What Trees Can, and Cannot Do for the Climate

We all know that trees (and other green plants) are a huge part of the natural carbon sequestration machinery that maintains the atmosphere at comfortable-for-life levels. It is both true that deforestation has played a significant role in the increased levels of carbon dioxide in the air, and that reforestation can play a significant role in reversing this. But how big of a role can tree planting really play in averting climate disaster?

How many trees would be needed for reforestation to make a difference?

What roles can reforestation play, realistically? We use some pretty big numbers here, millions, billions, trillions (see note 2 at end), but the picture comes clear. Let’s look at summary numbers:

  • A mighty oak tree can ... Walk in a forest, surrounded by giant trees—they’d have been growing for decades. A 50-year old oak may weigh 10 to 50 tons (depending on species, environment, etc., see note 3 at end), and that tree can sequester as much as 60 kilograms of atmospheric carbon per year, and keep doing that for decades. We ignore, for the moment, what happens when trees die, but in the long term that also needs to be considered.

  • At forest scale ... A healthy section of Amazonian rain forest contains from 400 to 800 mature trees per hectare—, These are large trees, so if each tree sequestered 80kg / year, each hectare could sequester 50,000 kg per year—50 metric tons per hectare. Notes 4 & 5.

  • What’s in the air ... The atmosphere has now surpassed 400 ppm (parts per million) of CO2, for a total of 121 million gigatons. That’s up from 320 ppm in the 1960s. And the amount of CO2 in the atmosphere rises by 33 to 40 gigatons per year. Note 6 (Amazingly: that’s nearly 5 tons per person per year.) Suppose, then, that we aimed that forests would draw down that amount—35 GT / yr. How many hectares of forest would that require?

  • Forests needed … The answer to that question is:

    • 35 billion tons divided by

    • 50 (tons / hectare)

    • = 700 million hectares (about 1,700 million acres) with about …

    • 500 billion more mature trees.

For comparison:

  • The area of the entire USA is about one billion hectares (2,430 million acres) so the increase in forested land would be equivalent to more than half of all the land in the USA. 

  • The entire area of all land on earth is 51,000 million hectares, and the amount that is considered to be forested today is 4 billion hectares (a half hectare per person alive, for comparison), so the increase is about 15% of what’s there.

  • There are about 3 trillion trees currently on earth. (That’s after humanity cut down at least a half trillion in the past century or so.) Note 7

Pragmatic solutions: what to do with this?

It does seem reasonable to expect to add something like this vast number of trees, but only reasonable to do so in a time scale of multiple decades: that’s how long it takes trees to mature, plus it’d take years to align the governmental and land-ownership and financial issues. And we don’t have that many years available to solve the problem—which will worsen during the intervening decades. Forests can be part, but only part, of solving the problem. (It’s important to note that adding more forests would solve other problems as well, including species diversity, moisture retention and soil enrichment.)

Collectively, we should be thinking about an earth that, by the year 2080 or 2090 has added at least 300 billion new trees. In subsequent explorations, let’s look at how to achieve that, and how to connect that goal to other goals—lower fossil fuel use and other means of carbon sequestration.

Unanswered questions: what to do next?

  • How many trees per person: if each adult in Europe and North America (combined total population of 950M, perhaps 700M adults) planted or sponsored planting a tree per day, what difference would it make? That’d total 700M trees * 365 = 255 billion. Fully half of the number of trees required to sequester the annual excess carbon.

  • Where can the trees go? There doesn’t seem to be enough room in the US and Europe? That’s partially or mostly true. But we do have the money, and many of the countries that have the space don’t have the money (rural Brazil, DRC, Indonesia, for example, along with other countries suffering rapid deforestation). 

  • These two points suggest that this is a matter of political or business will. And technological cost reduction, means of verification, trust. Solvable. Note 8

  • Does the type of tree matter? (Yes.) Does it matter where the tree is? (Mostly: no). What happens when the trees die? (This is a major issue, in at least two ways: large areas of forest stands now are entirely dead, because of the consequences of climate change, already; and because rotting trees do return some of their sequestered carbon to the atmosphere.)

Source notes and credits:

  1. Image - Greenpeace, John Novis (Congo River basin forest)

  2. Millions, billions … A million = a thousand times a thousand. A billion is one thousand times more = 1,000 x a million. A trillion = 1,000 x a billion (and also equals one million x one million). In scientific notation: one thousand is 10^3; a million is 10^6, a billion is 10^9, a trillion is 10^12. (Apologies: SquareSpace, the formatting engine we use, doesn’t easily support superscripts.)

  3. Missing, tbd

  4. About 600 trees per hectare in the Amazon rain forest. www.tropenbos.org/resources/publications/a+spatial+model+of+tree+alpha-diversity+and+tree+density+for+the+amazon Tree densities in temperate climates tend to be rather lower, with 100 trees / acre, 250 / hectare being considered healthy.

  5. Units: One hectare = 10,000 square meters, the area of a square 100 meters on each side. That’s about 2.5 acres; A metric ton is 1,000 kilograms, approximately 2,204 pounds. A standard US or British ton is 2,240 pounds. The similarity of metric and US / UK tons - they differ by less than 2% - makes it easy to use the ton for simple comparisons without specifying which ton you mean. 1 gigaton = 1 billion tons. 1 million gigatons = 1 million, billion tons.

  6. www.wri.org/blog/2019/12/co2-emissions-climb-all-time-high-again-2019-6-takeaways-latest-climate-data#

  7. https://www.scientificamerican.com/article/how-many-trees-are-there-in-the-world-video/ Some estimates are that the number of trees at the dawn of humanity may have been as high as 6 trillion.

  8. https://www.theatlantic.com/science/archive/2017/07/paying-people-to-preserve-their-trees/534351/ and we hope to dig in more on this subject in future notes.