The team, led by Rhidian Thomas of the National Centre for Atmospheric Science and the University of Reading, together with colleagues from the University of Edinburgh, the University of Reading, and NOAA’s Physical Sciences Laboratory, used a sophisticated technique built on the NOAA-CIRES-DOE Twentieth Century Reanalysis version 3, known as 20CRv3. A reanalysis system combines a numerical weather model with historical observations to reconstruct the atmospheric state of the past. In this case, the system assimilated more than a century-old archive of surface pressure readings from land stations, ships, and tropical cyclone reports, using an Ensemble Kalman Filter to blend those observations with the NOAA GFS atmospheric model. The result is an 80-member ensemble of three-hourly atmospheric fields covering all of 1903, a year that ranks as the third coldest in the 175-year HadCRUT5 temperature record, partly because of the cooling influence of the Santa Maria volcanic eruption of 1902.

The crucial innovation lies in what the researchers did next. They re-ran the entire data assimilation system, feeding it exactly the same pressure observations but with two deliberate changes: sea surface temperatures were uniformly raised by 2 degrees Celsius, and in a second, novel experiment, atmospheric carbon dioxide was lifted to 530 parts per million on top of the ocean warming. Because the same pressure data constrain the atmospheric flow in every experiment, all three simulations experience essentially identical weather patterns, the same storms tracking the same paths, the same anticyclones parking over the same regions. Any difference between the runs therefore reflects the pure thermodynamic response to a warmer, moister atmosphere, isolated from the uncertain changes in large-scale circulation that dominate disagreements between climate models.

This storyline approach, related to pseudo-global-warming and nudging methods used in event attribution, has a powerful advantage: it sidesteps the largest source of uncertainty in regional climate projections. Regional forecasts of temperature and rainfall depend heavily on how storm tracks, jet streams, and blocking patterns might shift, questions on which models diverge. By anchoring the analysis in weather patterns that demonstrably occurred, the reanalysis experiments answer a cleaner question: if the atmosphere’s circulation stayed the same, how would the same weather feel in a warmer world? The team focused on four regions where 1903’s pressure observations were dense enough to tightly constrain the circulation: northwestern Europe, the western Mediterranean, the East Coast of the United States, and southeastern Australia.

The temperature results are unambiguous. In the warmer experiments, 99 percent of days became warmer than in the original reconstruction, with typical days over land warming by roughly 2 to 2.5 degrees Celsius for a 2-degree ocean warming. But the warming was far from uniform across the temperature distribution. The largest increases, 4 to 5 degrees Celsius, fell on the coldest days, those dipping below freezing. The mechanism is a classic snow-albedo feedback: in the warmer simulations, less of the land surface is covered in snow, so proportionally more sunlight is absorbed by darker ground rather than reflected back to space, amplifying warming precisely on the days when snow would otherwise lie. Southeastern Australia, where freezing days are rare, was the lone region without this cold-day amplification.

Hot days warmed disproportionately too, with increases of 3 degrees Celsius or more on days above 30 degrees in the western Mediterranean, the US East Coast, and southeastern Australia. The underlying physics varies by region. On the US East Coast, the hottest days coincide with dry soils, and the simulations show a sharp reduction in evaporative cooling above the 97th temperature percentile, meaning the land surface loses its ability to shed heat through evaporation, a well-known mechanism that intensifies heatwaves. In southeastern Australia, by contrast, the amplification appears tied to reduced cloud cover and greater incoming solar radiation. Notably, the direct radiative effect of the added carbon dioxide contributed an extra 0.2 to 0.4 degrees of warming on typical days, and as much as a fifth of the total warming over the arid western United States, demonstrating that carbon dioxide warms the surface directly, not merely indirectly through ocean warming.

Rainfall told an equally counterintuitive story. Globally, total precipitation rose in the warmer experiments, but far more slowly than atmospheric moisture, which climbed at roughly 8.5 percent per degree of warming, in line with the Clausius-Clapeyron relation governing how much water vapor air can hold. The reason is the atmosphere’s energy budget: latent heating from rain must balance longwave cooling, and additional carbon dioxide suppresses longwave emission to space, blunting the precipitation increase. Regionally, the pattern echoed canonical model projections, with wetting across much of the extratropics and drying over the Mediterranean and subtropical oceans. Even in the western Mediterranean, where annual rainfall declined, daily rainfall variability increased, meaning larger swings from wet day to dry day, a signature consistent with emerging observational trends.

The most vivid finding concerns how the extra rain is delivered. Across all four regions, the number of days with light, drizzly rain decreased, while days with moderate to heavy rain grew exponentially more frequent as rain rates increased. The cutoff scale separating ordinary from heavy events shifted upward roughly in step with Clausius-Clapeyron scaling, yet the time-mean increase in rainfall was concentrated on fewer than 1 day in 10. In other words, even where total annual rainfall rises, most wet days actually become drier, and the annual surplus arrives in a handful of intense bursts. Dry days also increased in three of the four regions, likely because reduced relative humidity over land raises convective inhibition, lengthening the gaps between rain events.

October 1903 provides the study’s showpiece. That month remains the wettest calendar month on record for the United Kingdom, with 220 millimeters in the national series back to 1836. The reanalysis reproduced the observed geography of the deluge, and in the warmer experiments, rainfall over the UK and Ireland rose by 12 to 17 percent, equivalent to 6 to 8.8 percent per degree of local warming, well above the annual-mean scaling. On sub-daily timescales the amplification was even more dramatic: one intense three-hour event on 6 October intensified by 31 percent, around 15 percent per degree, exceeding Clausius-Clapeyron scaling as previous studies have found for short-duration extremes. Scaling to today’s roughly 1.5 degrees of warming since 1903, the authors estimate that the same weather pattern would now deliver around 240 millimeters across the UK, comfortably surpassing the previous record of 216.3 millimeters set in December 2015.

The study’s limitations are candidly acknowledged. The experiments are idealised, with uniform ocean warming, unchanged sea ice, and a single year of weather, and the assumptions that 1903’s patterns could occur in a warmer world, and that the assimilation remains consistent under perturbed boundary conditions, are supported but not proven; the warmer runs did reject somewhat more pressure observations, particularly over densely observed northwestern Europe. Circulation changes, if they occur, could amplify or modify these thermodynamic signals, and long-standing model biases in light rainfall temper confidence in the drizzle decline. Yet the fact that a single pair of counterfactual reanalysis runs recovers so many robust patterns seen across generations of climate models, amplified warming of hot and cold extremes, rising precipitation variability, fewer rainy days, and heavier downpours, makes this a powerful, computationally efficient new line of evidence. It suggests that the everyday weather of the past is already a preview of a different daily experience in our warming future, one written into every frost-free night and every intense burst of rain.

Subject of Research: Thermodynamic impacts of climate change on everyday temperature and precipitation, studied by reanalysis experiments translating the weather of 1903 into a warmer world

Article Title: Everyday weather in a warmer world

Article References: Thomas, R., Compo, G. P., George, S., Hegerl, G. C., Schurer, A., Shepherd, T. G., Slivinski, L. C., Thompson, V., & Hawkins, E. (2026). Everyday weather in a warmer world. Earth System Dynamics, 17(5), 1365-1380. https://doi.org/10.5194/esd-17-1365-2026

Image Credits: AI Generated

DOI: 10.5194/esd-17-1365-2026

Keywords: climate change, reanalysis, 20CRv3, extreme weather, precipitation, temperature extremes, Clausius-Clapeyron, storyline attribution, 1903, hydroclimate, snow albedo feedback, Mediterranean drying