A Problem for Sun-Blocking Cloud Geoengineering? Clouds Dissipate

Slashdot reader christoban writes:

In what may be an issue for Sun-obscuring strategies to combat global warming, it turns out that during solar eclipses, low level cumulus clouds rapidly disappear, reducing by a factor of 4, researchers have found.

The news comes from the science magazine Eos (published by the nonprofit organization of atmosphere/ocean/space scientists, the American Geophysical Union).

Victor J. H. Trees, a geoscientist at Delft University of Technology in the Netherlands, and his colleagues recently analyzed cloud cover data obtained during an annular eclipse in 2005, visible in parts of Europe and Africa. They mined visible and infrared imagery collected by two geostationary satellites operated by the European Organisation for the Exploitation of Meteorological Satellites. Going to space was key, Trees said. 'If you really want to quantify how clouds behave and how they react to a solar eclipse, it helps to study a large area. That's why we want to look from space....' [T]hey tracked cloud evolution for several hours leading up to the eclipse, during the eclipse, and for several hours afterward.

Low-level cumulus clouds — which tend to top out at altitudes around 2 kilometers (1.2 miles) — were strongly affected by the degree of solar obscuration. Cloud cover started to decrease when about 15% of the Sun's face was covered, about 30 minutes after the start of the eclipse. The clouds started to return only about 50 minutes after maximum obscuration. And whereas typical cloud cover hovered around 40% in noneclipse conditions, less than 10% of the sky was covered with clouds during maximum obscuration, the team noted. 'On a large scale, the cumulus clouds started to disappear,' Trees said... The temperature of the ground matters when it comes to cumulus clouds, Trees said, because they are low enough to be significantly affected by whatever is happening on Earth's surface...

Beyond shedding light on the physics of cloud dissipation during solar eclipses, these new findings also have implications for future geoengineering efforts, Trees and his collaborators suggested. Discussions are underway to mitigate the effects of climate change by, for instance, seeding the atmosphere with aerosols or launching solar reflectors into space to prevent some of the Sun's light from reaching Earth. Such geoengineering holds promise for cooling our planet, researchers agree, but its repercussions are largely unexplored and could be widespread and irreversible.

These new results suggest that cloud cover could decrease with geoengineering efforts involving solar obscuration. And because clouds reflect sunlight, the efficacy of any effort might correspondingly decrease, Trees said. That's an effect that needs to be taken into account when considering different options, the researchers concluded.


Another article on the site warns that 'Planting Trees May Not Be as Good for the Climate as Previously Believed.'
'The climate benefits of trees storing carbon dioxide is partially offset by dark forests' absorption of more heat from the Sun, and compounds they release that slow the destruction of methane in the atmosphere.'

Read more of this story at Slashdot.