Boulder scientists contribute to new UN climate report

Aug. 10—Human-caused climate change is resulting in immediate, harmful impacts on our world and requires rapid, large-scale reductions in greenhouse gas emissions to curb global warming in the near future, according to the first installment of a new United Nations scientific report published Monday.

The Intergovernmental Panel on Climate Change's Sixth Assessment Report, which included contributions by Boulder-based National Center for Atmospheric Research scientists, finds that global warming will exceed 1.5 to 2 degrees Celsius in the 21st century without deep cuts to carbon dioxide and other greenhouse gas emissions.

The Working Group 1 report addresses the physical science basis of climate change. Two future reports will address impacts, adaptation, vulnerability and mitigation.

Senior NCAR scientist Linda Mearns was a lead author of the Atlas chapter, which finds that North and Central America are projected to experience climate changes across all regions, with average and extreme temperature changes slated to be higher than the global average and to continue increasing.

Sea levels are projected to keep rising along most North American coasts and are associated with increased flooding and erosion, with some exceptions in the southern coast of Alaska and Hudson Bay.

The acidification of the ocean and marine heatwaves will likely increase, and severe weather like tropical cyclones and dust storms on the continent could become more extreme.

Loss of glaciers, permafrost and snow cover will most likely continue, while snow in the northern Arctic could get heavier.

It's likely that central and western North America will continue to see an increase in drought with further global warming. Western North America will also likely see an increase in fire weather, while extreme precipitation and flooding could also increase for both regions, according to the report.

"The problem has become more serious and there will eventually be nowhere to run and nowhere to hide," Mearns said. "You're not going to be able to get away from it, because it is everywhere."

Mearns, who has been involved with IPCC assessments since the '90s, said the sixth report represents a sea change in how IPCC scientists are communicating with each other and the public.

For example, previous reports have not included a designated Atlas chapter, instead weaving regional information throughout the entire report.

"One of the great advances this time around is the focus on regional information, which is what people really want and need to focus on how climate change is going to affect them and how they can react to it," Mearns said.

The new report is accompanied by an online, interactive map that allows users to see how different warming scenarios impact sea level rise, temperature extremes, precipitation extremes and more in their region.

The report shows that not only are more extreme climate events like hurricanes, flooding and wildfires occurring, Mearns said, but they're occurring at the same time and over larger and larger areas.

NCAR project scientist Angeline Pendergrass, an assistant professor at Cornell University, is a contributing author to the Water Cycle chapter.

The report finds that changes to the climate system increase in direct relation to increasing global warming, including increasing heat extremes, marine heatwaves, heavy precipitation, drought, tropical cyclones and reductions in Arctic sea ice, snow cover and permafrost.

Heavy precipitation will very likely intensify in most regions with further global warming, including daily precipitation and the proportion and wind speeds of intense tropical cyclones.

But Pendergrass said she hopes that people don't walk away from the IPCC report feeling hopeless. There are things people can do to create positive change, and less climate impact and fewer emissions means less warming.

Pendergrass also cautioned against climate fatigue.

"When we're trying to understand what's going on around us and how much of it is global warming, a lot of it is just weather and variations that would have happened anyway," she said. "We have to know some of what's happening is weather and some of what's happening is climate change."

When people experience climate fatigue, Pendergrass said, there's a concern they could become complacent.

"I worry that if we have one year with a lot of fires in the West and a lot of smoke and then the next year's there's less — because every year is not going to be worse than the last year, it's not a monotonic change like that — that people might get fatigued and let their guard down," she said.

Climate change is a slow process and it isn't happening in a straight line, Pendergrass said.

Speaking outside of her involvement with the IPCC report, Pendergrass said while there are some individual actions people can take to mitigate climate change, it is largely a societal problem.

"The main thing you can do is vote and participate in democracy and have influence on how we collectively do things together," she said. "This is a problem of infrastructure and how we get our food and how we heat our homes and those are things we have to address together. That's why we have a democracy, so we can decide these things together."

NCAR senior scientist Bette Otto-Bliesner is a contributing author to the report's technical summary and is an expert in paleoclimatology, or the study of Earth's climate history.

"The geological record provides natural experiments that challenge our understanding and modeling of what could happen on our planet this century and in future centuries," Otto-Bliesner said.

The report found that carbon dioxide in the atmosphere was higher in 2019 than it had been at any time in the last 2 million years, and concentrations of methane and nitrous oxide were higher than at any time in the last 800,000 years.

Looking at the past can help scientists predict what will happen in the future, Otto-Bliesner said.

"It's happening now and it's happening everywhere, and we need to consider how we can slow the change or stop the change," she said. "The earth isn't going to be the same as it was in the 20th century. It's obvious now that changes need to be made and we all need to be a part of that change."