NOAA GFDL - Geophysical Fluid Dynamics LaboratoryGFDL - Geophysical Fluid Dynamics Laboratory

Anthropogenic forcing and response yield observed positive trend in Earth’s energy imbalance

July 28th, 2021

Key Findings

  • It is exceptionally unlikely (< 1% probability) that the satellite-observed trend in Earth’s energy imbalance can be explained by natural variations in the climate system alone.
  • This trend is achieved only upon accounting for the increase in anthropogenic radiative forcing and the associated climate response.
  • The probable anthropogenic contribution to the observed trend is 65%-135% (66% confidence interval; 30%-170% for 95% confidence interval).
  • Increases in greenhouse gases, decreases in aerosols, and decreases in polar sea-ice were major contributors to the trapping of more heat on the planet during 2001-2020

Shiv Priyam Raghuraman, David Paynter, V. Ramaswamy. Nature Communications. DOI: 10.1038/s41467-021-24544-4

The observed trend in Earth’s energy imbalance, a measure of the acceleration of heat uptake by the planet, is a fundamental indicator of perturbations to climate. The study found that it is exceptionally unlikely (< 1% probability) that this observed trend can be explained by natural variations in the climate system alone. Using climate model simulations and by conducting a hierarchy of GFDL CM4/AM4 experiments, the authors estimated the anthropogenic and internal variability contributions to the observed trend during 2001-2020. The study concludes that the satellite record provides clear evidence of a human-influenced climate system.

The authors show that anthropogenic forcing and the associated climate response yield the significant positive globally-averaged observed trend. This trend is driven by a large decrease in reflected solar radiation and a small increase in emitted infrared radiation. This is because recent changes in forcing and feedbacks are additive in the solar spectrum, while being nearly offset by each other in the infrared.

A positive Earth’s energy imbalance manifests as many symptoms across the Earth system that impact society: increases in temperature, sea levels, flooding, droughts, and extreme events. This study increases confidence in the causes of the increase in Earth’s energy imbalance: anthropogenic activities. Knowing that human activity is responsible for the acceleration of planetary heat uptake implies the need for significant societal action to curb further increases in Earth’s energy imbalance.

Global-mean observed trend in Earth’s energy imbalance (EEI) obtained by anthropogenic forcing and the associated climate response. Trends in ΔEEI (TEEI) with 95% confidence intervals (CI) (Δ represents anomalies). Dashed lines indicate Clouds and the Earth’s Radiant Energy System (CERES) TEEI’s 95% CI derived from observational uncertainty and standard error of linear fit (internal variability). Shading indicates CERES TEEI’s 95% CI with internal variability uncertainty estimated from model-derived ε. The lower end indicates the minimum contribution by anthropogenic forcing and the associated climate response (ΔERF + λΔT ; see Eq. 1) to CERES TEEI. Conversely, the upper end indicates the maximum anthropogenic contribution to CERES TEEI. Geophysical Fluid Dynamics Laboratory Atmosphere Model 4 (GFDL AM4) ‘Historical’ value represented by the experiment with prescribed sea surface temperatures and sea ice and effective radiative forcing changes (forcing agents varying) (AM4 PSST+ERF; green). Control experiments denoted by light gray filled circles. Historical experiments denoted by green filled circles. CanESM5 = Canadian Earth System Model version 5, MIROC6 = Model for Interdisciplinary Research on Climate version 6, IPSL-CM6A-LR = Institut Pierre-Simon Laplace-Climate Model 6-Low Resolution, ACCESS-ESM1.5 = Australian Community Climate and Earth-System Simulator Earth System Model Version 1.5, GISS-E2.1-G = Goddard Institute for Space Studies (GISS) climate model.