Implications of a decrease in the precipitation area for the past and the future
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Number of Parts | 29 | |
Author | 0000-0002-5969-4508 (ORCID) | |
License | CC Attribution 3.0 Unported: You are free to use, adapt and copy, distribute and transmit the work or content in adapted or unchanged form for any legal purpose as long as the work is attributed to the author in the manner specified by the author or licensor. | |
Identifiers | 10.5446/39365 (DOI) | |
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00:00
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Transcript: English(auto-generated)
00:03
So, why do we see an increase in precipitation amounts, and more extreme rainfall? There are some obvious explanations. Warm air can hold more moisture. The rate of evaporation from Earth's surface increases with higher temperatures.
00:23
The clouds become taller. But could there also be some other explanations? The hydrological cycle works like a funnel. There is a continuous evaporation taking place over larger areas, such as the oceans.
00:41
The evaporation loads the air with moisture, but the moisture of the atmosphere is not changing much, on average, and the rain removes the moisture added. But it doesn't rain everywhere, or all the time. It only rains over small patches,
01:00
so the water evaporated over a large area is returned as rain that is concentrated over a smaller region. By how much is the rain concentrated? How much is the rainfall rate amplified by the differences in the area of evaporation, and the area of rain?
01:22
And how can I quantify these effects? There is some data that can give me an answer. Satellite measurements of daily rain rates. It is not quite global, and the quality is not perfect, but it provides some clue for rainfall over 77% of the Earth's surface,
01:44
between 50 south and 50 north. The satellite data, known as Tropical Rainfall Measurement Mission, or TRMM, have measurements going back to 1998. On a daily basis, these data suggest that it rains over 25% of the area,
02:04
implying an amplification of rains by a factor between 2 and 3 when the evaporation increases. When I studied the daily rainfall measurements from TRMM, I found an unexpected trait in the satellite data.
02:21
The area with daily rain decreased by 7% between 1998 and 2016. A decrease in the area of precipitation implies even stronger amplification of the rainfall intensity, making the rain more concentrated over a smaller area.
02:43
It also suggests more drought conditions, as the area with rain is reduced. I analyzed how the change in the area of daily rain varies with the global mean temperature, and found a connection. A reduction of 17 million square kilometer per degree centigrade warming.
03:04
If this connection is real, then projections for 2100 may indicate a 28% reduction in the daily rainfall area, given the emission scenario RCP4.5.