Why are new estimates of emissions from tropical deforestation lower?
- 26 Jun 2012, 14:25
- Freya Roberts
A
new study has suggested that emissions from tropical
deforestation could be lower than previously thought. So what's
changed? Were older estimates wrong, and if so, what might this
mean for the future of the planet's atmosphere?
New measurements from satellites
The new estimate comes from improved satellite measurements of how
tropical forest cover is changing. This ability to estimate forest
loss from orbit is relatively new, and allows scientists to
calculate emissions from deforestation with greater
certainty.
By comparing older and newer satellites images showing forest
cover, scientists are able to work out two things: forest cover and
the amount of biomass in an area. Assessing the amount of ground
covered by trees alongside the amount of biomass in an area tells
you about the amount of carbon stored in that area - the 'carbon
stock', and this lets researchers roughly work out the amount of
carbon released through deforestation.
This study, and others in recent years, calculate gross emissions
- making no assumptions about what the land is used for afterwards
and how this affects overall changes in emissions from
deforestation. This removes uncertainty in the calculation, but it
makes it tricky to compare new estimates to older studies, that did
look at land use change.
Older studies had different uncertainties
Instead of using satellite measurements, older estimates of
emissions from deforestation relied upon countries to report the
amount of deforestation that had occurred to the United Nations
Food and Agricultural Organisation (FAO), which collated the data.
Using this data and a computer model, researchers calculated the
emissions released from deforestation.
In practice, self-reporting often didn't work that well -
estimates were often
inaccurate and the data was collected inconsistently
by different countries.
Some of the assumptions
made to simplify calculation of emissions also introduced broad
margins of error. For example, models tried to work out what
happened to land after it was deforested, and feed that into the
carbon accounting, but there often isn't much data to show what
land is used for after deforestation, increasing uncertainty in the
estimates.
Satellite measurements help address some of these problems, and
avoiding looking at land use after deforestation may be a better
approach to a situation where there often isn't good data.
The new results in the context of other
studies
Compared to other studies, estimates of carbon emissions from this
new study are considerably lower, and it's not entirely clear
why.
As seen from the table below, the estimate calculated by the
authors is around a quarter of the Pan et
al. estimate, and around a third of the
Baccini et al. estimate.
Both of these earlier studies also use satellite images, but
combined them with FAO data. It may be that this introduced
inaccuracies into the calculations.
Estimates of deforestation emissions from studies using
satellite data. Source: reproduced from Harris
et al., 2012 (Supplementary Information Table S1). 1 petagram =
1,000,000,000 tonnes.
We contacted the lead author, Dr. Nancy Harris, to ask what could
explain the difference between the estimates.
She suggested it was down to the contrasting methods used to
calculate emissions. Using satellites to track emissions, she
argued, is a "more consistent methodological approach":
"Satellite data allowed us to pinpoint
more precisely where deforestation was occurring and what the
resulting emissions were."
Even so, the magnitude of the difference is still quite
surprising, and suggests more research to figure out what it is
that's causing the estimate to be so different.
Having accurate estimates of deforestation and the emissions it
causes is important, not least because there are a number of
climate policies structured around an assumption that we can
measure deforestation accurately. If research concludes that
emissions from deforestation aren't as high as we thought, it might
mean a rethink.