Hysteresis is a concept borrowed from physics and engineering, describing systems whose output depends not only on their current conditions but on the path they took to get there. In the climate system, it means that the world you get during a period of carbon removal may look very different from the world you had at the same CO2 level on the way up. For temperature, this asymmetry has been studied extensively in idealized model experiments. What has remained far less clear is how precipitation extremes, which depend on intricate interactions between moisture, circulation, and sea surface temperature patterns, behave when humanity eventually begins extracting carbon dioxide from the atmosphere at scale.

The research team tackled this problem using climate model simulations built around four different CO2 concentration thresholds and two distinct rates of emission reduction. In these pathway experiments, carbon dioxide concentrations first rise to a peak, then decline as emissions fall and removal proceeds, before stabilizing back at the initial concentration. By comparing the state of the climate during the ramp-up phase with the state during and after ramp-down at identical CO2 levels, the researchers could isolate the irreversibility signal from the direct response to greenhouse gas forcing. The design allowed them to test not just whether hysteresis occurs, but how its magnitude depends on the choices societies make about the timing and pace of decarbonization.

The results are striking. Extreme precipitation indices based on both intensity and frequency increase as CO2 climbs to its peak, which is exactly what physical understanding would predict, since a warmer atmosphere holds more water vapor and energizes storms through the Clausius-Clapeyron relationship. The surprise comes on the way down. When CO2 concentrations return to their initial values, the extreme precipitation indices do not. They remain elevated above their starting point, a residual enhancement that persists even though the greenhouse gas forcing that produced it has been nominally erased. In other words, the most dangerous aspect of East Asian climate change, the flooding rains of an intensified monsoon, would linger after the cause had been addressed.

Crucially, the study shows that this hysteresis is not a fixed property of the climate system but a variable one, amplified by how slowly humanity acts. When emission reductions are delayed or proceed at a slower rate, the lag between CO2 decline and precipitation recovery grows, and the irreversibility becomes more pronounced. This finding carries a direct policy implication: the sequencing of decarbonization matters as much as its endpoint. Two pathways that end at the same atmospheric CO2 concentration can leave East Asia with materially different risks of extreme rainfall depending on whether the peak was reached early and reduced quickly, or late and grudgingly. Carbon removal, in this sense, is not a clean slate but a partial one.

To understand why extreme precipitation gets stuck in its elevated state, the researchers traced the signal back to the tropical Pacific Ocean. The hysteresis in East Asian rainfall is linked to persistent changes in the pattern of tropical Pacific warming, which do not reverse in step with CO2 concentrations. Sea surface temperature patterns are notoriously sluggish, shaped by ocean heat uptake and circulation adjustments that operate on timescales far longer than the atmospheric response to greenhouse gases. Once the tropical Pacific settles into an altered warming configuration, that configuration continues to influence the atmosphere above it, generating teleconnections, vast waves of pressure and circulation anomalies, that propagate into the East Asian monsoon region and sustain enhanced mean monsoon precipitation long after the forcing has subsided.

The team then dissected the mechanism further using a moisture budget analysis, a technique that decomposes changes in precipitation into its thermodynamic and dynamic contributors. The thermodynamic factor concerns how much water vapor the air carries: warmer air holds more moisture, and in the early summer months the study finds that rises in specific humidity enhance the transport of moisture into East Asia through the mean atmospheric circulation. The dynamic factor concerns how the air moves: in late summer, the monsoon circulation itself intensifies, strengthening the winds that funnel moisture-laden air from the adjacent oceans onto the continent. Both effects contribute to the enhanced precipitation, and both are sustained by the hysteresis in the underlying ocean and circulation state, which explains why the extremes they feed remain elevated during the removal phase.

The regional stakes could hardly be higher. East Asia is home to some of the world’s densest populations and most productive agricultural systems, all calibrated to the rhythm of the monsoon. Extreme precipitation events in the region have repeatedly caused catastrophic flooding, landslides, and economic losses, and the study’s findings suggest that these risks would not simply unwind in parallel with a global decarbonization effort. Infrastructure designed, reservoirs managed, and flood defenses built around the assumption that climate risk scales linearly with CO2 would systematically underestimate the danger during the removal era. Adaptation planning, the research implies, must account for the possibility that the climate of the cleanup decades will be more hazardous than the concentration numbers alone would suggest.

More broadly, the work adds East Asian extreme precipitation to a growing list of climate impacts, from sea level rise to ice sheet loss, whose response to carbon removal is asymmetric and path-dependent. It underscores a message that has been building across the reversibility literature for years: preventing climate change is far more reliable than reversing it. The study’s comparison of emission reduction rates shows that every year of delay and every increment of slower action deepens the hysteresis and extends the period during which East Asia endures amplified extreme rainfall. The timing and pace of emission reductions, the authors conclude, play a critical role in determining regional climate outcomes, not merely the final concentration target.

