(On 2019-07-06, repaired a typo, and on 2019-07-16 linked in a post by Professor Stefan Rahmstorf at RealClimate.)
Jesse Reynolds at Legal Planet is on this.
But, as I noted at LinkedIn, even if I accept the entirety of the well-meaning paper by Bastin, et al admits that planting 500 billion trees, as they propose, will only solve 25% of the atmospheric CO2 problem. Actually, I believe they miscalculated that, but we’ll get to seeing how in a moment.
Let’s say the half trillion trees are planted, emissions of CO2 from human sources and other precursors, like CH4 are completely stopped (setting aside the challenge of how to get agriculture to stop emitting, too), and deforestation is stopped. Atmospheric CO2 is now about 414 ppm. The preindustrial baseline was 288 ppm. That means 126 ppm more CO2 is in atmosphere over pre-industrial. We probably don’t need to get to 288 ppm. 350 ppm will do. So that means we’re 64 ppm out, or so it seems. Atmospheric CO2 is increasing by about 2 ppm per year.
Bastin, et al estimate the half trillion trees will take out 200 GtC at maturity. is 0.127 ppm. So 200 GtC is a bit more than 25 ppm. That’s 39% of 64 ppm or 20% of 128 ppm.
Setting aside that this won’t happen overnight, or what the associated emissions of planting 500 billion trees are, this has another problem, alluded to above. Atmosphere only retains about 40% of total human emissions. That means 60% of human emissions (already) either go into soils or into the oceans. (In the long run, CO2 in oceans will turn into carbonates, but this is a very slow process.)
(Graphic courtesy of NASA. Click on it if you want to see a larger version in a separate window.)
Most importantly, oceans and soils are in equilibrium with atmosphere. This means if a ppm of CO2 is drawn from atmosphere, the partial pressure of CO2 in atmosphere will be lowered, and the entire climate system will come to a new equilibrium, drawing CO2 from soils and oceans. In the end, the total amount of CO2 to extract isn’t 128 ppm or 64 ppm, but 128/0.4 ppm or 64/0.4 ppm. These are 320 ppm and 160 ppm, respectively. Also, as Dr Steven Chu has pointed out, we’re not really at 414 ppm CO2 but, after considering the methane (CH4) in atmosphere, other hydrocarbon greenhouse gases which break down into CO2, and other greenhouse gases like N2O, 500 ppm CO2e. He also indicates we’re probably going to get to at least 600 ppm CO2e.
What does that mean? That means, even accepting Bastin et al uncritically, their 500 billion trees will do 7% of going from 500 ppm to 350 ppm CO2e (and that’s generous because they do little about, say, N2O), and 4% of going from 600 ppm to 350 ppm CO2e.
This is why Reynolds and I and others say the article is misleading. I also claim the Project Drawndown is highly misleading. These are at least wishful environmentalism if not greenwashing. If a political movement hangs its hat on the proposal, it is greenwashing.
The limitations of planting forests for this purpose are well known. For instance,
|L. Nave, G. M. Domke, K. L. Hofmeister, U. Mishra, C. H. Perry, B. F. Walters, C. W. Swanston, “Reforestation can sequester two petagrams of carbon in US topsoils in a century“, PNAS, 2018.|
A petagram of Carbon is a single GtC. (That’s because 1 Gt = 1 billion of a tonne = 1 billion of 1000 kilograms each = 1 billion of 1 million grams each = 10^15 grams = 1 petagram.) So, it’s not fast, either.
I’ve written about what it would take to really reduce CO2 in atmosphere at two blog posts:
- Getting back to 350 ppm CO2: You can’t go home again.
- You really can’t go home again: An update of getting back to 350 ppm CO2.
and I have written about the problem of convincing greenhouse gases to remain in soils even if they are put there through afforestation.
In short, (1) it’s far better and cheaper not to put the emissions up there in the first place, and (2), if we do, we’d better be prepared to live with the consequences, because CO2 in atmosphere is very long-lived. I think, too, in our personal lives, we need to be looking at what really contributes to global emissions: Consumption and its upstream emissions accounts for a lot! So do McMansions and does expansion of suburbs.
“‘Why hasn’t anyone told us of this before?‘”