AGU Journal highlights -- May 6, 2009

The following highlights summarize research papers that have been published or are "in press" (accepted, but not yet published) in Geophysical Research Letters (GRL) or the Journal of Geophysical Research – Atmospheres (JGR-D).

1. Cooling periods can occur within long-term global warming

Some Web sites, blogs, and media articles have pointed to the fact that average global temperature has not risen since 1998 as evidence against anthropogenic global warming. Although most climate scientists recognize the statistical insignificance of such short-term trends, Easterling and Wehner note that some segments of the public do pay attention to them. To quell such objections, they argue that such short periods are not meaningful in the context of long-term climate change. The authors analyze the observed globally averaged surface air temperatures for the period 1901� as well as several climate model simulations for the twentieth and 21st centuries, including anthropogenically forced models. They plot the probability distribution functions for decadal temperature trends and find that in the observed record and all models, decade-long periods of cooling can occur even within a strong overall warming trend. The authors expect that due to the natural variability of the real climate, in the 21st century there will probably be some multiyear periods of cooling or constant temperature within longer-term anthropogenic global warming.

Title: Is the Climate Warming or Cooling?

Authors: David R. Easterling: National Climatic Data Center, NOAA, Asheville, North Carolina, USA;

Michael F. Wehner: Lawrence Berkeley National Laboratory, Berkeley, California, USA.

Source: Geophysical Research Letters (GRL) paper 10.1029/22009GL037810, 2009; http://dx.doi.org/10.1029/2009GL037810

2. Scientists explore complexity of recent global dimming and brightening episodes

Several studies show evidence of a widespread decrease in solar radiation reaching the Earth's surface from the 1960s through about the early 1990s. Termed "global dimming," this decrease was followed by "brightening" during the late 1990s. Past efforts correlated brightening trends to decreases in atmospheric aerosol concentrations due to successful environmental regulations and recovery from the Mount Pinatubo eruption. To learn more about the mechanics of dimming and brightening, Long et al. study observations of shortwave radiation collected by the U.S. government at 12 surface sites across the United States. These data, aggregated into all-sky and clear-sky surveys, show that widespread brightening occurred over the continental United States during the past 12 years at rates higher than previously thought. The authors find that changes in aerosol concentrations and other direct effects cannot fully explain the observed changes in shortwave radiation. They show that decreases in cloudiness, which may be indirectly influenced by decreasing aerosol concentrations, have played a significant role in observed brightening patterns. Further, they suggest that the causes of global brightening are complex and are best studied locally or regionally, rather than on a global or continental scale.