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How much would planting 1 trillion trees slow global warming?

Posted on 24 February 2020 by dana1981

This post has been incorporated into the new rebuttal to the myth 'Planting a trillion trees will solve global warming' with the short URL

During his 2019 State of the Union address, Donald Trump announced that the United States will join the Trillion Trees Initiative. House Republicans plan to introduce legislation to plant 3.3 billion trees per year domestically over the next 30 years (an 800 million increase over the 2.5 billion per year that are already planted in the U.S.), as well as encouraging carbon capture and sequestration from power plants by providing research & development funding and creating a tax incentive for using the technology. This poses the question – how much impact would these carbon sequestration proposals (especially the tree planting initiative) have on atmospheric carbon dioxide (CO2) levels and global warming?

Bastin et al. (2019) sought to quantify the potential global tree restoration potential and the carbon sequestration associated with that reforestation and afforestation. The study concluded, “there is room for an extra 0.9 billion hectares of canopy cover, which could store 205 gigatonnes of carbon [GtC] in areas that would naturally support woodlands and forests.” For comparison, humans have emitted approximately 640 GtC, so this would represent a significant chunk of human emissions to date. 900 million hectares of land is approximately the size of the United States.

However, several comments identified flaws in the Bastin et al. estimate. Friedlingstein et al. (2019) noted that their estimate of the potential carbon storage of trees in each biome did not account for the carbon already stored in those regions, and thus concluded:

the potential carbon storage would be substantially lower than reported … Moreover, forests affect climate through biophysical feedbacks, such as changes in albedo or evapotranspiration, which can counteract the cooling effect from CO2 uptake … These biophysical feedbacks were not discussed in the article and could substantially reduce the potential of forest reforestation in some of the considered regions.

Veldman et al. (2019) in concluding that the true maximum tree carbon sequestration potential is closer to 42 GtC, noted:

Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of savannas, grasslands, and shrublands to be restoration.

Lewis et al. (2019) noted that the carbon sequestration rate used by Bastin et al. (0.22 GtC per million hectares) was twice that in previously published estimates. Using a variety of methods to roughly approximate the global tree carbon sequestration potential, Lewis et al. estimated the value between 89 and 108 GtC. Moreover, Lewis et al. note:

25% of the new tree cover [would be] in tundra and boreal regions, where warming from forests’ lower surface albedo can offset the cooling from new carbon uptake.

Let’s examine the carbon sequestration and global warming mitigation potential of planting trees in all available areas outside of tundra and boreal regions, where as Lewis et al. note, replacing relatively reflective land surface with dark tree canopy would offset the cooling from carbon uptake. Using the above referenced studies, as an approximation, let’s estimate that doing so could sequester 75 GtC, once the nearly trillion trees have reached maturity.

Humans currently release 10 GtC annually from fossil fuel combustion and other activities; therefore, continuing at current emissions rates would offset the carbon sequestration potential of this maximal tree restoration effort within 7 to 8 years. Thus it’s important to note that planting trees cannot replace the phasing out of fossil fuels, but it can complement it.

Under policies and commitments currently in place, global average surface temperatures are on a path for approximately 3°C in 2100 (though headed north of 4.5°C by the time a new equilibrium is reached, unless future policies bring emissions down to zero). This translates into approximately 1,650 GtC of human emissions between 2020 and 2100, or an atmospheric carbon dioxide concentration around 620ppm in 2100. Removing 75 GtC through tree planting would lower that CO2 concentration to around 585 ppm (see this post for useful conversions). To translate these numbers into global surface temperature changes, we can use the following formula related to the radiative forcing of CO2:

Where the left side of the formula represents the global surface temperature change once a new equilibrium is reached, RF is the radiative forcing (in this case, from increased CO2), lambda is the climate sensitivity parameter (approximately 0.8), C is the atmospheric CO2 concentration, and Co is the initial CO2 concentration (280 ppm pre-industrial).

