Climate Debate Runs Hot and Cold

We see occasional references in our discussions here to a “cooling phase” over the past decade or so. It’s often evoked in arguments against the case for global warming. Of course, in the centuries-long span of the Earth’s climate patterns, a decade or so is a mere blip on the screen.

Now a climatologist at Lawrence Berkeley National Lab has co-authored a paper that warns against using “periods of a decade or two” to argue the case one way or the other (we also often hear people use temperature data from recent hotter years as evidence for global warming).

Andrew Revkin summarizes the findings in his blog for the New York Times.

Wehner works in the Computational Research Division at LBNL and co-authored the paper with NOAA scientist David Easterling of the National Climatic Data Center, in Ashville, NC. Easterling wrote the Center’s FAQ page on global warming, in which he includes this answer to the question of whether the global climate is warming:

Global surface temperatures have increased about 0.74°C (plus or minus 0.18°C) since the late-19th century, and the linear trend for the past 50 years of 0.13°C (plus or minus 0.03°C) per decade is nearly twice that for the past 100 years. The warming has not been globally uniform. Some areas (including parts of the southeastern U.S. and parts of the North Atlantic) have, in fact, cooled slightly over the last century. The recent warmth has been greatest over North America and Eurasia between 40 and 70°N. Lastly, seven of the eight warmest years on record have occurred since 2001 and the 10 warmest years have all occurred since 1995.”

As for the geographic fluctuations, Robert Bornstein of San Jose State has produced data showing a general cooling trend along the California coast over the past 30 or 40 years. But he’s quick to point out that the anomaly is most likely a result of, not an argument against the broader global warming trend.

  • http://ncwatch.typepad.com/ Russell Steele

    Here is another perspective, Recent California climate variability: spatial and temporal patterns in temperature trends by Steve LaDochy, Richard Medina, and William Patzert, published in Climate Research, Vol. 33: 159–169, 2007

    Conclusion:
    Temperatures increased during 1950–2000 for much of the state, with minimum temperatures increasing faster than maxima, leading to reduced DTR. However there are great variations in these patterns within subregions that seem to point to land-use change as the main control. Some of the largest temperature increases occur in the vicinity of urban centers, particularly for minimum temperatures. Few rural stations show significant increases in minimum or maximum temperatures. An exception may be some agricultural sites, where minimum temperatures show increases comparable to some urban areas. While few stations showed temperature decreases for mean and minimum rates, there were substantial numbers of stations showing decreases in maximum temperatures, including some urban sites.

    If we assume that global warming affects all regions of the state, then the small increases seen in rural stations can be an estimate of this general warming pattern over land. Larger increases must then be due to local or regional surface changes. Using climatic division data, the fastest rates of warming were recorded in the southern California divisions, where urbanization has been greatest. The least warming occurred in the Central Valley, with the more irrigated south (San Jaquin drainage) having greater warming than the less irrigated north (Sacramento drainage). The NE Interior Basins division had cooling over the period of record, although most stations in the northern divisions had insignificant rates of change. It is unfortunate that the period of record is so short, especially since it practically overlays the switching from the cool phase to the warm phase of the PDO, which is highly correlated to temperatures statewide. Therefore, warming due to higher coastal sea surface temperatures since the shift around 1977 (Deser et al. 1996) should have increased temperatures, both minimum and maximum. SST rates offshore have increased at about half the rate of warming of the state, using climatic division data. The amount of this contribution interferes with calculating the background global warming signal and the magnitude of land-use change contributions.

    • Craig Miller

      Some have advanced the hypothesis that certain regional warming in California is a spillover from the “urban heat island” effect. Steele’s comment touches briefly on the fact that overnight lows (known in climate science circles as “T-minimums” are rising faster than daytime highs (T-maximums). It strikes me as one of the more insidious aspects of global warming and probably bears more discussion. BTW the “PDO” referred to is the Pacific Decadal Oscillation, a longer-cycle ocean phase similar to the more familiar El Nino and La Nina shifts.

  • Steve Bloom

    And of course the comment by Russ, far from being “another perspective” on the same subject matter, was just an attempt to confuse things for less-informed readers.

  • http://ncwatch.typepad.com/ Russ

    Mr Bloom,

    How are things over at the Sierra Club. Perhaps you can explain where I have confused the issue. The T-mins are rising faster because the concrete and black top of urban areas soak up the head during the day and give back to the atmosphere during he night. The T-mins are not increasing in rural area, except at specific sites. For example in 1976 the Nevada City Surface Station was move to the water plant. The T-min started climbing with that move. The sensor is only about 12 feet from a water mixing tank, and as we know water can hold a lot of energy over night. When you take a look at the specific stations with increasing T-min you can often find a logical reason that has nothing to do with increasing CO2.