Themes
The Science
LAST UPDATED on March 2, 2012
Global warming: what it is and what it does
Climate change mitigation via forests
Measuring and monitoring forest carbon
Growing trees and plants take up atmospheric carbon dioxide (CO2) through photosynthesis(1) and store it as carbon in biomass (living trunks, branches, leaves and roots), necromass (dead wood and litter), and soils. S ome of this carbon finds its way to the rivers and oceans. Some gets buried underground in the sediments and as fossil fuels such as coal, oil and gas. CO2 is in turn released into the atmosphere throug h plant and animal respiration and decomposition, soil respiration, burning of fossil fuels for energy and industrial activities, and through deforestation and forest degradation.
Figure 1. Forest Carbon Cycle.
Global warming: what it is and what it does
In the last century, humans have had an unprecedented impact on global climate through rapid industrial, urban and agricultural expansion which has been fed by burning fossil fuels, and forest exploitation and conversion to alternative land use. These activities have released high levels of greenhouse gases (GHGs)(2), particularly CO2, into the atmosphere. Global GHG emissions due to human activities have grown by 70% between 1970 and 2004 (Figure 2).
Figure 2(3). (a) Global annual GHG emissions from 1970 to 2004. (b) Share of different GHGs in total emissions in 2004 in terms of CO2-eq. (c) Share of different sectors in total GHG emissions in 2004 in terms of CO2-eq. (Forestry includes deforestation).
Antarctic ice sheets are formed through annual accumulation of snow over hundreds of thousands of years. Changes in past atmospheric CO2 concentrations can be determined by measuring the composition of air trapped in an ice core sample removed from an ice sheet. Such sample ice cores indicate that global atmospheric CO2 concentrations increased rapidly from a pre-industrial value of 280 parts per million in year 1750 to 379 parts per million in 2005(4) (Figure 3).
Figure 3(5). Atmospheric concentrations of key long-lived GHGs over the last 2000 years. Increases since about 1750 are attributed to human activities in the industrial era.
The recent rise was rapid (100 ppm or 36% over 250 years) when compared to the ~ 80 ppm rise in CO2 at the end of the past ice ages (Figure 4). Also current values are way outside the range of CO2 concentrations (180-300 ppm) for the past 650,000 years. Concentration units of parts per million (ppm) or parts per billion (ppb) indicate the number of greenhouse gas molecules per million or billion air molecules, respectively, in an atmospheric sample.
Figure 4(6). Variations of deuterium (δD; black), a proxy for local temperature, and the atmospheric concentrations of greenhouse gases CO2 (red), CH4 (blue), and N2O (green) derived from air trapped within ice cores from Antarctica and from recent atmospheric measurements. The stars and labels indicate concentrations at year 2000. The shading indicates the last interglacial warm periods. Time refers to the last 650,000 years. Benthic δ18O marine records (dark grey) is a proxy for global ice volume fluctuations.
CO2 and other GHGs blanket the earth, trap radiation or heat and keep it much warmer than it would be otherwise. They thus help sustain life on the planet. However the increased concentration of GHGs over the last century has resulted in what is now commonly referred to as global warming – significant warming of the earth’s surface and rising average temperatures.
Figure 5. The greenhouse effect.
The earth’s temperature has risen by about 0.6 degrees Celsius from the 1970s at a rate of 0.15-0.20 degrees per decade (Figure 6). This rate is unprecedented in the last 1000-year history of our planet(7). This seemingly small change has already had enormous impacts − 
melting of polar ice caps and mountain glaciers, significant sea level rise, increased frequency of hurricanes and extreme weather-related events, large-scale forest fires, decreased water availability in water-scarce regions affecting agricultural and pastoral production, and shifts in plant and animal distribution ranges(8).
Figure 6(9). Observed changes in (a) global average surface temperature; (b) global average sea level from tide gauge (blue) and satellite (red) data; and (c) Northern Hemisphere snow cover for March-April. All differences are relative to corresponding averages for the period 1961-1990. Smoothed curves represent decadal averaged values, circles show yearly values and the shaded areas are the uncertainty intervals.
