This press release below from Columbia University shown below suggests that irrigation cools the region undergoing irrigation. However, a study published three years ago of California’s central valley by Dr. John Christy suggests exactly the opposite. See this WUWT post from 2007, then read the Columbia story and decide for yourself.
From UAH: Irrigation most likely to blame for Central California warming
A two-year study of San Joaquin Valley nights found that summer nighttime low temperatures in six counties of California’s Central Valley climbed about 5.5 degrees Fahrenheit (approximately 3.0 C) between 1910 and 2003. The study’s results will be published in the “Journal of Climate.”
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From Columbia: Irrigation’s Cooling Effects May Mask Warming–For Now
If Water Runs Short, Some Regions May Suffer Significantly
Expanded irrigation has made it possible to feed the world’s growing billions—and it may also temporarily be counteracting the effects of climate change in some regions, say scientists in a new study. But some major groundwater aquifers, a source of irrigation water, are projected to dry up in coming decades from continuing overuse, and when they do, people may face the double whammy of food shortages and higher temperatures. A new study in the Journal of Geophysical Research pinpoints where the trouble spots may be.
“Irrigation can have a significant cooling effect on regional temperatures, where people live,” said the study’s lead author, Michael Puma, a hydrologist who works jointly with Columbia University’s Earth Institute and its affiliated NASA Goddard Institute for Space Studies. “An important question for the future is what happens to the climate if the water goes dry and the cooling disappears? How much warming is being hidden by irrigation?”
Scientists generally agree that in the last century, humans have warmed the planet about .7 degrees C (about 1.3 degrees F) by pumping vast amounts of carbon dioxide into the air. How much warmer earth will get depends not only on future carbon emissions but an array of other variables. For instance, earth’s oceans and vegetation have been absorbing a growing share of emissions, but recent studies suggest this uptake may be slowing. This could lead to more carbon dioxide in the air, and accelerated warming. On the other hand, humans are also cooling the planet to some degree, by releasing air-polluting particles that lower temperatures by reflecting the sun’s energy back into space. Pumping of vast amounts of heat-absorbing water onto crops is lowering temperatures in some regions as well, say the authors.
Scientists are just beginning to get a handle on irrigation’s impact. In a hundred years, the amount of irrigated farmland has grown four-fold, now covering an area four times the size of Texas. Puma and his coauthor, Benjamin Cook, a climatologist at Goddard and Columbia’s Lamont-Doherty Earth Observatory, are the first to look at the historic effects of mass watering on climate globally by analyzing temperature, precipitation and irrigation trends in a series of model simulations for the last century. They found that irrigation-linked cooling grew noticeably in the 1950s as irrigation rates exploded, and that more rain is now falling downstream of these heavily watered regions.
In warm, dry regions, irrigation increases the amount of water available for plants to release into the air through a process called evapotranspiration. When the soil is wet, part of the sun’s energy is diverted from warming the soil to vaporizing its moisture, creating a cooling effect. The same process explains why drying off in the sun after a swim at the beach can be so refreshing.
Globally, irrigation’s effect on climate is small—one-tenth of one degree C (about 0.2 degree F). But regionally, the cooling can match or exceed the impacts of greenhouse gases, say the scientists. For example, the study found some of the largest effects in India’s arid Indus River Basin, where irrigation may be cooling the climate up to 3 degrees C, (5.4 degrees F) and up to 1-2 degrees C in other heavily irrigated regions such as California’s Central Valley and parts of China. The study also found as much as .5 degree C cooling in heavily watered regions of Europe, Asia and North America during the summer.
The study suggests also that irrigation may be shaping the climate in other ways, by adding up to a millimeter per day of extra rain downwind of irrigated areas in Europe and parts of Asia. It also suggests that irrigation may be altering the pattern of the Asian monsoon, the rains that feed nearly half of the world’s population. These findings are more uncertain, the authors caution, and will require further research.
“Most previous modeling studies were idealized experiments used to explore potential impacts, but this is a much more realistic simulation of the actual impacts,” said David Lobell, a Stanford University scientist who studies climate impacts on agriculture and was not involved in the study. “Their results show some interesting differences by time period and region that will lead to more research and contribute to more accurate simulations of future climate, particularly in agricultural areas.”
