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So the record in July isn’t such a big deal, Mr. Trenberth said. “But the fact that the first seven months of the year are the hottest on record is much more impressive from a climate standpoint, and highlights the fact that there is more than just natural variability playing a role: Global warming from human activities has reared its head in a way that can only be a major warning for the future.”

Here are some more numbers unlikely to provide cold comfort. The coolest July on record was in 1915. The coldest month in U.S. history was January 1979 with an average temperature of 22.6 degrees.