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Reforesting an area the size of the US needed to help avert climate breakdown, say researchers – are they right?

Mark Maslin's picture
Professor and Director University College London

Mark Maslin FRGS, FRSA is a Professor at University College London and the Director of the London NERC Doctoral Training Partnership. He is a founding Director of Rezatec Ltd, a data product...

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  • Feb 1, 2021

Restoring the world’s forests on an unprecedented scale is “the best climate change solution available”, according to a recent study. The researchers claim that covering 900m hectares of land – roughly the size of the continental US – with trees could store up to 205 billion tonnes of carbon, about two thirds of the carbon that humans have already put into the atmosphere.  But it seems the real number is about about 55-60 billion tonnes of carbon.

While the best solution to climate change remains leaving fossil fuels in the ground, we will still need to suck carbon dioxide (CO₂) out of the atmosphere this century if we are to keep global warming below 1.5˚C. So the idea of reforesting much of the world isn’t as far-fetched as it sounds.

Since the dawn of agriculture, humans have cut down three trillion trees – about half the trees on Earth. Already 43 countries have pledged to restore 292m hectares of degraded land to forest worldwide. That’s an area ten times the size of the UK. But what the new study advocates is reforesting something like ten times that amount. This article examines the reforestation successes in Western China in the 1990s and the likelihood that we could reproduce this around the world and really plant 1 trillion tree.


Matt Chester's picture
Matt Chester on Feb 1, 2021

My question on reforestation is always on the scale of the solution vs. the problem-- the planting of trees definitely seems to only bring positives, but can it be relied on more than a drop in the bucket of attacking the problem? Reforestation surely can't replace decarbonization, but perhaps its advantage is buying us just a bit more time? 

Mark Maslin's picture
Mark Maslin on Feb 1, 2021

Matt you are absolutely wright - the only way we are going to keep climate change below 2˚C is to rapidly decarbonise the whole global economy.  But reforestation can play a part now in helping to get to net zero carbon as soon as possible.  My view of climate change is that we have left it so late to deal with it we now need to use every solution we have now.  There are no choices - we need to do everything.  Also if we want to reach 1.5˚C as opposed to 2˚C then we will need lots of negative carbon emissions in the second half of the century which means planning and starting to grow forests now.

Mark Silverstone's picture
Mark Silverstone on Feb 1, 2021

Presumably the cost of reforestation, aside from the side benefits, would be much lower than, say substituting wind for a coal generator. Any idea of the costs? To put it a different way, what would be the costs for this reforestation plan to remove 50-60 billion tons over, say, 80 years? 

A word of caution: Though reforestation might be great for removing CO2, it will not replace the biodiversity that is being lost every day from deforestation. Much of the biodiversity is gone forever.

Linda Stevens's picture
Linda Stevens on Feb 2, 2021

Planting trees helps in other ways in reducing energy usages. It is a great way to lower air-conditioning costs and it helps with soil erosion, etc. 

Bob Meinetz's picture
Bob Meinetz on Feb 2, 2021

Mark, in general, any academic study that begins with its own conclusion:

"Photosynthetic carbon capture by trees is likely to be among our most effective strategies to limit the rise of CO2 concentrations across the globe."

is a bad sign - that its author(s) are suffering from confirmation bias, i.e. "the tendency to interpret new evidence as confirmation of one's existing beliefs or theories."

In large part, the study is dedicated to showing how much of the Earth is suitable for growing trees and doesn't already have them. It finds a lot of it is - then leaps the yawning gulf to reforestation being "the best climate change solution available" by assuming actually doing it is a simple exercise, a matter of rolling up our shirtsleeves and getting to work. In so doing, it ignores practical matters of cost, of incentive, of labor, of politics. It ignores the 5.5 billion trees lost each year to biomass production, land clearing for crops, disease, and pests.

Most importantly, it ignores the fact that 2019 fossil fuel carbon emissions were 39 billion metric tonnes (GT), and are rising by 1 GT/yr.

No, planting trees is far from the best climate solution available. Like other holistic, "green" solutions it only adds to the existential problem of climate change by trivializing its scope and importance. Anyone who considers sequestering biomass the best way to fight climate change could be far more effective by helping to end de-forestation before re-forestation is necessary - not planting acorns while mighty oaks are toppling to the ground.

