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New evidence that climate change is caused by human activities

A new study led by the Woods Hole Oceanographic Institution (WHOI) has provided clear evidence of a human “fingerprint” on climate change, showing that specific signals from human activities have altered the temperature structure of our planet’s atmosphere.

While differences between tropospheric and lower stratospheric temperature trends have long been recognized as anthropogenic effects on the climate, scientists have largely ignored what happens from the mid to the upper stratosphere (about 25-50 kilometers above the Earth’s surface).

“Including this information improves the detectability of a human fingerprint by a factor of five. Enhanced detectability occurs because the mid to upper stratosphere has a large cooling signal from human-caused CO2 increases, small noise levels of natural internal variability, and differing signal and noise patterns,” the authors explained.

While in the troposphere, noise can include daily weather, interannual variability caused by El Niños and La Niñas, and longer-term natural fluctuations in climate, in the upper stratosphere such noises are smaller, making anthropogenic climate change signals easier to distinguish. 

“Extending fingerprinting to the upper stratosphere with long temperature records and improved climate models means that it is now virtually impossible for natural causes to explain satellite-measured trends in the thermal structure of the Earth’s atmosphere,” the scientists argued.

“This research undercuts and rebuts claims that recent atmospheric and surface temperature changes are natural, whether due to the Sun or due to internal cycles in the climate system,” said study lead author Benjamin Santer, an expert in Physical Oceanography at WHOI. 

“A natural explanation is virtually impossible in terms of what we are looking at here: changes in the temperature structure of the atmosphere. This research puts to rest incorrect claims that we don’t need to treat climate change seriously because it is all natural.”

This study is rooted in the earlier work of Suki Manabe and Richard Wetherald in 1967, who utilized a simple climate model to investigate the potential impact of CO2 emissions from fossil fuel combustion on atmospheric temperature. Their modeling revealed a distinct finding: an elevation in CO2 levels resulted in increased heat retention within the troposphere (the lowest layer of Earth’s atmosphere) and decreased heat dissipation in the stratosphere (the layer above the troposphere), consequently causing warming in the troposphere and cooling in the stratosphere. 

This projection of tropospheric warming and stratospheric cooling in response to rising CO2 levels has been consistently supported by more intricate models and validated through comparisons of model outcomes with global-mean atmospheric temperature measurements obtained from weather balloons and satellites.

While these earlier studies focused on global-mean temperature alterations in the middle and upper stratosphere, approximately 25 to 50 kilometers above the Earth’s surface, they did not delve into the intricate patterns of climate change within this particular layer. However, advancements in simulations and satellite data have now made it possible to examine this region more comprehensively. 

The current study marks the first attempt to identify discernible climate change patterns, often referred to as “fingerprints,” specifically attributed to human activities in the middle and upper stratosphere.

“The human fingerprints in temperature changes in the mid to upper stratosphere due to CO2 increases are truly exceptional because they are so large and so different from temperature changes there due to internal variability and natural external forcing. These unique fingerprints make it possible to detect the human impact on climate change due to CO2 in a short period of time (~10 – 15 years) with high confidence,” said co-author Qiang Fu, a professor of Atmospheric Sciences at the University of Washington.

“This study shows that the real world has changed in a way that simply cannot be explained by natural causes. We now face important decisions, in the United States and globally, on what to do about climate change. I hope those decisions are based on our best scientific understanding of the reality and seriousness of human effects on climate,” Santer concluded.

The study is published in the journal Proceedings of the National Academy of Sciences.

By Andrei Ionescu, Staff Writer

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