How Many New Trees Would We Need To Offset Our Carbon Emissions?
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Suddenly we are all being told khổng lồ plant trees. The hope is that they will save us from the worst effects of climate change.
The idea is everywhere. The Swedish climate activist Greta Thunberg has made a film arguing for extra protections for the world’s forests, và for the replanting of those that have been cut down. George Monbiot, a columnist in the UK’s Guardian newspaper, has founded a chiến dịch called Natural Climate Solutions, which advocates restoring forests & other ecosystems.
This is not just talk. The UK government has planted millions of trees over the last decade, và has pledged another million between 2020 & 2024. Others have attempted far more dramatic feats: in 2016 one Indian state planted 50 million trees in one day, while in July last year Ethiopia claimed lớn have planted 350 million in a day. Even the UK’s Daily Mail, a right-wing newspaper not known for its climate activism, has just launched a chiến dịch encouraging all its readers lớn plant a tree.
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Protecting existing forests & planting new ones are surely good things lớn do. However, scientists say we must not place too much faith in trees khổng lồ save us. In particular, last year one research group claimed we can plant a trillion extra trees và remove a quarter of the carbon dioxide currently in the air. These figures have been widely criticised as overhyped and unreliable. Trees will definitely help us slow climate change, but they won’t reverse it on their own.
The underlying problem is that our society is releasing greenhouse gases, especially carbon dioxide (CO2), that are warming the Earth’s climate lớn levels we have never experienced before. As a result the great ice sheets are melting, contributing to rising seas, và extreme weather events like hurricanes and droughts are becoming more severe.
The solution is to stop emitting all greenhouse gases, for instance by replacing fossil fuels with renewable energy sources lượt thích solar power. Deforestation is actually one of the biggest sources of carbon dioxide, because when trees are cut down much of the carbon stored within them escapes into the air – especially if the wood is burned. For instance, in 2017 land use changes – mostly deforestation – contributed four billion tonnes of CO2 emissions to the global total of 41 billion tonnes of CO2. In other words, if we stopped cutting down trees we would cut our annual emissions by about 10%.
However, simply stopping all our emissions is no longer enough. At this point we have emitted so much CO2, & left emissions cuts so late, that we are almost certain lớn miss our targets of limiting warming to 1.5C or 2C. That means we must also find ways to actively remove CO2 from the air.
All sorts of technological approaches have been proposed, but trees are an obvious contributor. New trees can either be planted in regions that have been deforested (reforestation) or in places that have never had them before (afforestation). As the trees grow they pull in CO2 through their leaves và convert it into carbohydrates, which they use to lớn grow. So long as a tree lives, that carbon stays within it – & trees can live for decades or centuries. Trees are a natural “carbon sink”. It follows that we should both stop chopping down forests – especially tropical ones like the Amazon, which store huge amounts of carbon – và start planting more.
By some estimates, trees can be an enormous carbon sink. A study published in July 2019, led by Thomas Crowther of ETH-Zurich in Switzerland, estimated the world has room for an extra 0.9 billion hectares of forest. Once those trees had matured, they could store 752 billion tonnes of CO2. Planting trees, the team wrote, is “one of the most effective carbon drawdown solutions lớn date”.
This finding has had immediate, fierce pushback from other climate scientists. In October 2019, the journal Science published four highly critical comments. These argued that the researchers had overestimated the carbon trees could store – by a factor of five. They also highlighted multiple mistakes. For instance, much of the land Crowther described as “available” for tree planting already has plants growing on it, all of them storing carbon, many of which would have to be removed, according to lớn Sonia Seneviratne of ETH-Zurich & her colleagues.
The criticism hit home and, in May 2020, Crowther"s team published an extensive correction, in which they admitted that some of their headline claims were "incorrect" & that the data contained "errors".
The criticism has hit home. In May 2020, Crowther"s team published an extensive correction, in which they admitted that some of their headline claims were "incorrect" and that the data contained "errors".
There are also deeper problems, because trees have more than one way to lớn affect the climate.
The first issue is that trees are dark, at least compared to lớn other things that might blanket the land, such as grass or snow. As a result, planting more trees typically makes the land darker. Since dark surfaces absorb more heat, a dark tree-covered surface will trap more of the Sun’s heat – và warm the local climate.
As a result, there is a delicate balance between trees’ ability to take in CO2, reducing warming, & their tendency lớn trap additional heat và thus create warming. This means planting trees only helps stop climate change in certain places.
Specifically, according lớn a 2007 study that has been repeatedly confirmed, the best place khổng lồ plant new trees is the tropics, where trees grow fastest & thus trap the most CO2. In contrast, planting trees in snowy regions near the poles is likely lớn cause a net warming, while planting them in temperate climates – like that of the UK, much of Europe & parts of the US – may have no net effect on climate.
“You have lớn be careful where you vị reforestation,” says David Beerling of the University of Sheffield in the UK.
Others say this problem is overblown. “They’re assuming that snow cover’s going khổng lồ stay there with warming,” says Beverly Law of Oregon State University in Corvallis. She points out that the polar regions are warming faster than the rest of the planet, so much of the snow may melt in the coming decades – in which case planting trees will not make the ground that much darker. “That’s been kind of a red herring that’s held out there a lot,” says Law.