At equilibrium, the temperature change associated with 620 ppm CO2 is 3.4°C, and for 585 ppm is 3.15°C (this doesn’t account for non-CO2 greenhouse gases or other forcings). By 2100 when the climate system will not yet be in equilibrium, the CO2-caused temperature changes would be closer to 2°C and 1.85°C, respectively. In short, maximal tree planting would offset around 0.15°C warming by 2100 and a quarter degree Celsius at equilibrum.

Domestically, the US emits about 5.3 Gt CO2 (1.4 GtC) per year. Planting another 24 billion trees (an additional 800 million per year over 30 years), at around 1,000 trees per hectare and 0.12 GtC sequestered per hectare, per Lewis et al. (2019), corresponds to about 5.3 GtC sequestered once those trees reach maturity. In short, the tree planting proposal would offset about 3–4 year’s worth of US carbon emissions at current rates. That’s a start, but only a start.

It’s worth noting that reforestation and afforestation occupy a number of high slots on the Project Drawdown list of top climate solutions, including #5 and #12. However, adding them all up accounts for just around 40 GtC. This is probably a more realistic number than our 75 GtC, since we can’t plant trees on every available hectare of land. Doing so would decrease agricultural production and thus increase food prices, for example. This highlights why Project Drawdown lists 100 different individual solutions. 

Planting trees won't be enough to solve climate change unless they're Ents and do to our fossil fuel infrastructure what they did to Isengard in Lord of the Rings.

There is no climate change silver bullet; planting trees helps, but it’s just one piece of silver buckshot among the many solutions needed to avert a climate crisis. 

With respect to the technological carbon capture and sequestration component of the House Republican plan, if a power plant were able to capture and permanently sequester all of its emitted carbon, it would produce nearly carbon-neutral power (with the exception of the extraction and transportation of the associated fuel, and energy associated with construction). But the technology would need widespread application, and methods of permanently sequestering large quantities of CO2 would be required. Determining whether subsidizing fossil fuel companies to install this technology would be an efficient use of resources and revenue would require economic analysis.

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Comments 1 to 19:

  1. "Their analysis inflated soil organic carbon gains, failed to safeguard against warming from trees at high latitudes and elevations, and considered afforestation of savannas, grasslands, and shrublands to be restoration."

    In addition, most grasslands are already used for cattle farming, so turning them into forests has considerable consequences,  and we have the Red Barons of the world mentioning that grasslands can be a useful carbon store if properly grazed. I just think the study on forests looks wildly optimistic, and is full of such obvious omissions you wonder what planet they were on when writing it. Of course forests do have some significant potential, just not the inflated potential some politicians like to portray.

    As to carbon capture and storage and its "economics" needing further analysis, according to this article "Plunging Prices Mean Building New Renewable Energy Is Cheaper Than Running Existing Coal". How can carbon capture and storage possibly compete with that?

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  2. USA NOT plant 800 million trees per year. This is manipulation, trick, fake, just lie. Finland good in planting, China in some part, others... But USA cut several times more than plant. We can look on google maps and see this.
    How many % from this "800 million" are still alive after 10, 20, 30 years?

    Plant  1 trillion trees - what about reality?

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  3. @2

    US started planting trees in the 1920's and there are at least as many now as the pilgrim days or more.

    What is different is the prairies. Most of them have been plowed under, especially the tallgrass prairie.

    You can look on Google Maps all you want, but just because you see an area without trees doesn't mean there was a forest there before. In most cases it was prairie.

    Unplowed Tallgrass Prairie: Rarer Than Old-Growth Forest

    This is important because grasslands are the planets cooling system, not forests. 

    Cenozoic Expansion of Grasslands and Climatic Cooling

    You are seeing the reason why right now in Australia. Above ground biomass always returns to the atmosphere eventually either by decay or my fire.

    Whereas a large % of the carbon (~40%) fixed by grasslands is stored deep in the soil profile. And a high % of that carbon (~70%) enters the geological long cycle and does NOT enter back into the atmosphere for thousands or even millions of years.

    Then there is also the albedo effects, where trees absorb much more radiation and grasses reflect more due to averaging a much paler green coloration. 

