The Global Carbon Project has released their preliminary estimate of CO₂ emissions in 2019.— Robert Rohde (@RARohde) December 4, 2019
The news is mixed.
CO₂ emissions set a new record high in 2019. ☹️
But, CO₂ emissions only increased by 0.6%, which is less than the rate of increase in 2017 and 2018. 🤔 pic.twitter.com/Wixipl3Qmm
I wonder how low the annual emissions level would have to be in order for the % of CO2 in the atmosphere to start to decrease.
Hi David. That's a good question, and I don't have time right now to research it. A 200 review paper by David Archer & Brovkin discusses a model where 1000 Gt carbon is released immediately into the atmosphere, all at once, and shows atmospheric CO2 peaking about a decade after. 5000 GtC peaks about 20-30 years later. The paper is below; see Figure 2b.
The millennial atmospheric lifetime of anthropogenic CO2
David Archer & Victor Brovkin
Climatic Change (2008) 90:283–297
However, the CO2 in our atmosphere will remain above the pre-industrial level essentially forever -- several hundred of thousands of years.
That's an ill-posed problem, because it depends on the time evolution of emissions and the ensuing slow feedbacks.
For example, looking at the CMIP5 scenarios:
Under RCP2.6, the global mean mixing ratio of CO2 starts to decrease (i.e., the 1-year change goes negative) in 2053, when CO2 emissions drop below 3.0 Gt C/year, and it continues decreasing consistently after that.
Under RCP4.5, emissions cross that 3.0 Gt C/year threshold in 2112, but the mixing ratio is still (slowly) increasing. It doesn't begin to decrease until 2121, when emissions are 2.6 Gt C/yr, and even then it barely decreases, oscillating between small negative and positive annual changes with a net decrease of <0.5 ppmv over the subsequent 30 years, even though emissions drop to 1.5 Gt C/yr.
In other words, the sooner we start to stabilize emissions, the faster the system will be able to respond, and we can then maintain a higher level of emissions. The longer we wait, the lower we'll have to drive emissions in order to achieve the same effect.
Every year of delay means that future generations will have to live with a *lower* maximum threshold of annual C emissions.
On further thought, I'm not sure that using the CMIP5 data is the right way to look at that, because the "scenarios" are "concentration pathways" and the atmospheric mixing ratios are dictated rather than evolving directly from emissions. This in turn means that carbon cycle feedbacks are effectively dictated by the scenarios as well, and that in turn means that we can't really use this to look at the effects of changes in the magnitude and timing of emissions.
tl;dr - I think the conclusion from my previous comment is still correct based on the literature elsewhere, but the evidence I used to support it here (CMIP5 emissions and concentration data) shouldn't be used to support it.
Ned, thanks for this good information.
Of course, if we stop fossil fuel emissions we'll have to also stop emissions from land use changes. Will that be harder than stopping FF emissions?
I'm no expert on this! But ... some things to keep in mind:
(1) We don't have to completely *stop* FF emissions. Obviously, lower is better, but AFAICT there's room for a low level of emissions (1-2 Gt C/yr?) on an ongoing basis. We're at 9-10 Gt C/yr now, so that is an 80-90% cut, but it's OK if it takes longer to get rid of the last and most difficult 10-20%.
(2) Land use emissions were larger in the mid-20th century, both in absolute #s and as a percentage of the total. At the moment they're less than 0.5 Gt/yr, so a very small part of the problem (but of course one should also factor in ecological and other environmental concerns associated with land use emissions).
(3) Note that cement manufacturing is also in the mix; in the CMIP5 database it's lumped in with the FF component.
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