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

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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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