Human influence on the seasonal cycle of tropospheric temperature
Study key findings & significance
An analysis of decades of satellite data has revealed how humans are changing seasonal cycles in the lower atmosphere.
The accumulation of greenhouse gases produced by burning fossil fuels has increased air temperatures in summer and caused larger annual temperature swings in the northern hemisphere.
Previous research has documented how global warming is altering seasons on the ground — causing winter snowpacks to melt earlier, shifting animal migrations and lengthening fire seasons.
This study, which draws on satellite temperature records from 1979 to 2016, is the first to pinpoint seasonal changes in the atmosphere.
The odds that natural climate variability can account for the magnitude of the temperature changes over the course of the satellite record are roughly five in a million.
“In the biological world, lots of people have been looking for and finding these changes, so we decided to take a look at the satellite data. What we see is profound evidence of the human impact on climate, not only in the annual temperatures but also in the seasonal cycle.
Benjamin Santer, lead author and atmospheric scientist at the Lawrence Livermore National Laboratory
We provide scientific evidence that a human-caused signal in the seasonal cycle of tropospheric temperature has emerged from the background noise of natural variability. Satellite data and the anthropogenic “fingerprint” predicted by climate models show common large-scale changes in geographical patterns of seasonal cycle amplitude. These common features include increases in amplitude at mid-latitudes in both hemispheres, amplitude decreases at high latitudes in the Southern Hemisphere, and small changes in the tropics. Simple physical mechanisms explain these features. The model fingerprint of seasonal cycle changes is identifiable with high statistical confidence in five out of six satellite temperature datasets. Our results suggest that attribution studies with the changing seasonal cycle provide powerful evidence for a significant human effect on Earth’s climate.