While those numbers are above historical averages (5, 7, 11, and 13 days, respectively) they are beneath consensus projections for the average July by just the middle of this century, assuming nothing is done to reduce pollution of heat-trapping gases. Under that scenario, Boston can expect an average of 12 July days above 90, New York can expect 16, Philadelphia 21, and Washington, DC 22.
Using a different measure, the monthly average temperature, July 2010 was also well above historical averages in northeast corridor cities, but about the same as temperatures projected for 2050. Claudia Tebaldi, a scientist for advocacy group Climate Central, conducted the analysis. More detail, illustrations and a description of methodology are available at http://www.climatecentral.org/gallery/collections/july_heat
High temperatures in a limited area for a limited time are not evidence for climate change but July's results come in the context of a report (http://www.noaanews.noaa.gov/stories2010/20100728_stateoftheclimate.html) just released by the National Oceanic and Atmospheric Administration (NOAA) concluding that the 2000's were the hottest decade since record-keeping began, with global indicators pointing toward unequivocal warming. In addition, independent analyses by NOAA (http://www.ncdc.noaa.gov/sotc/?report=global&year=2010&month=6&submitted=Get+Report) and NASA (http://data.giss.nasa.gov/gistemp/) both indicate that January through June 2010 was the hottest such period on record. Climate scientists project the long-term warming trend to continue and accelerate, in the absence of measures to reduce carbon emissions.
Heat stress is the largest weather-related cause of death in the United States. Hot summers also strain the electricity supply due to high demand for air conditioning, as widespread northeast brownouts this summer have testified. These problems would be expected to intensify with further warming.