Mark Silverstone's picture
Mark Silverstone on Feb 4, 2021

The writer really needs to learn to use a dictionary and primary reference materials. Frankly, I don´t think the authors need the writer´s tips on evaluating academic studies.

The article begins with a hypothesis, i.e.  an assumption or concession made for the sake of argument», a common practice in this and the two highly prestigious journals on which the post is based.

Glibly dismissing other people´s work that you don´t even read is not OK!

So, this publication refers to another one which draws from still others.

“the yawning gulf to reforestation being "the best climate change solution available" by assuming actually doing it is a simple exercise, a matter of rolling up our shirtsleeves and getting to work. In so doing, it ignores practical matters of cost, of incentive, of labor, of politics.»

One of the referenced studies goes into quite some detail regarding costs and other issues that you mention for this NCS (Natural Climate Solution):

«We explore the proportion of maximum NCS mitigation potential that offers a cost-effective contribution to meeting the Paris Climate Agreement goal of limiting warming to below 2 °C. We define a <2 °C “cost-effective” level of mitigation as a marginal abatement cost not greater than ∼100 USD MgCO2−1 as of 2030. This value is consistent with estimates for the avoided cost to society from holding warming to below 2 °C (7, 25).

The case is made that “Despite the large potential of NCS, land-based sequestration efforts receive only about 2.5% of climate mitigation dollars."

The authors base their conclusions on reasoning based partly on this:

We projected the potential contributions of NCS to overall CO2e mitigation action needed for a “likely” (greater than 66%) chance of holding warming to below 2 °C between 2016 and 2100. We compared this NCS scenario to a baseline scenario in which NCS are not implemented. In our NCS scenario, we assumed a linear ramp-up period between 2016 and 2025 to our <2 °C ambition mitigation levels reported in SI Appendix, Table S4. During this period, we assumed fossil fuel emissions were also held constant, after which they would decline. We assumed a maintenance of <2 °C ambition NCS mitigation levels through 2060, allowing for gradual pathway saturation represented as a linear decline of natural pathway mitigation from 2060 to 2090. We consider this a conservative assumption about overall NCS saturation, given the time periods we estimate before saturation reported in SI Appendix, Table S1.

If you want to argue, please argue with that.  I may also do so, but only after I take the time to understand the data and concepts described in the publications.   At the moment, some parts are still a bit over my head.   I do, however, understand the work to the extent that the authors make a strong case for NCS to help control climate change. 

Mark Maslin's picture
Mark Maslin on Feb 5, 2021

Dear Bob


I agree - the problem with the study was 1) it falsely suggested planting tree would solve climate change - but even if their numbers were right (and they are wrong) that would only take care of 5 years of global emissions.  So planting an area the size of the US saves 5 years pollution.  So rapid decarbonisation is the way forward. 2) Thier number is too large - mainly because they forgot that the land they want to plant trees on already has carbon in the soil and the vegetation and in Science they forgot to take away that existing carbon.

Hence we wrote the blog in a very careful away as we wanted to alert people that planting trees will not save us - but we wanted to ensure that everyone realised it is still very useful and has many additional benefits.




Rick Engebretson's picture
Rick Engebretson on Feb 5, 2021

Given "The Keeling Curve" ( ) we indeed know seasonal photosynthesis CO2 uptake can exceed output from ALL CO2 sources combined! Your reference follows other Keeling Curve discussions. ( eg. ) Let the blowhards argue against the only agreed data we have.

To cut losses of sequestered Carbon back into the atmosphere we can convert the "rottable" poly-glucoside cellulose into a more stable carbon form. Ancient cultures burned dead grass leading to carbon rich black dirt that feeds us today. Perhaps we can be smarter and not fuel open flame fires to perform this "biochar" chemistry.

Perhaps also we can consider not leaving the best plant growing land areas as bare ground waiting for the sun to warm the soil before seed planting. The data might be more emphatic, but we might starve.

Lastly, we can begin with an over-simplified photosynthesis growth rate kinetics relationship. K1[CO2]*K2[photons]*K3[water], where the Ks are some (not so constant) kinetics constants, and the bracket values are the reactant amounts. Plants are being CO2 fertilized and water starved in California and burn every year. While trees are pushing each other on to roads in Minnesota.

We need to respect agriculture and forestry and water scientists who have done a great job where their opinions are valued more than politicians'.

