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More Hot Days Are Coming With Climate Change. Our Choices Will Decide How Many

Research Report by Climate Central

Summer still has a month to go, but extreme heat has been a major storyline through June and July. Sweltering temperatures have grounded planes, sparked wildfires and set records from coast-to-coast.

These stories are becoming annual rites of passage as the world warms. And the number of hot days is projected to increase in the coming decades.

Climate Central has developed a new web-interactive tool that brings the reality of future heat to hometowns across the U.S.  Simply enter the name of your city, town or hamlet — or any place in the Lower 48 that piques your curiosity — to see how the number of days above summer temperature thresholds will change throughout the rest of the century. The interactive also shows how reducing greenhouse gas emissions can help reduce the heat.

For example, Phoenix has averaged less than one day above 115°F a year over the past 20 years. If the rate of global greenhouse gas emissions continues on its current trajectory, Phoenix may see as many as 60 days above 115°F each year by the end of the century (and a staggering 163 days above 100°F). Moderate emissions cuts bring the number down to about 40.

Or consider Yakima, Wash. Today, Yakima sees no days above 105°F on average. But if emissions continue on their current trajectory, Yakima is projected to experience 24 days — more than three weeks — above 105°F each year, on average, by 2100.

Climate Central’s analysis includes nearly 30,000 cities and towns, from New York City to Lost Springs, Wyo. (population 4). That means you can see what climate change means for summer temperatures anywhere in the Lower 48.

More extreme heat will have serious consequences on society and the infrastructure upon which we rely. In addition to direct health impacts like heat exhaustion and heat stroke, high temperatures exacerbate other conditions like asthma and cardiovascular disease. These health impacts are particularly dangerous to the very old and very young and other vulnerable portions of the population.

Prolonged exposure to extreme heat can also affect transportation and infrastructure. Highways and railroads can buckle, and the lower air density that comes with extreme heat means aircraft must work harder to take off (as we’ve seen this summer). Air conditioning demand also increases during heat waves, stressing the electric power grid and raising cooling costs. That can increase greenhouse gas emissions from power generation, disrupting the climate further. Air conditioners also release powerful greenhouse gases known as chlorofluorocarbons, turning up the heat even more.


Future days above 95°F, 100°F, 105°F, 110°F and 115°F in these cities

About Emissions Scenarios

The scenarios are representative of different levels of greenhouse gas emissions and warming and drive the model projections of future temperatures we used in this analysis. They were adopted by the International Panel on Climate Change as part of its fifth assessment report in 2014.

“Continue without emissions cuts”: This scenario assumes that few major changes are made in the amount of greenhouse gases we release, a scenario sometimes referred to as “business as usual.” This corresponds to a future greenhouse gas scenario called RCP 8.5, which has generally followed emissions over the past 10 years. Under RCP 8.5, global temperatures are projected to increase an average of 5.9°F above the 1985-2005 baseline by 2100.

“Moderate emissions cuts”: This corresponds to RCP 4.5, a scenario where annual emissions peak in 2040 and then decrease, stabilizing at roughly half of current levels. This reduction roughly corresponds to what would be needed to achieve the goal enshrined in the 2015 Paris Agreement of limiting average global warming to 2°C (3.6°F). Specifically, temperatures under RCP4.5 are projected to top out at 4.1°F above the 1985-2005 baseline by 2100. .

“Extreme emissions cuts”: This is a dramatic level of emissions cuts, which in 2017 is probably beyond reach realistically but is still technically possible. This option corresponds to RCP 2.6. Under this scenario, annual emissions peak in 2020, decline sharply to reach zero around 2070, and then would require sustained net negative emissions after that. Negative emissions would require engineered active removal of carbon from the atmosphere at a massive scale, a process that’s likely to be extremely difficult and expensive. Such extreme cuts would provide a good chance of limiting global warming to 2°F compared to the 1985-2005 baseline, well within the bounds set by the Paris Agreement.

Methodology

Data bars represent the average number of days in a year with high temperatures above the selected temperature threshold.

Data for each year in the interactive were calculated as the mean of the preceding 20-year period.

Average maximum daily temperature (Tmax) for 2016 is gridded historical data from Daymet.

Tmax for 2050, 2075 and 2100 is the median of the temperature output from a suite of 21 global climate models from the Coupled Model Intercomparison Project phase 5. These models are bias corrected and spatiotemporally downscaled to provide daily temperature projections at 1/8-degree geographical resolution.

Additional bias correction was done using a “delta method” where we applied the temperature change between the modeled temperature for 2016 and each future period to the gridded-historical Daymet data to arrive at projected Tmax values for 2050, 2075 and 2100.

Climate Central's James Bronzan contributed data analysis for this story.