How do climate scientists study causes of climate change?
BY BEN SANTER, PH.D., OPINION CONTRIBUTOR — 07/25/21 12:30 PM EDT
Much has been written about extreme events in the strange summer of 2021. There’s been much to write about. I won’t repeat the lists of , drought, and . Lists of extreme events in North America, Europe and Asia. Lists documenting human suffering and death. Property destruction. . Instead, I’d like to tell you about causation. How do climate scientists study the causes of climate change? And how do they investigate whether human-caused planetary warming is affecting extreme events?
I’ve done such attribution science for over 30 years. Back in 1995, I took part in an assessment of climate science conducted by the Intergovernmental Panel on Climate Change — the IPCC. I was in charge of the chapter that dealt with attribution science. After several years of work, our chapter concluded that “the balance of evidence suggests a discernible human influence on global climate..”
In retrospect, this was a cautious, even wimpy statement. Nevertheless, it marked a turning point. The international scientific community, in a major assessment report, had for the first time claimed detection of a human-caused global warming signal. Signal detection was not some future hypothetical. It had happened in our lifetimes. After 1995, humans could no longer plead ignorance of the climate disruption they were causing.
Attribution science was still a young science in 1995. It focused mostly on the 20th-century warming of Earth’s land and ocean surfaces. What caused this warming? To answer this question, attribution scientists looked at patterns of climate change. Geographical patterns. Slices through the atmosphere and ocean. Seasonal patterns. Patterns were powerful; different influences on climate had different “fingerprints.”
Computer models helped to reveal these differences. In model world, you could do the thought experiment we can’t do in the real world — you could change all the major human and natural influences on climate, one by one, to isolate the fingerprint of each influence. This helped scientists to separate natural changes in volcanic activity and the sun’s energy output from the fingerprints of human-caused fossil fuel burning, stratospheric ozone depletion, and deforestation.
Attribution science was not just about fancy pattern recognition techniques or complex computer models of the climate system. Attribution relied on evidence from studying climate fluctuations over “deep time,” before humans began industrial scale burning of fossil fuels. Attribution was informed by lab research revealing how greenhouse gases trap heat. Venusian and Martian atmospheres were another kind of laboratory, useful for studying the climates of planets with greenhouse gas levels very different from those of Earth. And observations of our home planet — from space, in the oceans, and on land — on global-scale climate change that scientists needed for attribution studies.
All these independent lines of evidence — from basic theory, “deep time,” planetary atmospheres, computer models, observations and pattern recognition techniques — pointed toward in recent climate change. The consistency of different lines of evidence was scientifically compelling. But at a personal level, the evidence was also deeply concerning. You know your species is changing the climate in a way that’s likely to impact present and future inhabitants. That disquieting knowledge is always there, a constant unwelcome companion.
The 1995 IPCC report didn’t devote much attention to figuring out whether human activities contributed to changes in extreme events. There were few relevant studies to assess. Things changed dramatically after the European summer heatwave in 2003. This catastrophic event led to — to tens of thousands of additional deaths that would not have happened without the heatwave. For many climate scientists, “climate change and extreme events” suddenly transitioned from the category of “academically interesting” to “critically important for us to understand.”
The 2003 European summer heatwave jump-started a field now called “event attribution.” Scientists in the U.K. . They asked a simple question: Had human-caused warming increased the likelihood of a heatwave of the size and scale of the 2003 event? They found that it had. Global warming increased the risk of an event like the 2003 European heatwave by at least . This type of calculation involved comparing two different worlds — a world with human-caused global warming, and a world without it. Climate modelers routinely perform such pairs of calculations. In each simulated world, one can determine the risk of some bad outcome, and then compare how the addition of human influence . A similar type of “risk ratio” is often used in evaluating the effectiveness of a particular drug or medical treatment.