New Infrared Satellite Concept Could Catch Secret Geoengineering Tests Within Days

Imagine a single nation, a private company, or even a well-funded startup quietly releasing tonnes of sulphur dioxide into the stratosphere to cool the planet, without telling anyone. It sounds like science fiction, but the technology to do it exists today, and the price tag is startlingly low. A new study published in the journal Atmospheric Measurement Techniques reveals a sobering truth: the international community currently has no satellite capability to detect such small-scale, potentially unilateral geoengineering experiments. The research, led by Pasquale Sellitto of the Laboratoire Interuniversitaire des Systèmes Atmosphériques in France, together with colleagues from Germany, Spain, the Netherlands, Belgium, and the United States, also offers a solution in the form of a proposed satellite technique called mid-infrared limb emission sounding, modelled on a European mission concept known as CAIRT.

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