Research papers and policymakers have largely overlooked the contribution that harvesting wood has on fueling climate change, but a closer look tells a much different story.
In a paper published in Nature, we estimate that global wood harvests will add 3.5 to 4.2 billion metric tons of greenhouse gases to the atmosphere annually over the coming decades. That’s roughly 10% of recent annual emissions of carbon dioxide. It’s also more than 3 times the annual emissions from aviation and roughly the equivalent of emissions from deforestation and conversion of pastures for agricultural expansion.
These emissions are around one third larger per year for the next 40 years than in 2010 due to a projected 54% rise in harvest levels. Producing this wood will require harvesting an area equal to the continental United States.
This finding is both bad and good news. The bad news is that emissions will rise to around 5 gigatons per year in 2050 absent efforts to reduce the effects of wood harvests. The good news is that our paper shows the potential to address climate change in ways that have received only sporadic attention.
Researchers have correctly estimated the total amount of carbon entering the atmosphere, but not the amount that people have contributed to it. If policymakers, consumers and the wood products industry can reduce the quantity of wood harvested or find more efficient ways to harvest or grow trees, global forests will absorb more carbon than they do currently and do more to curb climate change for decades.
Why Emissions from Harvesting Wood Have Been Overlooked
It’s easy to understand why there’s so much confusion about wood harvest impacts on the climate. On the one hand, California is paying landowners hundreds of millions of dollars for the carbon saved by reducing their wood harvest, which assumes the logic that harvesting wood emits carbon. Several academic papers also estimate the climate costs of harvesting wood in the tropics and call for reduced wood harvests there, particularly if using the wood for fuel.
On the other hand, many lifecycle analyses of timber or paper production treat the harvest of wood as having no effect on the climate — as being “carbon neutral” — so long as the harvest is sustainable. By this, these analyses usually mean that the quantity of wood harvested each year does not exceed the growth of wood in a large forest area — sometimes a whole country. This approach rests on the logic that if tree growth anywhere in the forest cancels out the effects of harvesting wood in parts of it, existing stores of carbon in the forest remain stable.
What this ignores is that if these new wood harvests did not occur, forests would continue growing and take more carbon out of the air. Your bank savings account provides a good analogy. If someone removes the amount of money you contribute to it each year, or is added by interest, you would be poorer than otherwise because your savings account would never grow. Similarly, harvesting wood makes us poorer in carbon savings because forests would grow more overall and store more carbon if wood were not being removed.
Accounting for Carbon Costs in the Northern Hemisphere vs. the Tropics
Not just lifecycle analyses, but research papers often use a similar approach and create an impression that harvesting wood in the tropics has a climate cost, but harvesting wood in the United States, Europe or other parts of the Northern Hemisphere does not.
The basic reason is that countries in the Northern Hemisphere, including China, tend to have regrowing forests because they cleared so much land in the past. As these countries switched from horses or other draught animals to cars and tractors, they freed up agricultural land previously used for feed. Europe and China also have been outsourcing some food production to the tropics, enabling their own reforestation but at the expense of deforestation abroad.
Forests are also growing faster everywhere because of climate change itself. Higher carbon dioxide concentrations in the atmosphere faster tree growth, and shorter winters in cold places allow trees to grow longer each year. These effects are large.
Overall, despite wood harvests, forests in the Northern Hemisphere are achieving net growth. Even researchers who count the effects of harvesting wood often report only these net changes in carbon in the forests. Since it is positive, the fact that forest harvests reduce the size of this growth and therefore effectively adds carbon to the air can be invisible.
Countries in the tropics, by contrast, have historically cleared less land, and are now both harvesting more forests and clearing more land for agriculture. Because these countries have fewer regrowing forests, they often have net losses of carbon from forests. In turn, because harvesting wood increases these net losses, their effects are more obvious. This bias explains why some research and policy focus on reducing wood harvests in the tropics even as they mostly ignore wood harvests in Northern countries.
The national climate accounting rules that countries use to report to the United Nations also can lead to this impression. Countries are allowed to report only the combined or “net” changes in forests considered “managed,” and that term applies to nearly all forests in the Northern Hemisphere. Since these forests are growing for the reasons explained, it can appear in national “inventory” reports that harvesting wood has no effect even though its effect even in the North is to reduce this forest “carbon sink.”