As nations weigh the role of carbon dioxide removal in future climate portfolios, this research offers a sobering technical caveat. Negative emissions, whether through afforestation, direct air capture, or enhanced weathering, are increasingly treated as a safety net that can compensate for overshooting temperature goals. But the East Asian monsoon system, coupled as it is to the slow memory of the tropical Pacific, does not reset on demand. The downpours that define the peak-CO2 world cast a shadow across the removal decades, and only faster, earlier emission cuts can shrink that shadow. The atmosphere’s carbon ledger may be reversible on paper; the monsoon, this study shows, keeps its own account.

Subject of Research: Hysteresis of extreme precipitation responses to carbon dioxide removal pathways in East Asia

Article Title: Hysteresis of extreme precipitation in East Asia under various CO2 removal pathways

Article References: Lee, M.-U., Park, J.-Y., Kim, H.-K., Kim, M.-K., Min, S.-K., Park, D.-S. R., Park, H.-S., Byun, Y.-H., Sung, H. M., Hong, J. G., Park, J.-S., & Jeon, W. (2026). Hysteresis of extreme precipitation in East Asia under various CO2 removal pathways. Nature Communications. https://doi.org/10.1038/s41467-026-78431-x

Image Credits: AI Generated

DOI: 10.1038/s41467-026-78431-x

Keywords: extreme precipitation, East Asia, CO2 removal, hysteresis, monsoon, tropical Pacific, climate modeling, moisture budget, emission reductions, climate reversibility, Nature Communications, atmospheric teleconnections

The research team focused on Tianjin, a sprawling port megacity on the Bohai Bay in northern China, during the period from 2017 to 2020. These years coincided with aggressive national emission reduction programs, making the city an ideal natural laboratory for separating the effects of pollution controls from those of weather. The scientists combined ground-based chemical observations with the WRF-NAQPMS modeling system, a coupled meteorology and air quality model widely used in China for forecasting and policy analysis. By running the model with fixed meteorology and varying emissions, and vice versa, they could disentangle how much of the observed change in each pollutant came from human activity and how much came from the atmosphere itself. This decomposition technique, often called a factor-separation or scenario approach, is one of the most rigorous tools available for attributing air quality trends, and the team applied it not just to annual averages but to distinct weather regimes defined by the sea-land breeze cycle.

The headline numbers from the study are striking in themselves. Emission reductions between 2017 and 2020 drove PM2.5 concentrations down by 7.1 micrograms per cubic meter, a decline driven primarily by reductions in carbonaceous components and sulfate, the chemical fingerprints of combustion and industrial sources. Yet over the same period, ozone rose by 24.5 micrograms per cubic meter according to the model, closely matching the net observed increase of 23.0 micrograms per cubic meter. Meteorology, averaged across the whole year, played a comparatively minor role, contributing a decline of 1.7 micrograms per cubic meter for PM2.5 and 1.4 micrograms per cubic meter for ozone. On paper, this looks like a familiar story: cleaner particles, dirtier air in a different form, with weather as a footnote. But the averages, the researchers found, were hiding the real drama.

When the team broke the data down by circulation regime, the modest annual meteorological contributions dissolved into wildly divergent day-by-day realities. On sea breeze days, when cool marine air pushes inland during the afternoon, meteorology was the only regime in which it actually increased ozone, adding 2.2 micrograms per cubic meter. More remarkably, the sea breeze offset 5.1 micrograms per cubic meter of the PM2.5 reductions that emission controls had achieved, effectively erasing a substantial fraction of the hard-won progress. During these periods, the researchers also observed concurrent increases in nitrate and elemental carbon, indicating that the marine air flow was not simply diluting pollution but actively reshaping its chemical composition, likely by transporting and transforming aerosol precursors along the coast. The sea breeze, in other words, is not a cleansing wind; it is a chemical conveyor.

The land breeze told the opposite story with even greater intensity. On land breeze days, when the nocturnal flow reverses and carries air from the city out toward the sea, interannual meteorological changes favored PM2.5 reduction with a contribution of 32.70 micrograms per cubic meter, a figure nearly five times larger than the annual mean decline. This meteorological assist was accompanied by substantial reductions in nitrate and elemental carbon, confirming that the ventilation effect was removing real pollution rather than merely redistributing it. Then there were the stagnant, stable days, the bane of every air quality manager. Under these conditions, meteorological factors were the primary driver of interannual PM2.5 increases, contributing a staggering 30.80 micrograms per cubic meter, and PM2.5 peaks exceeded 150 micrograms per cubic meter even with emission reductions in full force. On such days, the atmosphere itself overwhelmed the benefits of a decade of controls.

