Irrigation Has Masked Global Warming in Past

Irrigation Has Masked Global Warming in Past



Aug. 2007  - Expansion of irrigation has 
in the past masked global warming in California’s Central Valley, but 
irrigation will not make much of a difference in the future, new research 
reveals. 
"Throughout the major irrigated regions of the world, the cooling 
influence of irrigation on daytime maximum temperatures will be much 
smaller in the next 50 years than in the past century, and will likely not 
continue to curb the effects of greenhouse warming any more," said Celine 
Bonfils, lead author of the study from Lawrence Livermore National 
Laboratory and University of California-Merced. 
The research team, which includes Bonfils and David Lobell at Livermore 
Lab, first studied the net impact of widespread irrigation on local and 
regional climate in California, the top irrigating state in the United 
States, with 3.3 million hectares under irrigation. 
Based on observations of temperature and irrigation trends throughout the 
state, the authors demonstrated a clear irrigation induced cooling in 
agricultural areas, and showed that this effect has recently slowed down, 
according to their study published in the current edition of the 
"Proceedings of the National Academy of Sciences."


In highly irrigated regions of California's San Joaquin Valley, daytime 
temperatures relative to low irrigated areas have cooled by 1.8 degrees to 
3.2 degrees Celsius since irrigation began in 1887, the study shows. 
"In comparison, there was no clear effect of irrigation on temperatures 
over the 1980-2000 period when there was no net growth of irrigation," 
Lobell said. 
"This is not a model result, but something very clearly evident in the 
data," said Bonfils. "We also looked at other major irrigated regions in 
the world, and saw a very similar pattern." 
"Globally we derive 40 percent of our food from irrigated regions, so we’d 
like to be able to model future climate changes in these regions," she 
said. 
Irrigation cools the surface of the Earth by increasing the amount of 
energy used to evaporate water rather than heat the land. The more 
irrigated the land, the more intense the effect. 
"It was quite surprising how well we could distinguish a cooling trend 
that incrementally increases with the amount of irrigation," Bonfils said. 

Other major irrigation regions used in the study include the Aral Sea 
Basin, Eastern China, Thailand, the Indo-Gangetic Plains of India and 
Pakistan, and Nebraska, the second most irrigated state in the United 
States. 
In areas where irrigation development has been rapid, including Thailand, 
the Aral Sea Basin and Nebraska, the research team found the same cooling 
effect in summer daytime maximum temperatures.


In India, Pakistan and Eastern China, the temperature change due to 
irrigation is less clear because of the presence of aerosols in the 
atmosphere that also contribute to the observed cooling by reflecting or 
absorbing sunlight, they said. 
This study also shows that rapid summer nighttime warming observed in 
Central California since 1915 cannot be explained by irrigation expansion, 
as other research has claimed. 
In 2006, John Christy, director of the Earth System Science Center at The 
University of Alabama in Huntsville, published a study showing that 
irrigation of California's Central Valley, which turned it from desert to 
productive farmland, could be to blame for warmer summer nights that have 
been recorded in recent years. 
"What was once dry, light-colored soil that didn't absorb much solar 
warmth is now dark and damp and "can absorb heat like a sponge in the day 
and then, at night, release that heat into the atmosphere," Christy said. 
"Our results show that the expansion of irrigation has almost no effect on 
minimum temperatures and that irrigation cannot be blamed for this rapid 
warming," Bonfils said. 
Lobell said, "An increase in greenhouse gases and urbanization would best 
explain this trend, which exceeds what is possible from natural climate 
variability alone." 
In California, irrigation expansion is likely to end because of 
urbanization and an increase in the demand for water to serve the needs of 
urban populations. 
In the United States, irrigation has for the first time decreased by two 
percent from 1998-2003 and growth in irrigation has already slowed down in 
many other parts of the world. 
A study published in February by the American Geophysical Union's journal 
"Geophysical Letters" also showed the masking effect that irrigation has 
on global warming. 
Researchers Lisa Sloan, Mark Snyder, and Lara Kueppers from the Climate 
Change and Impacts Laboratory at the University of California-Santa Cruz, 
wrote, "Given our results for California and the global importance of 
irrigated agriculture, past expansion of irrigated land has likely 
affected observations of surface temperature, potentially masking the full 
warming signal caused by greenhouse gas increases." 

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