    The US easily plants as many or more trees than they log and have been doing this for about 100 years. So already many trees planted have already grown up and been logged again, and replanted again.

    However, as I said before, this does not mean the US isn't contributing to AGW. They certainly are. But the primary ways are because of fossil fuel emissions and plowing up and destroying the biome responcible for cooling the planet...grasslands.

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  4. Isn't the other problem with tree planting that it is only temporary carbon storage for a period of time that is equal to the length of time the tree lives?
    It falls and rots and returns all the carbon.

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  5. Manwichstick - the carbon is sequestered so long as the area planted remains in forest. Undisturbed, old trees are replaced by new one and the carbon sequestered by hectare of forest approaches an approximately stable equilibrium. Even if harvested or burnt, so long as forest regrows, then it is only temporary blib in the carbon stock.

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  6. "there are at least as many now as the pilgrim days or more."

    Not according to US Forest Service. Current forest looks to be 75% of what pre-European coverage was.

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  7. Scaddemp said, "Not according to US Forest Service. Current forest looks to be 75% of what pre-European coverage was." There is less acreage of forest but each acre has more trees than in pre-Euro times. In the US northeast, in pre-Europ times each acre may have had 40-100 trees per acre. Now most have several times that. The trees are much smaller and the total volume of wood is far less but it's growing fast and overall, US forests are sequestering more carbon than is being lost to harvesting, storms and fire. Trees are seldom planted in the northeast. In the US southeast, forests are intensely managed and trees are planted after most harvesting- more trees than will survive to the "final cut" due to periodic thinning.

    I  posted a comment in the new version of this thread. I'm certainly not qualified to debate climate science but I do know American forests.

    Joe Zorzin

    MA Forester License #261

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  8. Interesting info JoeZ. Do you have a source for carbon sequesteration per hectare in managed forest compared to virgin forest?

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  9. Scaddenp, it all depends. A true old growth forest (we foresters don't use the term virgin forest) might not be adding any carbon, it might be adding carbon in the soil if not in the trees (since some will be dying or decaying internally), it might be losing carbon (if old trees are dying frequently). Young forests will be sequestering carbon but it depends on the type of forest, where it is, how healthy it is. In the US northeast it might be roughly 1 ton/acre/year not counting soil carbon. From what I read in forestry literature- the forests of North American have been gaining carbon for decades faster than the carbon losses from harvesting and natural mortality and the frequent decay in living trees and conversion of forests to other uses. I don't have access to the current research. I have a lot of old text books but I'm too lazy to look up what they say. In summary, there is much less carbon in the forests now than pre-European times but despite a thriving timber industry and forests being lost to other uses the total amount of carbon in the forests is increasing. If forests are managed properly with concern for increasing carbon- we can harvest wood products while still increasing carbon. Meanwhile, there is a movement now happening (especially here in Massachusetts) to lock up all the forests so that their only purpose is sequestering carbon. No consideration is given to where we'll get wood for construction, furniture and paper producs- no consideration that using cement and steel for construction and plastic for furniture will have a higher carbon footprint. Major political wars are now occuring over how to manage or not manage forests- it's not just about  protecting the rain forests. One huge battle which I've been caught up in is over woody biomass for energy. Massachusetts is the epicenter of that battle. One last comment- there is now a movement to build very tall structures with wood and not steel. It's called cross laminated timber-

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  10. Joez, I am not much interested in how fast land is taking up carbon (because an equilibrium will be reached), but how much carbon per hectare total. On scale of hundred years, it doesnt matter too much what state the forest is in (if you clearfell, I think regrowth will suck CO2 from air faster than timber will release it) - what matters is how much in land is permanently allocated to forest as opposed to other land uses.

    There can be plenty of reasons other than CO2 sequestration to keep foresters from felling forests however. I wonder how many fights are really about that and CO2 sequestration is excuse?

    So again question is, in managed forest, how much carbon per hectare at point where forest is harvested compared to your "old growth" forest.

