Why is it that "With higher CO2, the dry gets drier and the wet gets wetter"?

Question

The Phys.org article CO2 dip may have helped dinosaurs walk from South America to Greenland quotes Dennis Kent, "adjunct research scientist at Columbia University's Lamont-Doherty Earth Observatory" in the following:

On Earth, areas around the equator are hot and humid, while adjacent areas in low latitudes tend to be very dry. Kent and Clemmensen say that on a planet supercharged with CO2, the differences between those climatic belts may have been extreme—perhaps too extreme for the sauropodomorph dinosaurs to cross.

"We know that with higher CO2, the dry gets drier and the wet gets wetter," said Kent. 230 million years ago, the high CO2 conditions could have made the arid belts too dry to support the movements of large herbivores that need to eat a lot of vegetation to survive. The tropics, too, may have been locked into rainy, monsoon-like conditions that may not have been ideal for sauropodomorphs. There is little evidence they ventured forth from the temperate, mid-latitude habitats they were adapted to in Argentina and Brazil.

The article references Dennis V. Kent el al., "Northward dispersal of dinosaurs from Gondwana to Greenland at the mid-Norian (215–212 Ma, Late Triassic) dip in atmospheric pCO2," PNAS (2021). https://www.pnas.org/cgi/doi/10.1073/pnas.2020778118

but currently the link to the article is not working; perhaps it's not actually been published yet.

Question: Why is it that "With higher CO2, the dry gets drier and the wet gets wetter"? Does this adage come from modeling, from data, or from both? Is it a well-accepted rule of thumb?

Answer 1

To my best knowledge, the paper by Held & Soden (2006) is the first paper that discussed the concept of the rich get richer. They used the climate change experiments generated for the Fourth Assessment of the intergovernmental panel on Climate Change or in other words, climate projections using climate models forced with different CO2 scenarios.

One of their findings is that:

Assuming that the lower-tropospheric relative humidity is unchanged and that the flow is un-changed, the poleward vapor transport and the pattern of evaporation minus precipitation (E - P) increases proportionally to the lower-tropospheric vapor, and in this sense wet regions get wetter and dry regions drier.

To explain, assuming warming earth leads to more evaporation and an increase in the air capacity to carry water vapor, therefore, relative humidity can be assumed unchanged in the lower-tropospheric. Simoustanly, with warmer mid and higher latitudes the act of the atmosphere (through General Circulation) to redistribute the heat is reduced. Thus, more water vapor remains in the regions of evaporation leading to wet regions get wetter and dry regions drier.

Yes, this result is widely accepted in the community.