Office of Biological and Environmental Research Weekly Report

April 20, 2009

 

Aerosol Effects on Thin Clouds - Not all Clouds are the Same.  Scientists in the DOE Atmospheric Radiation Measurement (ARM) Program used a cloud resolving model and found that aerosols affect thin and thick clouds differently.  Thin clouds cover approximately 30% of the globe and play an important role in the Earth’s radiation budget.  In thin clouds, if aerosols increase the total water content in these clouds, the reflection of sunlight would be increased and the clouds would cause cooling.  Thin clouds were studied for which rain did or did not reach the Earth’s surface.  Clouds from which rain reached the Earth’s surface had increased aerosols and increased water content.  Conversely, clouds from which rain did not reach the Earth’s surface had reduced aerosols but increased water content due to a differing microphysical process. Comparisons of these results with results from the literature for thicker and warmer clouds suggest that these two cloud types have different interactions with aerosols.  These results suggest that different approaches need to be considered in climate models for different cloud types to represent aerosol-cloud interactions to assess aerosol effects on clouds and hence climate.

 

Reference:

Lee, S. S., J.E. Penner, and S.M. Saleeby, 2008: Aerosol effects on liquid-water path of thin stratocumulus clouds, submitted to J. Geophys. Res.

 

Media Interest: No

Contact: Kiran Alapaty, SC-23.1, (301) 903-3175

 

Reducing Current Carbon Emissions Has Big Benefits for Climate.  In a paper being published this week in Geophysical Research Letters, Dr. Warren Washington and colleagues at the National Center for Atmospheric Research (NCAR) report that current patterns of global warming can still be greatly diminished if nations cut emissions of heat-trapping greenhouse gases by 70% this century.  Their analysis is based on assumptions about carbon emissions from the U.S. Climate Change Science Program Synthesis and Assessment Report #2.1.  This report cites 450 ppm carbon dioxide as an attainable target if the world quickly adapts conservation practices and new green technologies to cut emissions dramatically.  Left unchecked, emissions are currently on track to reach about 750 ppm by 2100.  The team’s results showed that if atmospheric CO₂ were held to 450 ppm, global temperatures would only increase by 0.6^oC by the end of the century. In contrast, the study showed that temperatures would rise by almost four times that amount, or 2.2^oC, if emissions are allowed to continue on their present course.  The more modest increase would partially avoid some of the most dangerous impacts of climate change, massive losses of Arctic sea ice and permafrost and significant sea level rise, in addition to lesser impacts on Arctic fisheries and mammals, and less global and regional changes of surface temperature and precipitation.  Such reductions in emissions would stabilize atmospheric CO₂ (and the climate system) by 2100 whereas the non-mitigation scenario would not result in stabilization in the present century.

 

Reference:  Washington, W. M., R. Knutti, G. A. Meehl, H. Teng, C. Tebaldi, D.

Lawrence, L. Buja, and W. G. Strand (2009), How much climate change can be avoided by mitigation?, Geophys. Res. Lett., doi:10.1029/2008GL037074, in press.

 

Media Interest: Yes

Contact: Anjuli Bamzai, SC-23.1, (301) 903-0294

Impacts of Tropical Clouds on Weather and Climate.  Scientists in DOE’s Atmospheric Radiation Measurement (ARM) Program used high-resolution radar data from two ARM sites in the tropical western Pacific and found that the daily and vertically averaged solar radiation absorption in the area between the Earth’s surface and the top of the atmosphere is same, whether tropical skies are cloudy or clear.  They also found that the presence and vertical location of clouds greatly influence where the energy gets absorbed in the atmosphere even though clouds do not affect the average absorption. These findings are significant for climate change because the amount and location of solar absorption in the atmosphere drives air movement, which affects weather and climate.

 

Reference: McFarlane, S. A., J. H. Mather, T. P. Ackerman, and Z. Liu. 2008. Effect of clouds on the calculated vertical distribution of shortwave absorption in the tropics, J. Geophys. Res., 113, D18203, doi:10.1029/2008JD009791. http://www.agu.org/pubs/crossref/2008/2008JD009791.shtml

 

Media Interest: No

Contact: Kiran Alapaty, SC-23.1, (301) 903-3175