Clouds and the Earth's radiation budget
The Earth's climate is determined by the amount of solar energy intercepted by the planet and the fraction of that energy that is absorbed. One quarter of the solar constant, or 343 watts per square meter (W/m2), is intercepted, and approximately 30% of this (the planetary albedo) is reflected back to space, leaving 70% or 240 W/m2 to be absorbed. Earth emits the same amount, 240 W/m2, to space as terrestrial radiation. About half the Earth's albedo is due to reflection of sunlight by clouds. If Earth's albedo changed from 30% to 29%, a 2° warming at the surface would occur (ice age was 6° colder). The global average cloud cover is ~60%. We get about half a degree of warming for every 1 W/m2 increase so 4 W/m2 increase yields 2° of warming.
The radiative properties of clouds are dependent on quite different variables in the two wavelength regimes. Short-wave albedo of clouds is controlled by cloud thickness, droplet sizes, and sun angle. Long wave emission, in contrast, is essentially controlled just by the cloud-top temperature. The relative importance of the two competing effects of clouds depends on the circumstances. The short-wave cooling effect is dominant for clouds over the ocean (and over other dark surfaces), in daytime, in summer, and for low clouds. The long-wave warming effect is dominant for clouds over snow (and over other bright surfaces), at night, in winter, and for high clouds.
Dimethyl Sulfide (DMS) as a source of cloud condensation nuclei (CCN): DMS-climate interactions, direct and indirect climatic effects of anthropogenic sulfate aerosols, and changes in cloud cover amounts
Clouds form on CCN, and the albedo of clouds is determined by the number and size of cloud droplets, so there is an effect on albedo if we change the number and/or size of CCN. Such a change could have climatic significance. What are the CCN and what could change the number of aerosol particles? They are mostly sulfates. Over the oceans, most of the sulfate particles (~80%) result from oxidation of dimethyl sulfide (DMS) of biological origin, so changes in biological productivity or changes in species abundance (diatoms, coccolithophorids, phaeocystis) could change the number of CCN and therefore cause changes in the size distribution of cloud droplets, affecting the albedo and lifetimes of clouds. There is a correlation between CCN and DMS in the remote ocean -- low in winter and high in summer during periods of high productivity up to a saturation point of 300 CCN per cubic meter. In unpolluted regions, there are more CCN when there are higher levels of DMS.
Because the albedo of clouds (and thus the Earth's radiation budget) is sensitive to CCN density, biological regulation of the climate is possible through the effects of temperature and sunlight on phytoplankton population and DMS production. To counteract the warming due to doubling of carbon dioxide, an approximate doubling of CCN would be needed. On the other hand, a reduction in DMS production might well exacerbate climate warming.
There are important implications of this hypothesis. If phytoplankton are responsible for keeping cloud droplets small, we'd better leave them alone and let them do their job. If we perturb or destroy phytoplankton, the Earth may overheat; there is potential for surprise. Are variations in DMS production responsible for part of seasonal and interannual variations of planetary albedo? Are phytoplankton involved in a climatic feedback? The scenario is as follows: sea surface temperature (SST) drops, DMS production drops, cloud droplet size increases, albedo falls, transmittance goes up, solar radiation at the sea surface and SST rise, resulting in a negative feedback.
Both the sign and the magnitude of this feedback are unknown. It would have to be negative to be consistent with the "Gaia Hypothesis." On the other hand, if it is positive, it could help explain the ice ages. It could be positive if a warmer ocean means less biological productivity. But even if all species increase DMS production when temperature or sunlight increases, the total production of DMS might still decrease if warmer SST and more sunlight favored diatoms over coccolithophorids and phaeocystis in species competition, because diatoms produce less DMS per capita.
The highest concentrations of DMS anywhere in the world are near the coast of Antarctica. Why do Phaeocystis pouchetii produce so much DMS? Their habitat is the sea-ice zone in both the Arctic and Antarctic. Perhaps they produce so much DMS to shield themselves from the high salinity of the brine pockets in sea ice, where they spend the winter. Colder temperatures and higher salinity could select for species that produce the most DMS. If sea ice declines due to global warming so that the habitat for these organisms shrinks, we could drastically reduce DMS production and ... surprise, a positive climate feedback!
Regarding the sensitivity of DMS production to climatic change:
(1) Seasonal cycle at Cape Grim (Tasmania) shows more DMS
production in summer (negative feedback?) and
(2) Ice-age cycle at Vostok (Antarctica) shows more sulfate and MSA
deposited during glacial period; this suggests that more DMS is
produced in a colder climate (positive feedback?) Different answers
result on different time scales.
There is now evidence that anthropogenic sulfate aerosols (from burning of sulfur-containing fossil fuels) can contribute significantly to the Earth's albedo even without nucleating cloud droplets.
How can aerosols compete with clouds for reflection of sunlight?
Uncertainties and Surprises Dr. Warren brought to AGCI Summer Session II from AGCI Summer Session I on Cloud Radiation Feedbacks and the Credibility of Atmospheric Models.
Uncertainties