Environment Counts | High quality satellite remote-sensing data lowers estimates of global emissions from deforestation by 40%
Author: Geoff Zeiss – Published At: 2016-01-18 22:26 – (949 Reads)
In 2007 the IPCC concluded that deforestation was the second largest contributor of anthropogenic carbon dioxide emissions, after energy. Over the past decade data from earth observation satellites have increasingly been used to clarify how deforestation is defined (for example, by providing more and higher quality data on forest degradation and forest regrowth) and to augment and check the national data compiled by the FAO. A 2012 paper by Harris et al. estimated carbon emissions due to deforestation using only satellite data, without recourse to any FAO data. The new higher quality satellite data and the work by Harris et al. (and other peer reviewed work) has resulted in a significant lowering of the estimated carbon emissions from deforestation by both the FAO’s most recent Global Forest Resource Assessment (FRA 2015) and the IPCC’s 2015 Assessment Review 5 (AR5). The new estimates have directly affected assessments of where to focus future climate remediation efforts.
Land use change is a major contributor of anthropogenic global emissions, but it is the most uncertain component of the global carbon cycle. Changes in agriculture, deforestation, and urban spread are the major contributors of anthropogenic sources of global emissions from land use change.
In considering the impacts of forests on global carbon emissions, not only deforestation, but also forest degradation, peatland degradation, and CO2 removals by forests and regrowth forests (after deforestation) need to be considered.
The latest estimates of emissions from deforestation show how the most recent satellite remote-sensing technology has not only provided data about previously inaccessible regions of the Earth, but more and higher quality data. It has also made it possible to identify and quantify forest classifications such as regrowth and degradation. In addition to providing higher quality data on global deforestation, modern satellite remote sensing has provided a way of independently and objectively checking the national statistics compiled by national governments and contributed to the FAO.
Global Forest Resources AssessmentÂ (FRA)
FAO has been monitoring the world’s forests at 5 to 10 year intervals since 1946. Every five years since 1990 FAO has published a Global Forest Resource Assessment (FRA) based on the self-reported national data. Until 2015 although the quality of the self-reported national data varied widely, there was no formal indication in the reports of the varying quality of the data.
Starting in 2015 with IPCC Assessment Report 5 (AR5) and FRA 2015 there has been a fundamental change and improvement in the extent and quality of the data on which these two reports are based. These changes are due to two factors.
- Steady advancement in the use of satellite based remote sensing, which is now used to assess forest resources globally for nearly all countries, instead of being limited to those countries with ready access to their own or other satellite data on forests.
- Stronger focus on data quality by the FAO, including the use of additional independent agents to verify the self-reported country data and augment it with additional satellite data. As a result, for the first time in FRA 2015, FAO now reports on and provides estimates of the quality of forest data for each country.
Together these two changes have led to a major improvement in global forest data and facilitated new analyses which have resulted in a significant reduction in the estimates of global emissions from deforestation.
Calculating carbon emissions from deforestation using satellite remote-sensingÂ data
Four important studies were published in 2011 and 2012.
- Saatchi et al. in 2011 mapped the total carbon stock in live biomass (above- and below ground). Their benchmark map of carbon stock provides a spatially refined and methodologically comparable carbon stock estimate for forests across 75 developing countries and improves upon previous assessments based on often old and incomplete national forest inventory data and earlier spatial products. To do that they used a combination of data from moderate resolution imaging spectroradiometer (MODIS), shuttle radar topography mission (SRTM), and quick scatterometer (QSCAT).
- Baccini et al. in 2012 used multi-sensor satellite data to map above ground live woody vegetation carbon density for tropical ecosystems with much greater accuracy than had been possible previously. By combining these estimates of above ground carbon stocks with regional deforestation rates and using the traditional carbon book keeping model Baccini et al. estimated the total net emission of carbon from tropical deforestation and land use.
- Pan et al. in 2011 included forest regrowth in their estimates of emissions due to tropical deforestation. They calculated that tropical regrowth forests represent approximately 30% of the total tropical forest area and estimated that the carbon sink (absorption of CO2) by tropical regrowth forests was a significant contributor to reducing CO2 emissions from deforestation. Here again satellite remote sensing also enabled researchers to identify and quantify forest regrowth as an important factor in calculating emissions due to deforestation.
- Harris et al. in 2012 reported a global estimate of tropical deforestation emissions using only satellite remote sensing data.
Deforestation estimates from remote sensing
Harris et al. combined spatially consistent data sets on forest loss and forest carbon stocks to match areas of forest loss with their carbon stocks before clearing to more accurately quantify gross emissions from deforestation in tropical regions. This allowed them to compute a systematic, spatially consistent, and transparent estimate of gross carbon emissions from deforestation between 2000 and 2005. This technique also enabled quantifying uncertainty in carbon emissions which was an advance over previous attempts that relied on qualitative estimates based on expert opinion.
The following table summarizes the different estimates of gross emissions and shows the dramatic drop in emissions estimates as a result of the application of satellite remote sensing.
Table 1 Gross emissions from deforestation
|Gross emissions from deforestation into atmosphere (Pg C/yr)||Methodology||Global 2000-2005||Uncertainty|
|Pan et al.||Includes tropical regrowth forests||2.9||2.4 to 3.4|
|Baccini et al.||Bookeeping model and FAO and satellite remote sensing data||2.22||NA|
|Harris et al.||Matching satellite remote sensing forest loss and carbon stocks maps||0.81||0.57 to 1.22|
The estimates from Pan et al. and Baccini et al. do not include globally significant emissions associated with the loss of carbon-dense tropical peatlands, mostly from Indonesia, where such losses are responsible for roughly half of all greenhouse gas emissions.
Revised global deforestation emissions from FAO (FRA2015) and IPCCÂ (AR5)
The large difference between the most recent remote sensing and earlier estimates have resulted in a reassessment of emissions from deforestation by the IPCC and FAO.
Federici et al. used the latest forestry data in FRA 2015 to revise estimates currently available through the IPCC AR5 and FAOSTAT for global and regional net CO2 emissions and removals from forest land. This includes estimates based on net forest conversion (used as a proxy for deforestation) and remaining forest. Federici et al. found that CO2 emissions from forest degradation have increased significantly, reaching one third of those from deforestation in 2011-2015.
Table 2 Global GHG forest emissions calculated from FRA 2015
|CO2 Emissions (PgC/yr)||1991-2000||2001-2010||2011-2015|
|CO2 removals by remaining forest||-0.91||-0.92||-0.86|
|Net forest emissions||0.46||0.47||0.22|
Based on these revised estimates, according to the latest IPCC AR5 released in 2015, while deforestation accounted for about a third of anthropogenic CO2 emissions from 1750 to 2011, they represent 12% of emissions in the period 2000 to 2009.
The latest estimates of emissions from deforestation show that the latest remote-sensing, especially satellite-based, has not only provided data about previously inaccessible forested regions of the Earth, but more and higher quality data. It has also made it possible to identify and quantify forest classifications such as regrowth and degradation. In addition to providing higher quality data on global deforestation, satellite remote sensing has provided a way of independently and objectively checking the national statistics compiled by national governments and contributed to the FAO.
New estimates of CO2 forest emissions and removals: 1990â€“2015, Sandro Federici, Francesco N. Tubiello, Mirella Salvatore, Heather Jacobs, Josef Schmidhuber, Forest Ecology and Management Volume 352, 7 September 2015, Pages 89â€“98