How hot will the United States get because of global warming?

Credit: NASA

Days after an absolutely sweltering heat wave, much of the U.S. experienced lovely summer weather in late July. Mercury-busting heat alternated with mild temperatures a few days later, and it felt like global warming was staved off for another season. So do we really have to worry about climate change, or not?

Simply put, weather varies. And regional variability is a major hurdle for scientists studying climate change. Nevertheless, all signs point toward a changing, warmer world as a direct result of human activities. The Earth will continue to warm even if humans curb their fossil fuel emissions, with major temperature increases in the Arctic, U.S. Northeast and other regions. Just this week, scientists at Stanford said the rate of climate change during the next 90-odd years will be at least 10 times faster than any climate shift since the demise of the dinosaurs.

So, how hot could it really get? Will we have Death-Valley-like 129-degree days in the Northeast in 100 years? This depends on so many factors that scientists can only make educated guesses. And variability is always an issue. But depending on where you are, the answer is that globally, it will be hotter — maybe just a little, and maybe a whole lot.

Average temperatures in the U.S. have risen about 1.5 degrees Fahrenheit since 1895, and more than 80 percent of this rise has happened since 1980. The decade of the 2000s was the warmest on record, according to the National Climate Assessment Development Advisory Committee. Just how warm it will get depends on how many fossil fuels we keep burning, and how much carbon dioxide this emits into the atmosphere.

To calculate this, climate scientists mix base numbers and laws of nature in the form of thermodynamic calculations. Computer models simulate future warming by changing inputs (like emissions) and seeing what happens. For example, climate models use a base period from 1951 to 1980 to calculate Earth’s average surface temperature across all land areas at 57 degrees Fahrenheit. Based on a range of emission estimates, those average temperatures will rise during the next century by 2 to 6 degrees Celsius, or 3.6 degrees to 10.8 degrees F. That’s averaged across the globe, so it could be much hotter in some spots, like the Arctic, or cooler in others, like the Pacific Northwest. Either way, there’s a big difference between the sweater-weather average of 57 degrees and the shorts-weather of 67.

 

In the U.S., most areas are expected to see about a 2 to 4-degree bump in the next few decades. All these numbers come from the National Climate Assessment Development Advisory Committee’s draft report, released in January as part of the country’s role in the forthcoming report from the Intergovernmental Panel on Climate Change. According to one study that examined clouds and humidity, climate models that predict a greater rise in temperature are probably more accurate.

By 2100, warming in the U.S. will be much like warming everywhere else, the NCADAC said. Based on a scenario where greenhouse gas emissions are reduced after 2050, the country will be warmer by an average of 4.5 degrees F by the end of this century. If emissions keep going up after 2050, and the concentration of carbon dioxide in the atmosphere is double what it is today, the continental U.S. will be 8 degrees warmer on average. Here’s a video that shows it in stark detail:

 

But climate models are not perfect: they’re all based on slightly different guesses, and numerical versions of the laws of nature. And they can’t account for random chance. As we’ve seen in the past few weeks, weather is variable. In a paper published last year, Clara Deser and colleagues from the National Center for Atmospheric Research ran some computer models to show the importance of this natural variability. They found that even with improved computer models, and even with better projections for greenhouse gas emissions, predicting future climate change is still an uncertain business.

Here’s an example from the study: During several model runs, called ensembles, winter and summer showed major warming in all locations. The biggest shifts happened in northern Canada and Alaska. But slight variations in one factor caused dramatically different outcomes, Deser and colleagues wrote. The model run with the most warming over the U.S. in winter showed an increase of 4 to 6 degrees Celsius over the northeast, and the smallest increase, of just 1 to 3 degrees C, over western Canada and Alaska. Then the team looked at a different model run, this one with the lowest warming during winter. Temperature changes across the contiguous U.S. were less than one degree, and some areas in the Pacific Northwest even experienced cooling. “These strikingly different climate outcomes occur despite both model runs being subjected to identical external forcings,” the authors wrote.

It shows that the role of natural variability is crucial to future climate changes, explained NCAR climate scientist Kevin Trenberth, who shared this paper with a group of journalism fellows (myself included) last month. "This is something that has probably not been adequately appreciated in the whole of climate change," he said. Variability — like the 30-degree swing from 98 degrees (with a 107 degree heat index) in New York July 19, to a comparatively brisk 68 on July 25 — definitely matters. But on the whole, the answer is simple: It will be hotter in the future.

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