Monday, February 17, 2014

Time Scales of Global Warming

My friend the AGW skeptic likes to argue that the last decade and a half or so has seen about 25% of the CO2 increase since pre-industrial times, but that the same period has seen little or no increase in temperature.  Even if we don't quibble about the latter point the argument is not a good one, because the time scale is just too short, mainly because the oceans have immense thermal inertia.

If my math is right, the oceans have a heat capacity of about 4 x 10^24 J/K.  The Earth absorbs something like 2 x 10^17 W from the Sun, scarfing up roughly one part in 2 billion of the big light bulb's total output, or 6.3 x 10^24 J/yr.  So even if every bit of that went into warming up the ocean, its temperature increase would only be about 1.5 K per year.  Of course almost all that absorbed energy is reradiated back into space (all of it over the long term).  A temporary heating due to an increase in greenhouse gases might drive a radiative forcing on the order of half a W/m^2.  Increasing the temperature of the ocean, even just the first few hundred meters, takes a long time.

Meanwhile, the natural dynamics of the oceans shuffles amounts of heat in and out (that are tiny for it, but not for the atmosphere) in poorly understood fashion, meaning that a decade or two is just too short of a time to expect a direct response of global temperatures to changes in radiative forcing. Based on the historical record of both CO2 and temperature, such dynamics superimpose a multidecadal noise signal on the warming trend.