The other thing trees do is emit volatile chemicals into the air. “That’s the pine-y smell you get when you walk through a forest,” says Dominick Spracklen of the University of Leeds in the UK. These chemicals stick together to khung tiny floating particles called aerosols, which have complicated effects.
For example, the aerosols create a faint haze. This scatters sunlight back into space, cooling the planet. “Probably the more important effect is those particles act as seeds for cloud droplets,” says Spracklen. This creates more low cloud, or thicker low cloud, which also bounces sunlight back to space.
However, the trees’ emissions can also lead khổng lồ warming if they react to form the greenhouse gas methane, or ozone, which is a greenhouse gas at low altitudes. For Nadine Unger of the University of Exeter in the UK, this is a major problem. “The mutual relationships between forests & climate are actually really rather more complex & not fully understood,” Unger told the James Lovelock Centenary conference at the University of Exeter in July 2019.
In năm trước Unger calculated that, by chopping down forests from 1850 khổng lồ the 2000s và thus preventing them emitting volatiles, we have created a cooling effect that slightly offset the warming from greenhouse gas emissions. Shortly afterwards she wrote an opinion piece for the new york Times headlined “To save the planet, don’t plant trees”.
However, other reforestation experts are critical of Unger’s findings. “The overall effect is quite small,” says Spracklen, who has studied the effects of aerosols. “Then the carbon storage blows all the rest out of the water.” Law agrees, saying the effects of aerosols are also “a red herring”.
So how much can trees really help us solve our climate problem?
In a 2017 study, researchers led by Bronsom Griscom, now at Conservation International, estimated the full potential of “natural climate solutions”. This includes restoring wetlands and other ecosystems, and minimising emissions from farmland, but the biggest contributors by far were preserving existing forests & reforesting degraded areas.
The team estimated that the natural climate solutions could lock up the equivalent of 23.8 billion tonnes of CO2 per year. That is a little over half our annual emissions, but they emphasise that many of the strategies they studied would not be cost-effective: a more plausible figure would be 11-15 billion tonnes of CO2 per year. This implies natural climate solutions could mop up about 30% of the CO2 we need to giảm giá with every year.
For Law, it is one of the best estimates published khổng lồ date. The researchers “really did a pretty good job”, she says.
The UK’s Royal Society came to lớn similar conclusions in a 2018 report on greenhouse gas removal technologies. They estimated that reforestation could remove three billion lớn 18 billion tonnes of CO2 per year. These are significant numbers.
Uncertainties bởi remain, however. For instance, the climate will keep changing for many decades, và this will affect trees’ behaviour and growth – but we don’t really know how yet. “There’s still a question mark,” says Beerling. “Will they be limited by nutrient availability or increased fire or increased drought?” Similarly, planting trees in dry areas can cause water scarcity because they suck up so much – as đài loan trung quốc has discovered.
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However, there are also surprise benefits of planting trees. For instance, a 2018 study suggested that large-scale tree planting in dry tropical regions would cause a shift in weather patterns, leading to lớn more rainfall on land – enabling more plant growth and therefore more carbon storage.
Also, planting trees is not just about stopping climate change. “As well as the climate emergency, we’re facing a biodiversity crisis,” says Spracklen. Planting trees can help with both, he says, “but only if we bởi it right”.
At the moment a lot of the trees being planted are monocultures of fast-growing commercial species lượt thích acacia or eucalyptus. These have “virtually no biodiversity benefits & may even replace something that was better”. It would be better khổng lồ restore species-rich forests, he says. In line with this, Law has highlighted that planting rich new forests can boost local biodiversity, as well as improving water availability.
The real uncertainties are not scientific, but socio-political. Put simply, where will people & nations allow the large-scale planting of trees? “As soon as you get down onto the land, there’s people living there & they have aspirations for how they want khổng lồ live their lives that maybe don’t involve tree-planting,” says Spracklen. “There’s virtually nowhere where land’s just lying idle và you can just come along và do that.”
He points to the Welsh hills, which are severely deforested & consequently lacking in wildlife – but which are politically difficult to lớn reforest because they are dominated by the sheep-farming industry. Similar conflicts over land use exist in all countries.
The message, then, is that trees can play a significant role in stopping dangerous climate change – provided we plant them in the right places. The challenge will be finding ways to fit huge new forests into our societies in such a way that people accept them.
* This article has been edited on 3 June 2020 to địa chỉ details of the correction issued in Science on the paper by Thomas Crowther & his team.
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Could planting trees make enough difference to lớn impact increased CO2 levels on a global level?
The idea seems simple enough. Since trees và plants "take in" carbon dioxide (CO2) during photosynthesis, planting more trees should reduce CO2 levels.
For reference, combined with oceans, the terrestrial biosphere including plants và trees already remove about 45% of the CO2 emitted by human activities each year.
Other scientists report that plants and trees globally are responding lớn elevated CO2 levels by taking up more CO2.
