Geoengineering: Effects on cirrus clouds and whitening of the sky Update July 18, 2013

Geoengineering: Effects on cirrus clouds and whitening of the sky

Update July 18, 2013

High and thin cirrus type clouds have an amplification effect on the greenhouse effect, at the same time they counteract the cooling effect of lower stratocumulus type clouds. [1] [2] Also the formation of cirrus clouds, product of the atmospheric dispersion of pollutants in aerosol form, as from aviation for example, is a very important factor in the worsening of droughts and floods. [3] [4]

[1] HIGH CLOUDS

Image and article from:

NASA Earth Observatory

http://earthobservatory.nasa.gov/Features/Clouds/clouds3.php

The high, thin cirrus clouds in the Earth's atmosphere act in a way similar to clear air because they are highly transparent to shortwave radiation (their cloud albedo forcing is small), but they readily absorb the outgoing longwave radiation. Like clear air, cirrus clouds absorb the Earth's radiation and then emit longwave, infrared radiation both out to space and back to the Earth's surface. Because cirrus clouds are high, and therefore cold, the energy radiated to outer space is lower than it would be without the cloud (the cloud greenhouse forcing is large). The portion of the radiation thus trapped and sent back to the Earth's surface adds to the shortwave energy from the sun and the longwave energy from the air already reaching the surface. The additional energy causes a warming of the surface and atmosphere. The overall effect of the high thin cirrus clouds then is to enhance atmospheric greenhouse warming.

[2] Aerosols and their Relation to Global Climate and Climate Sensitivity

http://www.nature.com/scitable/knowledge/library/aerosols-and-their-relation-to-global-climate-102215345
Image from Nature Education

How do aerosols affect the climate?

Extract:

All atmospheric aerosols scatter incoming solar radiation, and a few aerosol types can also absorb solar radiation. BC is the most important of the latter, but mineral dust and some OC components are also sunlight absorbers. Aerosols that mainly scatter solar radiation have a cooling effect, by enhancing the total reflected solar radiation from the Earth. Strongly absorbing aerosols have a warming effect. In the atmosphere, there is a mixture of scattering and absorbing aerosols, and their net effect on Earth's energy budget is dependent on surface and cloud characteristics. Scattering aerosols above a dark surface and absorbing aerosols above a bright surface are most efficient (see Figure 3a). Scattering (absorbing) aerosol above a bright (dark) surface are less efficient because the solar radiation is reflected (absorbed) anyway. (my emphasis) Absorbing aerosols are particularly efficient when positioned above clouds, which are a main contributor to the total reflection of solar radiation back to space.sun and the longwave energy from the air already reaching the surface. The additional energy causes a warming of the surface and atmosphere. The overall effect of the high thin cirrus clouds then is to enhance atmospheric greenhouse warming.
(BC black carbon) (OC organic compound)

Figure 3: The direct aerosol effect and the cloud albedo effect.

(a) The direct aerosol effect for low and high surface albedo, for scattering and absorbing aerosols.
A dark surface (low albedo) will already absorb a large portion of the solar radiation, and absorbing aerosols will thus
have a small effect. Scattering aerosols will instead amplify the total reflectance of solar radiation, since the
solar radiation would otherwise be absorbed at the surface. Over a bright surface (high albedo) scattering aerosols
have a reduced effect. Absorbing aerosols may, however, substantially reduce the outgoing radiation and thus have a
warming effect. (b) The cloud albedo effect (first indirect aerosol effect), cloud lifetime effect (second indirect aerosol
effect), and semi-direct effect.

[3] Air Pollution a Culprit in Worsening Drought and Flooding

Science Daily

http://www.sciencedaily.com/releases/2011/11/111114133742.htm

Extract:

Nov. 14, 2011 — Increases in air pollution and other particulate matter in the atmosphere can
strongly affect cloud development in ways that reduce precipitation in dry regions or seasons.
This while increasing rain, snowfall and the intensity of severe storms in wet regions or seasons,
according to results of a new study.The research provides the first clear evidence of how
aerosols--soot, dust and other particulates in the atmosphere--may affect weather and climate.

