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Atmospheric Sciences & Global Change
Research Highlights

June 2009

Clouding the Issue

Causes of global dimming and brightening more complex than previously thought

earth surface
Although brightening occurred throughout the continental U.S. from 1995 to 2007, more precise measurements revealed regional differences that may significantly test the abilities of climate models. The angle of the arrows indicates the amount of increase in brightening. Red arrows, in the Midwest and South, show the biggest change in brightening, followed by white arrows (average brightening) and blue (less brightening) in other parts of the country. Enlarge Image.

Results: Over time scales spanning at least a decade, the amount of sunlight that reaches the Earth's surface has varied. These variations affect the total amount of incoming and outgoing energy in the Earth system, which, in turn, affects climate change. Scientists refer to these variations as global dimming and brightening. Scientists had attributed much of the dimming and brightening to changes in the amounts of tiny particles, or aerosols, in the atmosphere. Analysis of new climate data shows the answer is not so simple.

A multidisciplinary team led by Pacific Northwest National Laboratory's Dr.  Chuck Long found that, at least in the continental United States, changes in clouds and cloudiness have a greater influence on brightening than any decrease in aerosol amounts alone. They further discovered that the magnitudes of dimming and brightening are regional, rather than global or continental phenomena.

The research appears in a special section of the Journal of Geophysical Research-Atmospheres and was one of the publication's five most downloaded papers for several weeks. The research also was also featured as an editor's highlight.

Why it matters: The quantity of solar radiation reaching the Earth's surface is the primary driver of the Earth-atmosphere system. The role of clouds in this equation is one of the big unknowns in climate change science. Current climate models do a poor job of simulating global dimming and brightening over decades. That is because just one "pixel" in most climate models covers an area at least the size of a 100-kilometer box. Yet many clouds are as small as a one-kilometer box. By taking a "close-in" view, the study revealed that the occurrence of clouds too small for climate models to detect individually can have a big impact on regional surface solar radiation and, therefore, on brightening or dimming.

The study also concluded that scientists should consider the brightening and dimming phenomenon on a local or regional scale, where small but important variations can be detected.

The results of the study have broad implications for understanding the earth's surface energy balance and for improving the accuracy of climate models. Researchers expect that adequately capturing the dimming and brightening phenomena will challenge every aspect of global climate models.

Method: To learn more about the amount and causes of dimming and brightening, the team analyzed incoming solar radiation collected at 12 surface sites across the United States from 1995 through 2007. The data came from several Department of Energy Atmospheric Radiation Measurement ARM Program sites and the National Oceanic and Atmospheric Administration Surface Radiation network.

Researchers used the Radiative Flux Analysis method developed by PNNL staff through the ARM Program to determine cloud quantities, and to analyze clear and cloudy sky conditions. By obtaining continuous estimates of how much surface solar radiation would have occurred had clouds not been present, they were able to determine the effect of clouds on the solar radiation reaching the surface.

What's next: The current work dealt with the influence of the quantity of cloud cover on global dimming and brightening. Future studies will investigate the impact of other properties of clouds.

Acknowledgments: PNNL is transforming the nation's ability to predict climate change and its impacts. This research was supported by the DOE Office of Science Biological and Environmental Research, ARM program; the NOAA Climate Goal and the NASA Radiation Projects Office; and the Swiss National Centre for Competence in Climate Research. The researchers also acknowledge the diligent and dedicated efforts of those responsible for the operation and maintenance of the instruments that produced the data used in this study.

Research Team: C. N. Long, S. A. McFarlane, and C. J. Flynn: Climate Physics Group, Pacific Northwest National Laboratory; E. G. Dutton and J. A. Augustine: Global Monitoring Division, Earth System Research Laboratory, U.S. National Oceanic and Atmospheric Administration, Boulder, Col.; W. Wiscombe: Brookhaven National Laboratory; M. Wild: Institute for Atmospheric and Climate Science, Eidgenossische Technische Hochschule, Zurich, Switzerland.

Reference: Charles N. Long, E. G. Dutton, J.A. Augustine, W. Wiscombe, M. Wild, S. A. McFarlane and C. J. Flynn. April 2009. "Significant decadal brightening of downwelling shortwave in the continental United States." Journal of Geophysical Research-Atmospheres, 10.1029/2008JD011263, 2009

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