If you live in the United States, it's time to stop rolling your eyes in despair every time an elderly person says, "it was never this hot in my day." Because not only are they right but they could be at risk. Summer in the United States is getting hotter. Moreover summer feels as though its getting hotter faster than it really is, because its also getting more humid. And, given that extremes of heat have been shown to have a greater impact on human health (particularly for people over 65 and under one month) than any other type of severe weather in the US, this particular change is about more than just nostalgia.
In what one climatologist describes as, "the first time anyone has shown systematic trends over such a large area in a climate variable that is directly related to human health," Dian Gaffen and Rebecca Ross, of the US National Oceanic and Atmospheric Administration report in Nature that between 1949 and 1995, mean temperature in July and August in the US increased by a third of a degree, Centigrade. Moreover, by combining effects of temperature and humidity (which has gone up several percent each decade) into a measure known as 'apparent' temperature - a widely-used measure of human health stress - they found that the mean summertime apparent temperature rose by a further tenth of a degree - at 0.42 degrees.
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