Corrected RCP Situation Removing Fractions

Reposted from Dr. Roy Spencer’s weblog

February sixth, 2020 by Roy W. Spencer, Ph. D.

Nicely, as I suspected (and warned everybody) in my weblog publish yesterday, a portion of my calculations had been in error relating to how a lot CO2 is taken out of the environment within the world carbon cycle fashions used for the RCP (Consultant Focus Pathway) eventualities. A number of feedback there mentioned it was exhausting to imagine such a discrepancy existed, and I mentioned so myself.

The error occurred by utilizing the incorrect baseline quantity for the “extra” CO2 (atmospheric CO2 content material above 295 ppm) that I divided by within the RCP eventualities.

Right here is the corrected Fig. 1 from yesterday’s publish. We see that in the course of the overlap between Mauna Loa CO2 observations (by 2019) and the RCP eventualities (beginning in 2000), the RCP eventualities do roughly match the observations for the fraction of atmospheric CO2 above 295 ppm.


Fig. 1. (corrected) Computed common yearly price of elimination of atmospheric CO2 above a baseline worth of 295 ppm from (1) historic emissions estimates in comparison with Mauna Loa CO2 information (crimson), (2) the RCP eventualities utilized by the IPCC CMIP5 local weather fashions Decrease proper), and (three) in a easy time-dependent CO2 price range mannequin compelled with historic emissions earlier than, and EIA-based assumed emissions after, 2018 (blue). Notice the time intervals change from 5 to 10 years in 2010. 

However now, the RCP eventualities have a lowered price of elimination within the coming many years throughout which that very same factor-of-Four discrepancy with the Mauna Loa commentary interval step by step develops. Extra on that in a minute.

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First, I ought to level out that the CO2 sink (elimination price) when it comes to ppm/yr in three of the 4 RCP eventualities does certainly enhance in absolute phrases from (for instance ) the 2000-2005 interval to the 2040-2050 interval: from 1.46 ppm/12 months throughout 2000-2005 to 2.68 ppm/yr (RCP4.5), three.07 ppm/yr (RCP6.Zero), and three.56 ppm/yr (RCP8.5). RCP2.6 is troublesome to match to as a result of it includes not solely a discount of emissions, however precise detrimental CO2 emissions sooner or later from enhanced CO2 uptake packages. So, the RCP curves in Fig.1 shouldn’t be used to deduce a lowered price of CO2 uptake; it’s only a lowered uptake relative to the atmospheric CO2 “overburden” relative to extra pre-Industrial ranges of CO2.

How Real looking are the Future RCP CO2 Removing Fractions?

I’ve been emphasizing that the Mauna Loa information are extraordinarily carefully matched by a easy mannequin (blue line in Fig. 1) that assumes CO2 is faraway from the environment at a relentless price of two.three%/yr of the atmospheric extra over a baseline worth of 295 ppm.

OK, now truly have a look at that determine I simply linked to, as a result of the match is amazingly good. I’ll wait….

Now, if I cut back the mannequin specified CO2 elimination price worth from 2.three to 2.Zero%/yr, I can’t match the Mauna Loa information. But the RCP eventualities insist that worth will lower markedly within the coming many years.

Who’s appropriate? Will nature proceed to take away 2.Zero-2.three%/yr of the CO2 extra above 295 ppm, or will that elimination price drop precipitously? If it stays pretty fixed, then the longer term RCP eventualities are overestimating future atmospheric CO2 concentrations, and because of this local weather fashions are predicting an excessive amount of future warming.

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Sadly, so far as I can inform, this example cannot be simply resolved. Since that elimination fraction is MY metric (which appears bodily cheap to me), however isn’t how the carbon cycle fashions are constructed, it may be claimed that my mannequin is simply too easy, and doesn’t include the physics mandatory to deal with how CO2 sinks change sooner or later.

Which is true. All I can say is that there isn’t a proof from the previous 60 years (1959-2019) of Mauna Loa information that the elimination fraction is altering…but.

There is no such thing as a means for me to win that argument.

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