Saturday, August 25, 2018

Global Carbon Sink Holding Up So Far

NOAA makes available data on CO2 concentrations on Mauna Loa in Hawaii that go back to 1959.  This is the famous Keeling curve, and the annual averages look like this:

The data is approximated to a very high accuracy by a quadratic curve, indicating that the increase in carbon going into the atmosphere each year is itself increasing linearly.  This next graph compares the annual increase in carbon in the atmosphere to fossil fuel emissions of carbon (from BP):

(You can convert the concentration of CO2 changes to absolute amounts if you know the mass of the atmosphere and the weight of CO2 molecules).  Since the amount of carbon in the atmosphere is increasing slower than carbon emissions, clearly some of the emissions are absorbed (by the ocean and by land plants) each year.  The fraction of emissions absorbed is almost constant, but has increased slightly over time:

But I think a more interesting way to look at the question is this.  Think of the sink as reflecting the fact that the CO2 in the atmosphere is not in equilibrium with the CO2 in the ocean and terrestrial ecosystems.  These latter components change slowly - the ocean is huge and takes around a thousand years to turn over, so changes in the atmosphere in the last few decades are far from fully equilibriated.  Likewise, changes in terrestrial ecosystems have only just begun.

From that perspective, we might expect the amount of carbon being absorbed by the ocean and biosphere to be proportional to how far the current atmospheric concentration is from pre-industrial concentrations (generally believed to be about 280ppm).  If we plot this - the size of the annual carbon sink vs the departure from pre-industrial, we see that it is indeed linear:

There are of course substantial year to year fluctuations depending on just how well global plants grew in any particular year (given weather).  But if you try fitting a quadratic to that data (allowing for the possibility of the sink degrading over time), it lies exactly on the linear curve - there is no indication of a tailing off.

This is somewhat reassuring with respect to  the "Inevitable Near Term Human Extinction" (INTHE) view of climate change.  One class of mechanisms that could lead to a runaway climate feedback would be if the biosphere were to start to turn from a net sink to a net source as a result of climate change - for example, forests burning, dying back from disease, the Amazon turning to savannah etc.  While all of these things are happening to a modest degree, the fact that overall, the global climate sink continues to behave in a predictable linear way suggests that this particular class of runaway feedbacks are not biting hard yet.

This is an update of a post of mine from six years ago.  The conclusion hasn't changed with six more years of data.

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