ucar Highlights 2007

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Winds of Change

Understanding the most critical environmental threat of our time—a climate in flux—is one of the prime goals of NCAR scientists and their university colleagues. Their work flows from NCAR’s rich and sustained program of basic research on the Earth system.

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The character of change | Top of the world | Shaping the atmosphere | Models and molecules |

How to talk about climate change | Oceans and carbon dioxide | A tropical tempest


web iconThe National Hail Research Experiment (article)


Shaping the atmosphere: Old dreams and new visions of weather and climate control

Humans have been inadvertently modifying climate for many years, most dramatically by adding greenhouse gases to the air. And for more than half a century, we’ve also tried to modify the atmosphere more consciously, sprinkling silver iodide or other particles into clouds in a bid to stimulate rain and snow where it’s most needed or wanted.

Weather modification remains a mixed bag, with success difficult to quantify. NCAR experts are working to ensure that rainmaking efforts are rigorously evaluated. Meanwhile, some of the center’s leading climate scientists are looking into the wisdom and feasibility of trying to engineer a cooler, more stable climate—a topic fraught with many of the fears and concerns raised by weather modification in the last half-century, but this time with the stakes raised to a global level
dan breed

Daniel Breed

A splash of reality

“We’ve found a niche,” says Daniel Breed, describing NCAR’s weather modification program. Instead of seeding clouds itself, the center gets support from states and other countries to help them plan and evaluate their own efforts. While there’s powerful motivation for weather modification sponsors and providers to put the best face on results, NCAR’s assessment team specializes in hard-headed analysis of what likely works and what doesn’t.

Take the Indonesian island of Sulawesi, which is prone to severe drought during El Niño events. “We found that the clouds are pretty efficient already, so cloud seeding wouldn’t have much of an effect. That wasn’t the answer they wanted,” says Breed. In the desert domain of the United Arab Emirates, a multiyear study led by NCAR also came to skeptical conclusions about the usefulness of weather modification. p> In other regions, seeding appears to bring benefits, but making the case airtight is exceedingly difficult. NCAR’s William Cooper and Roelof Bruintjes served as collaborators on a 1990s study in South Africa that showed real potential for enhancing rainfall by injecting hygroscopic (water-attracting) particles into clouds via aircraft. Several years later, NCAR assisted Mexico with a similar multiyear study. “The results almost identically matched South Africa’s,” says Breed, “but we erred in testing too many interrelated variables, so the statistics weren’t as solid.”

To truly prove the worth of cloud seeding, a study must be not only lengthy but randomized, with some clouds seeded, similar ones
proposed device

This proposed device could loft enough seawater spray to boost the sunlight-reflecting power of marine stratocumulus clouds, much as the 1991 eruption of Mount Pinatubo (below) blocked sunlight and cooled global climate.

unseeded, and everything observed carefully. “If you do aircraft measurements, you’ve got to be in every single cloud in the test region,” says Breed. Ideally, the seeding should be monitored remotely with tools such as a dual-polarization Doppler radar, which can distinguish rain from hail and provide other key microphysical data. The first major study to involve such a radar is taking shape in Australia, where severe drought has decimated agriculture and left millions of people water-poor.

Because cloud seeding relies on boosting the yield from rain-prone clouds, it’s better suited to increasing a normal year’s precipitation than to wringing out moisture in a dry year. This makes seeding a tempting option for places like the Rocky Mountains, where climate change is expected to shorten the abundant snow season on both ends and pinch the warm-season runoff vital for summer agriculture and urban water supply. “In that context, cloud seeding might help build up the snowpack and lengthen the melt period,” says Breed. NCAR and partners are now in the midst of a multiwinter study of a project in Wyoming aimed at boosting snowpack, which sustains hydroelectric power, water quality, agriculture, and recreation.

Can we engineer our way out of global warming?

When the 1991 eruption of Mount Pinatubo sent millions of tons of ash and sulfates into the stratosphere, it also cooled the globe for several years afterward. What if humans used rockets or

aircraft to inject a Pinatubo-sized helping of particles into the stratosphere every few years? NCAR’s Tom Wigley studied just this scenario in a 2006 paper. He found that this approach could stave off enough global warming to buy society as much as 20 years before the need for major emissions cutbacks—which he says would remain essential even after this artificially induced delay. “We’re already performing an uncontrolled experiment by adding greenhouse gases,” says Wigley. “We need to investigate all the options in an honest and comprehensive way.”

Research into geoengineering, the label for such schemes, has been going on for decades but has generally kept a low profile. Researchers feared sending a tacit message that global warming
Mt Pinatubo

The eruption of 1991’s Mount Pinatubo in the Philippines blocked sunlight and cooled global climate for more than a year.

could be reined in without emissions cuts. The quiet was broken in 2006 with a set of much-publicized papers in the journal Climatic Change, including one by Nobel Prize winner Paul Crutzen (Max Planck Institute for Chemistry). A workshop later that year, sponsored by NASA and the Carnegie Institution, pulled together more than 40 scientists for a frank discussion of how humans might “manage” the amount of solar radiation reaching Earth’s atmosphere—and whether or not this is even advisable.

“We had many bright people on hand with a lot of knowledge and good ideas,” says Carnegie’s Kenneth Caldeira, the workshop chair. “Just about everybody there would rather see large reductions in carbon dioxide emissions than deployment of geoengineering schemes.” However, he adds, “There is a lot of pessimism that cuts in carbon emissions will not be deep enough or come soon enough to ward off severe damage from climate change. Some of that pessimism is being channeled into serious consideration of geoengineering options.”

At the workshop, NCAR’s Philip Rasch presented initial results from his work using the NCAR Community Atmosphere Model to analyze the regional and seasonal fingerprints that stratospheric injections might leave on climate. Rasch found that even if global temperature could be stabilized, high-latitude winters might still warm up, and rainfall patterns in the tropics and Southern Ocean might be affected.

Artificial eruptions aren’t the only ideas on the table. In 1990, NCAR’s John Latham proposed altering the character of marine stratocumulus clouds. These are among Earth’s most extensive and reflective clouds, covering much of the subtropical ocean. If the number of droplets in marine stratocumulus could be roughly doubled, Latham argued, the cooling from their enhanced reflectivity would roughly balance the warming produced by a doubling of carbon dioxide.

Latham and colleagues from the Universities of Manchester, Leeds, and Edinburgh fleshed out the idea in a 2006 paper. One technique would be to use rotors 20 meters (66 feet) tall aboard wind-powered seagoing vessels (see illustration). The rotors would generate tiny salt-water droplets that could serve as cloud condensation nuclei. “Our recent papers, and positive responses to a lot of seminars and conference presentations, have now given us much more confidence and affirmation,” says Latham.
Some climate scientists worry about the wisdom of even considering geoengineering. For one thing, most of the proposed schemes still allow carbon dioxide emissions to increase, which raises the spectre of ocean acidification (see page 20). There’s also the law of unintended consequences. In a 2007 paper, NCAR’s Kevin Trenberth noted that the Pinatubo eruption was followed in 1991–92 by a global decrease in continental rainfall and snowfall and a record drop in runoff and river discharge. “These results suggest that major adverse effects, including drought, could arise from geoengineering solutions,” says Trenberth.

Whether they find merit or madness in studying geoengineering, nearly all climate scientists would like to see global warming attacked directly by reducing emissions significantly. In the words of NCAR’s Jeffrey Kiehl, “Treating the causes rather than the symptoms of climate change is the more appropriate approach to the problem.”

The character of change
| Top of the world | Shaping the atmosphere | Models and molecules |

How to talk about climate change | Oceans and carbon dioxide | A tropical tempest