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Environnement écologie

Etude: le climat se refroidit-il ou se réchauffe-t-il?
Study: Cool Spells Normal in Warming World
ADIT, avril 2009

Graphs provided by the authors, published by the AGU A climate simulation in a world facing “business as usual” increases in greenhouse gases still shows lots of periods with cool fluctuations.

     A valuable short paper that has been accepted for publication in Geophysical Research Letters (subscription required) makes a strong case against presenting any argument about human-driven global warming that's based on short-term trends (a decade or so). I've noted here before that climate campaigners who seek to use real-time events to engage the public can only retain credibility if they account for natural variability in framing their case and explain that the odds of such events are shifting. (Realclimate explored natural variability and warming last year, too.)
     The same requirement applies to the community of climate skeptics/contrarians/deniers/realists (depending on who's doing the labeling) who have made a mantra out of the "global cooling" since the 1998 peak in global temperature.

     Measured changes in global temperature show ups and downs, with some periods of a decade or more defying the long-term trend.
     The paper shows, both in recent records and projections using computer simulations, how utterly normal it is to have decade-long vagaries in temperature, up and down, on the way to a warmer world. The paper is titled simply, "Is the climate warming or cooling?" It is written by David R. Easterling of the National Climatic Data Center and Michael F. Wehner of the Lawrence Berkeley National Laboratory.
     The bottom line? "We show that the climate over the 21st century can and likely will produce periods of a decade or two where the globally averaged surface air temperature shows no trend or even slight cooling in the presence of longer-term warming," the paper says, adding that, "It is easy to ‘cherry pick' a period to reinforce a point of view."
     I asked Dr. Easterling why they pursued this effort, which somewhat replicates findings of the Intergovernmental Panel on Climate Change, but perhaps with a more pointed goal. Here's his reply:
    To show, in a peer-reviewed scientifically defensible way that there is no reason to expect the climate to warm in a monotonic type fashion, that there is natural variability along with anthropogenic forced warming and we shouldn't expect each year to be warmer than the next or even a run of 10 years always to show warming. That we can get a 10- or even 15-year period with no real change in globally averaged temperature even though in the end we have strong global warming.
     There was another useful effort by climate scientists and communication specialists this week, a letter to the journal Science, "Creating a Common Climate Language," urging international organizations to standardize basic terms in assessing climate science to gauge policy responses. (You can download the letter at Michael Mann's Web page.)
     The more work that the science community does along these lines, the better. There is a dizzying range of official definitions of the term "climate change" itself, for starters. Some assessments track only concentrations of carbon dioxide in the atmosphere; others consolidate the influence of all greenhouse gases into a "carbon dioxide equivalent" measurement. There's no common number for the globe's "pre-industrial" average temperature, etc. When entering any debate, a first step clearly is to settle on definitions.
I'll be doing more pieces on the climate basics soon, including a look at arguments that ocean cycles like the Pacific Decadal Oscillation are the dominant driver of recent climate change. I'll also be writing more on why sea levels do not rise uniformly (and may be falling in a few places) even as there is high confidence in rising seas in a warming world.