A new paper by John Christy, William Norris, Kelly Redmond and Kevin Gallo, in press with the Journal of Climate, entitled “Methodology and Results of Calculating Central California Surface Temperature Trends: Evidence of a Human-Induced Climate Change” offers important new insight to the role of landscape change in climate and in the interpretation of surface air temperature changes. The abstract of the paper reads,
“A procedure is described to construct time series of regional surface temperatures and is then applied to interior Central California stations to test the hypothesis that century-scale trend differences between irrigated and non-irrigated regions may be identified. The procedure requires documentation of every point in time at which a discontinuity in a station record may have occurred through (a) the examination of metadata forms (e.g. station moves) and (b) simple statistical tests.
From this we define “homogeneous segments” of temperature records for each station. Biases are determined for each segment relative to all others through a method employing mathematical graph theory. The debiased segments are then merged, forming a complete regional time series. Time series of daily maximum and minimum temperatures forstations in the irrigated San Joaquin Valley (Valley) and nearby non-irrigated Sierra Nevada Mountains (Sierra) were generated for 1910-2003. Results show that 20th century Valley minimum temperaturesare warming at a highly significant rate in all seasons, being greatest in summer and fall ( > +0.25 °C decade-1). The Valley trend of annual mean temperatures is +0.07 ±0.07 °C decade-1. Sierra summer and fallminimum temperatures appear to be cooling, but at a less significantrate, while the trend of annual mean Sierra temperatures is an unremarkable -0.02 ±0.10 °C decade-1. A working hypothesis is that the relative positive trends in Valley minus Sierra minima ( > 0.4 °C decade-1 for summer and fall) are related to the altered surface environment brought about by the growth of irrigated agriculture,essentially changing a high-albedo desert into a darker, moister,vegetated plain.”
The University of Alabama at Huntsville (UAH) has released a press release on January 18 on this paper (URL). Extracts from the news release state
“A two-year study of San Joaquin Valley nights found that summer nighttime
low temperatures in six counties of California¹s 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.
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.
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
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.”
The careful analysis of the data to minimize biases, as well as the protocol of comparing a set of data from valley and mountainous locations provides a robust assessment of how landscape processes affect the surface temperature trends. Landscape processes, with respect to how the land surface temperatures have been altered over time, has been a neglected issue in assessments such as the IPCC.