Adding to evidence attributing observed atmospheric changes to manmade influences, climate scientists leveraging satellite data from recent decades have identified a human "fingerprint" on Earth's atmosphere in a new place: the troposphere, or, the lowest region of the atmosphere. In this low atmospheric space, say the authors, human-caused warming has significantly affected the seasonal cycle of the temperature. The new study underscores human's influence on changes to the natural variability of local and seasonal cycles in Earth's climate system. Earth's climate is affected by many external factors, each having unique effects on the climate. Identifying the fingerprints attributable to human activity involves isolating their signals from the background noise of natural variability. Fingerprint studies have been used to determine much of what is known about anthropogenic effects on climate, however, most rely on annual or decadal averages, or focus on the drivers for climate change in individual seasons. Here, Benjamin Santer et al. used satellite measurements, which provide a continuous and near-global record of tropospheric temperature for nearly 40 years, to identify a fingerprint of human influences on the planet's seasonal cycles. Their satellite-based results suggesting a manmade influence were duplicated in model simulations that only featured anthropogenic forcing, further suggesting a human origin for the climate effects observed, say the authors. The authors' results show an increase in the amplitude of seasonal variability in the tropospheric temperature over most of the globe and most markedly, in the mid-latitudes. This effect is particularly true for the northern hemisphere where warming in the summertime is greater than in the winter, perhaps partially due to increased summer drying of the land surface. In a related Perspective, William Randel highlights the importance of both continuity and quality of satellite observations in understanding human influences in the climate system in the future.
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Science