Average global temperatures are expected to rise by 1.8 to 4.0 degrees Celsius by the end of this century(10). The consequences could be severe for future life on our planet as we know it. Parties to the Copenhagen Accord at the international climate change talks in December 2009 agreed that it was necessary to keep the global temperature increase below 2 degrees Celsius to avoid undue impact, and to adopt GHG emissions reduction targets in line with this goal.
At present forests cover just over 4 billion hectares or roughly 31% of the earth’s surface and sequester (absorb or remove from the atmosphere) and store large quantities of carbon. Forest ecosystems are estimated to store about 650 billion tonnes of carbon (44% in biomass, 11% in necromass and 45% in soils)(11) and absorb 8.5 billion tonnes of CO2 per year from the atmosphere(12).
However, deforestation (conversion to other land use) and forest degradation in the tropics through logging, fire and other anthropogenic disturbance results in substantial CO2 emissions. The UN’s Intergovernmental Panel on Climate Change (IPCC), using 1980s and 1990s-era forest surveys and satellite data, estimated that deforestation was responsible for around 17% of total annual global GHG emissions (see Figure 2). Using updated data (reduced deforestation and increased fossil fuel use in recent years), Guido van der Werf and colleagues scaled down the estimate to about 12% though allowing for a wide range of 6-17%(13). Winrock International’s ongoing study using LIDAR remote sensing data and field inventories suggests a much lower rate of around 8% for 2000 to 2005, with a possible range of 5-12%(14).
Estimates of forest contribution to GHG emissions and their potential role in mitigating (reducing) global warming vary widely because of incomplete country data, the complexity of monitoring forest cover and condition, the wide diversity of methods used, as well as changing land use and emissions patterns. However even at 8-12% contribution, forests still remain a significant source of global warming. Forests and the role they play and could play have become increasingly important in discussions and negotiations to mitigate climate change.
Climate change mitigation via forests
Forest carbon stocks could be potentially conserved and enhanced through a wide range of activities such as:
- Planting and/or regenerating trees on barren or non-forested land, in degraded forests, and in agricultural and urban landscapes. This includes concepts such as afforestation, reforestation, forestation, forest rehabilitation, forest restoration, agroforestry, urban forestry and enrichment planting.
- Conserving existing forests and avoiding their degradation or conversion to alternative land use. This includes concepts such as avoided deforestation, Reducing Emissions from Deforestation and Forest Degradation (REDD), and conservation of forest carbon stocks.
- Improved or sustainable forest management using options such as reduced impact logging (RIL), longer rotations, mixed ages and species.
- Managing harvested wood products.
- Soil (including peatland) conservation and rehabilitation.
- Use of forestry products for bioenergy to replace fossil fuel use.
- Tree species improvement to increase biomass productivity and carbon sequestration.
Measuring and monitoring forest carbon
A forest carbon sink project or national program has to demonstrate that net carbon gains have been made through the project or program activities and such gains are more than expected under a baseline scenario or reference emission level.
Carbon stocks and changes are to be estimated in five so called carbon “pools” or “reservoirs”:
- Below-ground biomass (roots),
- Above-ground biomass (tree trunk, branches, stems, leaves),
- Litter (dead leaves and other small fragments),
- Dead wood, and
- Soil organic carbon.
General principles: Estimates of emissions and removals should be demonstrable, as accurate as possible, complete, comparable, verifiable, and estimated consistently over time.
To meet the required quality standards, pioneer forest carbon offset projects have been developing detailed baseline and monitoring methodologies for approval and use by the Kyoto Protocol’s Clean Development Mechanism (CDM) and for the voluntary markets. CDM Afforestation Reforestation projects now have a choice of 18 approved methodologies for different situations and 15 tools for measuring additionality and estimating carbon stocks in deadwood among other things. Voluntary market standards are actively testing and registering methodologies not just for Afforestation-Reforestation but also avoided deforestation, improved forest management and other forest carbon project types.
At the same time, various countries have recently developed or are in the process of developing national-level forest carbon monitoring and accounting systems:
Developed countries that ratified the Kyoto Protocol have to report on net carbon removals or emissions from their afforestation/reforestation and deforestation activities (see Article 3.3). Many have in place rigorous monitoring protocols involving national forest inventories for deriving model data in all carbon pools, remote sensing of land cover change, and integrated data management and modeling systems for calculating the GHG emissions and removals over time.
