Is the Climate Impact of CO2 Increasing Disproportionately?

global warming

“Escalated Threat? The climate impact of carbon dioxide could increase disproportionately with rising CO2 levels, as a new analysis suggests. According to the analysis, the radiative forcing of CO2 does not increase consistently with its concentration but additionally rises by around 25 percent with each doubling of the initial CO2 values. This stratosphere-related effect could potentially intensify the future climate more than conventional climate models predict, as reported by researchers in Science.

How strongly does the Earth’s climate respond to an increase in greenhouse gases? This is a central question in climate research and crucial for our future. Climate sensitivity describes how much the radiative forcing of the Earth’s system—and thus the terrestrial energy surplus—changes in response to rising CO2 levels. Specifically, it indicates how many degrees the Earth’s temperatures will rise when CO2 levels double. In the 2021 IPCC report, this value was estimated to be in the range of 2.5 to four degrees.

Mysterious Discrepancies

However, the exact climate impact of CO2 is subject to varying results from different climate models. “Although these discrepancies have persisted for more than three decades, the reason for them has never been fully clarified,” reports Haozhe. He and his colleagues are from the University of Miami. The only consensus was that the radiative forcing of CO2 is constant; it increases linearly with CO2 concentration—this was the prevailing understanding.

However, this seems not to be the case, as he and his colleagues have now discovered. For their study, they combined comparative climate simulations with models from the Coupled Model Intercomparison Projects (CMIP), along with additional models that considered the state of individual atmospheric layers.

The Initial Value Matters

The findings revealed that, contrary to previous assumptions, the fundamental radiative forcing of CO2 is not constant but changes. Doubling the CO2 value from a high initial level results in higher climate sensitivity than doubling from an initially lower value. The direct radiative forcing for the doubling of CO2 is already around ten percent higher than pre-industrial times, according to the researchers.

“Our result implies that carbon dioxide becomes an even more potent greenhouse gas as the climate changes due to the increase in CO2,” explains senior author Brian Soden from the University of Miami. Specifically, the calculations showed that the direct radiative forcing of CO2 increases by approximately 25 percent with a twofold higher starting concentration.

Colder Stratosphere Intensifies Climate Effect

But what causes this deviation from common assumptions? As he and his colleagues found, the state of the stratosphere plays a crucial role. This atmospheric layer, beginning at around 15 kilometers in height, is known to react in the opposite way to climate warming than one would expect: when the lower atmosphere warms and CO2 levels rise, the stratosphere and the overlying mesosphere become colder.

This cooling of the stratosphere apparently underlies the disproportionate increase in CO2 radiative forcing, as reported by He and his team. When the stratosphere is colder, it enhances the greenhouse effect of CO2, as indicated by their models. Moreover, the stratosphere emits less long-wave infrared radiation into space, further intensifying the climate effect. “This supports the assumption that any factor affecting the temperature of the stratosphere also influences climate sensitivity,” says the team.

“These findings have significant implications for the interpretation of past and future climates,” says He. “They suggest that CO2-rich climate phases inherently respond more sensitively to greenhouse gases than CO2-poor phases.” Simultaneously, this explains why climate models provide different values for climate sensitivity; their results also depend on the chosen CO2 starting point.

Science, 2023; doi: 10.1126/science.abq6872