Global Warming

Human 'fingerprints' detected in Earth's seasonal temperature changes

For the first time, scientists have shown that human-caused climate change is affecting seasonal temperature cycles, a study released Thursday suggests.

The study shows that summers are warming more rapidly than the other three seasons as the planet's temperature rises, especially in portions of the Northern Hemisphere.

It concludes that there's no "natural" way the temperatures could have changed this way without the influence of rising atmospheric greenhouse-gas concentrations. Human-inflicted climate change is caused by the burning of fossil fuels such as coal, oil and gas, which release heat-trapping greenhouse gases such as carbon dioxide and methane into the atmosphere.

The findings deliver another blow against two refrains commonly repeated by climate deniers: that the satellite record doesn't show that the planet is warming, and that it's impossible to know how much warming is from nature and how much is from human beings. Both claims are wrong, say the authors of the study, published Thursday in the journal Science.

The study looked at the satellite record going back to the late 1970s to trace how warming is impacting seasons differently. They found that while year-round temperatures are rising, the rate of that temperature increase is happening faster in the mid-latitudes during the summer than it is during the winter. That's even more pronounced in the Northern Hemisphere.

The team found that at the mid-latitudes in the Northern Hemisphere, from about 40° North (close to the Kansas-Nebraska border) to about 60° North (mid-Canada), there is a gap between how much temperatures are rising in summer compared to how much they are rising in winter. That gap grew by roughly a tenth of a degrees Celsius each decade over the 38-year satellite record as the summers warmed faster.

The reason for this, the study explains, is that much of the world's land is in the Northern Hemisphere, as opposed to the Southern Hemisphere, which has more ocean. Ocean temperatures don't fluctuate as much and are slower to reflect change.

The mid-latitudes are also where many of the world's crops are grown, and as the temperature rises and the soil dries out, that could have major implications for food sources.

Above 60° North latitude—going into the Arctic—the scientists saw the trend reverse. There, the winters are getting warmer faster, giving seasonal sea ice less time to regrow each year.