The simplest thermodynamic equations make it clear that warmer air can hold more moisture than colder air: The Clausius-Clapeyron equation shows that for every 1°C temperature increase, Earth’s atmosphere can hold 7% more water.
The reality of global climate science, however, is often more complicated than the simplest thermodynamic equations.
Earth’s atmosphere is not uniform. Its composition is constantly changing, and it’s certainly not heating evenly everywhere—some places are even getting colder. Forecasting the likelihood of extreme precipitation events is therefore more challenging than adding numbers to a model. Still, the historic record, especially since anthropogenic warming took off in the 1900s, can provide insight into how Earth’s atmosphere responds to rapid warming.
In a new study, Papalexiou and Montanari use a novel technique to analyze historical data and investigate the likelihood that global warming was driving the frequency and magnitude of extreme precipitation events. The scientists collected their data from the Global Historical Climatology Network–Daily database. This data set includes measurements from approximately 100,000 precipitation stations across the world.
For their analysis, the researchers focused on the 1964–2013 period when global warming accelerated. They looked at how many complete years of data were recorded for a given station; then they chose to analyze that number of extreme precipitation events. So if a station provided 45 complete years of data, they analyzed the top 45 most extreme events. The authors argue that this analysis technique represents extreme rainfall events more accurately than simply looking at a series of annual maximum precipitation numbers because in the absence of some external force (such as rising temperatures) it should result in an even distribution. Stations with fewer than 5 complete years of data in each one of the 5 decades studied were excluded from the analysis, and after screening for a variety of other criteria, the researchers were left with a record from 8,730 stations from around the world, mostly clustering in North America, Europe, Russia, China, and Australia.
The researchers then constructed a time series for both annual frequency and average magnitude of the extreme rain events for each weather station. For the frequency data, the results were especially pronounced, with the occurrence of extreme precipitation events increasing significantly as time went on. In the last decade of data (2004–2013) the scientists found 7% more extreme precipitation events than they’d expect if no external force were skewing the distribution. The data related to magnitude were less pronounced but also indicated a slight uptick. Additionally, the researchers report they found no strong correlation between increasing frequency and increasing magnitude.
Finally, because each weather station is also tied to a geographical location, the researchers were able to analyze where the extra rain was falling, with Eurasia, northern Australia, and the midwestern United States absorbing the bulk of the new moisture.
The study suggests that as the planet continues to warm, extreme rainfall events will continue to become an increasingly common part of life for many heavily populated parts of the world. As land managers and policy makers fight to stay ahead of climate change, this type of data will become ever more informative and necessary. (Water Resources Research, https://doi.org/10.1029/2018WR024067, 2019)
—David Shultz, Freelance Writer