Regions with weakening water cycles and low soil moisture should be carefully tracked over the next few decades because they could become increasingly dry. That would make agriculture more difficult or require more irrigation.
In new research, scientists from the U.S. Geological Survey (USGS) showed that there has been an increase in the flow between the various stages of the water cycle over most the U.S. in the past seven decades. The rates of ocean evaporation, terrestrial evapotranspiration, and precipitation have been increasing. In other words, water has been moving more quickly and intensely through the various stages.
“As the planet warms we anticipate that the warmer air, which holds more moisture, will lead to more evaporation and precipitation,” said Tom Huntington, the study’s first author and a research hydrologist at USGS. “If those processes are increasing, it is evidence for an intensifying water cycle. But no one had really shown that trend quantitively.” Until now….
Huntington said these regions with weakening water cycles and low soil moisture should be carefully tracked over the next few decades because they could become increasingly dry. That would make agriculture more difficult or require more irrigation. On the other hand, too much rain or soil moisture storage, such as in the northeast U.S. or Texas, could lead to increased flooding. ..
In medieval times, the US Southwest was routinely struck by decades-long droughts. Those megadroughts stopped around 1600, but climate change could bring them back.
In a study published on Wednesday in Science Advances, researchers from Columbia’s Earth Institute used climate models to study what caused the megadroughts. Using historical climate data, they determined that two things were to blame: changing ocean temperatures and excess energy trapped inside the Earth’s atmosphere (called radiative forcing)….
“Having paleoclimatic evidence shows you what happened in the past,” lead author Nathan Steiger said over the phone. “It helps verify projections that say the American Southwest is almost assured to have a megadrought in the next few decades.” …
According to the study, the biggest driver of these historical megadroughts were La Niña events, which made the Pacific Ocean unseasonably cold, pushing the storm path north towards Washington and British Columbia. A warmer Atlantic played a smaller role, shifting a high pressure system that blocked storms from rolling over the continental US. “Both a warm Atlantic and a cold Pacific change where storms go,” Steiger said. “They both result in fewer storms going to the Southwest.”
In 2015, to combat the urgent threats posed by climate change, most of the world’s countries came together to establish the Paris Agreement: an ambitious plan to prevent the global temperature from rising 2°C above pre-industrial levels, and to work to further limit that temperature rise to 1.5°C.
These seemingly small numbers can mask the staggering impact and complexity that shifts in global temperature represent. For example, increased global temperature will also intensify the hydrologic cycle, significantly changing the frequency and intensity of rainfall. Flooding, droughts, mudslides, and food and water insecurity are just some of the many hazards of the resulting changes in rainfall patterns.
…. Another key finding was that the most extreme intensification would be about 10 times greater than the average intensification. “Our results suggest that extreme dry and wet events will increasingly co-occur, such as the switch from extreme drought to severe flooding we saw in California in the recent past,” says lead author Gavin D. Madakumbura. “At least in terms of disaster mitigation and water security, there would be significant benefits to limiting global warming to 1.5°C to dampen the intensification of event-to-event variability.”
A warming Arctic weakens the temperature difference between the tropics and the poles meaning less precipitation, weaker cyclones and weaker mid-latitude westerly wind flow — a recipe for prolonged drought.
Cody C. Routson, Nicholas P. McKay, Darrell S. Kaufman, Michael P. Erb, Hugues Goosse, Bryan N. Shuman, Jessica R. Rodysill, Toby Ault. Mid-latitude net precipitation decreased with Arctic warming during the Holocene. Nature, 2019; DOI: 10.1038/s41586-019-1060-3
When the Arctic warmed after the ice age 10,000 years ago, it created perfect conditions for drought. The temperature difference between the tropics and the poles drives a lot of weather. When those opposite temperatures are wider, the result is more precipitation, stronger cyclones and more robust wind flow. However, due to the Arctic ice melting and warming up the poles, those disparate temperatures are becoming closer.
“Our analysis shows that, when the Arctic is warmer, the jet stream and other wind patterns tend to be weaker,” says Bryan Shuman, a UW professor in the Department of Geology and Geophysics. “The temperature difference in the Arctic and the tropics is less steep. The change brings less precipitation to the mid-latitudes.”….