Climate Change Attribution

Climate change attribution is the science that links global climate change to specific weather events. This field of research lets scientists move beyond general statements—like that climate change will lead to more extreme heatwaves—to specific ones: that this heatwave, at this particular time and place, was worsened by climate change.

Usually, this is expressed in terms of probability: for example, that climate change made a wildfire twice as likely to occur. Sometimes, it is expressed in terms of intensity, like a hurricane dropping 10% more rain than we would expect if the world were not warming.

Climate and weather

Weather is highly variable. It’s normal to have some unusually warm, or cold, or rainy days, and occasional weather disasters have always been with us.

Climate, meanwhile, is the set of background conditions, like the makeup of the atmosphere and the geography of the planet, that create patterns of weather we can expect at a particular time and place. If the climate changes slightly—say, warming by 0.5° F on average—it will also have different weather. There will be more 90° F days and fewer 20° F days. But there will be few days, if any, so hot that they could not have happened before.

For many years, this meant that scientists were reluctant to blame any one weather event on climate change.

That changed in 2004, when the first climate change attribution study was published. A deadly heatwave the previous year had killed tens of thousands of people across Europe, and researchers concluded that climate change had made this heatwave at least twice as likely.¹

In large part, it was the steady advance of climate change itself that made this study possible. By 2003, the Earth was around 1° F (0.6° C) warmer than the historical average,² and the summer was, at the time, the hottest ever recorded in Europe. As the weather grew more unusual, it became easier to show a link to the world’s overall warming.

But the study also took new methods, developed to help scientists parse the signal of climate change from the noisy day-to-day turns of the weather.

How climate attribution is done

Climate change attribution relies on two types of evidence.

The first is weather records. In many places, daily weather records have been kept for over a century, letting scientists compare today’s weather patterns with those from a time when humans had barely begun to alter the climate. Other changes, like the growth of roads and the loss of tree cover, have also impacted the weather over this time. But by controlling for these changes, researchers can show by direct observation how local weather patterns change as the planet warms.

The second type of evidence comes from climate models. These computer programs simulate the physics of the Earth’s climate, predicting the range of weather patterns that are possible under different conditions. This means a climate model can be run both with and without the buildup of climate-warming greenhouse gases humans have added to the atmosphere over the past 150 years. This, too, shows the impact of climate change on regional weather.

Because these two types of evidence use different methods and data, they can be used to check each other’s results. When they agree, we can be very confident that climate change has altered the chances of seeing a given weather event, and by how much.

Some kinds of weather are easier to study than others. Climate change attribution is very good at dealing with temperature, because we have rich historical temperature data, and because the physics of temperature are well-represented in climate models. For this reason, many attribution studies look at heatwaves and cold spells.

Rainfall is harder. In most places, the average day has no rain at all, leaving weather records with fewer useful data points that show the amount of rainfall. Rain is also hard for climate models to simulate: it comes from clouds, which are smaller than the typical climate model can “see.”

Nonetheless, climate scientists continue to develop methods to study climate impacts on rainstorms and many other kinds of weather. Attribution studies have now been done on droughts, wildfires, hurricanes and more. This is, sadly, made easier by the continued warming of our planet, which has brought ever more extreme weather, in which the influence of climate change is easier to discern.

A valuable tool

At a high level, we already know the world will have to prepare for more extreme weather. But with climate change attribution, we can also link a particular storm or heatwave to climate change, highlighting that these are conditions that specific places will need to plan for. This is crucial information for city planners, policymakers, disaster responders, insurance companies, and everyone else making plans for our future climate.

Climate change attribution has even found a role in the legal system. In a recent suit in the European Court of Human Rights, Swiss citizens used attribution studies to argue that their government’s failure to take action on climate change had directly contributed to deaths from heatwaves. In April 2024, that court ruled that the Swiss government had failed to protect its people and must live up to its climate targets.

But for many of the scientists who do attribution studies, the highest aim of this science is to help the public appreciate how profoundly our world is changing. Ten years ago, even after the most extreme weather disasters, it was rare to hear the link to climate change discussed in any detail on the news. Today, reporters and weather forecasters can say, with strong evidence, when we are experiencing weather that is new to our world.

September 10, 2024