Whether or not global warming is being caused wholly or in part by human activity, and regardless of the mechanisms involved, here’s a graph demonstrating that the United States is getting hotter. Andrew Freedman writes at Climate Central:
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As the climate has warmed during the past several decades, there has been a growing imbalance between record daily high temperatures in the contiguous U.S. and record daily lows. A study published in 2009 found that rather than a 1-to-1 ratio, as would be expected if the climate were not warming, the ratio has been closer to 2-to-1 in favor of warm temperature records during the past decade (2000-2009). This finding cannot be explained by natural climate variability alone, the study found, and is instead consistent with global warming.
When you look at individual years, the imbalance can be more stark. For example, through late June 2012, daily record highs were outnumbering record daily lows by a ratio of 9-to-1.The study used computer models to project how the records ratios might shift in future decades as the amount of greenhouse gases in the air continues to increase.