Trees absorb carbon. Planting more trees will absorb more carbon from the atmosphere, and soak up the man-made emissions that are causing climate change. It is a simple, easy and attractive solution that would allow us to continue our high-emission business as usual and still stave off global warming. Brendan Mackey examines whether or not it would work.
What does counting trees have to do with climate change? Well, quite a lot, as it turns out. Trees store a large quantity of carbon, and when we destroy or degrade forests significant amounts of carbon dioxide are released into the atmosphere. Greenhouse gases like carbon dioxide cause global warming – an increase in the amount of solar energy which is kept within the Earth system. A little global warming is a good thing, as without any Earth's average surface temperature would be about 5°C rather than around 17°C. But now we are experiencing human-forced global warming, driven by the additional carbon dioxide being injected into the atmosphere above that due to natural processes. This accelerated global warming in turn causes changes to climatic conditions. The changes include altered rainfall regimes, the frequency and intensity of extreme weather events (heat waves, droughts and floods), along with ocean acidification, melting ice, expanding ocean waters and rising sea levels.
About 30% of the carbon dioxide in the atmosphere that has been added by past and present human activities is due to emissions from deforestation and degradation, and 70% is from burning fossil fuel (coal, oil, gas) for energy1. About 80% of our energy comes from burning fossil fuel2. Currently, about 10% of annual human emissions are from deforestation and degradation3. It is now clear that limiting warming requires cuts in greenhouse gas emissions, especially in carbon dioxide from burning fossil fuel. The latest Intergovernmental Panel on Climate Change (IPCC) report estimates that from now on, we (that is, humanity) can emit only about 269 Gt C (that is, billion tonnes of carbon) if we are to limit global warming to no more than 2°C above the pre-industrial average planetary surface temperature – the target agreed to by the international community4.
The issue I want to address here is the role of forest carbon in solving the climate change problem. Can planting trees offset fossil fuel emissions? And how much worse can the climate change problem get if we continue to destroy the world's remaining forests?
Global carbon cycle
In order to answer these two questions, I have to first explain how the global carbon cycle works. From a systems perspective, Earth is “open” to the flow of energy but is materially closed. Earth has a fixed amount of carbon – the total amount does not change, it just changes form: it can be gas (CO2); liquid (dissolved inorganic carbon in water) and ice; rock (carbonates); or biomass (organic carbon in living and dead plants and animals). Carbon flows between four major pools: the lithosphere (the crust and upper mantle); the ocean (including glaciers and the frozen waters of the north and south poles); the atmosphere; and ecosystems (especially forests).
Growing forests absorb carbon; but the rates of emission and absorption are crucial
Figure 1 illustrates the major carbon pools, the stock of carbon in each pool, and annual flows of carbon between the pools. Carbon naturally flows between the atmosphere and ecosystems, and the atmosphere and the ocean (there is also a small flow between the land and the ocean from river discharge, plus some natural degassing from volcanic activity). Plants grow by absorbing CO2 from the atmosphere through photosynthesis. Plants also respire about half of this carbon back into the atmosphere. Atmospheric CO2 dissolves into water as inorganic carbonates; the rate and the amount depend on water temperature, acidity, and the concentration gradient between the atmosphere and the ocean. Due to geographic and seasonal variations in these conditions, some oceanic zones absorb CO2 while other areas discharge CO2 back into the atmosphere. The only flow from fossil fuel is from burning it for energy; the oil, coal and gas stored in the lithosphere do not naturally degas into the atmosphere.
Note in Figure 1, however, the emissions coming from deforestation and degradation of ecosystems. Land use such as industrial logging mobilizes the carbon stored in the woody biomass of trees, the dead biomass, and soil carbon, depleting the ecosystem carbon stock, and releasing CO2 into the atmosphere at a rate above that of natural ecosystem respiration. This is an additional pulse of CO2 into the atmosphere; it comes from a different pool than that of fossil fuel carbon.
The tricky thing about the global carbon cycle is that the various processes illustrated in Figure 1 operate over vastly different time-scales. On an annual basis, around 200 Gt C is exchanged between the atmosphere and ecosystems and between the atmosphere and the ocean. Also on an annual time scale, about one-third of fossil fuel emissions are taken up by ecosystems and one-third by the ocean3. The remaining one-third stays in the atmosphere. The atmospheric concentration of carbon (in the form of CO2) is increasing because the rates of fossil fuel and land carbon emissions are greater than the rates of the two natural processes that take carbon out of the atmosphere; that is, nature cannot fix the problem faster than we are creating it.
If we were to stop using fossil fuel, a new equilibrium would eventually be reached as carbon works its way into deep ocean storage and the products of the weathering of rock are incorporated into ocean floor surface sediments. These physical processes operate over very long times. Global carbon models estimate that the lifetime of the airborne fraction of a pulse of CO2 (that is, how long it takes for it to be removed from the atmosphere and transferred to the deep ocean/sediment sink) is thousands of years. About 70% is removed in 300 years but the remaining 30% takes between 10 000 and 30 000 years. Practically, from a human perspective, fossil fuel emissions are “for ever” and will continue to interfere with the climate system for millennia.