Irrigation has increased because it boosts crop yields, supporting many millions of small farmers, said Upmanu Lall, head of the Columbia Water Center at the Earth Institute. But concern is growing that groundwater supplies in India and China may not keep up. “Near term and future climate predictions are essential for anticipating climate shocks and improving food security,” he said. “The study points to the importance of including irrigation in regional and global climate models so that we can anticipate precipitation and temperature impacts, and better manage our land, water and food in stressed environments.”
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NOTE: The scientific paper was not provided with the press release.
Richard Holle says:
September 9, 2010 at 2:29 am
Great!. “As Above, so Below”
savethesharks says:
Generally, real famine and pestilence occurs during cold times, not warm.
Chris here puts the emphasis where it belongs: on actual well-being. The AGW alarmists complain about what increases life or health; and praise what causes extinctions, suffering or hunger. Since so many people believe them, these econazis are a serious threat to your well-being and even your life. Not to mention “the environment.”
Let’s hope we see a followup article at WUWT from Dr Christy after he has had an opportunity to absorb this latest paper and has some time to write something. Nothing is as unsettled as real science. You have to go to Disneyland or Hollywood to find a clown who’ll tell you that all that is knowable is known about anything.
If this is true, then water feedback is certainly negative.
Christy mentions an increase in nighttime low temperatures by 5.5 F between 1910 and 2003. I would take this figure with a grain of salt if it relies on the same filtered data as NOAA, which as I understand it includes Sierra Nevada (high altitude, hence cool) temperatures in the early part of the century, which is removed from the data by the latter part. The NOAA valley temperature is derived from gridded data with interpolation from missing stations. The removal of NOAA data will introduce a fictional warming signal in central California. From that I don’t know what relative effects happen in the nighttime set, but it seems to me that it would be most extreme then, because as anyone familiar with high country will confirm, not only are temperatures cooler in general, but there is a larger daily variation at higher altitudes. Thus, nights are not only cooler because of the altitude but also because of a larger nighttime temperature swing.
Both of these studies appear to rely on cherrypicked data from regions with many peculiarities (a little less so in the latter case); it is a huge leap to isolate one particular causative factor out of many possible ones (such as urban development, drainage of swamps, deforestation, etc.) and attribute some climate factor change to that cause. Both studies, as far as I can tell from the articles, wave only vaguely at scientific principles that may explain such a connection but neither gives any actual theory demonstrating that, in principle, such an effect must occur.
I think a natural reply to both studies, even if one takes all data at face value, is “correlation is not causation”.
John-“That is from the 2007 presser you can access from the link above. So both studies agree on more daytime cooling the last century. No disagreement.”
Perhaps if you only look at the min and max separately and qualitatively, as you are doing, you might think these studies found the same thing. They did not. I can only explain this to you once more before I pull my hair out:
Christy found that the large min temp warming exceeded slight max temp cooling, so the mean trend was positive. But the study this post talks about says that the max cooling would exceeding the min temp warming, leading to net trends towards COOLING. Qualitatively these findings seem superficially similar, except in the trends in mean temps, but quantitatively they are very different.
R. Craigen-I am also only going to say this one more time, as I am tired of seeing people totally ignorant of what Christy did smearing himavailable that wasn’t previously! How dare you sir? by implying that he is using GISS or NOAA or GHCN data which is “flawed”. NO NO NO! Christy painstakingly assembled data himself. He does not MIX Sierra and Valley, he isolates them and uses data that none of the major groups use, PAINSTAKINGLY DIGITIZING THEM PERSONALLY (This is the thanks he gets for this hard, honest, careful book keeping?) where others simply ignored them. He did not “cherry pick” data, he went to great lengths to use all the data available and then to make data available that wasn’t already! READ THE DANG PAPER! UGH!
Water used per area of surface is being reduced by more efficient use such as drip/trickle feed, and monitoring fields with ability to control each sprinkler head independently. (Nothing like a shortage to motivate that – read The Doomsday Myth by Charles Maurice and Charles Smithson.)
But area may increase if the savings are still available for irrigation.