From NOAA (Caitlyn Kennedy):

At the heart of California’s water supply are atmospheric rivers—narrow, long ribbons of moisture that transport huge amounts of water vapor from the tropics toward the poles. When atmospheric rivers move inland and strike mountains, the air rises and cools, creating heavy rainfall. Atmospheric rivers are the source of 30-50 percent of precipitation along the U.S. West Coast, and they are a major driver of the region’s most serious floods. But the events are also called “drought busters” because, as these maps show, just one or two storms can help replenish the water system during dry spells.

Winter 2010 was a case in point. The maps at right show the drought status in California before and after two atmospheric river events in January 2010. Moderate to severe drought dominated California as of January 19 (image left). On January 20-21, an atmospheric river swept down the coast of California, followed by another event on January 24-26 that mainly impacted the northern half of the state. Over the course of the week, California averaged 3.63 inches of total accumulated rainfall, with some areas receiving as much as 21.8 inches (middle). By the end of the week (image right), only 19 percent of the state faced moderate drought conditions, a substantial decrease from 63 percent the previous week.

Persistent droughts often end as a result of the arrival of an especially wet month or a few very large storms—but how often do atmospheric rivers play a role? A 2013 study analyzing drought events along the West Coast over the last 60 years found that 33-74 percent of droughts were broken by storms delivered during atmospheric river events. These events broke up about two-thirds of the droughts in the Pacific Northwest and about one-third to 40 percent of all droughts in California. The remaining droughts in California were mostly broken up by rainfall resulting from local low-pressure systems.

Most of California’s rainfall occurs from November through May, and it’s very unusual for the region to get any rainfall in other parts of the year. A little more than half of California’s precipitation is generally concentrated in winter storms in December through February. This winter, however, an extreme lack of winter precipitation persisted, intensifying the deficit that had developed during the previous two water years. In February, a few storms finally found their way to the drought-stricken state, but unfortunately, they were not enough to alleviate the current drought emergency.

Water shortages during dry years affect the interests of all kinds of water users in California, and they leave water managers trying to strike a careful balance. For instance, Sonoma County, California, has received less than half of its normal rainfall for this water year to date and is currently experiencing its driest period on record. Water levels in Lake Mendocino—the smaller and more vulnerable of the region’s two major reservoirs—have dropped to a near historic low storage level for this time of year. If relief does not come soon, Californians will face significant cutbacks in water use to prevent reservoirs from going dry in the fall.

NOAA Climate.gov maps based on U.S. Drought Monitor data and PRISM precipitation data.

References

Dettinger, Michael D. (2013): Atmospheric Rivers as Drought Busters on the U.S. West Coast. J. Hydrometeor, 14, 1721–1732.

From The Washington Post (Shane Harris and Michael Birnbaum):

As the United States and nations around the world struggle to blunt the effects of rising temperatures and extreme weather, sweeping assessments released Thursday by the White House, the U.S. intelligence community and the Pentagon conclude that climate change will exacerbate long-standing threats to global security.

Together, the reports show a deepening concern within the U.S. security establishment that the shifts unleashed by climate change can reshape U.S. strategic interests, offer new opportunities to rivals such as China, and increase instability in nuclear states such as North Korea and Pakistan.

The reports emerge as world leaders prepare to gather in Glasgow, Scotland, next month for crucial U.N. climate talks. And the assessments suggest that the Biden administration is preparing to take on the national security consequences of global warming after four years of inaction under President Donald Trump. During his presidency, climate-related security assessments were routinely suppressed because they did not match his administration’s skeptical stance toward climate science.

Shortly after President Biden came into office, he ordered that climate change play a far more prominent role in U.S. security strategy.

The Pentagon report in particular marks a shift in how the U.S. military establishment is incorporating climate issues into its security strategy, analysts said. Until now, when the Defense Department has considered climate change, it has tended to focus on how floods and extreme heat can affect military readiness rather than the broader geopolitical consequences of a warming world. Now it is worried that climate change could lead to state failure.

