How do scientists know how climate change affects the weather?

As the climate warms, it increasingly affects our everyday weather. But the impact is not uniform or consistent across the world. In the last decade, enough data, understanding and computing power have developed to allow the scientific community to determine how much the warming climate has affected the weather on a particular day, or over a few days.

Scientists at the  World Weather Attribution team pioneered much of this research. Co-founded in 2015 by Dr. Geert Jan van Oldenborgh and Dr. Friederike Otto, they work with scientists across the world to review high-impact events such as heat waves, heavy rain, fires and droughts to determine how much climate change affected a specific event. Van Oldenborgh died in 2021, and Otto — now based at Imperial College London — has increasingly become the face of the program.

Using a combination of data from a specific weather event, historical climatology and mathematical simulations of the climate, both before and after the climate began to warm, the team can often determine the influence of the warming climate.

After a significant event, the team examines historical records to see if a similar event has happened in the past. Then, multiple simulations of the climate are examined, representing the atmosphere both with and without the observed warming of the last 50 years.

Sometimes, there is not much signal. But more often, the signal is strong. Most recently in North America, their analysis indicated that the warming climate more than doubled the likelihood of extreme fire weather conditions in eastern Canada in 2023. This indicates that the fires were able to spread more rapidly and widely compared to a couple of generations ago, regardless of how they got started.

The WWA continues to work to understand the role of climate change in areas outside of North America and Europe, where effects are worse and their impact lingers far longer. A study last month indicated that the recent meteorological drought in the Amazon River Basin is 10 times more likely now in our warmer climate. Worse, the agricultural drought there is now 30 times more likely. These analyses indicate that climate change may not be the root cause of an extreme event, but it does make it far more likely, like loading a pair of six-sided dice toward higher rolls.

On a more regular basis, a similar idea can be applied to daily high and low temperatures. Using all of the historical weather data at a site, scientists can determine the frequency of a particular temperature happening during a specific time of the year. For example, a high temperature of 85 degrees in St. Louis or Washington is common in July. Highs of 65 or 105 are possible, but each is rare.

But as the climate has warmed, that collection of temperatures is skewing warmer, meaning 105 degrees is becoming more likely and 65 degrees is becoming less likely.

With all of that historical data, scientists can now determine how much more likely a particular temperature is today versus the climate of the 20th century. This is the foundation of the Climate Shift Index developed by the scientific team at Climate Central.

In more recent examples, temperatures on Groundhog Day were above normal from Kansas City to Minnesota. In this particular case, the CSI indicated that those temperatures were 1.5 to 2 times more likely than during the climate of the 20th century.

And this winter continues the trend. There have been some colder spells, but no part of the United States is having a winter that is colder than average. Most locations from Minnesota to Maine, south to Missouri and Virginia, are having one of their 10 warmest winters on record.

The scientists will certainly stay busy in the years and decades to come.

Sean Sublette is the chief meteorologist for the Richmond Times-Dispatch in Virginia.