Monday, May 7, 2007

Long-Term Satellite Record Reveals Likely Recent Aerosol Trend

In the recent decade aerosols have gained much attention from a climate perspective. Researchers have been using both observations and modeling studies to address the question on radiative effects of aerosols and their role in global climate change. Ground based observations have provided unmatched understanding of aerosol optical and microphysical properties. Sophisticated remote sensing instruments such as MODIS, MISR, OMI, TOMS onboard various satellite platforms provide routine measurement of aerosol loading over the entire globe.

While much attention was being paid on characterizing aerosols and understanding their radiative effects not much speculation was done over their concentration trends over past decade or so until recently. An interesting paper appeared in Science on March 16, 2007 by Michael Mishchenko et al on “Long-Term Satellite Record Reveals Likely Recent Aerosol Trend “. They analyzed the Global Aerosol Climatology Project (GACP) data set to show a decrease in global tropospheric aerosol optical thickness by 0.03 during the period from 1991 to 2005.

Global Diming : Wikipedia

Comments on this paper also appeared in the same issue raising question on the dataset used, for example, the AVHRR observations that goes into GACP data set is questionable in the first place because AVHRR with limited channels was not really designed for aerosol retrieval. Cloud contamination was also in question. AVHRR retrieval was also the first question that came to both me and one of my colleagues Pawan Gupta. We discussed this issue and came up with the thought that MODIS and MISR being instruments meant to retrieve aerosol concentration (aerosol optical thickness => AOT) information should throw some light into this because they have been flying onboard Terra satellites since 1999. That gives a time series of 6 years (2000 – 2006). Mr Gupta generated the AOT trend plots shown below. The first figure overlays MODIS (red) and MISR (yellow) monthly mean AOT values over GACP AOT trend (blue). MODIS-MISR decreasing trends agree with each other both in trend and in magnitude though they do not agree in magnitude with GACP. But the key result is that all the three dataset do show a decreasing trend (see zero AOT line for reference).

A closer look annual mean AOT trends (figure below) separately over Land (from MISR) and Ocean (from MODIS) and global mean renders the same result – net decreasing trend in annual mean AOT.

These results along with study by Mishchenko et al., (2007) raise interesting questions on both global warming and global diming and their offset. How does this fit into what we observe around us? Is the decrease in aerosol concentration contributing to increase in global warming? What are some loop holes in understanding this debate and assimilating what data shows?

Acknowledgement : A special thanks to Mr Pawan Gupta for sharing some of his results (the two figures above) for this article.


Michael I. Mishchenko, Igor V. Geogdzhayev, William B. Rossow, Brian Cairns, Barbara E. Carlson, Andrew A. Lacis, Li Liu, and Larry D. Travis (16 March 2007)
Science 315 (5818), 1543. [DOI: 10.1126/science.1136709]

1 comment:

Harish Gadhavi said...

This reminds me of an article by my current advisor Pinker R. T. and others in science about estimating surface solar flux using satellite observations. They also found increasing trend in global surface solar flux, attributable to reduction in global aerosol loading. Following is the link for the article

Pinker et al, Science, 2005