Does Tree Planting Stop Global Warming?

Some experts say that tree-planting is the answer to climate change. Others disagree. They say that while more trees may absorb more carbon dioxide (CO2), they cannot compensate for the huge amounts of CO2 and other greenhouse gases being pumped into the atmosphere by power plants, cement factories, motor vehicles, agriculture and other sources. Nor can trees cope with the air pollution involved. We look at scientific studies and assess the role and effectiveness of trees in climate mitigation.
Most Trees Ever Planted: Record Ethiopia 2019
Ethiopia set a record by planting 350 million tree saplings over 12 hours in 2019. Photo: © PM’s Office – Ethiopia

Tree-planting has become a very positive public response to the global climate crisis, and provided they are the right species, their added presence must be beneficial. After all, trees and plants are hugely important to the carbon cycle since they act as the perfect carbon capture and storage system, which according to the IPCC represents a vital element in any climate change strategy.

Trees in tropical forests, like the Amazon Rainforest and the Congo Rainforest, exert an additional cooling effect thanks to their role in the regional water cycle between the Atlantic and the Andes. Of the estimated 39 billion tons of CO2 that we emit annually, about a quarter is removed by forests, 1 and another quarter by the oceans. Most of the remaining CO2 stays in the atmosphere. 2

Unfortunately, deforestation and global warming are steadily undermining the environmental benefits that trees have to offer. Deforestation in Brazil’s rainforests, for example, increased by almost 30 percent in 12 months (Aug 2018-Jul 2019), the highest rate since 2008, the country’s space agency reports. 3 Deforestation in Southeast Asia has been equally widespread.

At the same time rising temperatures in the Arctic Circle created the ideal tinderbox conditions for a huge series of Arctic fires which engulfed large areas of Canada, Alaska and Siberia, destroying 43,000 square km of boreal forest and creating smoke clouds covering 5 million square kilometers – larger than the size of the European Union. 4

In one month alone (June 2019), they emitted as much carbon dioxide (CO2) as the entire country of Sweden does in a whole year. 5 A smaller example makes the point more succinctly. A wildfire in a Scottish peat bog burned for 6 days in May 2019. It released almost three quarters of a million tonnes of CO2, effectively doubling Scotland’s greenhouse gas emissions for the six days it burnt. 6

Climate Change for Beginners
Climate Change for Students

How Many Trees Are There?

There are roughly three trillion trees on the planet and they play a major role in sequestering the CO2 we emit. 7 8 That said, researchers estimate that roughly 15 billion are cut down each year – a significant contributor to the amount of greenhouse gases that are driving up global temperatures. 7 However, trees draw carbon dioxide back out of the atmosphere as they grow, and have a significant cooling effect, especially in tropical zones. 9

Tree density varies considerably according to tree-type, climate and soil. The highest tree densities are found in the boreal forests of North America, Scandinavia and in the Russia taiga. These forests are typically tightly packed with relatively slim conifers and hold about 750 billion trees, roughly 24 percent of the world’s total. Tropical and subtropical forests, which occupy the greatest area of forested land, contain roughly 1.3 trillion trees, or 43 percent of the total. 

What’s the difference between afforestation and reforestation?
Afforestation is a way of creating an entirely new forest. It means the planting of trees where none existed before, or where forests have been absent for a long time. Reforestation is the replanting of trees in an area that was previously forested.

New Study Suggests We Plant One Billion Trees

Why not plant trees to help reduce our CO2 emissions? A recent study published in the journal Science tries to provide answers to this question by calculating the global potential of reforestation as a possible strategy for mitigating climate change. 10

It concludes that a worldwide program involving the planting of one billion trees could absorb two-thirds of all the emissions from human activities that remain in the atmosphere today. The study, published in the journal Science, explains the potential for tree planting but does not specify how a global tree planting programme might be paid for and implemented. Not surprisingly the study has garnered worldwide attention, as well as a robust response from within the climate industry. 11 12 13 14

How much carbon does a tree absorb?
On average, one acre of planted forest can absorb about 2.5 tons of carbon each year – that’s 9 tons of carbon dioxide. A young tree can sequester CO2 at a rate of about 6 kilos (13 pounds) per year. Trees reach their peak of carbon storage at about 10 years of age, when it’s estimated they absorb 48 pounds of CO2 per year. Planting 100 million trees could lock up around 18 million tons of carbon each year. This would save American consumers $4 billion annually on their utility bills. Of course, trees also reduce the greenhouse effect by shading and cooling buildings. This can reduce demand for air conditioning by as much as 30 percent. [Source: UrbanForestryNetwork.org]

Satellite Imagery & Artificial Intelligence

The international team of researchers, led by Jean-Francois Bastin of ETH-Zurich, used measurements of the tree cover from 80,000 high-resolution satellite images from Google Earth. Artificial intelligence software then combined this data with 10 key metrics concerning soil, topography and other climate factors, to produce a global map of where trees could grow. Put simply, it found that Earth’s ecosystems could support another 900 million hectares (2.2 billion acres) of forests, 25 percent more than we have now. By planting more than half a trillion native saplings, researchers say, we could capture about 205 billion tonnes of carbon, reducing atmospheric carbon by about 25 percent. This is the equivalent of 20 years of human-produced carbon emissions at today’s rate, or about 50 percent of all carbon emitted by fossil fuels since 1960.

“The research is excellent,” said Joseph Poore, an environmental researcher at Queen’s College, University of Oxford. “It presents an ambitious but essential vision for climate and biodiversity.”

This new quantitative evaluation shows that [forest] restoration isn’t just one of our climate change mitigation strategies, it is overwhelmingly the top one, says co-author Tom Crowther from the Swiss university ETH Zurich. “What blows my mind is the scale. I thought restoration would be in the top 10, but it is overwhelmingly more powerful than all of the other climate change solutions proposed.” 15

The scientists excluded all land used to grow crops as well as urban areas from their analysis. However, they did include grazing land, on which the researchers say a few trees can also benefit sheep and cattle. “Restoring trees at a low density is not mutually exclusive with grazing. In fact many studies suggest sheep and cattle do better if there are a few trees in the field” said Crowther.

Greenhouse Gas Emissions Need To Fall

Crowther stressed that it remains vital to reverse the still-rising levels of CO2 emissions from fossil fuels and from the effects of deforestation, and bring them down to zero. He said this is essential to prevent the crisis from worsening and also because the forest restoration envisaged would need 50-100 years to have its desired effect of removing 200 billion tonnes of carbon.

But at least tree planting is “a climate change solution that doesn’t require President Trump to immediately start believing in climate change, or scientists to come up with technological solutions to draw carbon dioxide out of the atmosphere”, Crowther said. “It is available now, it is the cheapest one possible and every one of us can get involved.”

According to Christiana Figueres, former UN climate chief: “Finally we have an authoritative assessment of how much land we can and should cover with trees without interfering with food production or living areas. This is a hugely important blueprint for governments and private sector.”

René Castro, assistant-director general at the UN Food and Agriculture Organisation, agreed: “We now have definitive evidence of the potential land area for re-growing forests, where they could exist and how much carbon they could store.” 15

If nothing is done, the study cautions, global forest cover may shrink by 223 million hectares by 2050, with the vast majority of losses happening in the tropics – the area most likely to benefit from tree-planting, and the area whose drying-out would be most dangerous for the planet.

7 Effects of Climate Change on Plants

Tree-Planting Is Carbon Capture Without The Technology

The Intergovernmental Panel on Climate Change (IPCC) calculates that in order to lower global temperature projections and cushion the impact of global warming, we need to remove somewhere between 100 billion to 1 trillion tonnes of carbon dioxide from the atmosphere by 2050. 16

But investment and development remain sluggish. As yet, only one experimental industrial facility is able to sequester CO2 directly from the atmosphere. 17 What’s more, very little electricity has been produced so far in power plants using carbon capture and storage technology (CCS).

One reason for this is the fact that CCS technology is expected to use between 10 and 40 percent of the energy produced by a power station, thus impacting considerably on efficiency and profitability. 18

Tree-planting may require greater co-ordination as well as international collaboration, but it uses proven technology and doesn’t affect anyone’s profits. See also: Which is the Largest Carbon Reservoir?

How Viable Is Tree-Planting As A Climate Strategy?

The IPCC made it abundantly clear in their Special Report on Global Warming of 1.5°C (2018) that the world needs to exploit every possible strategy in order to meet the goals of the Paris Climate Agreement.

Even so, planting trees is far from being problem-free. Sassan Saatchi, a senior scientist at NASA’s Jet Propulsion Laboratory in Pasadena, California, believes the idea has merit. But he says “it’s definitely not a solution by itself.” 19

Saatchi compliments the study on having established a reasonable estimate of global forest restoration potential and for having addressed the issue more directly than previous papers. Furthermore, he says the study’s findings on tree restoration are similar to the IPCC proposals in 2018, which suggested that 950 million hectares (2.3 billion acres) of new forests could help contain the increase in global temperature to 1.5-degrees Celsius (2.7 degrees Fahrenheit) by 2050. However, he cautions that “the devil is in the details.”

For example, before any sort of global reforestation effort is undertaken, Saatchi says, issues like the concept’s feasibility, scientific soundness and cost-efficiency need to be assessed. “We need to understand not only whether it’s possible to do such a thing, but whether we should do it,” he says.

Specific questions, he says include: how realistic are the study’s estimates of carbon sequestration? How long will it take to happen? Can grasslands and savanna ecosystems – the main biomes which the study plans to reforest – cope with and sustain increased tree cover? How much time, money and resources will be required to implement such a large project? How do the costs compare with the potential benefits? How much carbon might be released to the atmosphere by restoring forests? 

“Planting a billion hectares of trees won’t be easy,” he says. “Reforesting an area, the size of the United States and Canada combined (1 to 2 billion hectares) could take between one and two thousand years, assuming we plant a million hectares a year and that each hectare contains at least 50 to 100 trees to create an appropriate treetop canopy cover.”

Saatchi believes that within the next decade, better satellite observations and better climate models will likely enable us to assess whether a program of global reforestation will produce the carbon and climate benefits suggested, and therefore whether it should be undertaken.

Meantime, he says, we might study the tree-planting going on in China. Over the past decade and a half, China has planted millions of trees, producing millions of hectares of new forest. In southern China, for instance, forest cover increased by up to 20 percent, he says. As a result of creating this carbon sink, they almost doubled their carbon uptake.

In addition, China is a pioneer in the development of ‘sponge cities‘, involving the incorporation of more natural features (trees, grasses, plants, shrubs) in urban planning projects, to minimize flooding and reduce the urban heat island effect.

Other Reactions

Jean-Francois Bastin’s study has been contested by several scientists, including an international group of eminent ecologists led by Professor Joseph Veldman, who were invited by the editors of Science, to write a reply, now published under the title “Comment on ‘The global tree restoration potential.’” 14

Veldman’s article states that Bastin’s estimate that tree planting could sequester 205 billion tonnes of carbon is approximately five times too large. Furthermore, according to Giselda Durigan. a co-author of the Comment, Bastin and his collaborators made “extremely basic mistakes.” Such as, by including certain important sites (U.S. Yellowstone National Park, Los Llanos in Venezuela and the Cerrado in Brazil) as areas to be forested.

One of the biggest errors, according to Durigan was that Bastin proposed to plant trees in almost all areas of grassland and tropical and subtropical savanna in the world.

Grasslands and savannas are natural ecosystems, Durigan said, but are treated as degraded areas in the article. “They overlooked the fact that climate isn’t the only natural variable affecting biomass in ecosystems.”

 “The article has undermined a good idea by being overambitious and grandiose” said Durigan. It drew attention, she said, because it pleased large corporations and countries that benefit from fossil fuel burning to drive their economies.

