For most of the Earth’s lifetime, trees sufficiently sequestered carbon and kept the climate levels stable. Then, industrialization happened. Excessive amounts of greenhouse gases in the atmosphere caused global temperatures to rise. Many may wonder if by planting trees, climate change is a problem we can effectively tackle.
Global warming is causing climate changes that already result in catastrophic consequences. Precipitation patterns have become unpredictable in some places, and that has affected food production. In the coldest regions of the world, ice caps are rapidly melting; by 2050, the arctic sea may be completely ice-free. The intergovernmental panel on climate change warns that global temperatures should not exceed 1.5-degree celsius, but the current temperature is already above 1-degree celsius.
Would it not be easier to tackle rising temperatures if we just stopped emitting greenhouse gases altogether? Well, even though it would be very helpful, it could take decades before we see any difference in the environment. This is because greenhouse gases can stay in the atmosphere for up to 100 years or more. Besides that, much of the CO2 comes from human activities. Agriculture, transportation, industries, energy, and some everyday activities like cooking are sources of human-caused emissions.
A sudden and total stop of all human actions contributing to greenhouse gas emissions would cause massive job loss, financial difficulty, loss of modern conveniences, and even loss of lives. The world would be in chaos. Therefore the best approach is one that we can integrate into our way of living and incremental change in our practices.
Trees may be one way to curb global warming without disrupting our economy or reverting the progress humankind has made since the industrial revolution. The simple reason is that trees absorb carbon dioxide, which is the most significant amount of greenhouse gas in the atmosphere. The soil and oceans are also carbon sinks, but unlike trees, humans can not propagate them.
76% of GHG in the atmosphere is carbon dioxide; the others are methane, fluorinated gases, and nitrous oxide. It also accounts for 76% of global human-caused emissions. Burning fossil fuels, cutting down forests, agriculture, and other land-use activities release CO2. Once it’s released into the atmosphere it stays there for a very long time3. 40% remains after 100 years. Even after 10,000 years, 10% will remain.
The oceans are the world’s highest-performing carbon sinks; they absorb more than half of carbon dioxide emissions. The excess CO2 is reducing the ocean’s PH, making it acidic. Over the last 200 years, ocean water has become 30% more acidic. Shelled water animals are already suffering from the imbalance, and the acidity weakens their protective shells. Clearly, there will be worse consequences if we do nothing about this.
There are different schools of thought on whether tree planting can reverse global warming. Some think it is just what we need to stop climate change once and for all, but others are not so optimistic. Before examining all the different opinions on the climate-saving power of trees, let’s see how carbon sequestration works.
The process of photosynthesis through which plants increase their biomass requires carbon dioxide, amongst other things. CO2 is responsible for 33% to 60% of plant growth. So for plants to survive, they need carbon dioxide, and there is an excess amount in the atmosphere already. So forests absorb and store carbon, they also help our lands store it as well. In tropical forests, 50% of CO2 is stored in plant biomass, and the other half is in the soil. Trees all over the world help to maintain the CO2 balance of the planet. Not only do forests absorb CO2, but they also release it too. When we burn trees, cut them down, or they die and decay, the carbon escapes into the atmosphere.
Over the past four decades, forests have sequestered one-quarter of the CO2 emissions caused by humans. Scientists believe that they can do an even better job. The forests in the EU have an estimated 9.8 billion tons of carbon stored in their biomass. This means that the yearly carbon emissions of the EU are only about one-seventh of the amount already stored in the forests. Therefore, the forests in the EU are seen as a viable method of reducing global warming.
Scientific evidence shows that forests, both old-growth and managed, sequester up to 6 tons of carbon per hectare. Research theorizes that replanting trees on 2 billion hectares of degraded land can wipe out the yearly increase of atmospheric CO2.
Using trees to reduce global warming is not as straightforward as it may sound. This is because forests could also become carbon sources. Fires, pest outbreaks, and storms cause a massive return of carbon to the atmosphere. For instance, Canada's managed forests have done a great job as carbon sinks until recently. In the past decades, however, the forests were carbon sources at some points. This happened because of tree insect outbreaks, forest fires, and land use.
Another study advises that the tropics are the best place to plant trees2. Trees grow faster in these areas and therefore absorb CO2 faster. The study suggests that planting trees in snowy areas could become problematic. This is because the trees can create a warming effect, which is the issue we are trying to avoid in the first place. In temperate climates like much of Europe and some parts of the US, planting trees may have no significant effect on global warming. The study concludes that planting trees to solve global warming is not the worldwide solution presented by enthusiasts.
Professor Beverly Law of Oregon state university disagrees with the idea that more trees will do damage to the icy regions of the planet. In her opinion, the polar regions are warming at a much faster rate than the rest of the earth. It is not correct to assume that the snow cover will remain intact without trees despite the earth’s rising temperature. It is most likely that the snow will melt in the coming decades anyway. In that situation, trees will not create an albedo effect.
Then again, Nadine Unger, a professor at the University of Exeter, UK, warns against tree planting. She states trees could form the greenhouse gas methane or ozone through a chemical reaction as the chief reason. In her 2014 study, she calculates that deforestation from 1850 to the 2000s has created a cooling effect that slightly offsets the warming of greenhouse gas emissions. Her article “to save the planet, don't plant trees” was published in the New York Times. However, Professor Dominick Spracklen, who has studied the effects of aerosols, says such reactions are insignificant. And have a very minute impact on the climate of the earth.
Warming temperatures could also cause increased heterotrophic respiration and rapid decomposition of organic matter. This will reduce the carbon sinking capacity of the forests and make them sources instead. This study analyzed data from 244 structurally intact African tropical forests and 321 published plots in the Amazon for over 30 years. The researchers found that the CO2 intake of the African tropical forests has been stable at 0.66 tonnes per hectare for three decades up till 2015. When there began to be noticeable carbon loss. However, in the Amazonian forests, the decline in CO2 uptake has been in decline for a longer time.
Although both sites show increased tree growth, which is the expected net effect of rising temperature and atmospheric CO2, the carbon sinking capacity of these two forest sites is very different. The research concludes that the tropical forest carbon sink has already reached its limits1.
A 2000 publication by 18 climate specialists disagrees with the aforementioned statement. In their own opinion, the carbon sinking capacity of forests should increase by 10% to 20% because of Co2 fertilization. They predict that increased heterotrophic respiration will cause long-term saturation, contributing to strengthening forests as carbon sinks.
Individuals, organizations, and governments are throwing their weight behind tree planting. There are efforts to protect existing forests and plant new ones.
The Paris Agreement has a significant number of nations coming together to combat climate change. Although President Donald Trump announced intentions to pull out of the agreement, he endorsed the 1 trillion trees program at the 2020 world economic forum. The 1 trillion trees program is a multi-stakeholder effort by the world economic forum and its partners to grow, restore, and conserve 1 trillion trees. The United States representative, Bruce Westerman, also pushed the 1 trillion trees act. Although it garnered a lot of criticism, the president supports the 1 trillion trees act.
Nations under the United Nations Economic Commission for Europe have seen an increase in how much CO2 trees have taken up. This is because the policies and laws concerning forests in the area ensure that new trees are planted to replace the ones cut down. The total forest cover of this region amounts to 1.9 billion hectares.
Research led by professor Tom Crowther found that there are 1.7 billion hectares of land that could support new trees. This area is 11% of all land and equal to the size of the United States and China put together. The researchers did not include urban areas or crop fields in their analysis. They estimate that this worldwide tree planting scheme could remove 205 gigatons of human-caused carbon emissions in 50-100 years.
In professor Crowther's words
“... it is overwhelmingly more powerful than all of the other climate change solutions proposed”.
The researchers make it clear in an erratum later published that the statement in the earlier sentence does not mean to put planting trees above reduction of fossil fuel emissions. The study gives a clear blueprint of how much more trees we can plant without disrupting agriculture4. Although these proposed new forests will cover areas worldwide, over 50% of the forest restoration land area are in the United States, Brazil, Canada, China, Russia, and Australia.
The Edinburgh Centre for Carbon Management tried to examine if more trees are all that we need to stop global warming. They created two simulations; in the first, two situations are considered. The first is constant deforestation, no large-scale reforestation, and the Amazon sink becomes a carbon source. This situation showed atmospheric concentration exceeding 500ppm in 2050. In the second situation, they reduce deforestation and add extensive reforestation, but atmospheric concentration also exceeded 500 ppm in 2060. In the second simulation, they considered again the same situations but with a vast reduction of greenhouse gas emissions from the energy sector. In the first weak forestry situation, atmospheric concentration peaked in 2070. In the strong forest situation, however, critical atmospheric concentration was never reached. Rather it began to reduce in 2050.
This led the ECCM to conclude that tree planting alone can not solve global warming. It needs to be combined with a reduction in fossil fuel emissions. As Greta Thunberg says
“planting trees is very good, of course, but it is nowhere near enough”.
In addition to protecting the planet, increased forestry can be beneficial to people. Planting fruit and rubber trees can provide income for them. Also, in countries with large wood industries like Nigeria, Ghana, and Cameroon, a reforestation campaign could reduce pressure on their natural forests. An abundance of trees is equal to an abundance of wood that we could use to replace fossil fuels in some processes.
All over the world, people are planting trees to help tackle the climate crisis. Last year, Ethiopia claimed to have planted 350 million trees in one day. The UK government has planted millions of trees in the last decade and plans to plant a million more before 2024. Planting 1 trillion trees could help mitigate the effects of global warming, but tree planting programs are not without issues.
One of the biggest challenges to forest conservation and planting new forests is consumerism. Corporate commodity supply chains account for about four-fifth of the loss of tropical forests across Asia, sub-Saharan Africa, and South America. Trees and the land area they occupy are valuable economic resources. Often, government conservation and land use policies are relaxed to allow companies to use these resources to satisfy the increasing demand. In the United States, they have released 800,000 acres of protected public land for commercial development. There is also a presidential push for pro-logging in the Alaskan Tongass National Forest.
Sassan Saatchi suggests that planting the number of trees required to provide forest cover for over a billion hectares of land as recommended by this study could take between 1-2 thousand years. This is a relatively long time, especially because trees absorb CO2 as they grow and not immediately as we plant them. Mature trees sequester carbon much faster, and it takes about 100 years for trees to mature. The time factor is an issue that we can not do anything about.
The albedo and evapotranspiration of the earth's surface is also a concern in the scientific community. Scientists suggest that planting more forests could have a warming effect in the northern hemisphere during winter. They think the forest canopy could make the earth's surface darker, and as darker lands absorb more heat, this will warm up the local climate. Increased tree cover in the tropics will increase evapotranspiration and create a cooling effect.
More than one research clearly states that forest restoration can not be a monoculture. People need to replant degraded forests with as many different species as possible. Even at that, the new forests may not have the ecosystem as the previous naturally existing ones. Also, the soil health may have changed and may not support previous plant species the way it used to. Indiscriminate tree planting without considering biodiversity balance may only exacerbate the climate crisis.
Natural disasters like wildfires, storms, and pest infestation threaten the effectiveness of trees as reliable CO2 reservoirs.
The earth is in a state of climate emergency, and we need to take swift action to avoid catastrophic consequences. While scientists may disagree on trees being the solution to global warming, the general consensus is that trees can help. There are many ways to support trees for a better climate. One of the simplest ways is to plant one in your home. Planting trees, protecting existing forests, and positive land-use changes could support our fight against global warming.
Hubau, W., Lewis, S.L., Phillips, O.L. et al. Asynchronous carbon sink saturation in African and Amazonian tropical forests. Nature 579, 80–87 (2020). https://doi.org/10.1038/s41586-020-2035-0
Combined climate and carbon-cycle effects of large-scale deforestation. G. Bala, K. Caldeira, M. Wickett, T. J. Phillips, D. B. Lobell, C. Delire, A. MirinProceedings of the National Academy of Sciences Apr 2007, 104 (16) 6550-6555; DOI: 10.1073/pnas.0608998104
David Archer et al. (2009) Atmospheric lifetime of fossil fuel carbon dioxide. The Annual Review of Earth and Planetary Sciences.
The global tree restoration potential BY JEAN-FRANCOIS BASTIN, YELENA FINEGOLD, CLAUDE GARCIA, DANILO MOLLICONE, MARCELO REZENDE, DEVIN ROUTH, CONSTANTIN M. ZOHNER, THOMAS W. CROWTHER, SCIENCE05 JUL 2019 : 76-79