Indisputable evidence of human-caused climate change •
New research has provided unequivocal evidence of the impact of human activities on climate change by identifying a unique "fingerprint" that links human-induced alterations to the temperature structure of Earth's atmosphere. 

Indisputable evidence of human-caused climate change

New research has provided unequivocal evidence of the impact of human activities on climate change by identifying a unique “fingerprint” that links human-induced alterations to the temperature structure of Earth’s atmosphere. 

This groundbreaking study has significantly improved our understanding of how specific signals from human activities influence global temperatures, especially in the upper stratosphere, thereby challenging the claims that recent atmospheric and surface temperature changes are natural.

For years, scientists have been aware that differences between tropospheric and lower stratospheric temperature trends serve as a fingerprint of human effects on climate. However, this fingerprint had not considered data from the mid to upper stratosphere, located 25 to 50 kilometers above Earth’s surface. 

The inclusion of this information in the study has improved the detectability of a human fingerprint by a factor of five.

“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,” explained the authors in the journal Proceedings of the National Academy of Sciences (PNAS).

While the troposphere contains noise such as day-to-day weather, interannual variability from El Niños and La Niñas, and longer-term natural fluctuations in climate, the upper stratosphere exhibits less noise variability, and the human-caused climate change signal is larger. This makes it much easier to distinguish the human influence on climate change.

“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 paper states.

Lead author Benjamin Santer, an adjunct scientist in the Physical Oceanography Department at the Woods Hole Oceanographic Institute (WHOI) in Massachusetts, emphasized the significance of these findings: “This is the clearest evidence there is of a human-caused climate change signal associated with CO2 increases.”

Santer, who has been working on climate fingerprinting for more than 30 years, also addressed the implications of this research on common misconceptions about climate change: “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. 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 was inspired by earlier work conducted in 1967 by Suki Manabe and Richard Wetherald, who employed a simple climate model to investigate how CO2 emissions from fossil fuel burning might alter atmospheric temperature. 

Their modeling revealed a distinctive feature: increased CO2 levels led to more heat being trapped in the troposphere and less heat escaping into the stratosphere. This caused the troposphere to warm and the stratosphere to cool, thereby providing a foundation for the current research that clearly demonstrates the human fingerprint on climate change.

These findings have been made possible through improved simulations and satellite data, making it crucial to understand the critical role of carbon dioxide in climate change.

Earlier studies have examined global-mean temperature changes in the middle and upper stratosphere, roughly 25 to 50 kilometers above Earth’s surface, but detailed patterns of climate change in this layer have not been investigated. 

The latest research is the first to search for these human-caused climate change patterns, or “fingerprints,” in the middle and upper stratosphere.

Co-author Qiang Fu, a professor in the Department of Atmospheric Sciences at the University of Washington, explained the significance of these fingerprints: “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.”

Susan Solomon, Martin Professor of Environmental Studies at the Massachusetts Institute of Technology, emphasized the importance of understanding the role of carbon dioxide in climate change: “The world has been reeling under climate change, so being as confident as possible of the role of carbon dioxide is critical. The fact that observations show not only a warming troposphere but also a strongly cooling upper stratosphere is unique tell-tale evidence that nails the dominant role of carbon dioxide in climate change and greatly increases confidence.”

Lead author Benjamin Santer expressed both satisfaction and concern regarding the extension of fingerprinting higher up into the atmosphere to test the prediction made by Suki Manabe and Richard Wetherald in 1967: “As someone who tries to understand the kind of world that future generations are going to inhabit, these results make me very worried. We are fundamentally changing the thermal structure of Earth’s atmosphere, and there is no joy in recognizing that.”

Santer further highlighted the implications of these findings on decision-making: “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.”

The research was funded by the National Science Foundation, National Oceanic and Atmospheric Administration, U.S. Department of Energy, and the Francis E. Fowler IV Center for Ocean and Climate at Woods Hole Oceanographic Institution.

More about climate change and CO2

Carbon dioxide (CO2) is a greenhouse gas that plays a significant role in climate change. Greenhouse gases trap heat in the Earth’s atmosphere, which leads to a rise in global temperatures. This process, known as the greenhouse effect, is essential for maintaining life on Earth by providing warmth, but when greenhouse gas concentrations increase, this delicate balance is disrupted, resulting in climate change.

Here’s how CO2 contributes to climate change:

CO2 is released from various human activities

The primary source of CO2 emissions comes from the burning of fossil fuels like coal, oil, and natural gas for energy production, transportation, and industrial processes. Deforestation, cement production, and other land-use changes also contribute to CO2 emissions.

CO2 absorbs and re-emits infrared radiation

When sunlight reaches the Earth’s surface, some of it is absorbed and converted into heat. This heat is then radiated back into the atmosphere as infrared radiation. CO2 and other greenhouse gases absorb some of this infrared radiation and re-emit it in all directions, including back towards Earth’s surface. This process effectively traps heat in the atmosphere and causes temperatures to rise.

Positive feedback loops amplify warming

As global temperatures increase, natural feedback mechanisms can exacerbate the warming effect. For example, as ice melts at the poles, less sunlight is reflected back into space, causing more heat to be absorbed by the Earth. Additionally, higher temperatures can cause permafrost to melt, releasing even more CO2 and methane (another potent greenhouse gas) into the atmosphere.

Climate change impacts

The increase in CO2 levels and global temperatures leads to a range of climate change consequences, including more frequent and severe heatwaves, droughts, storms, and floods. These events can have devastating effects on ecosystems, agriculture, water resources, and human health. Rising temperatures also contribute to the melting of glaciers and ice sheets, which in turn causes sea levels to rise, threatening coastal communities and ecosystems.

Ocean acidification

The ocean absorbs a significant portion of the excess CO2 in the atmosphere. When CO2 dissolves in seawater, it forms carbonic acid, which lowers the pH of the water, making it more acidic. This process, called ocean acidification, can have severe consequences for marine life, particularly for species that rely on calcium carbonate to build their shells and skeletons, such as corals, mollusks, and some plankton.

Overall, CO2 plays a central role in driving climate change due to its heat-trapping properties, its abundance in the atmosphere, and its long atmospheric lifetime. Reducing CO2 emissions by transitioning to cleaner energy sources, implementing energy efficiency measures, and protecting and restoring forests are critical steps in mitigating the impacts of climate change.


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