Scientists at AGU 2012 discuss new insights into weather phenomena, and the degree to which a warming climate may shape them.

SAN FRANCISCO, CA, Dec. 4, 2012 — Heidi Steltzer, a biology professor at Fort Lewis College in Durango, Colo., said the alpine tundra high in the Rockies is the driest she’s seen in 20 years.

David Inouye, a biologist at the University of Maryland at College Park, said the Rockies are withering under the driest winter since 1973. It’s just weeks from Christmas, and the Rockies haven’t yet had significant snowfall.

The two researchers spoke Monday afternoon at the annual AGU meeting in San Francisco. Their talk about the stresses of drought and a progressively earlier spring in Colorado high country came during a handful of press conferences about weather phenomena and how continued warming around the globe might influence them in the decades to come.

Another press conference focused on new insights into the “pineapple express” — the atmospheric rivers of heavy moisture that flow from the tropics toward the west coast of California. The state was pounded by heavy rains and winds the week before the AGU meeting convened on Monday.

A third press conference discussed the potential for “Black Swan” cyclones in coming decades — megastorms that could make coastal residents on the Eastern Seaboard wish for another Hurricane Sandy.

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Back in Colorado, April mean temperatures have increased by 3.1 degrees Celsius since 1973, Inouye said. The mean temperature in April of this year was 3.4 degrees C above the 38-year mean.

Meanwhile, as snow melts earlier and more rain than snow falls, the consequences are transforming the highest mountain landscapes in the Rockies. Among them:

• Early buds are more vulnerable to frost damage, and fewer flowers results in less pollen for pollinators, and fewer seeds for insects, birds and mammals.
• Decreased seed production means plants aren’t replacing themselves as before.
• In one 12,000-foot alpine meadow studied by Steltzer, snowmelt came six weeks early and monsoon rains expected in July came late. The result was akin to the alpine meadow missing lunch and dinner — and a parched landscape. More than 50 percent of the plant species in the meadow, typically vibrant with color throughout the spring, became dormant earlier in the season.
• Dust storms — the causes of which is not clear and could have multiple drivers — are depositing particles on snowy peaks in the Rockies, contributing also to an earlier melting of snow, Inouye said.

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While Colorado dries, the west coast of California has been hammered by atmospheric rivers of moisture delivering huge amounts of rain — 15 inches in some spots since Thanksgiving. The pineapple express isn’t new, and it isn’t unique to the Pacific or California, but researchers discussed how they are learning more about these rapidly moving storm systems, and how they might change in the future.

A new coastal observation network is being installed up and down the state of California by the California Department of Water Resources and the Scripps Institution of Oceanography in La Jolla, CA to track the progress of atmospheric rivers. The network also will monitor soil moisture on the ground to help warn emergency managers of the potential for flooding.

These storms carry a huge amount of water to the west coast. The Colorado River transports about 15 million acre-feet of water annually, while a typical atmospheric river storm can deposit 10 million acre-feet of water in a day. Six to ten atmospheric river storms hit California every winter, and they are the primary source of rainwater in the mid-latitudes.

Continued warming in coming decades is expected to create a host of problems, researchers said. With warming, the atmosphere will hold more moisture and fuel the atmospheric rivers that make the pineapple express, said Mike Dettinger, a research hydrologist with the U.S. Geological Survey and a researcher with the Scripps Institution of Oceanography in La Jolla, CA. But continued warming is also expected to slacken the winds that drive these storms landward. Models show that atmospheric rivers on the west coast will become slightly more common — about 10-percent — and a little more intense by 2100, Dettinger said.

The models also show one or two atmospheric river storms carrying 50-percent to 100-percent more water than typically seen today. “Global warming is likely … to add in a few megastorms that we may not have encountered historically,” Dettinger said.

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If you live on the Eastern Seaboard, you know all about heavy rainfall, and the destructive power of another kind of storm system — like Sandy.

So-called “black swan” cyclones, extreme tropical storms that make landfall outside the tropics could generate surges of 16 feet in New York City — higher than Sandy. Computer models incorporate future projections of continued warming, said Princeton University researcher Ning Lin.