Blaming nature for the CO2 rise doesn't add up

Recently there’s been a resurgence in claims that CO2 rise is caused by 'natural' temperature rise rather than humans. Various arguments are used to make this sound sensible (see Paolo Soares, Murry Salby and Joe Bastardi), but they’re wrong.

I’m going to pick on just one of the claims Joe Bastardi made in a comment at Tamino’s blog and this is written as an explanation to Joe of why I’m convinced he’s wrong. I hope readers will be able to point out if I’ve made a mistake and work out which argument you think makes the most sense. Bastardi claims that:

“The answer is obvious. it is the earths temperature which is driving the co2 release into the atmosphere.”

I’m going to start with chemistry; most carbon in the atmosphere is in CO2 and it can move between the atmosphere and land/oceans through chemical reactions. Billions of tons of carbon don't magically turn into pixie dust.

Next some maths: if I have one ton of carbon and I add another ton of carbon then I have two. We can write this down as an equation for the change in atmospheric CO2 over a year, ΔCatm:

$\Delta C_{atm} = Emissions - Absorption$

This says that if I ‘emit’ a ton of carbon by, say, triggering a volcano then the atmosphere will gain a ton. If I ‘absorb’ a ton of carbon by growing a tree, then the atmosphere loses a ton.

We can expand the equation by counting human emissions (HE) and absorption (HA) and natural emissions (NE) and absorption (NA) separately.

$\Delta C_{atm} = NE + HE - NA - HA$

This works because carbon is additive. If a volcano emits a ton of carbon and a factory emits a ton then the atmosphere has gained two tons. This is a very simple balance sheet for the carbon cycle and fortunately there are ‘accountants’ who’ve measured some of these values for us.

Recently the amount of CO2 in the atmosphere has been rising at ~2 parts per million per year, or around 15 billion tons/year. Meanwhile  human emissions excluding land use change (like clearing or planting forests) are 30 billion tons per year. In billions of tons per year we have:

$\Delta C_{atm} = 15$

$HE = 30$

$15 = NE + 30 - NA - HA$

We can rearrange this:

$NE - NA - HA = -15$

Humans are also clearing rainforests and changing land use, but here I'll assume that human effects on absorption (HA) are not much different from zero, i.e.

$NE - NA = -15$

So Natural Absorption (NA) must be bigger than Natural Emissions (NE). Nature is absorbing more CO2 than it is emitting. It is not causing atmospheric CO2 to rise at all - in fact it is acting to try and reduce atmospheric CO2 and the long term rise is entirely because of humans.

Joe can demonstrate this is wrong by showing one of the following:

- Chemistry is wrong and CO2 magically disappears if humans put it into the atmosphere. Perhaps the equations should be updated with the Bastardi Pixie Dust Factor (BPDF)?:

$\Delta C_{atm} = Emissions - Absorption - BPDF$

- Addition is wrong, 1+1 does not equal 2 and 2+3 does not equal 5.

- Nature being a net absorber of CO2 from the atmosphere doesn't mean that it's taking CO2 away, and humans pumping CO2 into the atmosphere doesn't mean it's actually going into the atmosphere! Absorption is emission, emission is absorption and up is down.

- Human activity and land use changes are absorbing 30 billion tons of CO2 every year.

Or maybe, just maybe the rise in atmospheric CO2 is entirely human caused and without us CO2 levels would be much lower.

Posted by MarkR on Sunday, 14 August, 2011