Caroline Alden, BURN Contributor

Carbon credits are a hot commodity these days, but reliance on this system as a permanent offset of fossil fuel emissions may be dangerous. By nature, the land carbon stock is vulnerable and volatile, and policy makers and offset traders might need to reign in their expectations a bit. 

A recent article in Nature Climate Change clears up some common misconceptions about the land carbon sink, with the goal of making clear to those involved in climate policy and carbon offset markets that reducing emissions – not trading emissions credits – is the only way to stop global warming.

Land management is key to carbon markets, because protecting and rebuilding forests facilitates sequestration – the capturing of a tradeable commodity. But land management – and the offsets it achieves – while good for the environment (and, at least temporarily, for climate), can not stop climate change.

Here are the main points made in the article:

  • NEWSFLASH: Carbon offset policies and markets typically rely on the promise of carbon storage in land reservoirs of 100 years as sufficient for issuing a carbon credit. However, the lifetime of CO2 in the atmosphere is far, far longer than 100 years. Many think that lifetime is 100 years, and for good reason; even the Intergovernmental Panel on Climate Change botched this one in its first assessment report. This misunderstanding is literally an issue of semantics: an individual CO2 molecule will move from the atmosphere into another reservoir in about 100 years. But that’s not the point for global warming. What matters is how long high CO2 conditions (due to fossil fuel burning) will last, and that CO2 lifetime is many thousands of years.
  • PUT IT IN PERSPECTIVE: Reforestation offers some, but not a lot of leverage on the climate system compared with what we’re about to add to the atmosphere by burning fossil fuels, not to mention what more could be added if deforestation continues or if climate change degrades existing forests. Say humans managed to put back all of the carbon that we’ve released through deforestation and land use over the years… how much fossil fuel emissions would that ‘offset’? Such a colossal task would account for only a 40-70 ppm reduction in the 2100 CO2 concentration. Consider that conservative emission scenarios predict an increase in atmospheric CO2 of 170-600 ppm by 2100 (over 2000 levels), and that total global deforestation would increase CO2 levels by 130-290 ppm.
  • PUT IT IN GIGABYTES: “The land carbon stock can be described as a ‘buffer,'” the authors write, “by analogy with the term used in computer science to describe a device which temporarily stores data.” The image is excellent: we can fill the land carbon hard drive with a little extra carbon by planting trees, or release some to the atmosphere through deforestation, degradation, or burning. At the end of the day, though, a 500 gigabyte drive maxes out at 500 gigabytes of data. As for potential carbon storage in plants, scientists don’t know exactly what this maxing out point is. But they do know that there isn’t nearly enough storage on Earth to provide any real silver bullet place to put climate-warming excess CO2.
  • HARD DRIVES CRASH: Land carbon stocks are also not particularly reliable places to permanently store carbon. Fires and droughts can release massive amounts carbon back to the atmosphere within a season. Furthermore, climate change is shifting the landscapes for growth: some areas will increase biomass thanks to more rainfall and decreased evaporation. But other places will lose biomass due to increased drought and heat stress. A compilation of results from 13 climate-carbon cycle models shows that the net effect of climate change is likely to be destabilization and weakening of land carbon stocks, and a resultant boost in atmospheric CO2 concentrations. 
  •  EXTRA CREDIT: In the words of the authors, it must be recognized that forest conservation can avoid or reduce future carbon emissions, but does not in any meaningful sense offset continuing emissions from other sources. It must also be recognized that the capacity of the land buffer to remove and store CO2 from the atmosphere is strictly limited. However vigorous the measures taken to increase land carbon stocks, their total potential for carbon storage is minuscule compared with the stock of fossil fuels that could yet be burnt.

This is not to say that protecting forests is not vitally important for the health of the planet for many reasons, including climate change. But the language and metrics in carbon markets and policy forums should be both scientifically sound and self-consistent. Again, the authors:

As long as the right kinds of land management responses are implemented, the land carbon buffer can provide a valuable, cost-effective, short-term service in helping to reduce atmCO2, and slow the rate of anthropogenic climate change, bringing co-benefits for biodiversity and sustainable livelihoods, and giving us some time to develop a low carbon economy….

Consistent with our understanding of the lifetime of the airborne fraction of a pulse of CO2, the most effective form of climate change mitigation is to avoid carbon emissions from all sources.


Caroline Alden is a graduate student at the Institute of Arctic and Alpine Research in the Department of Geology at the University of Colorado at Boulder.