This makes sense. Here at windchimewalker.net, our customers report khổng lồ us that in controlled conditions like indoor greenhouses, doubling or tripling the CO2 levels can act as fertilizer, which increases the growth & productivity of plants.
Could enough trees be planted to impact CO2?
Unfortunately, not likely. Here"s why.
A typical hardwood tree can absorb as much as 48 pounds of carbon dioxide per year. This means it will sequester approximately 1 ton of carbon dioxide by the time it reaches 40 years old.
One ton of CO2 is a lot. However, on average human activity puts about 40 billion tons of CO2 into the air each year. This means we would theoretically have to lớn plant 40 billion trees every year, then wait for decades to see any positive effect. By the time 40 years had passed, the trees we had originally planted would only cancel out the increased CO2 levels today.
But others disagree.
For example, National Geographic says "An area the size of the United States could be restored as forests, with the potential of erasing nearly 100 years of carbon emissions." This is based off of the first study of its kind to determine how many trees the earth could actually support.
The other issue is the impact of CO2 on tree growth & the species of trees that thrive best on high CO2 levels.
For example, research from NASA indicates that the current increase in CO2 levels have resulted in a significant greening increase over the last 35 years. This increase in leaves on plants và trees is actually equivalent khổng lồ planting a forest twice the size of the continental United States. Yet, the shorter lifespans of these trees as the result of faster growth give them less time khổng lồ absorb CO2 than was anticipated. This implies that the trees will die sooner & before they"re big enough lớn store a significant amount of carbon from our atmosphere.
So should we really continue lớn plant trees?
Science Magazine published a report titled, "The global tree restoration potential" which concluded that there may seem khổng lồ be enough land to lớn increase the worlds forest areas by approximately one third. The downside lớn this is that the potential for land space can diminish quite quickly given global temperature rising. Additionally the report states, "Even if global warming is limited lớn 1.5 degrees Celsius, the area available for forest restoration could be reduced by a fifth by 2050 because it would be too warm for some tropical forests."
This same topic was researched in 2016, where a research group of 800,000 volunteers in India planted 50 million tree saplings in an effort khổng lồ re-green the country. While there are many good reasons lớn combat deforestation, this project would have to lớn be replicated 800 times to cancel out the CO2 created by humans.
This does not mean that there is still not some importance in doing so, however. Global CO2 levels could be reduced by planting trees as national geographic concludes, "If we act now we could cut carbon dioxide emissions by at least 25% these levels would not have been seen until almost a century ago".
Aren"t loggers required to replace the trees they cut down?
In many countries there are regulations that require logging companies to replace the trees they log. According khổng lồ Appalachian
Wood.org "Three quarters of all the trees planted in America last year were planted by forest hàng hóa companies & private timberland owners. Và logging companies pay a special fee lớn fund for replanting và reforestation when they buy the right to lớn harvest a section of timber on state or national forests." Americans plant at least 1.6 billion trees or about 6 trees for each one we use.
However, the same philosophy of forest management is not occurring in other countries. While their are conflicting data sets as to the rate of deforestation, all scientists agree that we are continuing khổng lồ have a net loss of forests year after year. According khổng lồ the Food và Agricultural Organization of the United Nations:
"Between năm ngoái and 2020, the rate of deforestation was estimated at 10 million hectares (38,000 square miles) per year, down from 16 million hectares per year in the 1990s. The area of primary forest worldwide has decreased by over 80 million hectares 310,000 square miles) since 1990."
For example, more than half the potential to restore trees can be found in the following countries: Russia (373 million); Canada (78 million); nước australia (58 million); Brazil (50 million); and china (40 million).
So the answer seems lớn be that planting trees, while a good idea, would not in itself cancel all of the effects of human production of CO2 và many trees would actually die off before they are large enough.
For more information on deforestation visit the State of Forests & Forestry website.
What is the best way lớn offset CO2 levels?
While planting trees are important, trees alone aren"t enough. As a single person, what can you do to help offset the rise in CO2 levels?
1. Reforestation. If you own land, plant trees on it. As the old Chinese proverb goes, "The best time khổng lồ plant a tree was trăng tròn years ago. The second best time is now.” Here are the trees that convert the most CO2:
2. Renewable energy. There are dozen"s of small ways you can take advantage of renewable energy around your home, from solar panels lớn roof-mounted solar water heaters lớn paying a few dollars extra lớn the utility company each month for their carbon offset program. Building a new house? Talk lớn your architect about using thermal mass walls khổng lồ retain both heating và cooling.
3. Community projects are a great way khổng lồ get involved and help the climate. The advantage of these projects is that you will spend time with like-minded people và see for yourself what works & what doesn"t in their homes và businesses.
4. Waste khổng lồ energy initiatives are programs that convert organic waste into energy. The only way these can happen is by supporting government officials and politicians who are also committed to lớn these goals.
5. Changing transport. Common sense says that taking a train instead of a plane doesn"t matter since the plane would make the trip with or without you. However, for personal transport, electric cars finally make sense. If everything you know about electric cars is over a year old, it"s time lớn research them again.
We can take steps to reduce CO2 emissions now, or we can wait & see what happens. Only good science và good data will give us a valid answer.
