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Are human-caused and natural global warming different? Study says yes.

A study suggests that human-caused and natural global warming episodes affect rainfall rates differently. The finding could help scientists better forecast what's ahead.

By Staff writer / January 31, 2013

Researchers calculate that global warming has made heat waves in Texas (O.C. Fisher Lake in San Angelo, Texas, is shown here) about 20 times more likely to happen during a La Niña year. La Niñas occur when the difference in temperature between the cooler water in the eastern tropical Pacific and the warmer water in the west intensifies.

Tony Gutierrez/AP/File

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Human-triggered climate warming appears to leave a unique fingerprint on global rainfall rates compared with natural warming, according to a new study.

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While rainfall rates increase whether the long-term warming trend is natural or not, the rate of increase appears to be higher during natural warming trends.

The result might help resolve a long-standing discrepancy between changes in rainfall projected in global climate models and changes projected by studying the historical record, researchers say.

The study suggests that "carbon dioxide has a fundamentally different mode of warming than natural climate change" – one that leaves a unique signature on rainfall rates, says Jeff Severinghaus, a climate researcher at the Scripps Institution of Oceanography in La Jolla, Calif. While the effect is most pronounced the the Pacific basin, the work "is about something more fundamental.... Natural and human-caused climate change really produce different effects."

"It will become increasingly clearer, as time goes on, that the rainfall patterns that we're seeing are not same ones we see in past warm periods," he adds.

In general, current climate forecasts project higher precipitation rates for the tropics and high latitudes, with already-dry areas in the subtropics experiencing additional drying.

Some trends in extreme precipitation already appear to be emerging. Since the 1950s, more regions of the globe appear to have experienced an increase in extreme precipitation events than have shown a decline – with the most solid evidence coming from North America, according to the Intergovernmental Panel on Climate Change.

But the global rate of change one might expect from a given amount of warming as greenhouse-gas concentrations rise and the climate warms, and how that plays out regionally, still generates lively discussions among climate researchers.

Going Medieval

To help answer such questions, the research team, led by Jian Liu, a climate researcher at Nanjing Normal University in China, looked at the tropical Pacific's past. The tropical Pacific and its interplay with the atmosphere play a key role in setting up rainfall patterns around the globe.

The team found that something unexpected happened there during a prolonged period of warming known as the Medieval Warm Period (950 to 1250 AD). Though the global average temperature was cooler than it is today, the rate of increase in rainfall was higher than today's rates.

This seemed to buck the well established principle that, as the atmosphere warms, it holds more water vapor – the raw material for precipitation of all types.

Based on its modeling studies, the team suggests that conditions in the tropical Pacific led to higher rainfall rates during the Medieval Warming Period because they were driven by higher than average solar activity and a lack of volcanic activity. Meanwhile, today's warming is dominated by greenhouse gases, which act in a different way and lead to lower rates of increase.

During the Medieval Warming Period, the sun's output and the low volcanic activity meant that direct heating of the ocean surface by sunlight was unimpeded by aerosols from volcanic eruptions. That added warmth to an already warm western tropical Pacific, increasing evaporation and the rise of warm, moist air into
 higher, cooler parts of the atmosphere.

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