Forests for storing carbon

One of the more annoying features of the climate change issue is the question of feedback, i.e. what are the consequences of what is inevitably going to happen? One important issue is whether we can fix carbon, at least temporarily, and the answer is, yes but . . .  The following illustrates some of the problems, based on a paper by McNicol et al. 2019. Environ. Res. Lett. 14 014004 .

We hear that forestry is a good place to store carbon. The first objection you hear will be that while the trees take carbon from the air, they eventually die and return the carbon to the air. Of course, if the forest is continuous, as the old trees die, new ones replace them, which means that if the trees are there, there is so much carbon dioxide taken from the atmosphere. New forests are net removers while they are growing; mature forests represent a constant fixed amount. Building things with the wood will add to the reduction of atmospheric carbon. Temperate rainforests can store up to 1500 t/ha of carbon, while something like 200 t/ha will commonly be stored in the top one meter of soil, particularly if there is plenty of rain. Peatlands store more carbon, as do deeper soils. Peatlands in the region can be up to six meters deep. The greatest concentration of organic carbon occurs where the ground is wettest, while slope is also important. To give some idea, the total mass of soil carbon calculated for the North Pacific coastal temperate rainforest was 4.5 billion tonnes of carbon. 

Soil carbon does not stay there. Soil is a rather remarkable mass of biological activity, the waste products of which return to the atmosphere as either methane or carbon dioxide. Increasing the temperature speeds this up, and an average increase of 1 degree Centigrade across the world could release 50 billion tonne of carbon into the atmosphere from this source alone by 2050. That is about five times as much as we produce annually through burning fossil fuels and through agricultural activities. On the other hand, if it rained more, an increase in water-saturated soil would lead to net storage. This is the issue of feedback I mentioned. Positive feedback would mean that as the temperature rose thanks to the carbon we have put in the air, the soil would put more there, and accelerate the heating. The negative feedback occurs where more rain falls in key places and holds more carbon in the soil. Which will it be?

The climate warming is inevitable, but what happens to the weather? One suggestion is that where it is already wet, it will get wetter, whereas where it is dry, it will get dryer. That makes Australia less of “a lucky country”, which it proclaims itself to be.

Of course, simple soil is not the only source of greenhouse gas. Most people will have heard of methane occluded in tundra that is gradually thawing. Something has to be done, but politicians prepared to do anything are thin on the ground, and those who have analysed and recognized what few schemes might actually work and make a significant contribution towards solving it are even thinner on the ground.  There are conflicting issues, thus to store carbon you want trees on flat land to store more in the soil, but that is where the food comes from. So maybe planting trees on hills is better, because that land is less useful for food. What would you do?