Carbon budget is close enough to exhaustion that how the remainder is spent really matters

The exact size of the carbon budget – the total CO2 emissions allowed in order to stay under a given global warming level – has proved harder to pin down. Over recent years, studies have each used slightly different approaches to come up with their estimates.In a guest article this week, scientists Dr Joeri Rogelj and Prof Piers Forster – from Imperial College London and the University of Leeds, respectively – unpacked their new Nature paper that aims to reconcile these estimates. They lay out five main factors that explain the differences between budget estimates, such as the assessed amount of warming to date and assumed warming from non-CO2 gases. Following their own framework, they estimate “a remaining carbon budget from 2018 onwards of 580bn tonnes of CO2 for a 50% chance of keeping warming below 1.5C”. This is less than 15 years of global emissions at current rates, they note. Rogelj and Forster also provide an “overview of the various ways in which carbon budgets can be presented in scientific literature” and a “checklist of key information that future studies should provide” so that new estimates can be put into context alongside existing ones.

The latest data from the US National Oceanic and Atmospheric Administration (NOAA) shows that June 2019 was the hottest June in their 140-year global record, reports Reuters. It continues: “The report said the average global temperature in June was 1.71F [0.95C] above the 20th-century average of 59.9F (15.5C) and marks the 414th consecutive month in which temperatures were above the 20th-century average.” “Nine of the 10 hottest Junes on its 1880-2019 record have occurred in the past nine years”, says BBC News. Forty-one countries may have set their record warmest June, notes USA Today, including Bangladesh, Hungary, Iraq, Nicaragua, the Philippines and Turkey. Meanwhile, the Guardian reports on a new study that warns the number of extremely hot days around the US could more than double this century. “By mid-century, an average of 36 days a year could feel like 100F (37.7C) or hotter. Toward the end of the century, 54 days a year could feel that hot,” it says.

Detection of a climate change signal in extreme heat, heat stress and cold in Europe from observations (Geophysical Research Letters) In the last two decades Europe experienced a series of extreme heat events. This study examined trends in temperature extremes in Europe, finding that on average across Europe the number of days with extreme heat and heat stress has more than tripled and hot extremes have warmed by 2.3C from 1950‐2018. Days with extreme cold temperatures have decreased by a factor of 2‐3 and warmed by more than 3C. Cold and hot extremes have warmed at about 94% of stations, a climate change signal that cannot be explained by internal variability.

To understand how climate change increases the frequency of heat waves, it helps to think of the Earth’s temperature as a bell curve said Michael Mann, the director of the Penn State Earth System Science Center.

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Climate change is shifting that bell curve toward the hotter part of the temperature scale. Even a tiny shift in the center means that more of the curve touches the extreme part of the temperature scale.

“So you know, a warming of 1 degree Celsius, which is what we’ve seen thus far, can lead to a 10-fold increase in the frequency of 100 degree days in New York City for example,” said Dr. Mann. According to the U.S. Global Change Research Program, since the 1960s the average number of heat waves — defined as two or more consecutive days where daily lows exceeded historical July and August temperatures — in 50 major American cities has tripled.How Much Hotter Is Your Hometown Than When You Were Born?See how days at or above 90 degrees Fahrenheit have changed in your lifetime and how much hotter it could get.Aug. 30, 2018

The program used historic lows because the most serious impacts of extreme heat tend to come when nighttime temperatures don’t cool off. By the 2010s, the average number of heat waves had risen from an average of two per year in the 1960s to the current average of nearly six per year.A number of people collapsed from heat at a May rally for President Trump in Pennsylvania. CreditTom Williams/CQ Roll Call, via Getty Images

A number of people collapsed from heat at a May rally for President Trump in Pennsylvania. 
A number of people collapsed from heat at a May rally for President Trump in Pennsylvania. CreditTom Williams/CQ Roll Call, via Getty Images

There’s another way that climate change worsens heat waves: by changing the jet stream. Those air currents in the atmosphere help move weather systems around and are driven by temperature differences, which are shrinking. So when heat waves arrive, they stay in place longer.

“We’re warming up the Arctic faster than the rest of the northern hemisphere,” said Dr. Mann. “So that’s decreasing that temperature contrast from the subtropics to the pole, and it’s that temperature contrast that drives the jet stream in the first place.”

At the same time, under certain circumstances the jet stream can get “stuck” between an atmospheric wall in the subtropics, and at the Arctic, trapping weather systems in place.

“That’s when you get these record breaking weather events,” said Dr. Mann, “either the unprecedented heat wave and drought, to wildfires and floods.”

This accounts for last summer’s European heat wave, as well as the recent European heat wave, he says, and is behind the current North American heat wave.

Nationwide, the time period in which heat waves might be expected to occur is 45 days longer than it was in the 1960s, according to the U.S. Global Change Research Program.

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Heat-related causes had killed 116 people in Chicago by July 16, 1995.  
Heat-related causes had killed 116 people in Chicago by July 16, 1995.  CreditBrian Bahr/Agence France-Presse — Getty Images

According to the Centers for Disease Control and Prevention, which uses methods most in accordance with global standards, currently, cold weather kills more people than hot weather does.

But as global temperatures increase, the number of deaths associated with extreme cold are predicted to decrease. At the same time, the number of deaths associated with extreme heat will increase. And those deaths, according to the National Climate Assessment, will exceed the decline in deaths from extreme cold, meaning an overall increase in mortality.

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It’s important to note that not everyone suffers equally when temperatures soar. In addition to the vulnerable groups, like elderly people, it also matters where you live. Researchers at the University of California, Berkeley, analyzed data from the 2000 census and found that people of color were up to 52 percent more likely to live in the hottest parts of cities.

Similarly, Eric Klinenberg, the director of the Institute for Public Knowledge at New York University, found that during the 1995 Chicago heat wave that killed more than 700 people, the death tolls were highest in places that were not just poor and segregated, but what he calls “institutionally depleted.”More on Extreme HeatRed Cross to World’s Cities: Here’s How to Prevent Heat Wave DeathsJuly 16, 2019A Heat Wave Tests Europe’s Defenses. Expect More.July 1, 2019

“In a heat wave and many climate events, it’s social isolation that proves to be truly dangerous,” he said. “If you’re home and alone in a heat wave when you’re old and frail you’re more likely to die if you don’t have air conditioning.”

The solution is reining in greenhouse gas emissions, said Dr. Mann.

If we don’t, he said, “think about the most extreme summer heat you’ve ever experienced in your lifetime. That will become a typical summer day by the middle of this century, if we continue on the path that we’re on.”

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Kendra Pierre-Louis is a reporter on the climate team. Before joining The Times in 2017, she covered science and the environment for Popular Science. @kendrawritesA version of this article appears in print on July 18, 2019, Section A, Page 18 of the New York edition with the headline: Climate Change Bolsters The Threat of Heat Waves. Order Reprints | Today’s Paper | Subscribe