Various developing countries are seeking carbon financing for protecting their forests as part of the evolving UNFCCC REDD+ negotiations. They will over time have to develop similar national-level monitoring systems for the forest carbon activities they include. The IPCC 2006 Guidelines for the AFOLU (Agriculture, Forestry and Other Land Use) sector recommend a tiered approach for estimating GHG emissions and removals based on data availability and national circumstances.
- Tier 1 uses equations and default parameter values for different pools and forest types.
- Tier 2 derives country or region-specific estimates from existing or new inventories and studies.
- Tier 3 provides more accurate estimates for key categories through detailed inventories and models repeated over time and high-resolution land cover change data.
Brazil has one of the most advanced forest monitoring systems among the developing countries. It has conducted wall-to-wall monitoring of deforestation in the Amazon for the last 20 years and recently started reporting on carbon gains with reduced deforestation. In Asia, India has a long-running national forest cover monitoring system and field inventories (since 1987) to which it is adding forest type mapping and reporting of forest-based carbon emissions. Other countries such as Indonesia, Vietnam, the Philippines and Laos are currently working towards establishing appropriate national MRV systems.
1Photosynthesis: Process by which plants, algae and some bacteria use energy from sunlight to convert carbon dioxide into organic compounds (sugars) and release oxygen as a by-product. This process provides us with life-sustaining oxygen and usable chemical energy or food. 6CO2 (carbon dioxide) + 6H2O (water) →C6H12O6 (sugars) + 6O2 (oxygen)
2Greenhouse gases (GHGs) include carbon dioxide CO2, methane CH4, nitrous oxide N2O, perfluorocarbons PFC, sulfur hexafluoride SF6, hydrofluorocarbons HFC and chlorofluorocarbons CFC among others. CO2 is the most common and abundant (76%) of these greenhouse gases in the earth’s atmosphere and GHG emissions are often referred to as carbon or CO2 emissions.
3Figure 2.1 in: Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Core Writing Team, Pachauri, R.K. and Reisinger, A. (Eds.). IPCC, Geneva, Switzerland. pp 104.
4Climate Change 2007 “The Physical Science Basis”. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, 2007. Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor and H.L. Miller (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
5FAQ 2.1, Figure 1. Climate Change 2007 “The Physical Science Basis”. Contribution of Working Group I to the Fourth Assessment Report of the IPCC, 2007.
6Figure 6.3. Climate Change 2007 “The Physical Science Basis”. Contribution of Working Group I to the Fourth Assessment Report of the IPCC, 2007.
7A paleo perspective on global warming. Paleoclimatic data for the last 2000 years. NOAA satellite and information service. National Climatic Data Center, US.
8Climate Change 2007 “The Physical Science Basis”. Contribution of Working Group I to the Fourth Assessment Report of the IPCC, 2007.
9Figure 1.1 in: Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the IPCC.
10Climate Change 2007: Synthesis Report. Contribution of Working Groups I, II and III to the Fourth Assessment Report of the IPCC.
11Global Forest Resources Assessment 2010: Main Report. October 2010. FAO Forestry paper 163, Rome. 340 pages. ISBN 978-92-5-106654-6
12Land observation based estimate for gross residual terrestrial sink for the 1990s. Nabuurs, G.J., O. Masera, K. Andrasko, P. Benitez-Ponce, R. Boer, M. Dutschke, E. Elsiddig, J. Ford-Robertson, P. Frumhoff, T. Karjalainen, O. Krankina, W.A. Kurz, M. Matsumoto, W. Oyhantcabal, N.H. Ravindranath, M.J. Sanz Sanchez, X. Zhang, 2007: Forestry. In Climate Change 2007: Mitigation. Contribution of Working Group III to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [B. Metz, O.R. Davidson, P.R. Bosch, R. Dave, L.A. Meyer (eds)], Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA.
13CO2 emissions from forest loss. 2009. G. R. van der Werf, D. C. Morton, R. S. DeFries, J. G. J. Olivier, P. S. Kasibhatla, R. B. Jackson, G. J. Collatz & J. T. Randerson. Nature Geoscience 2, 737 – 738.
14Deforestation not so important for climate change. December 2010. Fred Pearce. New Scientist.