Scientists are increasingly concluding that changes in low level cloud cover, not CO2, are what govern the surface radiation budget in the polar regions, driving and determining the retreat of the ice sheets.
It is considered “established science” that “the greenhouse effect of clouds may be larger than that resulting from a hundredfold increase of the C02 concentration of the atmosphere” (Ramanathan et al., 1989, 1,647 citations to date).
Indeed, satellite observations indicate that the Earth’s “radiation budget changes are caused by changes in tropical mean cloudiness” (Wielicki et al., 2002), not CO2 concentration changes.
Consequently, scientists have increasingly concluded that the driving mechanism that has governed and determined the melt of the polar ice sheets during recent decades has been the natural decadal-scale changes in cloud cover.
Decreasing cloud cover drives the recent
mass loss on the Greenland Ice Sheet
“The Greenland Ice Sheet (GrIS) has been losing mass at an accelerating rate since the mid-1990s. … We show, using satellite data and climate model output, that the abrupt reduction in surface mass balance since about 1995 can be attributed largely to a coincident trend of decreasing summer cloud cover enhancing the melt-albedo feedback. Satellite observations show that, from 1995 to 2009, summer cloud cover decreased by 0.9 ± 0.3% per year. Model output indicates that the GrIS summer melt increases by 27 ± 13 gigatons (Gt) per percent reduction in summer cloud cover, principally because of the impact of increased shortwave radiation over the low albedo ablation zone. The observed reduction in cloud cover is strongly correlated with a state shift in the North Atlantic Oscillation promoting anticyclonic conditions in summer and suggests that the enhanced surface mass loss from the GrIS is driven by synoptic-scale changes in Arctic-wide atmospheric circulation. … Th[e] strong correlation between summertime NAO index and the MAR-based cloud cover could be used to forecast whether the observed reduction in cloud cover during summer, and the associated increase in GrIS melt, is likely to continue.”
“[Our results] also indicate that the sudden decline in Greenland’s (surface) mass balance is not primarily a direct response to the local increase in atmospheric temperature, because anomalies in downwelling longwave radiation have contributed less energy to the increase in melt of the GrIS than SWD [summertime shortwave forcing] anomalies. This is contradictory to previous analyses that have focused on the increase in temperature as the main cause of GrIS melting (15, 16), as well as on the longwave warming effect of clouds (10). Climate warming is instead altering large-scale circulation patterns.”
“The radiative effect of clouds has attracted increasing attention; for example, it was found that decreasing cloud cover drives the recent loss of mass from the Greenland ice sheet by enhancing the melt-albedo feedback (Hofer et al., 2017). Thus, enhanced albedo effect from increasing cloud cover in southwest China during the early Holocene could have caused a reduction in summer temperature. … From 1960 to 2005, total cloud cover decreased over southwest China, including Yunnan Province (Zhang et al., 2011b) … [as] summer temperature increased [1961-2007] (Liu et al., 2010). This negative relationship between cloud cover and summer temperature was also found in India during the period 1931-2002 (Roy and Balling, 2005).”
“The surface radiation budget of the Arctic Ocean plays a central role in summer ice melt and is governed by clouds and surface albedo. … Longwave and shortwave radiation are primary drivers in the surface heat budget during summer melt (Persson et al., 2002). The surface radiative balance consists of contributions from incoming shortwave radiation, reflected shortwave radiation, incoming longwave radiation, and outgoing longwave radiation. Clouds have a major impact on both incoming longwave and shortwave radiative fluxes. … Future impacts on net radiative balances will depend on both ice and cloud conditions. As the sea ice cover evolves towards more first year ice, greater melt pond coverage, and more open water, the area-averaged albedo will be less than the break-even albedo for much of the summer. This implies less melting under cloudy conditions than sunny. However, the net radiative balance will still likely be less under sunny skies at the beginning of the melt season in May and early June.”
“During the short Antarctic summer, strong onshore winds may by themselves raise the ice sheet’s surface temperature (Ts) up to the melting point(through exchange of sensible heat), especially at low elevations. However, Ts [surface temperature] is ultimately controlled by the full surface energy budget (SEB), being the net of radiative (short- and longwave) and turbulent (sensible and latent) heat fluxes. Clouds exert an important influence on the SEB by modulating the radiative fluxes, primarily by enhancing downwelling longwave radiation and attenuating incoming solar radiation. In particular, low-level liquid-bearing clouds can have a determinant role in either causing or prolonging melting conditions over ice sheets.”
“Clouds are an essential parameter of the surface energy budget influencing the West Antarctic Ice Sheet (WAIS) response to atmospheric warming and net contribution to global sea level rise. … Owing to perennial high-albedo snow and ice cover, cloud infrared emission dominates over cloud solar reflection and absorption leading to a positive net all-wave cloud radiative effect (CRE) at the surface … The annual-mean CRE [cloud radiative effect] at the WAIS [West Antarctic Ice Sheet] surface is 34 W m−2, representing a significant cloud-induced warming of the ice sheet. … In summer, clouds warm the WAIS by 26 W m−2, on average, despite maximum offsetting shortwave CRE.”
via NoTricksZone
August 30, 2018 at 08:37AM
Iowa Climate Science Education 