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

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"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.

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