How do climatologists study the causes of climate change?

Much has been written about the extreme events of the weird summer of 2021. There has been a lot to write about. I will not repeat the record lists of heat, drought, floods and wildfires. Lists of extreme events in North America, Europe and Asia. Lists documenting human suffering and death. Destruction of property. The devastation of the ecosystem.

Instead, I’d like to talk to you about causation. How do climatologists study the causes of climate change? And how do they study whether human-caused global warming affects extreme events?

I have been doing such attribution science for over 30 years. In 1995, I participated in a climate science assessment conducted by the Intergovernmental Panel on Climate Change – the IPCC. I was in charge of the chapter which dealt with the science of attribution. After several years of work, our chapter concluded that “the balance of evidence suggests a discernible human influence on the global climate”.

In retrospect, it was a cautious, even tearful statement. Nevertheless, it marked a turning point. The international scientific community, in an important assessment report, had for the first time claimed the detection of a signal of global warming of human origin. Signal detection was not a future hypothesis. It had happened in our lifetime. After 1995, humans could no longer plead ignorance of the climate disruption they caused.

The science of attribution was still a young science in 1995. It focused mainly on the warming of land and ocean surfaces in the 20th century. What caused this warming?

To answer this question, attribution scientists looked at models of climate change. Geographic models. Slices through the atmosphere and the ocean. Seasonal reasons. The models were powerful; different influences on the climate had different “fingerprints”.

Computer models have helped reveal these differences. In the model world, you could experience thinking that we cannot do in the real world – you could change all the major human and natural influences on the climate, one by one, to isolate the imprint of each influence. . This model information has helped scientists separate the natural changes in volcanic activity and the sun’s energy production from the fingerprints of human-made fossil fuel combustion, stratospheric ozone depletion. and deforestation.

The science of attribution was not just about sophisticated pattern recognition techniques or complex computer models of the climate system. The attribution was based on evidence from studying climate fluctuations over a “deep time” before humans began to burn fossil fuels on an industrial scale. The attribution was informed by lab research revealing how greenhouse gases trap heat. The Venusian and Martian atmospheres were another type of laboratory, useful for studying the climates of planets with very different levels of greenhouse gases than on Earth. And observations of our home planet – from space, in the oceans, and on land – provided the hard data on global climate change that scientists needed for attribution studies.

All of these independent sources of evidence – from basic theory, “deep time”, planetary atmospheres, computer models, observations, and pattern recognition techniques – pointed to a dominant human role in recent climate change.

The consistency of the different data sources was scientifically convincing. But on a personal level, the evidence was also deeply concerning. You know your species is changing the climate in ways that can impact current and future inhabitants. This disturbing knowledge is still there, a constant and unwelcome companion.

The 1995 IPCC report did not devote much attention to determining 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 resulted in significant excess mortality – tens of thousands of additional deaths that would not have occurred without the heatwave. For many climatologists, “climate change and extreme events” suddenly moved from “academically interesting” to “extremely important for us to understand”.

The European heatwave of summer 2003 launched an area now called “event attribution”. British scientists were pioneers in this field. They asked a simple question: Did human-caused warming increase the likelihood of a heat wave the size and magnitude of the 2003 event?

They found it to be. Global warming has increased the risk of an event like the 2003 European heatwave by at least a factor of two.

This type of calculation involved comparing two different worlds – a world with man-made global warming and a world without. Climate modelers regularly perform such pairs of calculations. In each simulated world, one can determine the risk of a bad outcome, and then compare how the addition of human influence changed the risk. A similar type of “risk report” is often used to assess the effectiveness of a particular drug or medical treatment.

Today, event attribution has dozens of practitioners around the world. Increasingly, these “climate CSI” teams are using a sophisticated array of computer modeling, statistical approaches and observations to calculate changes in risk after a specific extreme event.

A good example is the assessment of the human contribution to the extraordinary record heat wave that hit much of the Pacific Northwest in July 2021. Based on the analysis of temperature observations, it is estimated that it was a unique event in a millennium. Analysis of many different model simulations showed that the Pacific Northwest heat wave was virtually impossible without man-made warming.

At the frontier of the science of event attribution, the focus is now more on trying to understand the specific weather situation that accompanied an extreme event.

Is the significant warming over the Arctic systematically changing atmospheric circulation patterns, the location and ripple of the jet stream, and the likelihood of unusual heat domes? While the jury is still out on these more nuanced questions, they are extremely important questions that need to be answered – and answered quickly. But it’s pretty clear that the man-made warming of Earth’s climate is already affecting our weather patterns.

What are the prospects for all of us? No more extreme extremes. The millennial heat wave is becoming something that we experience many times in our lives. More serious impacts of extreme events on health, life, all aspects of our economy and global ecosystems. Daily demonstration of human vulnerability to a rapidly changing climate.

But it doesn’t have to be.

Faced with a global pandemic, scientists have worked at unprecedented speed to develop multiple vaccines. The vaccines are remarkably effective. They save lives. The vast majority of those who die now from COVID-19 are not vaccinated. In the United States, where Moderna, Pfizer, and Johnson & Johnson vaccines are widely available, vaccination is now a personal choice of life or death.

We face equally serious choices in tackling man-made climate change. There are several clean energy “vaccines”. They work. They reduce our dependence on fossil fuels. They reduce greenhouse gas emissions. We can choose to deploy low carbon power generation systems quickly and at scale. Or we can choose to ignore climate science, follow a “business as usual” strategy of using fossil fuels – and remain unvaccinated against the ever-increasing risks of severe climate disruption. It’s a choice.

One lesson learned from COVID-19 is that getting a vaccine isn’t helpful when you’re already in intensive care, intubated because you’re having trouble breathing. It also won’t help if real action on climate change is delayed for decades – as we all struggle to breathe air that is not tainted by smoke from wildfires.

Ben Santer, Ph.D., is thatlearned and John D. and Catherine T. MacArthur Fellow. HI was also the main author of VSChapter 8 of the 1995 IPCC Report and was a to contributeor the six IPCC reports.

About Roberto Frank

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