Can planting trees offset fossil fuel emissions?
Given the seriousness of the climate change problem, it is no wonder that people look hopefully to forests as a way of offsetting fossil fuel emissions. As noted, there is a gross carbon exchange of around 200 Gt per year between the atmosphere and ecosystems. This is an order of magnitude greater than annual fossil fuel emissions – so perhaps we can solve the problem simply by planting more trees and using better forest management?
Photosynthesis is the biochemical process used by plants to produce new biomass from the CO2 absorbed from the atmosphere through their leaves and the nutrients and water drawn up by their roots; it is powered by solar energy. About half the carbon assimilated through photosynthesis is released by plants back into the atmosphere from respiration. The rest of the carbon is turned into biomass, partitioned between plant parts as woody stems, branches, leaves and roots. When plant parts die, some of the dead biomass carbon is incorporated into the soil carbon pool. The maximum amount of carbon a forest ecosystem can grow and store is regulated primarily by prevailing climatic conditions (though the life history attributes of plants, such as the longevity of tree species, is also important). Temperature and wetness determine rates of both plant growth and respiration; and the difference between them (that is, carbon in minus carbon out) over decades to centuries determines the size of the carbon stock stored in the ecosystem.
The exchange of carbon between the atmosphere and ecosystems is an order ofmagnitude greater than fossil fuel emissions
Young trees and saplings of course contain carbon; but most of the biomass carbon in a mature forest is stored in the stems, branches and roots of big old trees5. When a forest is subject to industrial logging the stock of carbon is therefore greatly depleted6. If the forest is allowed to regrow then only the same amount of carbon that was in the forest before it was logged can be restored. This is helpful from a climate change perspective because it reduces atmospheric CO2. However, it is best understood as simply repaying the “carbon debt” from when the forest was logged. Useful as this is, it does not offset any of the additional fossil fuel emissions humans have added to the atmosphere7.
What about planting additional trees where forests do not naturally grow – would that help? It is an attractive suggestion. It comes, though, with a problem: we would need to provide the trees with all the inputs to photosynthesis that in other places nature provides – especially water. Planted forests (plantations) in arid lands need irrigation. Plantations are expensive to establish and maintain and the trees are cut down every 5–15 years as input to manufactured woody fibre products – which is an insignificant time in terms of reducing atmospheric concentrations of CO2. The argument is sometimes made that logging forests, and even converting natural forests to plantations, is good for climate change because the harvested carbon is stored in long-lived wood products like violins or dining room tables. However, the facts are somewhat different8. Global paper consumption in 2007 was 510 million cubic metres and solid wood products (sawn timber and wood panels) was 688 million cubic metres. The lifetime of carbon in pulp and paper products is only around 1–10 years, and a mere 4% of the carbon in a forest tree ends up in longer-lived timber products of 30–100 years’ lifespan9. Logging accelerates land carbon emissions and the carbon temporarily stored in pulp, paper and wood products is at best a delayed emission.
Does it matter if we cut down the forests?
It seems that we cannot solve the climate change problem by planting trees and letting logged forest regrow. Does it then matter if we continue to log and clear natural forests? How much worse can the climate change problem get if deforestation continues? About half the world's natural forests have already been cleared for crops, ranching and human settlement, leaving about 4 billion hectares of natural forest cover. Of this only 1.4 billion hectares is primary forest that has not been logged to some degree and retains its carbon carrying capacity10. Agricultural land has lost most of its natural biomass carbon but retains some, albeit depleted, soil carbon stocks. Also, there are substantial stocks of organic carbon stored in peat bogs of boreal tundra landscapes. Adding all sources together, there is estimated to be about 2400 Gt C in natural ecosystems, with around 289 Gt C in living trees.
Half the world's natural forests have already been cleared. Only about a quarter of what is left retains it carbon-carrying capacity
Estimating how much how climate change would be caused if all the world's ecosystem carbon was released into the atmosphere is not straightforward. A useful rule of thumb, according to the Carbon Dioxide Information Analysis Center (http://cdiac.ornl.gov), is that 1 ppmv (part per million by volume) of CO2 in the atmosphere is equivalent to 2.13 Gt C. Given this, if all 280 Gt C in living trees were logged and the biomass carbon emitted into the atmosphere as CO2 then atmospheric concentrations could increase by 131 ppmv. To arrive at an accurate estimate, however, requires accounting for the differently scaled processes illustrated in Figure 1. Using this approach, it has been estimated that complete global deforestation would increase atmospheric concentrations by about 130–290 ppmv11. Currently atmospheric concentrations are about 400ppmv, which is around 112 ppmv above pre-industrial levels. Conversely, if all the carbon so far released by land-use changes (mainly deforestation) could be restored through reforestation this would reduce atmospheric CO2 at the end of the century by 40–70 ppmv. Ignoring the fact that reforestation simply repays the land-use carbon debt, 70 ppmv equates to only 19 years of current global fossil fuel emissions (7.8 Gt C per year; 3.66 ppmv equivalent) which under a “business as usual” scenario could rise to 25 Gt C per year by 21004. This comparison illustrates the climate change significance of the potential emissions from deforestation and the limited extent to which planting trees can offset fossil fuel emissions.
