Forests: The Impossible Mitigation Task

The misunderstood complexity of complex systems

Is it a good idea to fight global warming using forests? Johan Rockström argues in a recent comment (see below) that this is not possible. He is right, and let me explain why.

The whole story pivots around the capability of forests to remove carbon dioxide from the atmosphere. They can do this through two different mechanisms: one involves increasing in area and volume, and the other involves storing carbon in the soil in stable forms.

Regarding the first mechanism, storing carbon as tree biomass, a little work with the numbers shows that to offset the current emission of approximately 11 Gt/y of carbon into the atmosphere, we would need to increase the forest area by 400,000 km² every year. An area larger than the whole of Italy. It means having to reforest the whole Sahara Desert (about nine million km²) in some 20 years. But it is not just a question of planting trees: to have a forest, you must wait for saplings to become trees, and that takes decades. Reforesting 400,000 km² is simply an impossible task.

The other mechanism, sequestration in the soil, is a different matter. A healthy forest does that all the time, even if it doesn’t grow. But the amount is small in comparison with human emissions. According to a paper by Harris et al., nowadays global forests provide a gross carbon removal of 4.25 GtC per year, which is less than half of the total human emissions (approximately 11 GtC/y). And the net sink, taking into account other effects that return CO2 to the atmosphere, is about half that, 2.1 GtC/y. This means that to compensate for the current human emissions, we should increase the global forest area by at least five times. The Earth’s surface covered with forest is about 46 million km²; Earth’s land area is 149 million km². It just can’t be done.

Note also how focused the debate is on CO2 alone, never mentioning the other factors that affect forests' climate impact: the atmospheric heat pump (cooling), water vapor emissions (warming), cloud formation (slight cooling), and albedo decrease (warming). These effects are not easily quantifiable, and we don’t know what fraction of the current warming may be attributed to deforestation or other effects. A significant fraction, surely, but how much exactly? And even if the atmospheric heat pump is more important for climate than we currently believe, it only affects temperatures. It doesn’t affect the chemical effects of CO2; ocean acidification, metabolic damage to living beings, and other consequences. Didn’t I say we are dealing with a complex system? Indeed.

In any case, the debate is often stuck in a narrow view that sees forests as a miracle solution to global warming. If you just pay a little money to organizations that claim to be planting trees, then the problem is solved. Maybe they do (but we can’t be sure), and, in any case, these planted trees are not forests and can’t do much, even assuming that they survive for a certain time. Just look at the numbers, and you’ll see that we are asking forests to perform an impossible task.

Here is Rockström’s post.

Forest carbon budgets and Finnish climate policy: explaining my position

Johan Rockström Potsdam Institute for Climate Impact Research. Professor Earth System Science, University of Potsdam

March 11, 2025

Finland domestic climate target is to be climate neutral 2035, by reducing the national Greenhouse gas (GHG) emissions - today of approximately 41 million tons of CO2eq/yr - to equal carbon sequestration on land (AFOLU), i.e., uptake in forests. Up until 2014 this amounted to an uptake of approximately 30 million tons (average 1990-2014), i.e., fossil-fuel emissions and other GHGs had to be cut by less than half to meet the target in the Finnish Climate Act (423/2022). The problem is that the carbon sink in Finnish forests has, over the past decade, collapsed. The past three years, the Finnish forest sector has been a net source (see Figure above and detailed data on source and sinks).

The situation is similar in Sweden, which has adopted a climate law stipulating the country to reach "net-zero 2045". The "net" part though, allows for expected forest carbon sinks to compensate for remaining emissions in other sectors. The reasoning (in Sweden, Finland, but also in the EU) is that the entire land sector (forests but also grassland, wetlands, peatland, farm land), the so-called LULUCF sector, constitutes "an important part of the green transformation" where the large uptake of CO2, which correctly has been the case historically, has (to quote the Swedish EPA) "the potential to increase and compensate for emissions that remain in other sectors".

Forest carbon sinks cannot be offset against mitigation targets

My argument is that this compensation does not work, even if the forest is healthy and operates at a high carbon uptake level. A logic which becomes even less scientifically justifiable, when the forest carbon sinks start to collapse, as in Finland, but also in Sweden and Norway.

