Reforestation and Climate Change

by Gordon Maclean, Ph.D.

If carbon dioxide (CO2) is a greenhouse gas and causing climate change and trees use CO2 to live, let’s just “plant a bunch of trees.”

A very simple statement, and on the surface a true statement, the concept is called “green sponging.” Green sponging is the idea that we can use trees to soak up all the excess CO2 like a giant global green sponge. But as with most simple statements, it ignores a LOT of hidden issues and paradoxes.

Greenhouse gases are primarily released from the burning of fossil fuels. If you remember our previous discussion on carbon cycling, plants use CO2 along with sunlight in the process of photosynthesis. If a plant dies and cannot decompose, the stored CO2 eventually, by geologic processes, becomes coal, oil, or natural gas. Fossil fuels are the stored material of millions of years of plant life that has been locked away underground for hundreds of millions of years. All the coal, oil, and gas we have been using for the last 150 years is draining out that giant sink of CO2 and putting that back into the atmosphere.

I will save how greenhouse gases work for another time. I want to focus on what we can do to reverse CO2 levels in the atmosphere since this is critical to reversing climate change.

First of all, we need to stop using fossil fuels wherever possible—a lofty goal. Even if we could do that tomorrow, the effects of the CO2 already added to our atmosphere would not change our climate’s course for decades.

Fossil fuel burning and fires emit about 11 gigatons of carbon per year; forests, fields, grasslands, and oceans absorb about six gigatons. That means every year, about five gigatons of CO2 gets added into the atmosphere.

OK, what’s a “gigaton” you ask. The prefix giga means “one billion,” so one billion tons of something is a gigaton. What weighs a “gigaton?” Well, if you could fill up New York’s Central Park with water to a depth of 200 feet, that would come close. That’s one gigaton; we are putting five of those into the air every year after taking into account what the earth can currently handle. If you made a balloon out of pure CO2 emissions, it would be almost 10 miles in diameter each year.

Let’s talk about planting trees. According to a 2019 study in the journal Science, planting one trillion trees could store about 225 gigatons of carbon or about two-thirds of the carbon released by humans into the atmosphere since the Industrial Revolution began.

One trillion of anything is difficult to imagine. If you were to plant trees spaced ten feet apart, one trillion trees would require about 4,000,000 square miles, more land area than the entire United States. Unfortunately, we have other requirements for much of our land, cities, farms, highways, and we have a lot of desert land and grasslands simply not capable of growing trees. It is estimated that prior to European colonization, only 46% of the United States was forested (mostly east of the Mississippi River and along the west coast.)

The amount of forested land in the US reached its low point around 1955, and since 1990 we have been reforested about 30,000 square miles of land in total. By the way, “reforestation” does not include lands where trees have been harvested but remain in a “forestry” use. That is, the intent is for the forest to grow back.

Elsewhere around the globe, we have to look for other places to plant our one trillion trees. But before we can do that, it would be necessary to halt deforestation practices taking place in some of the most vibrant ecosystems on the planet, especially the Amazon and Indonesia. Since 1970, over 270,000 square miles of the Amazon have been deforested, an area the size of Texas. If the goal is to plant one trillion trees to reforest four million square miles, it would first make sense not to make the job more difficult by adding to what we need to replant.

The logistics of managing a global operation of planting one trillion trees face the political aspect of climate change policies. Amazon deforestation is not done for generating tree products but done to gain access to tillable soil for farms or pasture. A process that we in the United States were guilty of throughout the colonization process, primarily from the 18th through early 20th centuries. It is hard to ask other nations, with their own societal issues, to “don't do that" when you only recently stopped yourself. More tillable land and pasture are needed to feed our still burgeoning global population. For most third world countries, the choice of planting trees versus feeding your people is not really up for debate.

Let's say we do find a way to plant one trillion trees. The world will be a much greener place, but what happens in 50-80 years when these trees mature? Remember our old friend the carbon cycle? If the trees die and fall to the ground, a small part of that carbon will be incorporated into the soil, but most of the trees will decay, and that CO2 will go right back into our atmosphere.

Making the concept of a "green CO2 sponge" workable would require that those trees be harvested and sequestered in some semi-permanent form. Housing perhaps? I don't know. I have not seen any long-term plan for what to do with all the atmospheric CO2 turned into tree biomass.

"Green sponging" our way out of climate change could work but will require a considerable amount of global cooperation. It will only truly make sense as part of a comprehensive plan that includes eliminating the creation of vast quantities of greenhouse gases and the need for the sponge. We have to live within the global carbon cycle limits, whether we like it or not.