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Adapted from an article by Santer et al. in Science, 20 July 2018; Map: Axios Visuals

Human activities are altering Earth's seasons in a way that is creating a greater contrast between summer and winter in much of North America, Europe and Eurasia, a new study finds.

Why it's important: The research, published Thursday in the journal Science, is the first to find a human "fingerprint" on the seasonal cycle of temperatures, adding another global trend that is formally attributed to human emissions of greenhouse gases.

Other trends already pinned on human activities include an increasing amount of heat held in the world's oceans, the sharp decline in Arctic sea ice cover, and changes in the water cycle that are leading to heavier downpours as well as severe drought events.

Based on computer models and basic physics, scientists had an idea of how temperatures in the troposphere — the layer of air extending from near Earth's surface to about 35,000 feet and where most weather occurs — should be varying from one season to the next as the world warms overall. The new study put these model simulations — with and without increased greenhouse gas emissions — to the test against observations.

How they did it: Researchers from Lawrence Livermore National Laboratory, Remote Sensing Systems, MIT and other institutions combined 38 years of satellite observations of the troposphere and computer model data.

  • They asked: How much do summer temperature changes differ from wintertime trends?
  • They then looked at the pattern of temperature changes and performed different tests using model simulations and statistics to determine if natural variability could explain their findings.

What they found: The researchers found that human emissions of greenhouse gases have altered the seasonal cycle around the world, as measured in the troposphere, but with ramifications for conditions at the surface.

"There’s compelling evidence from this work that human activities are now changing the seasonal cycle of temperature and that’s something that we really care about because the difference between summer and winter affects a lot of our lives."
— Benjamin Santer, study author from Lawrence Livermore National Laboratory

Specifically, summer is warming faster than winter in the northern and southern midlatitudes, leading to a greater contrast between the hottest and coldest times of year.

  • One exception to this is at the highest latitudes, where the difference is actually decreasing, which is related to the loss of Arctic sea ice in the far north, and ozone depletion above Antarctica.
  • Interestingly, the findings run counter to surface temperature trends in northern midlatitudes, which show that winters are warming faster than summers, which is a discrepancy that has yet to be fully explained.
  • The study found that it's extremely unlikely the trends detected are from natural climate variability. For annual mean mid-to-upper tropospheric temperatures, the odds are roughly "5 in 1 million" of getting the annual mean signal-to-noise ratios due solely to natural variability, the study states.

What they're saying: Scientists who were not involved in the new study told Axios that its findings are both credible and consequential.

"It’s one thing to know that human intervention is causing the annual-average temperature in their location to change.  It’s another, and even more sobering thing to know that human intervention is also impacting the swing in their local temperature from summer to winter," said John Fyfe, a senior research scientist at the Canadian Center for Climate Modelling and Analysis, via email.

What's next: According to Bill Randell, a scientist at the National Center for Atmospheric Research, future studies will likely identify more trends from satellite data as the period of record lengthens. How the tropospheric changes are reflected at the surface "will likely be a topic of follow-on research," he said.

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