This makes it very difficult - if not impossible - for Finland to meet its climate targets, unless something drastic is done to reduce the logging of trees by the forest industry. The problem though is that the reason for the collapse of the carbon sink in Finnish forests is not only intensive logging. It is also climate induced stress, caused by droughts and heatwaves, manifested as slow growth - due to water scarcity - and disease outbreaks e.g. spruce bark-beetle. Here, Finland is not an isolated case. The same trend, with reduced forest carbon uptake, is experienced in Sweden, Norway and Canada.

This abrupt change in forest carbon uptake in Finland has triggered a political debate, particularly focused on what responsibility does the forest industry have in contributing to reduce the abrupt fall in carbon uptake. It is in this context that a talk I gave in Helsinki, hosted by the Tiina and Antti Herlin foundation in 2023, followed by an interview in Helsingin Sanomat (in Finnish, paywall), that has been misinterpreted in the Finnish debate. I would therefore like to clarify what I said at the time, a position I still strongly hold.

Climate models and the reality of global carbon budgets

My point was this. I explained that the climate modelling scenarios (e.g., in the IPCC WG3 of the AR6) that align with the Paris Agreement of holding global warming "well below 2°C and aim for 1.5°C", have all, already factored in large and optimistic assumptions on

1. that AFOLU (i.e., managed land in agriculture and forestry) will transition from source to sink, and

2. that nature on land (which includes natural forests/ecosystems and managed forests, i.e., part of AFOLU and what is defined as LULUCF - Land Use and Land Use Change and Forestry) will continue to take up ≈ 25% of global carbon emissions (globally).

These two criteria (together with similar assumptions for ocean uptake and CDR technologies) are required in order to enable the world to have a remaining global carbon budget for fossil-fuel emissions. So, when Finland, Sweden (and the EU) translate the science to climate policy aiming to cut emissions by half by 2030 (e.g., EUs Fit for 55) and reach net-zero by 2050 (Finland by 2035), all these scenarios have already factored in assumptions on forest carbon uptake. It is already built into the models, and is a key reason why a carbon budget for an "orderly phase-out" of fossil-fuels exists in the first place.

The problem is that we are not making progress on fossil-fuel phase out. So, today, less than 200 billion tons of CO2 remains in the global carbon budget to have a 50% chance of limiting warming to 1.5°C, moreover, after overshoot. This leaves no room for "compensation" or "offsetting". Oil, coal and gas simply have to be phased out to as close to zero as fast as possible. For countries like Finland and Sweden the "zero" point for fossil-fuel phase out is 2030-2040. Add to this the uncertainty with regards to the carbon sinks in nature (forests etc) and we have a problem.

A dual obligation: phasing out fossil fuels and protecting carbon sinks

The solution? Act with maximum efforts on both fronts simultaneously - both fossil-fuel phase out, and sustainable (resilient) forest management to avoid loss of carbon uptake. Therefore, my position (in the talk and in the interview) was that it is not possible, and therefore not advisable, to integrate the carbon reporting from fossil-fuels with emissions from forests. These are two different carbon budgets, which both need to be "delivered upon" i.e., reduced to zero for fossil-fuels and maximised for forest carbon uptake. There is no room to substitute between the two, or - as is done on the voluntary carbon market - allow for carbon offsetting between trees and oil/coal/gas.

In order to have a chance of limiting warming at 1.5°C requires that the world is net-negative from 2050 onwards. This means that major investments are required to safe-guard and increase carbon sinks and stocks in forests and all land based and marine ecosystems. Finland, like all rich developed countries, has a particular responsibility in leading the way towards a safe net-negative future.

In summary: Mitigation is key, i.e., to phase out anthropogenic GHG emissions. Sustainable forest management is key, i.e., maximize carbon uptake and sustain carbon stocks, in managed, and old-growth natural forest systems. Both have to occur simultaneously. And, they are equally urgent, not least when (as in Finland) the forest carbon sink is collapsing, not due to natural causes, but due primarily to human causes. Nobody, neither politicians, nor forest industry executives can escape from this reality.