by Richard Houghton
Globally, fossil fuels contribute to about ninety percent of the carbon emissions from human activity. If we are to stabilize the concentration of carbon dioxide in the atmosphere, these carbon emissions must be reduced to zero in the long run. But only about fifty percent of the reductions need to occur right now, and while it is not a panacea, forest management can play an important role in helping achieve this reduction.
The suggested reduction is based on the global carbon budget. During the first decade of the twenty-first century, average global emissions of carbon were about 9 billion metric tons (BMT) per year, with about 8 BMT per year from fossil fuels and 1 BMT per year from land management, largely deforestation in the tropics. Of these total emissions, about 4 BMT per year remained in the atmosphere, while 2.4 BMT per year were taken up by the land and by the ocean. The uptake by land and ocean was driven by the CO2 concentration gradient between atmosphere and ocean and land sinks, not by the annual emissions. This understanding is important because if we were to reduce global carbon emissions by 4 BMT per year, the remaining amount could be taken up by land and ocean, at least for a while. As the surface ocean equilibrates with the atmospheric CO2, oceanic uptake would decline, and the emissions would have to be reduced further. But the stabilization might only work for a few decades.
Could we reduce emissions by 4 BMT per year with forest management? It doesn’t seem likely if stopping deforestation and forest degradation completely would only reduce emissions by 1 BMT per year. We would still need an additional reduction of 3 BMT per year—or an additional uptake of the same amount by land, since reducing emissions or increasing uptake are equivalent in terms of carbon budgeting.
But since ending deforestation accounts for only a fraction of this potential for reduction, we can also explore other opportunities. Reforestation of about 500 million hectares of degraded and unused land that was once forested would take an additional 1 BMT per year out of the atmosphere and store it in trees and soils. And that uptake doesn’t count the carbon that might be taken up by 5 billion hectares of previously degraded grassland soils, according to the UN Convention to Combat Desertification. To be sure, 500 million hectares of new forests is a huge area, about half the land area of the United States or China, one-third of the area in croplands today, or one-fifth of the area in grazing lands. But while this is a massive effort, it is certainly not unimaginable.
Stopping deforestation in combination with the establishment of 500 million hectares of new forests reduces emissions by about 2 BMT per year. But where can we find the other 2 BMT per year needed to stabilize global carbon dioxide concentration levels? The answer lies in forests that are recovering from harvests and from agricultural abandonment. These forests are currently taking up between 1-3 BMT per year. Adding that uptake to the changes already described gets us to a total of 3-5 BMT per year, or within reach of the total needed to stabilize carbon dioxide concentrations right now.
But this window of opportunity is closing. Because the above calculations are based on emissions between 2000 and 2010, subsequent increases could make forest management more difficult. The good news is that there is a natural carbon sink of 2.4 BMT per year in forests that are not managed. The not-so-good news is that global warming may change the carbon balance of natural as well as managed ecosystems. And questions still surround the stability of these carbon sinks as the Earth warms.
The scale of the problem and these solutions are enormous. Ending deforestation and establishing massive new forest areas would not be easy, particularly since they would require an international agreement. Furthermore, these are just estimates of what could happen and they come with a number of caveats.
Altogether, these three activities could lower carbon emissions enough to stabilize the carbon dioxide concentration for the next forty to fifty years. But this reduction wouldn’t entirely suffice as grown forests remove less carbon from the atmosphere than young forests. The idea that we can stabilize the carbon dioxide concentration by managing forests is only an interim strategy, not a permanent solution. It buys us the time the world needs to convert from fossil to renewable energy. And perhaps the greatest impediment to managing forests is the prohibition on harvests of timber, fuelwood, and biofuel, since the above calculations are based on the continued growth of forests.
Are the threats of climatic disaster great enough that people would forego harvesting forests for energy? Will the price on carbon exceed the opportunity costs of harvesting? To complicate the economic issue, this trade-off is largely a north-south issue, with fossil reductions in the north being offset by terrestrial sinks in the south. Where is this once-forested land that is not now being used? And is the land fertile enough to support forests again?
About the Author
Dr. Richard A. Houghton is the Acting President and a senior scientist at the Woods Hole Research Center in Falmouth, Massachusetts. Dr. Houghton has studied the interactions of terrestrial ecosystems with the global carbon cycle and climate change for about thirty years, in particular documenting changes in land use and determining the sources and sinks of carbon attributable to land management.