Document Type

Working Paper

Publication Date



Environmental Law | Law


Sabin Center for Climate Change Law


Globally, the ten warmest years on record have all been since 1998, with the four warmest years occurring since 2014. In the contiguous United States, average annual temperatures are about 1.8°F higher than they were over the period from 1895-2016. This is expected to increase by about 2.5°F before mid-century, regardless of what happens to greenhouse gas levels. If, at the end of this century, greenhouse gas emissions are at the Intergovernmental Panel on Climate Change’s high scenario (termed “RCP 8.5”), average U.S. temperatures could go up by as much as 11.9°F by 2100, with emissions at a middle scenario (RCP 4.5), and temperatures perhaps as little as 2.8°F. This shows that greenhouse gas levels will not make much of a difference to what we experience over the next few decades, but will make a huge difference toward the end of the century. Even if the world meets the goal set at the Paris Climate Conference in 2015 of keeping rises well under 2°C (3.6°F) above pre-industrial levels, which seems increasingly unlikely, the average summer high temperature in the United States is expected to rise from a historical average of 74°F to an average of 81°F by 2100; with high emissions, that could be 91°F.

All the above numbers are just about averages, but it is the extremes in time and place that have the greatest impacts. Extreme temperatures in the United States are projected to increase even more than average temperatures. For every 1°C (1.8°F) that the temperature goes up, there may be around twenty-one more heatwave days in the western United States and twenty-six in the eastern states. At high emissions levels, heatwaves that now occur once every twenty years could become annual events, and the average number of extremely hot days could more than triple by the end of the century.

The worst heatwave in modern history occurred in Russia in 2010; its rare combination of extreme temperatures and long duration killed an estimated 55,000 people. Under an RCP 8.5 scenario, comparable heat waves could occur every two years in the eastern United States by the end of the century, and by then in Europe, the legendary heat wave of 2003 “would be classed as an anomalously cold summer relative to the new climate.” These increased temperatures and heat waves are not just occurring randomly. The science is clear that human activities – mostly greenhouse gas emissions and deforestation – are the major cause.

This article discusses the impacts of rising heat; the methods available to cope with heat (principally air conditioning and the reduction of the urban heat island effect through cool roofs and enhanced vegetation); the existing regulations that deal with heat; and legal tools that can be adopted to enhance resilience to heat.