Anthropogenic and natural warming inferred from changes in Earth's energy balance

The Earth’s energy balance is key to understanding climate and climate variations that are caused by natural and anthropogenic changes in the atmospheric composition. Despite abundant observational evidence for changes in the energy balance over the past decades1–3, the formal detection of climate warming and its attribution to human influence has so far relied mostly on the difference between spatio-temporal warming patterns of natural and anthropogenic origin4–6. Here we present an alternative attribution method that relies on the principle of conservation of energy, without assumptions about spatial warming patterns. Based on a massive ensemble of simulations with an intermediate-complexity climate model we demonstrate that known changes in the global energy balance and in radiative forcing tightly constrain the magnitude of anthropogenic warming. We find that since the mid-twentieth century, greenhouse gases contributed 0.85 C of warming (5–95% uncertainty: 0.6–1.1 C), about half of which was offset by the cooling effects of aerosols, with a total observed change in global temperature of about 0.56 C. The observed trends are extremely unlikely (<5%) to be caused by internal variability, even if current models were found to strongly underestimate it. Our method is complementary to optimal fingerprinting attribution and produces fully consistent results, thus suggesting an even higher confidence that human-induced causes dominate the observed warming. The optimal fingerprint detection and attribution framework provides a rigorous, statistical method to quantify the contributions of different external forcings and internal variability to the observed climate changes7. In essence, it is based on a regression of the observations onto model simulated patterns and relies on the spatio-temporal response patterns from different forcings being clearly distinct. The assumptions are that climate models simulate the spatial patterns reasonably well and that regional responses from different forcings can be scaled and combined linearly. The global energy budget is not necessarily conserved and observed changes in the energy budget are not considered. Previous studies showed that observed patterns of surface air temperature provide a constraint on the human contribution to the observed warming4. Here we demonstrate that the global energy balance provides a further strong, comprehensive andphysicallymotivated constraint. In equilibrium, the Earth emits as much energy by outgoing longwave radiation at the top of the atmosphere as it receives net shortwave radiation from the sun. Robust evidence for recent deviations from that equilibrium comes from a variety of observations and model simulations1,8. The most likely value of the current net radiative forcing F is estimated at 1.6Wm−2, compensated by further outgoing longwave radiation λT and energy uptake of the planet Q: