How many trees would I have to plant to solve Global Warming?

Question

According to NASA, causes of the Earth's greenhouse effect include water vapor, carbon dioxide, methane, nitrous oxide, and CFCs. Carbon dioxide gets the most press, and NASA's page says:

Carbon dioxide (CO₂). A minor but very important component of the atmosphere, carbon dioxide is released through natural processes such as respiration and volcano eruptions and through human activities such as deforestation, land use changes, and burning fossil fuels. Humans have increased atmospheric CO₂ concentration by a third since the Industrial Revolution began. This is the most important long-lived "forcing" of climate change.

An article by NC State University says that a healthy tree can store 13 pounds of carbon per year. As I understand it, carbon dioxide is processed by the tree: the carbon is stored, and the oxygen released.

Given that, how many trees would I need to plant to solve the global climate change crisis? Should I optimize for a specific type of tree, or would pine work as well as oak or black walnut?

Answer 1

In 2010 anthropogenic emissions (not including land use change) were approximately 9167 million metric tonnes. Your data on trees holding 13 lbs (5.9 kg) of carbon per year equates to 169.6 trees per metric tonne of emissions.

So to take up all of the emissions from 2010 you would need 1,545,000,000,000 trees. A mature forest has only about 100 trees per acre (400 per hectare), so you would need 15,545,000,000 acres of mature forest. This equals an area of 24,290,000 mi² (62,910,000 km²). This is approximately the land area of Asia, Europe, and Australia combined!
The surface area of land on the planet is about 150,000,000 km², so in principle we would need to add cover onto 42% of the current land (or we could take soil from deep ocean floors to landfill 1/5th of the oceans!) in order to plant enough trees to solve the problem.

This also assumes that the 13 pounds (5.9 kg) of carbon figure is for mature forests, rather than for growing trees (see comments for further discussion). If this value is for young developing trees, it would indicate less be attainable in the longrun. If the 13 pounds is for a developed forest, perhaps a greater amount could be removed in early years as a temporary quicker fix.

There is also the problem that forested land is likely to have a lower albedo than the land surface that it covers, and hence the planet will reflect less sunlight back into space which would lead to some extra warming, so we would also need to compensate for that somehow. Apparently about 26% of Earth's land is already covered with forests. I rather doubt over half of the uncovered land surface is suitable for new forests, the continents have large bands either side of the equator that are generally too arid, and the regions close to the poles are too cold.

In short, it isn't going to work, even with the most generous assumptions about forest CO₂ exchanges (unless of course I have made an arithmetic error, which is definitely a possibility).

Answer 2

This question is from 2014 with answers from 2015. Just to add the point of view of some research that has been done since.

In essence, new calculations show that NCS (natural climate solutions: a combination of land management, forestation, etc):

...can provide over one-third of the cost-effective climate mitigation needed between now and 2030 to stabilize warming to below 2 °C.

Source: Griscom et al (2017), Natural climate solutions, PNAS, https://doi.org/10.1073/pnas.1710465114

However!

Turns out that trees, other than sequestering carbon, also emit volatile organic compounds (VOC) that act as greenhouse gases. Therefore, the answer isn't as simple as planting trees.

Quoting some recent articles on the subject:

Many scientists applaud the push for expanding forests, but some urge caution. They argue that forests have many more-complex and uncertain climate impacts than policymakers, environmentalists and even some scientists acknowledge. Although trees cool the globe by taking up carbon through photosynthesis, they also emit a complex potpourri of chemicals, some of which warm the planet. The dark leaves of trees can also raise temperatures by absorbing sunlight. Several analyses in the past few years suggest that these warming effects from forests could partially or fully offset their cooling ability.

Source: Gabriel Popkin (2019), How much can forests fight climate change?, Nature, http://doi.org/10.1038/d41586-019-00122-z

and

That doesn’t mean that all forests cool the planet, however. Researchers have known for decades that tree leaves absorb more sunlight than do other types of land cover, such as fields or bare ground. Forests can reduce Earth’s surface albedo, meaning that the planet reflects less incoming sunlight back into space, leading to warming. This effect is especially pronounced at higher latitudes and in mountainous or dry regions, where slower-growing coniferous trees with dark leaves cover light-coloured ground or snow that would otherwise reflect sunlight. Most scientists agree, however, that tropical forests are clear climate coolers: trees there grow relatively fast and transpire massive amounts of water that forms clouds, two effects that help to cool the climate.

Source: Gabriel Popkin (2019)

and

Atmospheric chemist Nadine Unger, then at Yale University in New Haven, Connecticut, conducted one of the first global studies examining one part of this exchange: the influence of volatile organic compounds, or VOCs, emitted by trees. These include isoprene, a small hydrocarbon that can warm the globe in several ways. It can react with nitrogen oxides in the air to form ozone — a potent climate-warming gas when it resides in the lower atmosphere. Isoprene can also lengthen the lifetime of atmospheric methane — another greenhouse gas. Yet isoprene can have a cooling influence, too, by helping to produce aerosol particles that block incoming sunlight.

Source: Gabriel Popkin (2019)

and

Plants take up carbon dioxide and release volatile organic compounds (VOCs), in a similar way to how other organisms breathe in oxygen and exhale CO2. These VOCs are oxidized in the atmosphere and then contribute substantially to the burden of tiny particles suspended in the air, which are known as aerosols. Aerosols produced from VOCs are known as secondary organic aerosols (SOAs), and affect both air quality and Earth’s climate. The total rate of SOA production was thought to be the sum of the individual rates associated with the oxidation of each VOC. But writing in Nature, McFiggans et al.1 show that a more accurate description is needed to improve the representation of SOAs in computational models of air quality and climate.

Fangqun Yu (2019), Atmospheric reaction networks affecting climate are more complex than was thought, Nature, https://doi.org/10.1038/d41586-019-00263-1

Sources of the articles:

one; two; three

Answer 3

At our current rate of global emissions - around 10 billion tonnes per year, PWC's 2013 report estimated that we will exceed the "safe" limit of 2 degrees warming by 2034. Trees planted in tropical areas sequester an average of 22.6 Kg of carbon per annum. NASA estimates that there are currently 400 billion trees globally. The addition of just 7 billion trees (one for every person on Earth) would therefore give us a further 16 years of safe climate at our current rate of emissions. During this time one would hope we will be able to increase renewable energy use, energy efficiency etc so as to reduce our current emissions to sustainable levels. 7 billion is vastly less than the 1,555 billion quoted above and could make all the difference.

Answer 4

Question; Given that, how many trees would I need to plant to solve the global climate change crisis? Should I optimize for a specific type of tree, or would pine work as well as oak or black walnut?

First, you need to think it as a forest. In a forest there are also trees which lose the game of evolution in addition to trees that win.

Second, you need to look where (in terms of climate) your forest is, and what's its soil. This defines the tree type.

Third, you need to cut your forest on time to optimize it' growth rate. 26 years is optimum, so roughly 4 times a century.

This way you can reach growth rates like 4.8 tn/ha/year in Finland or
5.16 tons/acre/year in the USA. I am not sure about the unit used in this source, but if their ton is 907 kg and an acre is 4046.85 m$^2$, this results in 11.56 tn/ha/year.

If the used carbon amount is $9.2 \times 10^9$ tons, and half of wood weight is carbon then you need $1.6 \times 10^9$ ha or 16 000 000 km$^2$ of forest.

There is 39 500 000 km$^2$ of forest in the world. If 40% of it is used optimally, it would be enough at present rates. The CO$_2$ concentration is not a problem; human population growth is.

Answer 5

None, except tropical young trees. Overall, trees are carbon neutral. Wood/biomass has to be geologically sequestered to store carbon in the long term. That requires a tree with

• a long life span (sequoia, cypress, etc)
• and growing in swampy conditions (to hold the remnants in sediment)