How much global warming pollution comes from tropical deforestation?

A joint study by two of the world’s leading research groups provides an answer: 3.0 billion tons of carbon dioxide a year.

This estimate is the consensus of teams from Winrock International and Woods Hole Research Center, which have published detailed studies in scientific journals (Baccini et al. 2012, Harris et al. 2012) based on sophisticated use of satellite imagery, ground-based inventories, mathematical models, and other information.

Initially their estimates appeared to be quite different — by a factor of three — and therefore they worked together for several months to figure out the reasons for the apparent divergence. They found that, when they used comparable methods and definitions — compared mangos to mangos, to use a tropical analogy — they actually were in close agreement. They presented this agreement in a policy brief released at the international climate negotiations in Qatar in December 2012.

The consensus figure was a yearly average of 3.0 billion tons of carbon dioxide (Gt CO2) for the period 2000-2005. This figure is for gross deforestation, not net deforestation, meaning that it does not subtract out the carbon sequestration that is taken up by forest growth.

How big is three billion tons?

Three billion tons of anything is a lot, but it’s hard to grasp just how much — particularly when it’s tons of CO2, which we don’t have any everyday experience in weighing.

One way to look at it is that the average U.S. car emits about 5 tons of CO2 a year from the tailpipe, so three billion tons is the equivalent of 600 million cars — about twice as many as there are in the whole United States.

Another way of expressing it is that this is the equivalent of about 13 million railcars full of coal, which would stretch about 125,000 miles (half the distance to the moon).

It's also equal to the total emissions from Western Europe, including Austria, the Czech Republic, Slovenia, all the Scandinavian countries, and Finland.

What’s the percentage?

That’s useful, but most often we tend to get a better sense of such large numbers by converting them to percentages — putting them as a fraction of the total emissions of the whole world. Thus, the last assessment of the Intergovernmental Panel on Climate Change (IPCC) in 2007 used the figure of 17 percent to explain the fraction of CO2 that came from deforestation, and a consensus of scientists and organizations released at the 2009 Barcelona climate conference, based on newer research, estimated that the proportion was about 15 percent. So what percentage of global emissions corresponds to the new 3.0 Gt figure?

This turns out to be a little more complicated than just dividing it by a denominator and multiplying by 100, as we all learned in school. The question is, what denominator should be used? Should it be just CO2 emissions, or all greenhouse gases, including methane and nitrous oxide? Should it be the total for 2000-2005, which is now a decade old, or the most recent figure available?

There are some alternatives for the numerator, too. The WI/WHRC consensus number is only for gross deforestation. It doesn’t include forest degradation — activities such as selective logging, fires in the forest understory, and shifting cultivation — which maintain a forest canopy but result in the loss of some of its carbon. Nor does it include the CO2 from the breakdown of carbon-rich peat soils in Indonesia and Malaysia, which can continue to decompose for decades after the forest over them is cleared. Nor does it include any CO2 uptake from regrowing secondary forests or recovering logged forests. The uncertainty, however, about these additional forest-related emissions is very large. Should they be included?

Taking all the combinations of numerator and denominator into account, there are quite a few ways to calculate the percentage — sixteen, by our reckoning. What are the most reasonable ones?

Two ways to do the math

There are two different approaches we might take. On the one hand we could make sure that the numbers we use for both numerator and denominator are as certain as possible and the most closely comparable in what they measure — deforestation only, CO2 only, and from the same date (2000-2005). Using this conservative method, the numerator is 3.0 and the denominator is 30.2 Gt CO2, so the percentage turns out to be 9.9 percent.

On the other hand, instead of certainty and comparability, we could put the emphasis on using the figures that are most complete and most up-to-date. This would mean including in the numerator not only deforestation but also forest degradation and peat emissions, which the WI/WHRC Policy Brief estimates as being “as much as 2.2 Gt CO2.” In the denominator, we would include all greenhouse gases, not just CO2, and use the most recent figures available — for 2012 for fossil fuels and cement emissions, and for 2010 for non- CO2 gases. (It would also be desirable to include non-CO2 emissions from deforestation in the numerator, but we lack up-to-date estimates of these.)

Using the second approach, both the numerator and the denominator are larger, but it turns out that the increases nearly balance out. So this method turns out to give almost exactly the same percentage as the first approach: 10.0 percent.

As I mentioned, there are other possible ways to do the math, and the 14 other ways we’ve examined that give values ranging from 6.0 percent to 16.0 percent. Of the sixteen ways, seven of them give values below 9.9 percent, two are exactly 9.9 percent, one is exactly 10.0 percent and six are above it.

So, although there’s some variation, the message seems clear. The most reasonable current estimate for the percentage of emissions that come from tropical deforestation is 10 percent.

Why the difference from previous percentages?

This figure is considerably below the 17 percent of the 2007 number or even the 2009 estimate of about 15 percent. What explains the decrease?

Part of it comes from improvements in the science, and part of it can be explained by differences in methods — comparing mangos to pineapples. But a considerable amount is due simply to the fact that fossil fuel and non-CO2 emissions have been growing, while deforestation emissions have not. Thus, the reduction in the percentage reflects bad news about its denominator — that overall, greenhouse gas emissions are large and continue to grow larger.

It’s not that there isn’t any good news at all about deforestation. In fact, there is some, but it doesn’t yet figure in the percentage.

The good news

The 3.0 Gt CO2 numerator is an estimate for 2000-2005, which means it is now a full decade old. Since that time, deforestation has declined, according to several different sources of data. These include the FAO’s 2010 Global Forest Resources Assessment, the Center for Global Development’s FORMA project, and three datasets on deforestation in the Amazon, which is the planet’s largest area of tropical forest.

These studies differ somewhat in their numbers, and none of them estimate emissions, only the area that was deforested. But they all agree that deforestation has dropped, not just relatively but also in absolute terms, since the first half of the decade of the 2000s. This is most dramatic for Brazil, where gross deforestation in the Amazon region is down by more than two-thirds, but there is some evidence of progress in other tropical forest regions as well.

Over the next few years we will get new estimates of emissions from deforestation, and they are quite likely to show progress compared to the 3.0 Gt CO2 figure. That will mean that the percent will drop further from today’s best estimate of 10 percent. But that reduction, unlike the recent changes, will actually reflect progress in dealing with global warming pollution, and toward the eventual end of deforestation.