Ned Ford's picture
Ned Ford on Feb 5, 2021

Since the 1980's there has been a general acceptance of the science which says that 10 - 12% of all anthropogenic CO2 is from biomass - the rest is from fossil fuels.  I find that a little dubious, since in the 80's and 90's there was no recognition of the toll on grasslands and soil carbon, and yet we use the same value today.  Maybe the value is based on isotopic evidence, but anyways...

if 10 - 12% of the current atmospheric loading, which is actually just a fraction of the biosphere loading we have caused, is from deforestation, then it is absurd to suggest that more than 10 - 12% of what is in the atmosphere today can be removed by restoring it. 

Well, that's not exactly right.  IPCC has two camps of scientists.  One camp gained dominance in the Fifth Assessment report - the folks who believe that atmospheric CO2 will drop back by about 200 ppm in the next five hundred years or so, once we stop emitting it.  The other camp makes more sense to me, because they understand that the processes which remove CO2 from the atmosphere today are not going to persist when we stop raising the level of CO2.   I don't want to put too much of the detail here, but these folks think we may be looking at a very slow return to pre-industrial levels - three to five thousand years.

That stuff is important for this discussion because if you want to see atmospheric CO2 reduced you need to understand the issues.   And I've never seen anyone who cares about reforestation speak knowingly about the context.

What I tell people is that one to three trillion trees will help.  The most important thing which happens when we stop using fossil fuels is that methane returns to about pre-industrial levels in about forty years.  It will probably start dropping a decade or so before we stop using fossil fuels, if we are making steady reductions.   75% of methane emissions are from fossil fuels, and most of that is probably from fracking today.  That's the only rational explanation for why atmospheric methane started dropping in 2007, and then resumed growing.

When we stop burning fossil fuels, the elimination of soot and smog may permit rainfall patterns to return to those prior to 1940.  This, all by itself might allow the world's deserts to shrink by 10%, and go a very long way to making those three trillion trees start growing.   Remember too, that grasslands sequester carbon about as well as forests do, and are much more prevalent in the pre-industrial world.

So I think that efforts to improve soil carbon content in agricultural lands is also going to be easier than tree planting.  There is an economic structure in place to make it happen - paying farmers.   Since these practices seem to increase farm income, they are not an uphill battle.

So all these things help.   None of them will restore pre-industrial CO2 in less than several centuries.  Some of the carbon which has moved from the fossil fuels to the atmosphere to the ocean will stay there.  I think less than the most recent IPCC report suggests.  I'd love to discuss this, but not in this post.  When we stop adding more CO2, about 40% of what is in the atmosphere remains in the ocean and will come back out, if anything else takes it out of the atmosphere.  The debate - the two camps I mentioned, are disputing whether that is just 40%, or more like 80% - we all agree that about half of fossil fuel and biomass emissions go into the ocean and the other half remain in the atmosphere.   The debate is about the various forms of that carbon in the ocean.  20% is certainly there for good.

The single most useful thing we on this discussion board can do is speed the end of fossil fuels by doing everything we can to increase efficiency and speed up the growth of wind and solar.  I find the items about nuclear power "re-emerging" to be sadly humorous.  The nuclear industry never did understand economics.  The single most important thing for efficiency is 15% shares of verified net savings as incentives to the utilities.   I don't know a simple phrase that would help renewables, but I think that telling people that wind and solar are now about half what they cost ten years ago, and are now cheaper than 90% of the world's fossil and nuclear generation is a pretty good proxy for such a phrase.

We aren't the people who will plant the trees or change the farming practices.  The most we have to offer there is a little correct perspective, and a promise to end fossil fuels as fast as possible so we have less sequestration to ponder.

Doubling global renewables construction rates next year, the way wind construction in the U.S. doubled last year, will cut a decade's worth of emissions out of the atmosphere.   That's how fast things are moving.  We literally need 2.25years of doubling the current rate of wind and solar construction to end fossil fuels in 15 years.

Rick Engebretson's picture
Rick Engebretson on Feb 5, 2021

The plant growth rate (chemical kinetics) equation "starting point" given above implies 280 parts per million "pre-industrial" atmospheric CO2 levels might not be adequate to feed the current global human population. Agriculture is stretched pretty thin as it is.

Most certainly, agriculture (and likely energy technology) doesn't  need a bunch of photo-voltaic and windmill concrete, roads, wires, batteries, pesticide and herbicide killing stuff around political monuments.

The two camps of environmentalists you describe are more a living environment or a dead environment camp.

Mark Maslin's picture
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