Jun 06 2023

How Much Carbon do Living Things Store?

Since we are in a “all hands on deck” situation when it comes to climate change, we need to take a look at all potential strategies for delaying and blunting global warming. The game at this point is all about peak warming – how much will the Earth warm before temperatures peak and then start to come down again (assuming we eventually drastically reduce our collective carbon footprint). This is not a win-lose, all-or-nothing scenario. It is very much a sliding scale – the higher peak warming is, the greater the chance of hitting some nasty tipping points and the greater the disruption to life and civilization.

Current evidence indicates that if we keep at 1.5 C warming or below (we are currently at 1 C warming) then we will probably be OK. If we exceed 2.5 C then there will very likely be disastrous results. Between 1.5 and 2.5 there is uncertainty. We don’t know precisely where the tipping points are (where will the Antarctic glaciers fail and essentially slide into the ocean), or exactly how bad things will get at each degree above 1.5. The problem is, we will almost certainly land somewhere between 1.5 and 2.5 C. So every bit we do to reduce that peak warming will likely have measurable benefits. For any intervention, therefore, the question is not whether or not it will prevent global warming, but rather how much will it likely reduce peak warming and lower the odds of the really bad stuff happening. This is why many experts believe we need an “all of the above” strategy. Every little bit helps.

With that in mind, what is the potential for reducing peak warming by maximizing living carbon sinks? Often this question is framed with respect to the most obvious living carbon sink – trees. Can we “prevent” global warming by planting a trillion trees, which should be framed as, how much does planting trees contribute to reducing peak warming. We can also frame it as, can we keep at or below a certain level (such as 1.5 C) if we plant enough trees?

The world’s forests store about 861 gigatonnes of carbon (about 3 trillion trees). Individual trees are about 50% carbon by dry weight, with trees on average capturing about 40 pounds of carbon per year. Global CO2 emissions are about 36 gigatonnes per year.  By my calculations we would need 1,983,600,000,000 (almost 2 trillion) trees to completely offset global CO2 emissions. This assumes that the current biosphere, trees included, is in a steady state when it comes to carbon, with carbon cycling through living things without any net carbon being added or subtracted from any compartment, such as the water or atmosphere. If we planted 2 trillion new trees this would temporarily shift the carbon cycle into this new storage for about 50 years or so until a new steady state is achieved. This means planting about 250 trees for every person on the planet.

Of course, our ecosystem is not in steady state. We are cutting down about 8 billion trees per year through deforestation – we are going in the wrong direction. If we stopped and even reversed deforestation, let’s say planting 100 billion trees per year, that would offset about 5% of current emissions cumulatively per year (10% in the second year, 15% in the third, etc). At baseline the world plants about 1.9 billion trees per year, so this has to increase about 20 fold. This is the logic behind the trillion tree initiative. The goal is to plant 1 trillion new trees by 2030 to help offset global warming. Again, the rough calculations show that this could be a significant contributor to reducing CO2 in the atmosphere. At one trillion trees we would be sequestering about half of current global emissions.

The good news is, there is support for this initiative across the political spectrum. The oil industry likes such initiatives because it means they can keep pumping oil – just plant more trees. For those not concerned about global warming, planting trees can be presented as simply conservation, and creating new resources for future generations.

But there is also a reality check to this strategy. Critics argue that we simply don’t have enough land to pull this off. You also can’t just plant trees, you need to restore forest ecosystems. Many of the planted trees die for this reason. By some calculations we would need to halve meat production to free up grazing land for reforestation. There is therefore a lot of debate about what is practically achievable. I return, therefore, to my opening premise – this is not an all-or-nothing game. We should essentially plant as many trees as we can, as smartly as we can, as one strategy among many. This is not a way to keep burning fossil fuel – we also need to phase that out as quickly as possible. But each tree we plant buys us a sliver of time to transition our economy to low carbon.

There are also carbon stores in nature other than trees. About 3.9 gigatonnes of plant carbon is transferred each year to fungus, mostly under the ground. A lot of carbon sinks to the deep ocean each year. These systems are all in steady state as well, but perhaps there are ways to temporarily shift more carbon into these stores, buying us a few more years. At the very least we need to look carefully at activity which is reducing the store of carbon in the ecosystem.

Meanwhile, if you own land, plant a few trees.

No responses yet