Several attempts have been made to
explain this very short-term pattern in such a way as to show we might be out
of the woods on global warming, mainly by weakening the “sensitivity” of our
climate models. Hey – those sensitivity factors were inferred from a lot of
real data covering centuries and millennia – it makes no sense to me to start
fiddling with them based on 10 years worth. I don’t believe in free lunches.
In fact, the recent (and very short by
ecosystem timeframes) leveling of the mean global air temperature is likely a
case of borrowing from Peter now, only to have to pay Paul back later. I like
this explanation because (a) it makes sense, and (b) doesn’t involve a free
lunch.
According to a paper in the American
Geophysical Union's Geophysical Research Letters, the reason the air temps
aren't rising is that the global warming energy has gone into heating the
deeper ocean waters. Normally, that doesn't happen - the surface water's warm
first and since warm water floats over cold water, the deeper ocean is slow to
heat. But - enter the Pacific Decadal Oscillation (PDO), which during the past
decade has been pushing colder water up from the depths, and circulating the
warm water down. With the cold water on top, the ocean has been absorbing more
of the heat energy of global warming, leaving the surface air temperatures
unchanged.
Trouble is, when the next PDO cycle
starts, that pattern will reverse, and much of that absorbed energy will
re-enter the atmosphere, and we'll pay for this decade of "flat"
temperatures with a decade of rapidly increasing temperatures. In fact, say the
scientists, this is exactly what happened in the late 80s and 90s (1998
remaining one of the hottest years on record).
That's the trouble with looking at one
number (mean global surface temperature) as a proxy for the complex phenomenon
of climate change. We have to remember that “Global Warming” refers to the
entire 3D globe, not just the surface.
We're coasting now, but hang on to your
hat!
Note 1: The graph
from NOAA (see the original at http://data.giss.nasa.gov/gistemp/graphs_v3/)
Temperatures are shown as a departure (“anomaly”) from the 1951-1980 average. So
over the last 10 years or so, the mean surface temperature has been holding
steady at 0.6 degrees Celsius (1.08 degrees Fahrenheit) over the 1951-1980
average.