What happened to global warming? - Science Fair - USATODAY.com
What happened to global warming?
This week's heat wave notwithstanding, scientists have been puzzled as to why global warming has occurred at a slower pace since 1998, following decades of increasing temperatures.
A new study out today in the journal Science reports the cause could be an increase in the amount of aerosols – tiny, airborne solid and liquid particles from both natural and man-made sources – high up in the stratosphere.
In the study, scientists from the National Oceanic and Atmospheric Administration (NOAA) and other agencies found that an increase in stratospheric aerosols decreased the global warming that would have otherwise occurred by 25 percent since 1998.
"There was less warming than you would have had without the aerosols," says study co-author John Daniel of NOAA's Earth System Research Laboratory in Boulder, Colo.
Sea salt, dust and volcanic ash are three common types of natural aerosols; these airborne particles can also come from man-made sources from the burning of fossil fuels.
"Most of the global warming of the past half-century has been driven by continuing increases in anthropogenic greenhouse gases," the study reports, "but natural aerosols from particular 'colossal' volcanic eruptions have significantly cooled the global climate at times, including for example the 'year without a summer' experienced after the eruption of the Tambora volcano in 1815 and notable cooling after the Pinatubo eruption in 1991."
Daniel added that he wouldn't have thought that the aerosols would still be a factor now, this long after the 1991 volcanic eruption of Pinatubo.
The stratospheric aerosol increase could also be due in part to human emissions of sulfur precursors (such as sulfur dioxide from burning coal), the authors point out in the study.
This study follows another study earlier this month in the Proceedings of the National Academy of Sciences, which found that China doubled its coal consumption from the years 2003 to 2007, leading to a huge increase in sulfur emissions that may have had a cooling effect on the planet. The researchers in that study suggested that this cooling effect may have counteracted ongoing warming due to increased carbon dioxide concentrations, permitting natural forces to predominate the planet's temperature.
Will there be a point in the future at which the impact of aerosols on global temperatures will be less of a factor than it is now?
"What happens in the future depends on the cause of the aerosols," says Daniel. "If it's volcanic, it depends on what volcanoes do. If its sulfur, it depends on what our pollution is."
The paper does not address how man-made versus natural activities contribute to aerosol creation, which they say is a question to be explored in further studies.
As for aerosols' impact on climate models used to estimate future global warming, according to the study, "climate model projections neglecting these changes would continue to overestimate the … global warming in coming decades if these aerosols remain present at current values or increase."
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