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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 a list of 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 the days in an average August exceed 95°F are Phoenix and Dallas; by the 2050’s, 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 2050’s, they could be joined by Atlanta, Denver, Indianapolis, Miami, and Philadelphia. And, by midcentury, a dozen cities could average more than one day over 100°F per August, where today only three share that dubious distinction.
These 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 be become more frequent, more intense, and longer lasting (see US Climate Change Science Program report).
Climate Central used established scientific methods to take results averaged from twelve 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 twelve 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.
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 (Weather and Climate Extremes in a Changing Climate).
Events such as the Chicago heat wave of 1995 and the 2003 European heat wave, which killed an estimated 40,000 people, have proven especially deadly to vulnerable populations, such as the elderly and persons with respiratory illnesses (The Report on Excess Mortality in Europe During Summer 2003 [PDF]).
Other societal impacts of extreme heat include livestock mortality, increases in peak energy demand, crop damage, and increased demand for water, as detailed in a report of the US Global Change Research Program.
This table is based on historical data (for the 1980's & 1990's) and best-guess climate change projections (for the 2050's) under a scenario of greenhouse gas emissions lower than the current trend—thus a conservative scenario. Climate Central used established methods to apply regional results averaged from twelve major global climate models to the cities and measures listed here. Table values are projections of long-term averages, not predictions for any given year; actual outcomes will vary significantly from year to year due to natural variability. Furthermore, because the modeling and methods used involve uncertainty, table values 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.