Evaluating Twenty‐Year Trends in Earth's Energy Flows From Observations and Reanalyses

Abstract

Earth's energy flows encompass the exchange of energy between Earth and space and between Earth's atmosphere, ocean, lithosphere, and cryosphere. These exchanges occur over a range of time and space scales and influence weather and climate at any given location and time. A thorough understanding of Earth's energy flows is thus necessary to project how regional and global climate will change in response to radiative forcing. Observations of Earth's energy flows are essential for evaluating and improving the climate models used to make these projections. Ideally, the observations must provide accurate descriptions of the mean state of Earth's energy flows as well as their variations on seasonal, interannual, and decadal time scales.Efforts aimed at quantifying Earth's mean energy flows date back to the early twentieth century (Hunt et al., 1986). So-called "radiation budget diagrams" of global mean values of shortwave and longwave radiation within the climate system first appeared in 1908 (Abbot & Fowle, 1908). These diagrams were later extended to include non-radiative contributions (Dines, 1917;London, 1957). Energy budget diagrams were further refined following the launch of the first orbiting satellites, which included instruments designed to observe Earth's radiation budget (ERB;House et al., 1986). A key advance was made by Kiehl and Trenberth (1997), who used adjusted global mean top-of-atmosphere (TOA) radiative fluxes from the Earth Radiation Budget Experiment (ERBE), surface radiative fluxes derived from radiative transfer calculations, surface latent heat flux inferred from estimates of global mean precipitation, and sensible heat flux determined as a residual ensuring a global energy balance at the surface. Subsequent studies by Trenberth et al. (2009), Stephens et al. (2012), Wild et al. (2013) and L'Ecuyer et al. (2015) further refined the global mean energy budget diagram using increasingly more sophisticated datasets and analysis techniques.Early efforts aimed at quantifying energy transports within the climate system focused primarily on meridional transports (e.g.,

Arctic as the ‘radiator fins’ of Earth in a warming climate

“…In comparison, the global mean OLR trend is found to be 0.17±0.18 W m −2 decade −1 , resulting from the nonrobust trends in other regions such as the tropics (table 1). The insignificant global mean trend found here is in consistency with several previous studies [8,13,14]. Such comparison bears an interesting and important question-why does the Arctic region exhibit a strong OLR increase while the other regions do not?…”

The changing nature of Earth's reflected sunlight

“…We suggest, however, further scrutiny and evaluation of these observational trends is essential, including assessment of alternative explanations for them such as those based on arguments of instrument degradation (e.g. [36]).…”

Robust acceleration of Earth system heating observed over the past six decades

“…In the context of the heating rates, a positive value indicates that the Earth system is experiencing warming, while a negative value is associated with a cooling. estimates of warming acceleration (bars with black contours) are computed from a linear OLS regression accounting for autocorrelation 17,46 , over global TOA net radiative flux (white bar) and near-global TOA net radiative flux (which excludes latitudes higher than 60°, represented by the hatched white bar).…”