NATURE GEOSCIENCE | VOL 5 | OCTOBER 2012 | www.nature.com/naturegeoscience 691 E arth’s climate is determined by the ﬂows of energy into and out of the planet and to and from Earth’s surface. Geographical dis- tributions of these energy ﬂows at the surface are particularly important as they drive ocean circulations, fuel the evaporation of water from Earth’s surface and govern the planetary hydrologi- cal cycle. Changes to the surface energy balance also ultimately control how this hydrological cycle responds to the small energy imbalances that force climate change 1 . e seminal importance of Earth’s energy balance to climate has been understood for more than a century. Although the earliest depictions of the global annual mean energy budget of Earth date to the beginning of the twentieth century 2,3 , the most signiﬁcant advance to our understanding of this energy balance occurred aﬅer the space age in the 1960s. Among the highlights obtained from early satellite views of Earth was the measurement of Earth’s albedo (the ratio of outgoing ﬂux of solar energy to incoming ﬂux from the Sun) at approximately 30% (ref. 4), thus settling a long-standing debate on its magnitude — values ranged between 89% and 29% (ref. 5) before these measurements. e sign and magnitude of the net eﬀect of clouds on the top-of-atmosphere (TOA) ﬂuxes 6 was also later established with the space-borne observations of the scanning instrument on the Earth Radiation Budget Experiment (ERBE) 7 , which better delineated between clear and cloudy skies. ERBE, and later the Clouds and the Earth’s Radiant Energy System (CERES) 8 and the French Scanner for Radiation Budget 9 , conﬁrmed that the global cloud albedo eﬀect was signiﬁcantly larger than the green- house eﬀect of clouds. Although this was a major advance at the time, determining the inﬂuence of clouds on atmospheric and sur- face ﬂuxes had to wait until the recent satellite measurements of the vertical structure of clouds became available from the A-train 10 . An update on Earth’s energy balance in light of the latest global observations Graeme L. Stephens 1 *, Juilin Li 1 , Martin Wild 2 , Carol Anne Clayson 3 , Norman Loeb 4 , Seiji Kato 4 , Tristan L’Ecuyer 5 , Paul W. Stackhouse Jr 4 , Matthew Lebsock 1 and Timothy Andrews 6 Climate change is governed by changes to the global energy balance. At the top of the atmosphere, this balance is monitored globally by satellite sensors that provide measurements of energy ﬂowing to and from Earth. By contrast, observations at the surface are limited mostly to land areas. As a result, the global balance of energy ﬂuxes within the atmosphere or at Earth’s surface cannot be derived directly from measured ﬂuxes, and is therefore uncertain. This lack of precise knowledge of surface energy ﬂuxes profoundly aﬀects our ability to understand how Earth’s climate responds to increasing concentrations of greenhouse gases. In light of compilations of up-to-date surface and satellite data, the surface energy balance needs to be revised. Speciﬁcally, the longwave radiation received at the surface is estimated to be signiﬁcantly larger, by between 10 and 17 Wm –2 , than earlier model-based estimates. Moreover, the latest satellite observations of global precipitation indicate that more precipitation is generated than previously thought. This additional precipitation is sustained by more energy leaving the surface by evaporation — that is, in the form of latent heat ﬂux — and thereby oﬀsets much of the increase in longwave ﬂux to the surface. The global annual mean energy balance e current revised depiction of the global annual mean energy balance for the decade 2000–2010 is provided in Fig. B1. Although the ﬂuxes given are meant to be an average for that decade, the net ﬂux at the TOA (the diﬀerence of incoming minus outgoing ﬂuxes) varies on a variety of timescales 11,12 that include relatively large but episodic changes by volcanic eruptions and a much smaller, more systematic increase associated with increases in ocean heat storage as Earth warms. For the decade considered, the average imbalance is 0.6 = 340.2 − 239.7 − 99.9 Wm –2 when these TOA ﬂuxes are con- strained to the best estimate ocean heat content (OHC) observations since 2005 (refs 13,14). is small imbalance is over two orders of magnitude smaller than the individual components that deﬁne it and smaller than the error of each individual ﬂux. e combined uncer- tainty on the net TOA ﬂux determined from CERES is ±4 Wm –2 (95% conﬁdence) due largely to instrument calibration errors 12,15 . us the sum of current satellite-derived ﬂuxes cannot determine the net TOA radiation imbalance with the accuracy needed to track such small imbalances associated with forced climate change 11 . Despite this limitation, changes in the CERES net ﬂux have been shown to track the changes in OHC data 16,17 . is suggests that the intrinsic precision of CERES is able to resolve the small imbalances on interannual timescales 12,16 , thus providing a basis for constrain- ing the balance of the measured radiation ﬂuxes to time-varying changes in OHC (Supplementary Information). e average annual excess of net TOA radiation constrained by OHC is 0.6±0.4 Wm –2 (90% conﬁdence) since 2005 when Argo data 14 became available, before which the OHC data are much more uncertain 14 . e uncer- tainty on this estimated imbalance is based on the combination of both the Argo OHC and CERES net ﬂux data 16 . 1 Center for Climate Sciences, Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA, 2 Institute for Atmospheric and Climate Science, ETH Zurich, Universitätsstrasse 16, CH-8092, Zurich, Switzerland, 3 Phyiscal Oceanography Department, Woods Hole Oceanographic Institution, 266 Woods Hole Road, Massachusetts 02543, USA, 4 NASA Langley Research Center, 21 Langley Boulevard, Hampton, Virginia 23681, USA, 5 Department of Atmospheric Sciences, University of Wisconsin, Madison, Wisconsin 80523, USA, 6 UK Met Oﬃce, FitzRoy Road, Exeter, Devon EX1 3PB, UK. *e-mail: graeme.stephens@jpl.nasa.gov PROGRESS ARTICLE PUBLISHED ONLINE: 23 SEPTEMBER 2012 | DOI: 10.1038/NGEO1580 © 2012 Macmillan Publishers Limited. 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