Two studies of rainfall change during the Paleocene-Eocene Thermal Maximum (PETM) 56 Million years ago - similar to our predicted warming - indicate that dry land innundated by occasional torrential rain is in our future. The first was a floodplain field study in Spain.

Global warming: Worrying lessons from the past

Based on the analysis of sediments from the southern slope of the Pyrenees, researchers measured the impact of this warming on river floods and the surrounding landscapes. Their conclusions show that the consequences of such global warming may have been much greater than predicted by current climate models.

... the amplitude of floods increased by a factor of eight -- and sometimes even by a factor of 14 -- and vegetated landscapes may have been replaced by arid pebbly plains. 

... thousands of ancient river pebbles were measured in the field. Step by step, thanks to the direct relationship between the size of the pebbles and the slope of the rivers, researchers were thus able to calculate their flow velocity and discharge. They have therefore unveiled the whole history of these rivers, and that of the spectacular changes that have affected them.

The landscape changed completely

"With global warming, the landscape changed completely. The channel-forming floods, which occur on average every 2 to 3 years and whose flow we have been able to measure, went up to 14 times greater than before when climate was cooler,"...

During the PETM, rivers constantly changed course, they no longer adapted to increased discharge by incising their bed but instead they widened sometimes dramatically, from 15 to 160 meters wide in the most extreme case. Instead of being trapped in the floodplains, the alluvium was transferred directly towards the ocean, and the vegetation seemed to disappear. The landscape turned into arid extensive gravel plains, crossed by ephemeral and torrential rivers.

One degree of temperature rise implies a 7% increase in the atmosphere capacity to retain moisture, and this ratio is generally used to assess the increase in precipitation. "But our study shows that there are thresholds, non-linear evolutions that go beyond this ratio. With a ratio of 14 for flood magnitude, we face effects that we do not understand, which can perhaps be explained by local factors, but also by global factors that are not yet incorporated into current climate models. Our study proves that the risks associated with global warming may be far greater than we generally think,"  [emphasis mine]

image sources:flood,baby (text is paraphrase)

Episodic and intense rain caused by ancient global warming

The second study used climate models, and found that " much of the world experienced more intense and episodic (or 'flashy') rainfall events."

"The tropics became wetter and the incidence of extreme events increased, by as much as 70 percent in some tropical regions.

"In other places, total annual precipitation and the number of extreme events became decoupled; in other words, they became drier, with less frequent but more extreme events. 

"In many places, river systems that had been transporting silt or sand became associated with fist-sized rocks or even boulders; and more sediment was transported to and buried in coastal margins. In some locations, the rate of sediment accumulation increased by a factor of ten. But at the same time, there is also evidence that these systems became more arid.

"Our climate simulations reconcile this for many locations, showing an increase in aridity with fewer but more intense rainfall events." [emphasis mine]

If you can't imagine a silt-and-sand streambed suddenly hurling boulders during flood, check out this 2002 event.

Geologist investigates canyon carved in just three days in Texas flood

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