New research provides clear evidence of a human ‘fingerprint’ on climate change and shows that specific signals from human activities have altered the temperature structure of Earth’s atmosphere.
Variations between temperature trends in the troposphere and lower stratosphere have long been recognized as a fingerprint of human influences on climate. However, this footprint neglected information from the middle of the stratosphere up, 25 to 50 kilometers above the Earth’s surface.
“Including this information improves the detectability of a human footprint by a factor of five. The improved detectability occurs because the middle to upper stratosphere has a significant cooling signal from anthropogenic carbon monoxide2 increases, small noise levels of normal internal variations, and differing signal and noise patterns,” according to the journal article,”The stratosphere’s exceptional contribution to human imprints on atmospheric temperature,” published in Proceedings of the National Academy of Sciences (PNAS). Noises in the troposphere can include daily weather, fluctuations between years arising from El Niños and La Niñas, and long-term natural fluctuations in climate. In the upper stratosphere, the contrast noise is lower, and the signal of human-caused climate change is larger, so the signal is more easily distinguishable.
“Extending the fingerprints to the upper stratosphere with extended temperature records and improved climate models means that it is now virtually impossible for natural reasons to interpret satellite-measured trends in the thermal composition of Earth’s atmosphere,” the paper states.
“This is the clearest evidence of a human-induced climate change signal linked to carbon dioxide2 increases,” according to lead author Benjamin Santer, an assistant scientist in the Department of Physical Oceanography at the Woods Hole Oceanographic Institution (WHOI) in Massachusetts.
“This research undermines and refutes claims that recent changes in atmospheric and surface temperature are natural, whether due to the sun or due to internal cycles in the climate system. A natural explanation is almost impossible in relation to what we are looking at here: changes in the temperature structure of the atmosphere,” he added. Santer, who has worked on climate fingerprinting for more than 30 years. “This research clears up false claims that we don’t need to take climate change seriously because it’s all natural.”
The research was prompted by earlier work by Soki Manabe and Richard Weatherald, who in 1967 used a simple climate model to study how carbon dioxide was used.2 Burning of fossil fuels may alter the temperature of the atmosphere. Their modeling found a very distinctive feature: an increase in carbon dioxide2 The levels led to more heat being trapped in the troposphere (the lowest layer of Earth’s atmosphere) and less heat escaping upward into the stratosphere (the layer above the troposphere), thus heating the troposphere and cooling the stratosphere. This predicted tropospheric warming and stratospheric cooling in response to increased carbon dioxide2 It has been confirmed many times by more complex models and verified by comparing model results with observations of globally averaged atmospheric temperature from weather balloons and satellites.
Although these previous studies looked at global average temperature changes in the middle and upper stratosphere, approximately 25 to 50 km above the Earth’s surface, they did not look at the detailed patterns of climate change in this layer. This region can be studied better now due to improved simulations and satellite data. The new research is the first to look for patterns of human-caused climate change — also called “fingerprints” — in the middle and upper stratosphere.
“Human signature changes in mid- and higher-stratospheric temperatures due to carbon dioxide2 The increases are really exceptional because they are so large and so different from the temperature changes there due to internal variance and normal external influence. These unique fingerprints make it possible to detect the human impact on climate change due to carbon dioxide2 In a short period of time (~10-15 years) with high confidence,” said co-author Qiang Fu, professor in the Department of Atmospheric Sciences at the University of Washington.
“The world has been reeling under the influence of climate change, so being as confident as possible in the role of carbon dioxide is critical,” said co-author Susan Solomon, MIT’s Martin Professor of Environmental Studies. “The fact that the observations show not only a warming of the troposphere but also a very cold upper stratosphere is unique evidence that establishes the dominant role of carbon dioxide in climate change and greatly increases confidence.”
Santer said that while it’s intellectually interesting to be able to extend the fingerprints higher into the atmosphere to test Manabe and Wetherald’s predictions, it’s also deeply unsettling.
“As someone who’s trying to understand what kind of world future generations will live in, these findings make me very concerned. We’re fundamentally changing the thermal structure of Earth’s atmosphere, and there’s no fun in admitting that,” said Santer.
“This study shows that the real world has changed in a way that cannot simply be explained by natural causes,” added Santer. “We now face important decisions, in the United States and globally, about what to do about climate change. And I hope those decisions will be based on the best scientific understanding we have of the reality and severity of human impacts on climate.”
The study was funded by the National Science Foundation, the National Oceanic and Atmospheric Administration, the US Department of Energy, and the Francis E. Fowler IV Center for Oceans and Climate at Woods Hole Oceanographic Institution.