|
Press Release 04-108
Falloff in Freezes: Study Projects Decrease in Frost Days
August 25, 2004
ARLINGTON, Va.—Days when the air temperature dips below freezing will become increasingly less common across much of the world by the late 21st century, according to a modeling study by scientists at the National Center for Atmospheric Research in Boulder, Colo.
The reduction in days with freezes (frost days) is projected to be most dramatic across the western parts of North America and Europe. The study is the first to examine trends in frost days using a global climate model.
In a paper published in the August 20 online edition of Climate Dynamics, NCAR scientists Gerald Meehl, Claudia Tebaldi and Doug Nychka examine the factors that have led to a reduction in frost days in many areas over the past 50 years. The authors then use the Parallel Climate Model, developed by NCAR and the U.S. Department of Energy (DOE), to simulate day-to-day temperature changes across the globe for the years 2080 to 2099. NCAR's primary sponsor, the National Science Foundation, and the DOE funded the study, with additional support from NCAR's Weather and Climate Impact Assessment Initiative.
Says Cliff Jacobs, program director in NSF's division of atmospheric sciences, which funded the research, "Based on the model simulations, this study reveals the complex relationship between various environmental factors that influence the formation of frost. An analysis of data from these model simulations and observations is likely to yield new insight into the broader impacts of climate change."
Over the past half-century, many weather stations across the western United States reported a decrease of 10 or more frost days per year, mostly related to warmer conditions in springtime. Little change in frost-day frequency has been reported across the upper Midwest and Northeast.
Until now, most modeling studies of future climate change have focused on average temperatures rather than day-to-day variations, says Meehl.
"For many years, a lot of the models didn't even have diurnal [24-hour] cycles," Meehl says. With increased computer power and more complex software, scientists can now simulate and study changes in potential day-to-day weather events far into the future.
Meehl and colleagues found the frost-day trends over the past 50 years intensifying during the next century. Nearly all of the United States and Canada show losses in frost days in 2080-2099 compared to 1961-1990.
"In general, there is a gradient from west to east across the continent, with greater decreases in frost days in the western regions," says Meehl. The biggest decrease is from the Great Plains westward, where the model produces more than 20 fewer frost days in a typical year by 2080-2099. More than 40 fewer frost days per year are projected along and near the Pacific coast from Washington State north into British Columbia.
The factors at work
The cause of this pattern, according to the authors, is a shift in atmospheric circulation.
- In northwestern North America, low-level winds are projected to blow more frequently from the Pacific, bringing relatively mild air during the winter.
- Eastern North America is projected to receive more wintertime flow of cold Canadian air. This partially cancels out the decrease in frost days that results from overall climate warming.
- A similar pattern produces greater reductions in frost days across western and northern Europe than over northeast Asia.
Although the model warms polar regions the most (as measured by average daily minimum temperatures), these areas do not show the largest decrease in frost days. The difference is in part because winter nights are so cold at high latitudes that even a century of warming fails to bring the averages above freezing.
Changes in soil moisture and cloud cover can also affect overnight low temperatures. However, the changes in atmospheric pressure and circulation were the dominant factor in producing the frost-day reductions for all locations the authors studied.
Implications for agriculture
Frost days are obviously related to the growing season, but a reduction in frost days does not necessarily mean a longer season for crops. For example, the average dates for the first and last freeze might not change even if there are fewer intervening freezes.
The authors compared the 2080–2099 model results to 1961-1990 data for one common measure of the growing season: the number of days between the first six-day spring stretch with average daily temperatures above 41°F and the first six-day autumn period with averages below 41°F. Although the length of the growing season increased in all regions examined, the authors found only a weak statistical relationship between this result and the changes in frost days.
"This tells us that other factors, in addition to frost days, contribute to the length of the growing season," says Meehl. The NCAR team plans to examine growing seasons in more detail in a future study.
The frost-day study, as well as a related effort examining heat waves, were produced in preparation for the next assessment by the Intergovernmental Panel on Climate Change (IPCC), which is expected in 2007.
-NSF-
Media Contacts
Cheryl Dybas, NSF (703) 292-7734 cdybas@nsf.gov
Anatta , NCAR (303) 497-8604 anatta@ucar.edu
Program Contacts
Cliff Jacobs, NSF (703) 292-8520 cjacobs@nsf.gov
The National Science Foundation (NSF) is an independent federal agency that supports fundamental research and education across all fields of science and engineering. In fiscal year (FY) 2009, its budget is $9.5 billion, which includes $3.0 billion provided through the American Recovery and Reinvestment Act. NSF funds reach all 50 states through grants to over 1,900 universities and institutions. Each year, NSF receives about 44,400 competitive requests for funding, and makes over 11,500 new funding awards. NSF also awards over $400 million in professional and service contracts yearly.
 Get News Updates by Email
Useful NSF Web Sites:
NSF Home Page: http://www.nsf.gov
NSF News: http://www.nsf.gov/news/
For the News Media: http://www.nsf.gov/news/newsroom.jsp
Science and Engineering Statistics: http://www.nsf.gov/statistics/
Awards Searches: http://www.nsf.gov/awardsearch/
| 
