Estimated Magnitude of Persistent Carbon Sink in World’s Forests

Forests are important in absorbing a significant amount of CO2 in the earth’s atmosphere. It is necessary to understand how much of an effect they have in order to set limitations on greenhouse gas emissions and better understand the effects of climate change. The Intergovernmental Panel on Climate Change (IPCC) reports a wide range of data on the C uptake by terrestrial ecosystems, stating that uptake could range from less than 1.0 to 2.6 Pg per year. More recent models on climate change report a C sink range of 2.0 to 3.4 Pg per year. Understanding and constraining these limitations is crucial in understanding the future effects of climate change. In this report, the authors carried out a bottom-up estimation of C stocks and changes based on recent data and long-term field observations coupled with statistical modeling. The C pools in forest include measurements from dead wood, harvested wood, living biomass, litter, and soil. Data from different countries, regions, and continents was compared to understand trends across geographic boundaries. The area examined contains 3.9 billion ha, which accounts for 95% of the world’s forests. —Taylor Jones
Pan, Y., Birdsey, R. A., Fang, J., Houghton, R., Kauppi, P. E., Kurz, W. A., Phillips, O. L., Shvidenko, A., Lewis, S. L., Canadell, J. G., Ciais, P., Jackson, R. B., Pacala, S. W., McGuire, A. D., Piao, S., Rautiainen, A., Sitch, S., Hayes, D., 2011. A Large and Persistent Carbon Sink in the World’s Forests. Science 333, 988-993.

          Yude Pan and colleagues report the estimated amount of carbon stock in the world’s forests at 861 ± 66 Pg C of which 44% resides in soil, 42% in live biomass, 8% in dead wood, and 5% in litter. Breaking down the C sink by types of forests, tropical forests account for 471 ± 93 Pg C, boreal forests 272 ± 23 Pg C, and temperate forests 119 ± 6 Pg C. Tropical and boreal forests store most of the earth’s carbon, however tropical forests have 56% in above ground biomass and boreal forests only have 20% in above ground biomass. This difference is crucial in determining what precautions should be taken to preserve certain parts of forests in different regions. The annual change in C stock shows an uptake of 2.5 ± 0.4 Pg C per year for 1990 to 1999 and 2.3 ± 0.5 Pg C per year from 2000 to 2007. Despite the overall C uptake in these two time periods, regional differences are apparent. For example, temperate forests increased C sink by 17% more from 2000 to 2007 compared to 1990 to 1999, while tropical forests decreased C sink by 23% during the same time period. By subtracting the C sink losses from forest degradation in tropical areas, the net forest C sink is estimated at 1.0 ± 0.8 and 1.2 Pg C per year from 1990 to 1999 and 2000 to 2007, respectively.
          Examining C sink by region and biome, boreal forests had an estimated C sink of 0.5 ± 0.1 Pg C per year for the past two decades. Some regions, such as Asian Russia, account for a significant portion of the total sink, but experienced no overall change. Other areas, like European Russia, experienced a 35% increase in C sink. The authors suggest this increase could be due to increased forest area after agricultural abandonment, reduced harvesting or several areas of forest progressing to later stages in the plant life cycle. The C sink in Canadian forests reduced by half during the same time period, largely due to wildfires and insect outbreaks. As a result of the increases and decreases mentioned above, the overall C sink did not experience a net change.
          Temperate forests contributed 0.7 ± 0.1 and 0.8 ± 0.1 Pg C per year for the past two decades. This increase in C sink is likely due to increases in density of forest biomass and an increase in forest area. The C sink in the U.S. on average increased by 33% during this time period due to the growth of forest area resulting from previous agriculture and harvesting. However, the western U.S. has experienced forest mortality due to drought stress, insects, and fires. The C sink in Europe remained constant, but the C sink in China increased by 34%, likely due to newly planted forest area and reforestation programs.
          Tropical forests account for about 70% of the world’s forests and in this study, the authors collected data from intensive monitoring of Africa and South America, and used these trends to estimate the data for Southeast Asia. The total C sink in tropical forests is estimated at 1.3 ± 0.3 and 1.0 ± 0.5 Pg C per year for the past two decades. The total C sink during this time period accounts for about half of the total C sink. Tropical land use, including clearing forests for agriculture, timber and pasture areas, accounts for a carbon release second to the amount produced by fossil fuels. Deforestation accounted for about 40% of global fossil fuel emissions in the past two decades, but this is often overlooked because it was offset by a large uptake in C due to forest regrowth. Pan and colleagues estimate that C uptake was stronger in regrowth forests compared to previously intact forests due to simultaneous rapid increases in biomass. The authors also suggest that the state tropical forests has a significant impact on total C sink and better monitoring techniques and increased understanding of C cycling in these areas should be a priority in the future.
          Dead wood, litter, soil, and harvested wood account for about 35% of the world’s forest C sink and are important factors that should not be overlooked, however they remain the most difficult to measure. This measurement could also be too low as it does not include deep soil beyond 1 meter and improved measuring techniques would be necessary to account for this. Dead wood is more vulnerable to fires than other sources of C and harvested wood in boreal areas experienced a decrease over the past two decades, mainly due to decreased Russian harvesting.
          Pan and colleagues recognize critical data gaps in their study including a substantial lack of data for North America (mainly Canadian unmanaged forests and Alaska), and for the C flux in tropical forests, which may account for a 10–20% error in estimates. The authors suggest that in order to attempt to combat these uncertainties, land monitoring should be increased, globally consistent land-sensing is necessary, and scientists need better tools to measure below ground, dead wood and litter sources of C.
          Forests have a crucial role in absorbing atmospheric C and will continue to maintain strong control over atmospheric CO2 levels. The factors that affect C levels in the atmosphere are complex and it is necessary to adopt better monitoring systems to separate their impacts and determine the effects of climate change. The authors note that although a large amount of CO2 humans place in the atmosphere is sequestered by forests, deforestation significantly contributes to C losses and relying on forests to absorb C is not without risk.

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