AR5 Carbon Cycle and the Bern Model



Double The Atmospheric CO2? Fuggeddaboutit!

Willis Eschenbach


March 27, 2016


Now, there are many things of interest in this graphic, but what particularly interested me in this were their estimates of total fossil fuel reserves. Including gas, oil and coal, they estimate a total fossil fuel reserve of about 640 to 1580 gigatonnes of carbon (GtC). I decided to apply those numbers to both the Bern Model and the simple exponential decay model.


Now, the Bern model and the simple exponential model are both exponential decay models. The difference is that the simple exponential decay model uses one value for the half-life of the CO2 emissions. On the other hand, the Bern model uses three different half-life’s applied to three different fractions of the CO2 emissions, plus 15% of the emitted CO2 is said to only decay over thousands of years.


My interest was in finding out what would happen, according to the two CO2 models, if we burned all of the fossil fuels by 2100. For the smaller case, burning 640 GtC by the year 2100 implies a burn rate below current emissions, that is to say about  7.5 GtC per year for the next eighty-five years.


For the larger case, 1,580 GtC implies a burn rate that increases every year by 1.1%. If that happens, then by the end of this century we’d have burned 1,580 gigatonnes of carbon.


Figure 2. CO2 projections using the Bern Model (red and blue) and a single exponential decay model (purple and light green).

Single exponential decay model uses a time constant tau of 33 years. Note that this graph has been replaced, the original graph showed incorrect values.

Now, there are several things of interest here.  First, you can see that unfortunately, we still don’t have enough information to distinguish whether the Bern Model or the single exponential decay model is more accurate.

Next, the two upper values seem unlikely, in that they assume a continuing exponential growth over eighty-five years.  This kind of long-term exponential growth is rare in real life.

Finally, here’s the reason I wrote this post. This year, the atmospheric CO2 level is right around four hundred ppmv.  So to double, it would have to go to eight hundred ppmv … and even assuming we could maintain exponential growth for the next eight decades and we burned every drop of the two thousand gigatonne high-end estimate of the fossil reserves, CO2 levels would still not be double those of today.

And in fact, even a fifty percent increase in CO2 levels by 2100 seems unlikely. That would be six hundred ppmv … possible, but doubtful given the graph above.

Short version? According to the IPCC, there are not enough fossil fuel reserves (oil, gas, and coal) on the planet to double the atmospheric CO2 concentration from its current value.



Section for a video or follow-on comment

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