Properly Counting the Effects of Wood Harvesting
Although prior wood harvests climate change and forest regrowth due to agricultural land abandonment do not alter the climate consequences of new wood harvests, what legitimately complicates the climate accounting is that newly harvested forests can also regrow in the future.
Of course, forests that are not harvested also continue to grow. But a few years after planting or regeneration, younger forests usually grow at a faster rate than older forests. After long periods of time, and if these regrowing forests are not harvested again, this regrowth can eventually pay back most of the lost carbon — the so-called “carbon debt.” Unlike the growth of other forests, this at least partial “payback” is a consequence of new wood harvests and must be factored into any evaluation of their climate “costs.”
This eventual payback means that the largest climate consequence of harvesting trees each year is the increased carbon in the atmosphere for the many decades it takes harvested forests to catch up toward unharvested forests. A proper accounting of the effects of wood harvests should credit this regrowth but also factor in the climate consequences of higher emissions and increased warming for decades. These increases in emissions both cause additional damage during these decades and reduce the time the world has to fully address climate change before the most catastrophic effects occur.
Our new paper calculates the “costs” of harvesting wood by placing a value on these additional emissions for decades. Technically, our new model (called the Carbon Harvest Model or “CHARM”) applies a discount rate to changes in emissions and removals of carbon to and from the air so that earlier changes have a higher value than later changes. The model also accounts for the fact that not all harvested wood is lost immediately. Tree roots, for example, only decompose over several years in the forest. And some wood can stay in furniture or construction for decades. The model factors all that in, estimates how much additional carbon will be added to or removed from the air each year following a harvest and ultimately calculates a cost to the climate in a currency we all can understand: the equivalent cost to a quantity of permanent emissions if they all occurred in the year of the harvest.
Perhaps surprisingly, so long as any discount rate is applied, the estimates remain mostly the same. This is the technical way of saying that even if society has only a modest preference for mitigating climate change earlier rather than later — and it should have a strong preference — the results will be similar.
Although analyzing global harvests has many data limitations and other uncertainties, our research recomputes the estimates with a range of different assumptions, and the results do not fundamentally change. Our overall estimate of climate costs is also more likely low than high because it ignores the likely adverse effect that wood harvests have on carbon stored in soil. (The details of these effects are too uncertain to incorporate yet in a global model.) Our estimate also excludes the indirect effects of harvesting wood, such as the various invasions of forests that tend to occur after construction of forest roads. Other papers have estimated these effects at several times greater than the direct effects.
How to Reduce Emissions from Wood Harvests
Our paper explores some ways the world can reduce emissions from harvesting and using wood. For example, tens of millions of hectares of “forests” are already intensively managed plantations, some of which are more properly characterized as “tree farms” than forests. They tend to have very low biodiversity but can produce a greater amount of wood per area of land. Steadily increasing the yields of existing plantations by 50% over 40 years –– a real possibility –– would reduce global emissions by 600 million tons per year on average during this time. More carefully harvesting tropical forests, where many adjacent trees are typically killed when harvesting big trees, would increase those savings by another 200 million tons.
Another option is to reduce the harvesting of wood for fuel and instead replace it with solar power, wind or just use it in more efficient cook stoves. Wood for fuel is the dominant use of wood (and energy) in many poor countries. It is also still heavily used in Europe and other wealthier countries, where such use is even growing due to flawed incentives to increase wood in power plants and industrial facilities. If instead of growing by an estimated 20% this wood fuel use could decline by roughly 40%, global emissions would decline by another 500 million tons.
These various mitigation strategies, which represent only some of the options, when added together can lead to significant gains in the fight to combat climate change. Our hope is that better understanding the effects of harvesting wood can spur creative new efforts to harvest, reuse and use wood more efficiently to keep more trees growing.
If the world could reduce its harvests of wood, more of the world’s forests might eventually reach an old age and remove less carbon per year. However, they would continue keeping carbon stored away from the atmosphere. And this additional forest growth for many decades would also help limit the heat waves, rising water levels, fierce forest fires and other immediate damages from climate change while providing the world a little more time to fully implement other solutions that rein in warming.
