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.

Tony Gutierrez/AP/File
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.

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

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.

As the water vapor condensed, it released as heat the energy that had turned it into a vapor. This heating fueled further convection, boosting the build-up of thunderstorms with intense rains. The collective action of powerful updrafts feeding the thunderstorms strengthened the surface trade winds that blow from east to west. Those prevailing winds encouraged stronger upwelling of deep, cold ocean water along the South American coast, and that sharpened the contrast in sea-surface temperatures between the eastern and western tropical Pacific.

Greenhouse gases' new wrinkle

Dr. Liu's team showed that greenhouse gases have thrown a wrinkle into the process.

Instead of heating the surface of the western tropical Pacific – as happened the Medieval Warming Period – greenhouse gases heat the lowest layer of the atmosphere, or troposphere. This reduces the temperature contrast between the ocean surface and the air above it, easing the pace of convection and throttling back on the level of thunderstorm activity.

The trade winds relax, the upwelling slows, and the temperature contrast between the eastern and western tropical Pacific surface waters ease. The relaxed trade winds also allow the warm pool to migrate east, shifting its position along the equator in ways that allow its convective activity to alter atmospheric circulation patterns far from this center of action.

The contrast in sea-surface temperatures in the eastern and western tropical Pacific – and the location of the warmest water – can significantly shift regional rainfall patterns beyond the tropics.

Reconciling contradictions

Previous research into what should happen in the tropical Pacific as greenhouse gases rise had yielded opposite conclusions.

One model found that the temperature difference between the cooler eastern tropical Pacific and the warmer west intensified in a warmer climate, according to a study by Mark Cane, a climate researcher at Columbia University's Lamont-Doherty Earth Observatory in Palisades, N.Y., and a member of the team reporting the new results in the current issue of the journal Nature.

But a different, more detailed model came to the opposite conclusion that the temperature contrast would ease, according to a study by Jerry Meehl at the National Center for Atmospheric Research.

"You'd think, 'OK, we've had increasing greenhouse gases for quite awhile, and we should be able to have observations to show us how the tropical Pacific is responding,' " Dr. Meehl says. "But the observations are not that good going back into the earlier parts of the last century."

Now, the new study suggests that "we are all right," he says.

The results also may provide fresh reasons to be wary of "geoengineering" – attempts to stem global warming by changing the earth, for example building an umbrella of tiny aerosol particles high into the atmosphere to reflect sunlight back into space.

"One conclusion from this work would be: Don't fool yourself into thinking that will simply undo the damage that extra greenhouse gases do," says Dr. Cane.

If the only goal is to reduce global average temperatures, "you could probably do it," he says. But, he adds, attempts control the amount of radiation reaching Earth's surface "will not restore everything back to where it was" – in this case, precipitation patterns.

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