Best mitigation option is avoided emissions
There is a tendency when it comes to environmental problems for people to want to keep doing what they are doing and to find ways to mitigate the harm without making any major changes to their business operations. So, the idea that we can offset fossil fuel emissions by planting trees is appealing. Given the seriousness of the climate change problem, however, the time has come to accept that we have to focus on how we can avoid emissions. Carbon is quick to be emitted but drawing it back down from the atmosphere and keep it securely stored so that it is never again released into the atmosphere is very difficult, costly and up to now neither technically nor economically feasible. The best option therefore is to avoid emitting it in the first place.
When it comes to avoiding emissions from deforestation and degradation we face difficult challenges. Forests are cut down for a reason. The land is where people live and work, it is where we grow our food and fibre, and where we mine most of our energy and minerals. And all of these activities take up land at the expense of natural ecosystems like forests. It is inevitable that the human endeavour has a large land carbon footprint. The question before us now, as we head toward a world population of 9 billion people by 2050, is how much of the world's remaining forests we can leave protected from logging and other modern land uses and in so doing avoid substantial carbon dioxide emissions. The more deforestation and degradation, the more land carbon emissions are added to the fossil fuel emissions that are accumulating in the atmosphere, and the harder the mitigation problem becomes to solve.
Drawing carbon back down from the atmosphere is difficult, costly and technically and economically unfeasible. Instead we must avoid emitting it
Variability, uncertainty and attribution
The various carbon values I have cited in this article (including Figure 1) are but estimates that carry with them varying degrees of uncertainty. We know with a high degree of accuracy the atmospheric concentration of CO2 as this gas is well mixed and adequately sampled in space and time. The estimate of fossil carbon is usually defined in terms of “known reserves” as exploration continues to reveal new deposits. I think our estimates of land carbon – the organic carbon stored in the living and dead biomass and soils of ecosystems – are probably only accurate to around +30%. The carbon density of ecosystems varies by up to an order of magnitude, depending on local environmental conditions, especially climate, as plant growth is a function of water availability, temperature, light and nutrients12. The proportion of organic carbon stored above and below ground also varies with climate: tropical forests have far more carbon in living trees, whereas boreal forests – the subarctic, evergreen coniferous forests of northern Eurasia and Canada – have more held below ground in decomposing dead biomass12. Extant carbon stocks also vary, depending on land use and land-use history. As noted, most of the carbon above ground is stored in the woody biomass of large old trees, so industrial logging results in a significant (30–60%) reduction in biomass carbons stocks compared to unlogged forests13. Ecosystem carbon stocks are therefore highly variable, reflecting both natural heterogeneity in environmental conditions and the patchwork impact of land-use activities. In most countries, both the natural and human-induced variability is under-sampled, leading to relatively large uncertainty in estimates of organic carbon stocks.
A major scientific challenge is to discern natural variability in the climate system from human-forced climate change. The latest IPCC reports do this by running virtual experiments with global climate change models. Earth climate system simulations are run from the start of the industrial revolution to 2100 or beyond, with and without human forcings (mainly emissions from fossil fuel and deforestation). Current climatic conditions can only be replicated when human forcings are included. Of course, climate change per se can only be empirically demonstrated retrospectively given that “climate” is defined as the characteristic weather conditions over a standard period (typically 30 years).
As we peer into the future, the estimate of how much carbon can be emitted before we exceed the “2°C warming” commitment (∼269 Gt C) also comes with a degree of uncertainty. Unknowable contingencies aside, global climate models have their own internal errors14 reflecting, among other things, imperfect Earth system process knowledge, inadequately calibrated empirical functions, and the range of possible future societal responses which will determine the level of anthropogenic emissions and the strength of human climatic forcing. These uncertainties notwithstanding, however, it is clear that Earth system models are now of sufficient accuracy that their estimates must be taken seriously by policy-makers. We need no longer invoke the precautionary principle to call for action, as it has been scientifically established that “Warming of the climate system is unequivocal” and “It is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century”15. Note that the IPCC term “extremely likely” equates to an assessed likelihood of an outcome of 95–100%. The world community can no longer plead ignorance and must take urgent action to achieve the deep cuts needed in greenhouse gas emissions from all sources – fossil fuel and land carbon – if we are to avoid a level of global warming beyond human and natural adaptive capacities and that will cause grave harm to future generations.
Most of the carbon above ground is stored in the trunks of large old trees; replacing unlogged forests with plantations results in a significant (30–60%) reduction in stored carbon
Thanks to Clive Hilliker for preparation of the global carbon cycle figure.