“Climate change is altering the strategic landscape and shaping the security environment, posing complex threats to the United States and nations around the world,” Defense Secretary Lloyd Austin said in a statement that accompanied the Pentagon report. “To deter war and protect our country, the [Defense] Department must understand the ways climate change affects missions, plans, and capabilities.”

The shift in Washington comes as militaries and security agencies around the world are accounting for global warming in their planning. At NATO, Secretary General Jens Stoltenberg earlier this year made climate change a major focus of the defense alliance as it overhauls its strategic plans. The British military this spring unveiled a sustainability report that counsels a top-to-bottom overhaul of military operations to prepare for far more climate-related deployments in the coming decades.

The release of the U.S. assessments “sends a warning message ahead of next month’s U.N. summit of the grave risks that we’re facing and why it’s so critical. These reports are overdue,” said Erin Sikorsky, director of the Center for Climate and Security and a former senior U.S. intelligence official focused on climate issues.

The new National Intelligence Estimate (NIE) on climate, a first-of-its kind document by the Office of the Director of National Intelligence, builds on other grim warnings from national security officials about how a changing climate could upend societies and topple governments.

“We assess that climate change will increasingly exacerbate risks to U.S. national security interests as the physical impacts increase and geopolitical tensions mount about how to respond to the challenge,” the document states. It also concludes that while momentum to reduce global emissions of greenhouse gases is growing, “current policies and pledges are insufficient” to meet the goals that countries laid out in the landmark Paris climate accord.

A former senior intelligence official lauded the document’s contribution to understanding the security implications of climate change…

The Pentagon warns that disruption to fisheries could spark conflict over food security. Unpredictable rainfall might increase tensions over access to rivers that cross national boundaries, such as the Nile and the Mekong. Even efforts to combat climate change could lead to unintended consequences, such as conflicts over access to the rare minerals that are needed to build circuitry and wind turbines.

The report says the Defense Department should ready itself to provide humanitarian assistance in climate crises, incorporate climate-related issues into its war-games — and also work on “countering malign actors who seek to exploit climate change to gain influence.” Some of the most specific analyses remained classified.

The White House report on migration, which examines the way climate change is driving human movement around the world, notes that drought and other extreme weather can spark conflicts and force population displacements — and that countries such as China and Russia are poised to take advantage…

It advocates expanding asylum and refugee programs to better take into account climate-driven migration. And it says that U.S. policymakers need to be ready to direct funding and resources toward regions that are facing influxes of migrants driven to move by extreme weather, droughts and climate-related conflicts. It cites one report that estimates that by 2050, up to 143 million people in Latin America, sub-Saharan Africa and South Asia could move for climate-driven reasons…

To a significant degree, China will influence how quickly and how much global temperatures rise. The NIE notes that it accounts for about 30 percent of emissions globally, the largest single source.
But “modest reduction targets” in China’s long-term plans raise doubts about whether it will meet its reduction goals, the NIE finds.

“China has not publicly articulated detailed plans for meeting its 2060 net-zero emissions target; to do so, we assess that Beijing would need to follow through on President Xi Jinping’s pledge at the U.S. Climate Summit in April to phase out coal consumption,” the NIE said.

And that will be hard to do. China, along with India — the world’s fourth-largest emitter — are incorporating more renewable and low-carbon sources of energy, the NIE says, “but several factors will limit their displacement of coal.”

The NIE concludes that geopolitical tensions are likely to rise in the coming decades as countries struggle to deal with the physical effects of climate change — which scientists say already is producing more devastating floods, fires and storms — as well as the political ones. Mitigating climate-related disasters may call for solutions that some countries cannot afford and political will that some leaders cannot muster.

The physical effects are likely to be most keenly felt in parts of the world already being reshaped — such as the Arctic — and in regions and countries that are particularly vulnerable because they experience extreme climate events, such as hurricanes or droughts, and because their governments are ill-equipped to manage the fallout.

The NIE identifies 11 countries in that category of acute risk: Afghanistan, Colombia, Guatemala, Haiti, Honduras, India, Iraq, Myanmar, North Korea, Nicaragua and Pakistan.

