheric Platforms Programme, with its 18 funded teams and £70 million budget, is betting that the stratosphere can become a routine operating environment — a physical backbone for next-generation communications and a path to connectivity for underserved regions. The energy to make that happen, the Bath team argues, is already there, rippling invisibly through the sky, waiting to be navigated.

Subject of Research: Exploiting atmospheric gravity waves to extend the endurance of stratospheric high-altitude pseudo-satellite aircraft

Article Title: Bath receives £1.9 million funding to unlock energy hidden in the sky

Article References: Bath receives £1.9 million funding to unlock energy hidden in the sky. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: atmospheric gravity waves, stratosphere, HAPS, high-altitude pseudo-satellites, University of Bath, ARIA, STRAT-NAV, long-endurance flight, atmospheric modelling, soaring, renewable energy, connectivity

The jet stream is a narrow, meandering current of air near the tropopause, the boundary between the troposphere and the stratosphere, typically located several kilometres above the surface and flowing from west to east at speeds that can exceed 50 metres per second in winter. Over the Atlantic–European sector, this current is maintained by the temperature contrast between the warm subtropical ocean and the cold polar region, a contrast that generates pressure gradients at upper levels of the atmosphere. Because the jet acts as a kind of atmospheric conveyor belt, guiding storm systems from the ocean toward the continent, even modest shifts in its strength or position can reshape winter weather across Britain, Scandinavia, central Europe and the Mediterranean.

Understanding how the jet responds to greenhouse warming has been one of the more contested questions in climate dynamics. One influential line of argument holds that rapid Arctic amplification—the fact that the high northern latitudes are warming several times faster than the globe as a whole—reduces the equator-to-pole temperature gradient near the surface. A weaker gradient, in this view, should sap the energy available to the jet, producing weaker winds, larger meanders and a greater tendency toward persistent blocking patterns that lock cold air over Europe for weeks at a time. Another line of argument emphasizes that the upper troposphere warms more strongly than the surface in the tropics and mid-latitudes, which increases the vertical shear of the winds and can actually strengthen the jet aloft while shifting it poleward.

The new analysis addresses this debate by looking not only at the average behaviour of the Atlantic–European jet in winter but at the full distribution of its variability, including the extreme tail. Using observational and reanalysis datasets—long records of atmospheric conditions reconstructed from weather observations and satellite data—the researchers tracked jet strength over recent decades and examined how frequently the jet reached its most intense values. The central result is a nuanced one: the typical winter jet has become stronger, consistent with the expectation that upper-level warming and changes in the temperature gradient enhance the mean flow, but the extremes of jet strength have not kept pace. In relative terms, the jet is becoming a steadier current rather than a more violent one.

This distinction between the mean and the extremes matters for how scientists and society interpret jet stream change. A stronger mean jet is generally associated with a more vigorous storm track, which can bring more frequent passages of Atlantic low-pressure systems and the mild, wet, windy weather they deliver to northwestern Europe. Extreme jet episodes, by contrast, are often linked to exceptional weather: an unusually intense jet can coincide with explosive cyclogenesis, in which storms deepen rapidly and cause damaging windstorms, while an unusually weak or displaced jet can coincide with prolonged cold spells or drought. If the mean strengthens while the extremes moderate, the net effect on hazardous winter weather may be less dramatic than either a simple strengthening or a simple weakening story would suggest.

The physical reasoning behind such a divergence can be traced to how different components of the climate system respond to warming. The mean jet strength is governed largely by broad-scale temperature gradients and by the vertical structure of tropospheric warming, both of which evolve smoothly and predictably with rising greenhouse gas concentrations. Extreme jet episodes, however, often depend on transient processes: the phasing of planetary-scale Rossby waves, the life cycles of individual baroclinic storms, and episodic coupling with the stratospheric polar vortex. Some of these transient drivers may weaken or become less variable under warming, damping the upper tail of the jet distribution even as the background flow intensifies. The study’s results are consistent with such a mechanism, in which variability about the mean contracts even as the mean itself rises.

The findings also speak to a long-running scientific controversy about Arctic influence on mid-latitude weather. Over the past decade, a vigorous debate has played out in the journals over whether sea-ice loss and Arctic warming make European winters more prone to blocking and severe cold. Proponents of this view point to episodes such as the cold winters of recent decades and to modelling experiments in which reduced sea ice favours a wavier jet. Critics counter that the observational record is short, that internal variability is large, and that model projections more often show a strengthened, poleward-shifted jet with reduced waviness in winter. By documenting a strengthening yet less extreme jet, the new work lends weight to the second camp, while underscoring that the answer may differ by season, by region and by which aspect of the jet is measured.

Methodologically, the study illustrates the value of going beyond simple averages when assessing climate change signals. Jet stream behaviour is notoriously noisy: individual winters can differ enormously, and the Atlantic–European sector in particular is influenced by modes of variability such as the North Atlantic Oscillation, which swings between phases that strengthen or weaken the westerlies on timescales from weeks to decades. Separating a forced climate signal from this internal noise requires careful statistical treatment, long datasets and, in many cases, large ensembles of climate model simulations in which many parallel realizations of the same warming scenario are run to isolate the common response. The researchers’ focus on the distribution of jet strength, rather than a single metric of average speed, allows a more complete picture of how the flow is evolving and reduces the risk of drawing conclusions from a few memorable extreme winters.

The implications extend to practical forecasting and adaptation. Winter storm risk assessment for Europe depends on assumptions about how often severe windstorms strike, and those assumptions are typically based on historical statistics. If the character of the jet is changing—stronger on average but with moderated extremes—then the historical record may be an imperfect guide to the coming decades. Insurers, infrastructure planners and emergency services all have an interest in knowing whether the tail risks of winter weather are growing or shrinking. The new results suggest that for jet-driven wind extremes, at least, the most catastrophic outcomes may not intensify as rapidly as the mean conditions do, although the researchers caution that other hazards, such as precipitation extremes associated with a moister, warmer atmosphere, continue to worsen independently of the wind field.

There remain open questions. The observational record of the upper atmosphere is only a few decades long, and reanalysis products carry uncertainties, particularly in the earlier satellite era and in the data-sparse regions of the North Atlantic. Disentangling the roles of tropical upper-tropospheric warming, Arctic amplification, stratospheric variability and ocean circulation in shaping the jet will require further modelling work. Nevertheless, the study offers a clear and somewhat reassuring refinement of the picture: the Atlantic–European winter jet is not collapsing into weakness and chaos, nor is it becoming uniformly more ferocious. Instead, it is strengthening as a background current while its wildest excursions become relatively less extreme—a reminder that climate change rarely moves every aspect of a system in the same direction, and that the most useful science is often found in the details between the averages and the extremes.

Subject of Research: Winter strengthening and reduced extremity of the Atlantic–European jet stream

Article Title: A strengthening yet less extreme Atlantic–European jet during winter

Article References: Brönnimann, S., Brugnara, Y., & Kallabis, P. (2026). A strengthening yet less extreme Atlantic–European jet during winter. Nature Geoscience. https://doi.org/10.1038/s41561-026-02069-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02069-z

Keywords: jet stream, Atlantic–European jet, winter climate, North Atlantic Oscillation, Arctic amplification, storm track, climate variability, reanalysis, stratosphere, extreme weather, climate change, geoscience