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Nationwide: August Days over 90 Degrees

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.

For additional information, please see the August Heat feature.