An NIE is a unique document in that it reflects the consensus view of all the U.S. intelligence agencies. Traditionally, producing the documents can take months, and they present the most comprehensive analysis of significant national security concerns. The NIE released publicly is unclassified, but a classified version will be provided to policymakers, officials said.

Using satellite data, scientists are documenting the inexorable melting of South America’s glaciers and ice fields, with Andean glaciers thinning by nearly three feet a year since 2000. The loss of ice poses a threat to water supplies and agriculture from Bolivia to Chile.

In recent decades, nearby residents of the Cordillera Vilcanota have watched in dismay as the Colquepunco and surrounding glaciers have steadily shrunk. Now, researchers in Germany and France have quantified just how rapidly ice in Peru and throughout the Andes is disappearing. Using high-resolution data generated by satellites and a 2000 Space Shuttle mission to create three-dimensional representations of Andean glacier change over time, the researchers calculated that the area covered by glaciers in Peru shrank by nearly a third from 2000 to 2016…

Across the Andes, glaciers have lost nearly 3 feet in thickness annually since 2000, according to Etienne Berthier, a glaciologist at the Laboratory of Geophysical Studies and Oceanography in Toulouse, France, who recently published his findings in Nature Geoscience. Warming temperatures also have caused glaciers to swiftly recede, particularly in the southern Andes, where some glaciers have retreated 5.5 miles in the past century. Ninety-eight percent of Andean glaciers have shrunk this century.

Glaciers are vital resources for communities in and around the Andes, where meltwater is used for drinking, irrigation, and hydroelectric power — especially in arid regions and during periods of drought. “The disappearance of glaciers will have an impact on the cities, but not just cities — locals, farmers, and people who do agriculture more broadly,” says Francou.

The loss of Andean glaciers also has global repercussions. Nearly all the world’s ice is locked up in the vast ice sheets of Antarctica and Greenland, with lower-latitude mountain glaciers and ice caps making up only 4 percent of the world’s land ice area. But because the world’s mountain glaciers — including in the Andes, the Himalayas, the Tibetan Plateau, the Alps, and various Alaskan and Russian ranges — are melting so rapidly, they have been responsible for a disproportionate share of global sea level rise in recent decades. No mountain region has lost more ice, relative to its size, than the Andes.

Until recently, information regarding the speed and quantity of Andean ice loss was generally restricted to more easily accessible sites, with scientists manually planting stakes in glaciers and recording changes in their mass over the years, says Berthier. But the recent satellite studies have greatly expanded scientists’ ability to track melting glaciers in the Andes and around the globe.

Patagonia’s ice fields account for 83 percent of all ice loss in South America.

“Our study, and the one from Etienne Berthier, are the first studies that cover the whole [South American]continent based on measurements everywhere,” says Thorsten Seehaus, a glaciologist at Friedrich-Alexander-Universität Erlangen-Nürnberg, who recently published his findings in Nature Climate Change.

Berthier and his team were able to obtain data covering most sub-regions of Andean glaciers, giving the researchers a more accurate picture of the pace of glacial retreat and enabling them to better forecast how quickly glaciers will recede in the future.

What the new data shows is that while Andean glaciers overall are receding, they are doing so at varying rates in different regions. In the Desert Andes, for example, a small number of glaciers are actually expanding or holding steady, says Seehaus — though these account for only 1.3 percent of the glaciers studied.

The overall trend, though, is abundantly clear. Andean glaciers — from the small icy regions of Colombia and Venezuela in the north all the way to Patagonia’s glaciated expanses in the south — are rapidly shrinking.

The south Patagonian ice fields are the fastest-melting on the continent, thinning by an average of nearly 3.3 feet a year, according to Berthier’s study. Together with the northern Patagonian ice fields, these regions account for 83 percent of all ice loss in South America. The reason for this, explains Francou, is that the low-altitude glaciers of Patagonia make them particularly vulnerable to rising air temperatures.