A few folks have mentioned to me over the years that they thought we haven't been getting as many frosts and freezes as in the past, that mosquitos were worse than in years past, and that it seems more humid than it used to be.
Of course the reaction could be to say "Global Warming". But you'd be surprised (as I was) to learn that there may be another reason. Irrigation. Rice Fields, cotton fields, nut orchards, and other agricultural enterprises have grown (pardon the pun) dramatically in the San Joaquin and Sacramento Valley in the last century. A University of Alabama study found that Irrigation was likely the cause of increased overnight temperatures.
Intrigued, I decided to lookup and plot the minimum temperature data for Chico University Farm, which is the station of record for climate here to see if it showed the same trend. It did. Here is the results of data from 1900-2000:
And to be consistent, I also plotted the maximum temps too, which was surprising:
And even more surprising was the Mean Annual Temperature:
If anybody wants to check my data, I'll gladly make it available. Here's the article in its entirety:
Irrigation most likely to blame for Central California warming From: University of Alabama, Hunstville
http://www.uah.edu/News/newsread.php?newsID=293
The same irrigation that turned California's Central Valley from desert into productive farmland is probably also to blame for summer nights there getting noticeably warmer.
Irrigation has turned much of the San Joaquin Valley's dry, light-colored soil dark and damp, says Dr. John Christy, director of the Earth System Science Center at The University of Alabama in Huntsville (UAH). While the valley's light, dry desert ground couldn't absorb or hold much heat energy, the dark, damp irrigated fields "can absorb heat like a sponge in the day and then, at night, release that heat into the atmosphere."
That means the region's summer nighttime temperatures don't get as cool as they did before irrigation came along.
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."
The study area included six California counties: Kings, Tulare, Fresno, Madera, Merced and Mariposa.
While nighttime temperatures have risen, there has been no change in summer nighttime temperatures in the adjacent Sierra Nevada mountains. Summer daytime temperatures in the six county area have actually cooled slightly since 1910. Those discrepancies, says Christy, might best be explained by looking at the effects of widespread irrigation.
"The San Joaquin Valley is a desert," he said. "Historically that desert has been cool at night due to two factors: Dry air and dry, light-colored soil. In its natural state the soil in the Central Valley absorbs and retains very little energy from the sun, so it has little heat to release at night.
"Another factor is the dry air, something common to all deserts. Water vapor is a powerful greenhouse gas. Desert air lacks water vapor. The air turns cold at night because it doesn't retain much warmth from the daytime and it can't trap what little heat might rise from the ground at night."
Evaporation from irrigated fields adds water vapor to the air -- a process that cools summer days but traps heat rising from the damp soil at night.
"If there is anything I've learned in Alabama, it is that humidity can make summer nights very warm," said Christy, a Fresno, Calif., native who has lived in Alabama since 1987.
Since the early 20th Century irrigation has helped to convert much of California's Central Valley desert -- including more than two million acres in the study area's six counties -- into a dark, moist, vegetated plain.
Irrigation has not spread into the nearby mountains, Christy said, and that might be why summer nighttime temperatures there haven't warmed.
(Increased humidity also reduces the effectiveness of evaporative "swamp coolers" once in widespread use in homes and businesses across the Central California desert.)
Curious after hearing complaints from family and friends about the warm nights, Christy looked at Fresno weather reports and noticed that some temperatures were warmer than any he remembered from his youth. He turned his curiosity about the Fresno weather into a research proposal that was funded by the National Science Foundation.
With help from UAH's William Norris, Dr. Kevin Gallo, a NOAA scientist at the U.S. Geological Survey's National Center for Earth Resources Observation and Science in Sioux Falls, S.D., and Kelly Redmond at the Western Regional Climate Center in Reno, NV, Christy spent two years studying the valley's climate record, hand-entering into the database information from 1,600 pages of daily temperature reports back to 1887 from some stations. He ended up with 18 valley and 23 mountain stations to study.
The conflicting temperature trends in the valley and the mountains reduce the likelihood that the valley's warmer summer nights might be caused by large-scale or global climate change due to enhanced greenhouse gases, especially carbon dioxide, in the atmosphere, Christy said. "If this was related to large-scale climate change, you would expect all elevations to be affected."
Most theories about human-caused global warming predict that nights should warm more than days (with winter nights warming the most) as concentrations of carbon dioxide and other greenhouse gases in the atmosphere increase.
In the California study area, however, nighttime low temperatures warmed faster in the summer and fall than they did in the winter and spring ‹ 5.5 degrees vs. 3.8 degrees Fahrenheit (3.0 vs. 2.1 C).
There are one or two possible explanations for this, says Christy -- assuming that the Central Valley warming is caused by irrigation. For one thing, winter air in the valley was damp before irrigation arrived.
"It's much more humid in the winter time," he said. "Fog can last for a week there and you don't see the sun. It's naturally wetter, so it was already trapping some heat. Adding water that evaporates from irrigation to already moist winter air and soil isn't going to change the energy equation as much as it does when the air and ground are both dry.
"Plus, it's cooler in the winter so there isn't going to be as much water evaporating from canals and irrigated fields."
Computer models used to forecast climate change also typically predict that in California the effects of global warming due to increased carbon dioxide levels should warm temperatures in the Sierra Nevada mountains faster than in the nearby valleys. The UAH study, however, found that from 1910 to 2003 night and daytime temperatures in the nearby mountains did not climb.