The diurnal asymmetry the study documents adds another layer of complexity. Daytime sea breezes dominated ozone production, because the marine air arriving over the sun-heated land encounters a shallow, turbulent boundary layer in which photochemistry runs at full throttle. Precursor gases trapped near the surface are efficiently mixed and cooked into ozone under intense solar radiation. At night, the land breeze stage favored pollutant accumulation and secondary formation, as weak winds and temperature inversions allowed emissions to pool near the ground while heterogeneous chemistry continued to manufacture secondary aerosols in the dark. This means that a single 24-hour cycle in a coastal city contains two chemically distinct pollution environments, each responding differently to the same emission inventory. A policy calibrated against daily averages will inevitably misjudge both.

Why does ozone rise even as its precursors fall? The answer lies in the nonlinear photochemistry of the troposphere. Ozone formation depends on the ratio of nitrogen oxides to volatile organic compounds, and in many Chinese cities the chemical regime is what atmospheric scientists call volatile organic compound limited or transitional. When nitrogen oxide emissions drop, ozone destruction by nitrogen oxides weakens faster than ozone production, so ozone concentrations climb even though the ingredients for smog are diminishing. This VOC-limited behavior has been documented across the North China Plain and is a central reason why the co-control of PM2.5 and ozone has become the defining challenge of Chinese air quality policy. The Tianjin study adds a crucial spatial dimension to this problem: the sea-land breeze modulates the local chemical regime on a daily cycle, meaning the effective sensitivity of ozone to emissions can flip between morning and afternoon.

The implications for policy are profound. The study’s central conclusion is that regime-specific meteorological contributions dwarf the annual averages used in most policy evaluations, and that sea-land breeze circulation therefore fundamentally modulates pollutant responses to emission controls. The authors argue for circulation-specific co-control strategies in coastal megacities. In practice, this could mean scheduling stricter temporary emission curbs on forecast sea breeze days, when ozone formation will be amplified and particulate reductions partially offset, while relaxing controls on land breeze days when natural ventilation does much of the work. It could also mean targeting volatile organic compound reductions more aggressively than nitrogen oxides in coastal zones, to push the photochemical regime toward one where further nitrogen oxide cuts actually lower ozone. Cities from the Pearl River Delta to the Yangtze River Delta, where sea breeze effects on coastal ozone are already documented, could apply the same framework.

There is also a warning embedded in the findings about the future. Related research has shown that ocean warming and urbanization are weakening sea-land breezes in coastal megacities, which means the natural ventilation and chemical transport patterns that the Tianjin study quantified are themselves changing. A control strategy optimized for today’s breeze climatology may misfire as the circulation shifts. Moreover, the study’s demonstration that stagnant conditions can push PM2.5 above 150 micrograms per cubic meter despite emission reductions underscores that meteorological variability remains a first-order driver of extreme pollution events, not a second-order correction. Climate change is expected to alter the frequency and persistence of such stagnant episodes across many densely populated coastal regions, adding yet another moving part to an already intricate system.

What makes this study resonate beyond atmospheric science is its reframing of a familiar frustration. Residents of coastal cities have long sensed that some breezy days feel fresh and others feel thick with haze, and now there is a quantitative framework explaining why. The daily dance between sea and land is not background noise in the air quality record; it is a key modulation, capable of amplifying or erasing the effects of policy at scales that rival the policies themselves. For the growing number of megacities clustered along the world’s coastlines, from Tianjin to Mumbai to Lagos, the message is clear: to manage air pollution in a coastal city, you must first learn to read the wind.

Subject of Research: How sea–land breeze circulation modulates PM2.5 and ozone responses to emission reductions in coastal megacities

Article Title: Key modulation of PM2.5–O3 responses to emission reductions by sea–land breeze in coastal megacities

Article References: Liu, K., Wu, L., Lu, M., Tang, X., Kong, L., Zhang, Y., & Wang, Z. (2026). Key modulation of PM2.5–O3 responses to emission reductions by sea–land breeze in coastal megacities. Air Quality, Atmosphere & Health, 19(10), Article 220. https://doi.org/10.1007/s11869-026-02038-w

Image Credits: AI Generated

DOI: 10.1007/s11869-026-02038-w

Keywords: PM2.5, ozone, sea–land breeze, emission reductions, coastal megacities, air quality modeling, WRF-NAQPMS, Tianjin, photochemistry, meteorology, co-pollution, co-control strategies