Researchers at the University of Miami have found that the greenhouse effect of carbon dioxide increases as its concentration in the atmosphere increases, calling into question long-held beliefs about its ability to continuously trap heat. The new study, led by scientists at the University of Miami's Rosenstiel School of Oceanic, Atmospheric and Science, published in the journal Science, comes as world leaders are in Dubai, United Arab Emirates, this week for COP28, the United Nations climate change conference.

"Our findings mean that as the climate responds to increases in carbon dioxide, carbon dioxide itself becomes a stronger greenhouse gas," said Brian Soden, senior author of the study and a professor of atmospheric sciences at the Rosenstiel School. "This is further confirmation that curbing carbon emissions as early as possible is necessary to avoid the worst impacts of climate change."

In the study, researchers used state-of-the-art climate models and other tools to analyze the impact of increased carbon dioxide on the upper region of the atmosphere, known as the stratosphere. They found that cooling of the stratosphere caused subsequent increases in carbon dioxide to have a greater heat-trapping effect than previous increases, making carbon dioxide stronger as a greenhouse gas. Source: NASA

It has long been thought that the proportional increase in heat trapped in the atmosphere by carbon dioxide (what scientists call radiative forcing) is a constant that does not change over time.

"This new finding shows that radiative forcing is not constant but changes as the climate responds to increases in carbon dioxide," said Ryan Kramer, a physical scientist at the National Oceanic and Atmospheric Administration's (NOAA) Geophysical Fluid Dynamics Laboratory and a Rosenstiel College alumnus.

Carbon dioxide contributes to global warming by trapping heat energy in the climate system. The study's first author, Haozhe He, completed the research while pursuing his doctoral degree at the Rosenstiel School. He believes: "This new understanding has great significance for interpreting past and future climate changes, and means that high carbon dioxide climates may be inherently more sensitive than low carbon dioxide climates."

The work was conducted using a suite of climate model simulations provided by the Coupled Model Intercomparison Project (CMIP), which provides a coordinated series of experiments with dozens of the world's most comprehensive climate models to support IPCC assessments. To take their work beyond the simulated world of climate models, the team also performed a number of "offline" radiative flux calculations using high-precision radiative transfer models and analytical models.

The study, "State dependence of carbon dioxide forcing and its implications for climate sensitivity," was published in the December 1 issue of Science. Nadir-Jewanji from the National Oceanic and Atmospheric Administration's Geohydrodynamics Laboratory is also a co-author of the study.