Climate Change Is Intensifying the Water Cycle, The Intergovernmental Panel on Climate Change,
The 1,300-page paper is the most comprehensive, up-to-date report yet on the physical science of climate change,
synthesizing the findings of thousands of recent publications.
The report paints an alarming picture of the future of fresh water. It concludes that man-made contributions to a warming planet are far-reaching. It finds more evidence that severe weather events are linked to carbon in the atmosphere and are becoming more extreme.
And it shows that certain trends such as rising seas
and shrinking ice sheets will continue even if carbon pollution were halted immediately.
But it also indicates that by swiftly and drastically cutting greenhouse gas emissions,
the worst effects of climate change can be prevented, avoiding worst-case outcomes for water availability.
“I used to say, when I was talking about climate change, that climate change is serious, certain, and soon,” said Linda Mearns,
a senior scientist at the National Center for Atmospheric Research.
“But this is no longer accurate. Now it is very serious, very certain, and now.”
Extreme droughts affecting agriculture and ecosystems are already more frequent and intense than they were last century.
This trend will continue as glacial melting, decreased rainfall, and a “thirstier” atmosphere jeopardize sources of freshwater in some parts of the globe.
The report makes a distinction between four types of droughts: those that affect precipitation, streams, farming, and ecosystems.
Agricultural and ecological drought
It states with medium confidence that climate change is leading to an increase in droughts that impair agricultural production and ecosystems, due to the drying of soils.
Agricultural and ecological drought events that used to occur, on average,
every 10 years are already 70 percent more frequent than they would be absent human influence.
After 2 degrees Celsius of warming, which is the current trajectory unless emissions fall rapidly in the next few years, these events become 2.4 times as frequent.
This is happening both because less rain is falling in drier areas,
and because a warmer atmosphere is “thirstier” than in the past, evaporating more water out of soils.
These changes are seen most clearly today, the report states, in western North America and the Mediterranean.
“If emissions continue, then there is a very good chance [for the western United States] that we’re going to see a level of drought and aridity that we haven’t seen in at least a thousand years,” said Jessica Tierney, an associate professor of geosciences at the University of Arizona.
Another trend is that snowpacks and glaciers that many rely on for fresh water are disappearing.
As winter months become warmer and the global mass of glaciers and snowpack shrinks,
downstream regions will see less water in summer months.
Droughts are now worse mainly in dry areas, such as the Mediterranean basin, western North America, and southern Africa. But some wet regions, like the Caribbean and the Amazon, are expected to experience more severe droughts as well. These patterns are expected to further strain groundwater, which is already being depleted in many areas.
Heavy rainfall will become more common and more intense.
Atmospheric warming makes heavy rainfall even heavier: the warmer the atmosphere,
the more moisture it can hold, which means more rain. Today,
with atmospheric warming of just over 1 degree Celsius, 10-year rain events are 30 percent more frequent
and 7 percent more intense than they would be absent human influence.
The intensity and frequency of such storms increases with more warming.