By Eric Roston | Bloomberg

The largest wildfires in New Mexico’s history belong to the same troubling increase in conflagrations, exacerbated by drought and rising temperatures, that have burned vast areas of the western United States in recent years.

William Anderegg, an associate professor at the University of Utah who studies forests and climate, has spent years tracking and projecting these changes. In research published last month in the journal Ecology Letters, he and colleagues assembled the history of three climate threats — fire, drought and insects — across 112 types of forest and outlined three scenarios for how they may rise through the century.

Their conclusion? The Southwest saw 0.19% of its forested area burned in 2018, in a historical scenario that estimates previous years’ risk. In their three future scenarios for 2099, that figure rises to 0.9%, 1.8% and 2.9%. The differences among those potential outcomes are enormous, underscoring the influence that humanity’s decisions to act or not act will have on global heating, Anderegg said.

This transcript has been edited for length and clarity.

What’s your reaction when reading about an event like the fires in New Mexico?

It’s eerily consistent with what our models project. New Mexico and other parts of the Southwest, into California, are some of the most vulnerable places in the US to wildfire, and just, year after year, we keep seeing these record-breaking fires, like what’s happening in New Mexico right now.

Have fires ever interrupted your research?

In 2018, we had fires in southwest Colorado that got within half a mile of burning some of our long-term research plots. It was wild, and it was scary. There were times when we had to just evacuate. At a certain point, the US Forest Service closed the national forest because they were worried that they couldn’t evacuate people in time. It had pretty large impacts on the research, unfortunately, because that was the peak of the drought stress, when we really wanted to be measuring the trees to see if the drought was going to kill them. We were able to get back after the fires quieted down later in the summer.

The study has two parts, a look back at what’s happened and a look ahead at what might, through “scenarios.” What does that mean?

These are different paths that the future can take. They’re entirely determined by human decisions and policy for greenhouse gases and technology. We looked at three of these different scenarios: a very high climate-change scenario, a high scenario, and then a moderate-to-low scenario. That’s SSP5-8.5, 3-7.0 and 2-4.5 in technical terms.

Why do scientists stick with technical names? Why not just use small, medium and large? Or maybe tall, grande and venti?

It’s partially because so many assumptions and decisions go into each one. It’s everything from population to economic development to technology transfer, to energy, the carbon emissions from a given society and land use as well. It’s a really complicated area. I try to talk about high climate change, moderate-high, moderate-low. We didn’t end up looking at any very low climate-change scenarios, because there’s not a lot of evidence that we’re anywhere near those.

Especially looking out towards the last 50 years of the 21st century, the differences between the highest and lowest scenario that we examined were just massive for forests. You could see something like a 10-fold increase in burned area in our models in the highest scenario, and maybe it’s only a two-to-three-fold increase in the lowest scenario. That’s a huge difference in the risks a forest may face.

Which path are we on?

This is actually an area of quite a bit of scientific debate right now. For a lot of the 2010s it looked quite a bit like we’re on the very highest scenario. Recent evidence suggests maybe we’ve started to come off that, and are even going somewhat below the high scenario — but not yet necessarily on track for low scenarios. It’s a little hard to see at this point. It seems like we may have moved off the highest scenario for now.

What are the main takeaways of the research?

What humanity decides to do about climate change is just fundamental. It’s first-order to the future of Earth’s forests and the future of US forests.

Also, there are a huge number of government, corporate and NGO actions, focused on net-zero commitments that are trying to leverage forests and their ability to store carbon. Forest offsets are one of the big mechanisms here. And this really highlights that it may be a fraught and dicey assumption. Forest offsets that you invest in today may not necessarily be around in 20 or 40 or 80 years. That’s a huge problem. In order for them to work for the climate, they have to store carbon over the long term — we tend to think at least a century or more.

How did you end up doing this? 


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