Published on March 22nd, 2018 | by Jesper Berggreen0
International Day Of Forests — How Many Trees Are Needed To Cool The Planet? A Lot!
March 22nd, 2018 by Jesper Berggreen
The Food and Agriculture Organization of the United Nations calls the 21st March 2018 “International Day of Forests,” and they state the following accompanying the video below:
More people live in cities than ever before and by 2050, 6 billion people or as much as 70% of the global population is expected to live in urban areas. But rapid urbanization does not need to result in polluted urban sprawl. Trees and urban forests can make our cities greener, healthier and happier places to live by cooling the air, filtering out harmful pollutants and mitigating the effects of climate change.
OK, but let’s also look at the big picture of how many trees are needed globally to suck up all the excess carbon we’re adding to our common biosphere, which potentially leads catastrophic climate change. Note: the following calculations are speculative at best and erroneous at worst, but give me a hint in the comments if you know better.
First let’s try to figure out how much wood it takes to bind the carbon we emit now. Logically, an amount of (extra) trees that absorb as much carbon as we emit could halt the rise of CO2 in the atmosphere. The concentration of CO2 in the atmosphere is currently 410 ppm, which corresponds to the thickness of the blanket that keeps the planet warm and cozy. However, if we are to avoid further heating, the blanket must of course be made thinner. Thus, we need to store more carbon than we emit (an atmospheric concentration of 350 ppm would — according to experts — be adequate).
Let’s begin by defining a standard fictitious tree: It can grow to 25 meters in height and has a lifetime of about 50 years. Its wood mass fully grown can bind 2.5 tons of CO2 equivalent. An average inhabitant of this planet emits roughly 5 tonnes of CO2 equivalent per year through the consumption of food, housing, and transport.
So let’s look at how much space is needed if we use our standard tree to offset our fossil fuel based living. Let’s assume forests of our standard tree will be able to store 18 tonnes of CO2 equivalent per hectare per year, not taking different climate zones into consideration.
The world’s available land area is approximately 130 million square kilometers, or 13 billion hectares (1 hectare = 10,000 m2). According to the World Bank, 30.8% of the Earth’s surface is already covered by forest, so we will just leave that alone for now. 37.5% of the Earth’s land area is used for agriculture, and it is claimed that if all of us were vegetarians that would be sufficient to feed 17 billion people. However, if we all ate steaks every day, there would only be enough land to feed a couple of billion people. So, we would need to keep approximately 15% of farmland for vegetables and fruit. In any case it’s goodbye to the steak, otherwise we will need a few more planets! Like I said, speculative… but fun right?
So, we have 37.5% – 15% = 22.5% of 13 billion hectares, which leaves us with approximately 3 billion hectares to cultivate new forest on. Let’s see how much carbon we can soak up every year with that much space using our standard tree:
3 billion hectares x 18 tonnes of CO2 equivalents per hectare per year = 54 billion tonnes of CO2 equivalent per year.
Wow! That’s a lot of carbon! Let’s take into account how much we — 7 billion people — emit:
7 billion x 5 tonnes of CO2 equivalents per year = 35 billion tonnes of CO2 equivalent per year.
OK, so that’s an excess of 19 billion tonnes of CO2 equivalent per year. Not bad! But there’s a catch: we have reached 410 ppm CO2 in the atmosphere already, and although we would — in theory — be able to store more CO2 than we emit in this tree-planting frenzy, how long would it take before we got down to the safe 350 ppm threshold? The atmosphere went past the 350 ppm mark 30 years ago, and since everything above 350 ppm is changing our climate for the worse, then we must get down to 350 ppm again as fast as possible.
But even with an absurdly unrealistic plan to stop using all livestock tomorrow, and start planting trees everywhere, it will take decades to reach 350 ppm. The trees also need time to grow, and moreover it is necessary that no wood get burnt or left to rot. The wood must be stored for at least a few hundred years in buildings, furniture and paper etc. Forget about burning wood and being “CO2 neutral.” We must wait to be neutral until we reach 350 ppm.
If we simultaneously expand renewable energy and clean transport at a furious pace, like giant offshore wind farms, solar panels on all roofs and massive investment in electric transportation and energy storage, we could — maybe — prevent the emission per capita of the world to increase, despite rising population. The fact that we all need to become vegetarians to make room for trees will of course help reduce average emissions.