    Situation here (NZ) is very different. We have large plantation forests of tightly managed exotic trees (mostly Pinus Radiata which can be ready for harvest in under 30 years in our climate) but also large areas of slow growing native forest, much of it virgin. Native forest is largely protected with very limited amounts of forestry. Plantation gets clear-felled and immediately replanted. Very selective logging (sometimes with helicopter) is norm in native forest.

    Mature native forest contain 258 tonnes carbon /hectare compared to  192 tonnes per hectare for plantation at maturity. Radiata however sucks up carbon far faster than native so much preferred for carbon farming.

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  11. Scanddenp,

    "what matters is how much in land is permanently allocated to forest as opposed to other land uses" You got that right!

    "There can be plenty of reasons other than CO2 sequestration to keep foresters from felling forests however. I wonder how many fights are really about that and CO2 sequestration is excuse?" Right on that too- at least, there are other not so good reasons- like, some people just can't stand the sight of a forest that has seen some silvicultural work which includes thinning and clear cutting- they consider it a rape of the land- despite their own love of their wood home, wood furniture, and paper products- like the people who think milk comes from the supermarket, not a cow. Forestry work is also beneficial for wildlife becaue many species prefer young forests or thinned forests. Biodiversity is not maximized by letting all the forests grow old.

    "in managed forest, how much carbon per hectare at point where forest is harvested compared to your "old growth" forest." I don't have the numbers in front of me- but forestry researchers have the numbers which vary depending on forest type and where it is. As I mentioned, here in New England, it might be something like a half ton per acre per year- whether it's just been logged or not. Old growth forest should be fairly stable in sequestered carbon- but the total will be substantially greater than any managed forest. But as many have noted- stopping all logging or slowly down the amount of logging isn't going to solve the problem of climate change and it will result in economic problems for some and the loss of a fine, low carbon footprint raw material. Unfortunately, as a non academic, I don't have good access to the research. Most such research is behind a paywall- especially in America from The Society of American Foresters- which have no need to join due to expense. There is literally tons of forestry research from all over the planet. I'm just a guy who has been working in the forests- not spending a great deal of time on top of the research so my knowledge is more personal- and localized to New England- which has many forests types due to elevation and geologic variations.

    Saving forests from a "land use change" is a great idea- stopping all or most logging to save the Earth from climate change is foolish. I should think a fair amount of logging even in the rain forests would be a good idea- if done right, which of course is the issue- because it probably wouldn't be.

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  12. Joe Z says "Forestry work is also beneficial for wildlife becaue many species prefer young forests or thinned forests. Biodiversity is not maximized by letting all the forests grow old."

    This is the kind of statement that deserves citations from science papers for substantiation. My experience of forestry was limited to the African rainforest, where, as far as I can remember, the highest biodiversity is found in old growth forests, where the canopy is continuous and so little light reaches the ground that it is easy to circulate on foot because of the lack of undergrowth. These forests are very rich in 3 dimensions, from the top of the canopy to their soil, which often can be thin (hence the buttresses shown by many tropical species) and tends to wash away without the tree cover due to the heavy rains from repeated thunderstorms (a daily occurrence in the rainy season).

    I wholheartedly agree, however, that good forest management is possible, in any environment. It is possible only if based on sound scientific evidence and free from undue influences like corruption or the maximization of profits at any cost.

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  13. Dana, note the typo in US emissions: It is not 5.4 GtC, but 5.4 GtCO2. Otherwise the US would emit more than 50% of global (=10 GtC).

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  14.  @RedBaron   "US started planting trees in the 1920's" Rome started planting trees 2000 years ago, and What is ...? In our region started planting trees 300 years ago, becouse almost desert. and What?
    "and there are at least as many now as the pilgrim days or more." Lie. Americans killed many forests. For example wonderful sequoias.
    "Deforestation in the United States. Deforestation refers to the long-term or permanent loss of tree canopy cover and the conversion of this land for other purposes. A 10 percent loss of canopy qualifies for this term. United States deforestation has caused the destruction of virgin forests by 75% percent since 1600." Google tells. So you just lied.
    USA export wood to China MORE than Russia, in dollars.

