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WIREs Clim Change
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Decadal changes in radiative fluxes at land and ocean surfaces and their relevance for global warming

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Anthropogenic interference with climate occurs primarily through modification of radiative fluxes in the climate system. Increasing releases of greenhouse gases into the atmosphere lead to an enhancement of thermal radiation from the atmosphere to the surface by presently about 2 W m−2 per decade, thereby causing global warming. Yet not only thermal radiation undergoes substantial decadal changes at the Earth surface, but also incident solar radiation (SSR), often in line with changes in aerosol emissions. Land‐based observations suggest widespread declines in SSR from 1950s to 1980s (‘global dimming’), a partial recovery (‘brightening’) since mid‐1980s, and indication for an ‘early’ brightening in 1930s and 1940s. No similar extended observational records are available over oceans. However, modeling studies, conceptual frameworks and available satellite‐derived records point to the existence of decadal SSR variations also over oceans. SSR changes overall match with decadal variations in observed warming rates, suggesting that SSR variations may effectively modulate greenhouse gas‐induced warming. Specifically, on the Northern Hemisphere, the lack of warming from 1950s to 1980s and its subsequent acceleration in the 1990s fits to the trend reversal from dimming to brightening and associated changes in air pollution levels. From the 1950s to 1980s no warming was also observed over Northern Hemispheric Oceans, in line with conceptual ideas that subtle aerosol changes in pristine ocean areas, effectively amplified by aerosol–cloud interactions, can substantially alter SSR, thereby modulating Sea Surface Temperatures. On the Southern Hemisphere, the absence of significant aerosol levels fits to the observed stable (greenhouse gas‐induced) warming rates since the 1950s. WIREs Clim Change 2016, 7:91–107. doi: 10.1002/wcc.372

Annual mean surface solar radiation (in W m−2) as observed at Potsdam, Germany, from 1937 to 2014. Five year moving average in blue. Distinct phases of inclines (1930s–1940s, ‘early brightening’), declines (1950s–1980s, ‘dimming’), and renewed inclines (since 1980s, ‘brightening’) can be seen. Units W m−2. In addition, a stabilization since around 2010 can be noted.
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Annual 2‐m temperature anomalies observed over (a) Northern Hemispheric oceans and (b) Northern Hemispheric land surfaces. Observations from CRUTEM4 (land) and HADSST3 (oceans), anomalies with respect to 1960–1990. Linear trends over the dimming phase (1950s–1980s) in blue, over the brightening phase (1980s–2000s) in red. Units °C.
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Annual 2‐meter temperature anomalies observed on the (a) Northern and (b) Southern Hemispheres. Observations from HadCRUT4, anomalies with respect to 1960–1990. Linear trends over the dimming phase (1950s–1980s) in blue, over the brightening phase (1980s–2000s) in red. On the polluted Northern Hemisphere, no warming is observed during dimming with strong aerosol increase whereas rapid warming is observed during subsequent brightening with aerosol decrease. On the more pristine Southern Hemisphere, with greenhouse‐gases as sole major anthropogenic forcing, observed warming is similar during both periods. Updated from Wild. Units °C.
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Annual downward thermal radiation at the surface measured at the BSRN site South Pole. A linear regression analysis has been applied. As at the majority of the BSRN sites, an increase can be seen since the initiation of the BSRN network starting from the early 1990s. Units W m−2.
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Annual mean anomalies of diurnal temperature range averaged over Northern Hemispheric land surfaces, from 1900 to 2013. Variations of diurnal temperature range may provide a useful proxy for variations in surface solar radiation over extended temporal and spatial scales. Data source: CRU TS3.22. Anomalies are shown with respect to 1961–1990 mean. Units °C. Updated from Wild to 2013.
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