From the Big Apple to the Big Coconut?Climate Central’s analysis found that New York City will see a three-fold increase in the number of 90-degree-plus days in August by midcentury.Photo illustration by Tom Twigg / Grist

By some measures, the Chicago and New York of tomorrow are likely to be hotter than the Atlanta of today — at least in August.

Climate Central’s analysis of projected midcentury August temperatures for 21 major American cities, under a fairly conservative warming scenario, suggests some startling changes ahead. Today, the only cities on the list where more than half of the days in an average August exceed 95° F are Phoenix and Dallas; by the 2050s, Houston, Sacramento, Tampa Bay, and Orlando could join them. Today, seven cities break 90° F on at least half of the days of a typical August; by the 2050s, they could be joined by Atlanta, Denver, Indianapolis, Miami, and Philadelphia. By midcentury, a dozen cities could average more than one day over 100° F per August, where today only three share that dubious distinction. (See below for a list of detailed results for all cities analyzed.)

Climate Central logoThese patterns match a broad finding in climate research that what seems to be a small amount of general global warming could have a large effect on weather extremes–including extreme heat events, which are forecast to become more frequent, more intense, and longer lasting (see U.S. Climate Change Science Program report).

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Extreme weather and climate events can cause significant damages, and heat waves are considered public health emergencies. Hot temperatures contribute to increased emergency-room visits and hospital admissions for cardiovascular disease, and can cause heat stroke and other life-threatening conditions.

Events such as the Chicago heat wave of 1995 and the 2003 European heat wave [PDF], which killed an estimated 40,000 people, have proven especially deadly to vulnerable populations, such as the elderly and persons with respiratory illnesses.

Climate Central used established scientific methods (more detail on the next page) to take results averaged from 12 major global climate models and apply them to 21 American cities. The resulting projections should be taken not as concrete predictions but rather as best guesses within a range of uncertainty. However, all 12 models used are unanimous in projecting more hot days by the middle of the century than we have today. For its projections, Climate Central used a moderate-high scenario of greenhouse-gas emissions. The scenario and resulting projections of risk currently appear to be conservative, since global emissions have exceeded the scenario in recent years.

Find out more and watch city-specific videos at Climate Central.


Average number of days each August over …

90°F 95°F 100°F
City 1980’s & 1990’s Projection: 2050’s 1980’s & 1990’s Projection: 2050’s 1980’s & 1990’s Projection: 2050’s
Atlanta 11 25 2 14 0 3
Boston 3 8 0 3 0 0
Chicago 4 14 1 6 0 2
Cleveland 2 10 0 3 0 0
Dallas 26 29 19 26 10 19
Denver 7 22 1 12 0 3
Detroit 2 14 0 5 0 1
Houston 23 29 6 20 1 4
Indianapolis 5 18 1 8 0 2
Los Angeles 6 14 2 6 1 2
Miami 17 30 0 11 0 0
Minneapolis 2 8 0 3 0 1
New York 4 12 1 4 0 1
Orlando 19 29 1 18 0 1
Philadelphia 6 20 1 9 0 2
Phoenix 31 31 30 31 27 30
Sacramento 20 26 12 20 7 12
San Francisco 0 1 0 0 0 0
Seattle 1 5 0 1 0 0
Tampa Bay 22 30 2 22 0 2
Washington, DC 10 19 3 10 0 3

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Analysis methods

Climate Central’s analysis is based on recent data from weather stations, regional-scale outputs from climate projection models, and a common technique for deducing best-guess local climate projections from regional ones. In its essence, this method involves calculating differences between current and future global climate model simulations, and applying them to observed climate data from the same vicinity.

Climate Central first identified weather stations closest to each city on the list, and then found the nearest regional location with model projections available.

For current patterns, Climate Central looked at August daily high temperatures at each weather station for 1981-2000, counting the total days exceeding 90°, 95° or 100° F over those 20 years, dividing by 20, and rounding off. The answers for each city were the average number of days reaching above each temperature in one August during the 1980s or 1990s — roughly today’s climate. (Any given August might have many more or fewer hot days.)

For future projections, Climate Central averaged outputs from 12 major climate models at the available regional location nearest to each city for 2046-2064, and for 1981-2000 (simulated values, not actual ones). We then calculated changes in the 20-year average monthly maximum temperature between these simulated future and current climates, and added the differences to the actual 1981-2000 weather station data from each city (already described).

This last step created new, simulated data for each city for 20 Augusts in the middle of this century. We then applied the same method that we used with actual 1981-2000 temperatures to estimate the average number of days over each temperature threshold in this future scenario.

The resulting projections give long-term averages, not predictions for any individual year; actual outcomes will vary significantly from year to year due to the natural variability of climate. Furthermore, because the modeling and methods used involve uncertainty, the projections should be taken as best guesses within a range of uncertainty. True long-term averages will likely prove somewhat higher or lower than the projections here. However, all twelve models are unanimous in projecting increased hot days from the present by the middle of the century.

All model outputs used were based on a medium-high greenhouse gas emissions scenario called “A1B” by the Intergovernmental Panel on Climate Change. Carbon dioxide and other greenhouse-gas emissions this decade have exceeded the A1B scenario thus far. This makes the projections here conservative, matching a future in which emissions are reduced compared to the current trend.

The techniques and general climate projections used are well established in the scientific literature (for example, see here and here).