    Americans at first have to stop killing forests, and AFTER this we will speak about 1 trillion new trees.

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  15. Recommended supplemntal reading:

    The Congo rainforest is losing ability to absorb carbon dioxide. That’s bad for climate change. by Daniel Grossman, Climate & Environment, Washington Post, Mar 4, 2020

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  16. gws @13 - thanks good catch, correction made!

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  17. Philip, you're right that amongst the array of forest types/ages, an old forest will have the most biodiversity in terms of numbers of species. But, there are some species of wildlife that prefer younger forests. Younger forests whether managed or not- will have different niches than an old growth forest. Here in New England, it's been noted by wildlife professionals and even Massachusetts Audubon- that some bird species prefer young forests and dense, brushy areas- and that they won't be found in older forests. Here, deer, rabits and other mammals prefer young forests- often heavily cut forests because there is more accessible browse. Though this is true, this ecological fact is often an excuse to cut heavily- when, in my opinion, those loggers/foresters aren't really interested in wildlife, they're interested in higher profits to be made from heavy logging. I've always prefered fairly light harvest in my 47 years as a forester.

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  18. There are several characteristics of forests that aren't mentioned as far as I can see in the article or the posts.  (1) Mature forests sequester a large amount of carbon in soil micro-biology mainly through large amounts of mycorrhizal fungi and their creation of humates.  This will build over time and remain there.  (2) Tree leaves represent laminate surfaces containing water so their temperatures change slower than air around them, so they condense water as dew and in many places condense large amounts of water. (3) Roots and dead tree matter particularly on slopes reduces runoff and rapid evaporation so water helps cool forests (4) Forests change the amount of water in an area in many other ways.  Rainfall and snow doesn't evaporate as easily in shade as in fields or parking lots so it is released slower and tends to contibute to more vegetation growth. The difference is large in hot areas. (5) As I understand it, forests don't emit mid and far-range infrared as much as an adjacent area of earth, rock, pavement, etc. because they don't get hot and cool themselves through transpiration.  This increases water vapor in the air along with nucleating pollen, that contributes to rain down wind which encourages more biomass.  Water vapor becomes clouds which can create albedo themselves, rain for more plants, lower temperatures blocking sun, but of course they also operate as greenhouse gases so there are some questions there. But water that goes up also comes down.

    The work of Prof. Walter Jahne (online) is interesting on this and there were at least two Science magazine articles on the modeling 20 years ago of rain, vegetation, and climate alteration.  Which is to say that the effect of forests on global temperatures is, just like climate, a very complicated issue.  But I would like to know what anyone with expertise in both forest ecology and climate thinks about these issues (if there is such a person) and its relation to the important topic of the crisis we face and what to do about it.

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  19. I am a climatologist, not a forest ecologist, but I have dealt with forest carbon cycles and monitoring. My main familiarity is with boreal forests - and I know enough to know that different types of forest have different characteristics.

    Much of what you say makes sense to me. In boreal forests, much of the stored carbon is in the soil, not the biomass (living). It gets into the soil via root decay, plus leaf litter (annual input from deciduous trees) and fallen branches and trees trunks ("detritus"). These decay over time, and are gradually incorporated into soidl carbon - and gradually transformed to more stable compounds.

    Boreal forests are dominated by fire regimes. A cleared area can lose a lot of carbon as detritus and soil carbon decay faster with warmer soil temperatures. It takes quite a bit of time for new biomass growth to overtake the increased decay, so disturbed forests lose carbon for a while before the become a sink again.

    One item I'd disagree with in the details: leaf temperatures will not differ significantly from air temperature. Air near the ground is heated by the input from the surface (soil, shrubs, trees, etc.), so the difference is minimal. Trees do represent a method to bring soil water up to the surface (transpiration), so the water vapour flux is indeed enhanced by vegetation.

    Any vegetation cover adds organic matter to the soil and enhances soil structure in ways that reduce runoff and alter the hydrology.

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