The findings have important implications for the availability, management and use of water
resources in regions across the United States and around the world.

"Using a 10-year dataset of atmospheric measurements, we have uncovered the long-term,
net impact of aerosols on cloud height and thickness and the resulting changes in precipitation
frequency and intensity," says Zhanqing Li, an atmospheric scientist at the University of
Maryland and lead author of a paper reporting the results.

More...

[4] June 6, 2013

Pollution in Northern Hemisphere helped cause 1980s African drought

By Hannah Hickey University of Washington

http://www.washington.edu/news/2013/06/06/pollution-in-northern-hemisphere-helped-cause-1980s-african-drought/

Extract:

Decades of drought in central Africa reached their worst point in the 1980s, causing Lake Chad,
a shallow lake used to water crops in neighboring countries, to almost dry out completely.

The shrinking lake and prolonged drought were initially blamed on overgrazing and bad
agricultural practices. More recently, Lake Chad became an example of global warming.

New University of Washington research, to be published in Geophysical Research Letters,
shows that the drought was caused at least in part by Northern Hemisphere air pollution.

Aerosols emanating from coal-burning factories in the United States and Europe during
the 1960s, ’70s and ’80s cooled the entire Northern Hemisphere, shifting tropical rain bands
south. Rains no longer reached the Sahel region, a band that spans the African continent just
below the Sahara desert.

When clean-air legislation passed in the U.S. and Europe, the rain band shifted back, and the
drought lessened.

Related research by the UW researchers and their collaborators shows that global warming is
now causing the land-covered Northern Hemisphere to warm faster than the Southern
Hemisphere, further reversing the pre-1980s trend.

Previous research has suggested a connection between coal-burning and the Sahel drought,
but this was the first study that used decades of historical observations to find that this drought
was part of a global shift in tropical rainfall, and then used multiple climate models to determine
why.

More...

Original post March 2013

From:
20 reasons why geoengineering may be a bad idea
BY ALAN ROBOCK
http://www.thebulletin.org/files/064002006_0.pdf

6. Effects of cirrus clouds. As aerosol particles injected into the stratosphere fall to Earth, they may seed cirrus cloud formations in the troposphere.[11]

Cirrus clouds affect Earth’s radiative balance of incoming and outgoing heat, although the amplitude and even direction of the effects are not well understood.

While evidence exists that some volcanic aerosols form cirrus clouds, the global effect has not been quantified.[12]

7. Whitening of the sky (but nice sunsets). Atmospheric aerosols close to the size of the wavelength of light produce a white, cloudy appearance to the sky.

They also contribute to colorful sunsets, similar to those that occur after volcanic eruptions. The red and yellow sky in The Scream by Edvard Munch was inspired by the brilliant sunsets he witnessed over
Oslo in 1883, following the eruption of Krakatau in Indonesia.[13]

Both the disappearance of blue skies and the appearance of red sunsets could have strong psychological impacts on humanity.

References:

[11] Richard P. Turco et al., “A Study of Mesospheric Rocket Contrails and Clouds Produced

by Liquid-Fueled Rockets,” Space Solar Power

Review, vol. 3, pp. 223–34 (1982); V. A. Mohnen,

http://ntrs.nasa.gov/search.jsp?R=19830033300

“Stratospheric Ion and Aerosol Chemistry and

Possible Links With Cirrus Cloud Microphysics—

A Critical Assessment,” Journal of Atmospheric

Science, vol. 47, pp. 1,933–48 (1990)

http://journals.ametsoc.org/doi/pdf/10.1175/1520-0469(1990)047%3C1933%3ASIAACA%3E2.0.CO%3B2

[12] K. Sassen et al., “The 5–6 December 1991

FIRE IFO II Jet Stream Cirrus Case Study: Possible Influences of Volcanic Aerosols,” Journal of

Atmospheric Science, vol. 52, pp. 97–123 (1993).