Observed trend in Earth energy imbalance may provide a constraint for low climate sensitivity models

Increases in greenhouse gases, especially CO₂, causes the Earth to warm. Although the physics of global warming is well understood, scientists have been uncertain about exactly how much warming to expect from increases in CO₂ and other greenhouse gases. The largest source of this uncertainty is how clouds will respond as the climate warms - clouds are complex and can respond to a warming in complex ways which likely amplify the warming caused by greenhouse gases, but by an uncertain amount. 

In a stable climate, the Earth emits as much energy into space as it receives from the sun. However, as greenhouse gas concentration increases, this balance is changing. Satellite measurements since 2001 clearly show significant changes in both solar radiation absorbed by the Earth and the outgoing thermal radiation from Earth. Climate models are an important tool for understanding how the atmosphere including clouds, respond to the warming from greenhouse gases. The latest generation of climate models still have a relatively large range in the amount of global warming we can expect from the same increase in greenhouse gas concentration. 

A new study led by CICERO Center for International Climate Research, with co-authors from NASA Langley Research Center and Priestley Centre for Climate Futures (University of Leeds), shows that climate models that give a low warming from increases in greenhouse gases do not match satellite measurements. This means that models with a stronger warming response to greenhouse gas increases are more realistic. In the paper now published in Science, the authors highlighted that this in turn will increase estimates of global warming this century if the world continues along its current emission trajectory. Future warming will likely be worse than thought unless society acts.  

“We therefore need to cut greenhouse gas emissions even further to have a chance of keeping global warming below 2 degrees”  Gunnar Myhre, research director at CICERO and lead author of the study, said. 

Climate sensitivity: The most common way to assess how much warming we can expect from a given increase in greenhouse gases is to estimate the temperature increase from a doubling of atmospheric CO₂ concentration. The Earth warms in direct response to the co₂ increase but this warming in turn drives other changes such as increases in atmospheric water vapour, snow and ice melting and cloud changes. These changes are termed climate feedbacks and they all add to the initial warming. Cloud changes, in particular, contribute significantly to the uncertainty in how sensitive the climate system is to greenhouse gases. The latest UN climate report (IPCC, 2021) estimates a most likely climate sensitivity of 3°C. It is very likely between 2 and 5°C, and likely between 2.5 and 4°C. These estimates are based on theoretical understanding, reconstructions of past climate conditions (such as ice ages and warm periods), and observations since 1850. 

Science study: The CERES satellite measures the Earth's energy imbalance—specifically, how much solar radiation is absorbed compared to how much heat (longwave) radiation is emitted back into space. The data show a significant increase in absorbed solar radiation, partly due to reduced snow and ice cover, but also because of changes to clouds. At the same time, the Earth is emitting more heat, driven by rising surface temperatures. 

The satellite measurements have been compared with results from 37 climate models. The study shows a clear connection between climate sensitivity in the models and the ratio between increased absorbed solar radiation and increased heat radiation from the Earth. Climate models with low climate sensitivity show small changes in the energy imbalance in the individual contributions from absorbed solar radiation and increased terrestrial radiation from the Earth, and are less able to reproduce what is measured from satellite data. This indicates that a climate sensitivity in the lower half of the IPCC range is less likely and a higher climate sensitivity is more likely. This would suggest that weaker degrees of global warming response to greenhouse gases can be ruled out, and that estimates of stronger future warming, for a given change in greenhouse gases, become more likely. 

Models with climate sensitivity lower than 2.9°C show significantly smaller increases in absorbed solar radiation than what is measured by CERES. For models with climate sensitivity lower than 2.5°C, it is not possible to replicate the satellite observations – even when taking into account the possibility that the models underestimate the effect of reduced emissions of polluting particles. This demonstrates how important satellite data, particularly CERES data (https://ceres.larc.nasa.gov/), are for monitoring and understanding of current climate change. 

 

Observed trend in Earth energy imbalance may provide a constraint for low climate sensitivity models (link to article)