Artificially planted forest
Planted Forest with native trees and ferns, Algyoe, Hungary. Reforestation is highly beneficial for global climate, since it removes carbon dioxide emissions from the atmosphere via photosynthesis. Photo: Takkk/CC BY-SA 3.0

Tree-Planting Must Be Good

Bastin’s article is not “the” solution to global warming, but it’s an interesting attempt to get people thinking. It wasn’t offered as an alternative to reducing CO2 emissions, or as a take-it-or-leave-it proposal, or as a magic bullet. And who seriously believes that Bastin or his colleagues are driven by a desire to please big corporations? Climate experts will continue to debate what is the best approach to mitigate global warming, but this shouldn’t entail personal attacks on those with whom they disagree. If we are to overcome our climate crisis, we have to recognize that the real opposition are those people who don’t care what climate science says. Such as those driving the ongoing deforestation in the Amazon Rainforest. In a nutshell, tree-planting must be good, even if it is only part of the solution.

References

  1. Ocean-Atmosphere CO2 Exchange.” NOAA. []
  2. Five Reasons the Earth’s Climate Depends on Forests.” Climate and Land Use Alliance. []
  3. “Brazil’s Amazon deforestation highest since 2008, space agency says.” BBC News. 18 November 2019. []
  4. “Arctic wildfires spew soot and smoke cloud bigger than EU.” The Guardian. Jonathan Watts. Aug 12, 2019. []
  5. Unprecedented wildfires in the Arctic.” World Meteorological Organization. July 12, 2019. []
  6. “Huge Flow Country wildfire doubled Scotland’s emissions.” Kevin Keane. BBC Scotland. 18 November 2019. []
  7. “Global count reaches 3 trillion trees.” Rachel Ehrenberg, Nature. September 2, 2015. [][]
  8. Crowther, T. W. et al. Nature. []
  9. Climate effects of global land cover change”. Geophysical Research Letters. 32: L23705. S. G. Gibbard; K. Caldeira; G. Bala; T. Phillips; M. Wickett; Lawrence Livermore National Laboratory; Carnegie Institution of Washington (2005-10-29). []
  10. The global tree restoration potential,” Jean-Francois Bastin, Yelena Finegold, Claude Garcia, Danilo Mollicone, Marcelo Rezende, Devin Routh, Constantin M. Zohner, Thomas W. Crowther. Science 05 Jul 2019: Vol. 365, Issue 6448, pp. 76-79. []
  11. “Exaggerating how much CO2 can be absorbed by tree planting risks deterring crucial climate action.” Professor Duncan McLaren. The Conversation. July 12, 2019. []
  12. “Reforesting an area, the size of the US needed to help avert climate breakdown, say researchers – are they right?” Professor Mark Maslin, Professor Simon Lewis. The Conversation. July 4, 2019. []
  13. “When tree planting actually damages ecosystems. The Conversation. Professor Kate Parr, Senior Lecturer Caroline Lehmann, July 26, 2019. []
  14. Comment on “The global tree restoration potential,” Science (2019). Joseph W. Veldman et al. [][]
  15. “Tree planting ‘has mind-blowing potential’ to tackle climate crisis.” The Guardian. Damian Carrington. 4 Jul 2019. [][]
  16. IPCC Special Report on Global Warming of 1.5°C (2018) Headline Statements. []
  17. “Silver bullet to suck CO2 from air and halt climate change ruled out.” The Guardian. Damian Carrington. 1 Feb 2018. []
  18. “Carbon capture and coal consumption: Implications of energy penalties and large-scale deployment”. Energy Strategy Reviews. 7 (4): 18–28. Thorbjornsson, A; Wachtmeister, H; Wang, J; Hook, M (2015). http://www.cup.edu.cn/peakoil/docs/20151224153459586713.pdf []
  19. Examining the Viability of Planting Trees to Help Mitigate Climate Change.” Alan Buis, NASA. November 7, 2019. []
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