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The 'Hydro-illogical cycle'

15 Apr 2020 | Science Notes

Alas, we cannot take credit for that title, which was coined by hydrologists a long time ago to describe the tendency of experts and others to target flood research on recent disasters, skewing data collection in the process. In a recent study of the global hydrological cycle and its behaviour under climate change, Greek climate expert Demetris Koutsoyiannis wrote “A recent large flood in a catchment will often lead to funding a study on the flood history of that catchment which will find there was a large flood at the end of the record.” Repeat the process often enough and voila, you have a collection of evidence showing floods are increasing. But, Koutsoyiannis added, a wider perspective reveals a different picture: “based on a list of world record point precipitation measurements... the highest frequency of record rainfall events occurred in the period 1960-80; later the frequency was decreased remarkably.” Seems rainfall patterns aren’t following the illogical predictions of climate models.

According to conventional models, as the climate system warms, the air should absorb and retain more water vapour, so average (aka “specific”) humidity should be rising. With more moisture in the air available to form clouds and rain, the hydrological cycle should also be intensifying, leading to more frequent and severe heavy rainfall events. If not, well, you face a painful choice between assumptions and evidence again.

In his paper, Koutsoyiannis gathers the largest global data sets, combining ground- and satellite-based measures, which together give us a fairly complete picture of what is actually happening in the atmosphere, and compares them to what climate models predict. He finds that while there are fluctuations on short- and long-term time scales, humidity is only increasing by about one-third the rate predicted in climate models, and overall hydrological intensity is going down, not up.

He also adds that even if the model projections were proved to be mostly accurate, they still imply fluctuations that are very small compared to… natural cycles. “In this respect, even if the established climatic hypotheses of an intensifying hydrological cycle with rates of the order of 1% (never reaching that of 10%) were validated, hydroclimatic concerns would not be justified. In older times such rates of change would not be discussed at all.” Lately it has seemed people talk about little else.

One comment on “The 'Hydro-illogical cycle'”

  1. One degree C increases the vapour pressure of water by 7% which means 7% more water molecules in the air immediately above the liquid water. Since our atmosphere is probably half rising and half falling, one can make the argument that there should be 3.5 % more rainfall. Except if your average year has say 15 inches of rainfall, that is really a range from 6 inches in a dry year to 24” in a wet year….and an increase of 3.5% or half an inch….is just noise in your measurement error….or an afternoon thunder shower that missed your rain gauge.

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