Chapter 6 Saturation of the Terrestrial Carbon Sink Josep G. Canadell · Diane E. Pataki · Roger Gifford · Richard A. Houghton · Yiqi Luo · Michael R. Raupach Pete Smith · Will Steffen most of the biological sinks will eventually level-off and 6.1 Introduction subsequently declined to zero (hereafter referred as “sink saturation”) whereby no further C will be removed from There is strong evidence that the terrestrial biosphere the atmosphere. has acted as a net carbon (C) sink over the last two and Coupled with this sink decline, global warming and half decades. Its strength is highly variable year-to-year deforestation have the potential to destabilize large bio- ranging from 0.3 to 5.0 Pg C yr–1; an amount of signifi- spheric C pools (hereafter referred as “vulnerable C pools”) cant magnitude compared to the emission of about which would add CO2 to the atmosphere. This C source 7 Pg C yr–1 from fossil fuel burning (Prentice et al. 2001; component will further diminish the net gains of C sinks Schimel et al. 2001; Sabine et al. 2004). Uncertainties as- and could even diminish the sink strength beyond zero, sociated with C emissions from land-use change are large. thereby moving from being a C sink to a source during On average, the terrestrial C sink is responsible for re- this century. Such an eventuality would put further pres- moving from the atmosphere approximately one third sure on society to select higher targets of CO2 emission of the CO2 emitted from fossil fuel combustion, thereby reductions from fossil fuel burning. slowing the build-up of atmospheric CO2. The ocean sink In this paper, we first introduce briefly the current is of similar magnitude (Sabine et al. 2004). Given the understanding of the global distribution of C sources and international efforts to stabilize atmospheric CO2 con- sinks. Second, we discuss processes thought to be respon- centration and climate (i.e., Kyoto Protocol, C trading sible for the current terrestrial C sink and their likely markets), the terrestrial C sink can be viewed as a sub- future dynamics highlighting their potential contribu- sidy to our global economy worth trillions of dollars. tion to saturation of the terrestrial C sink. Third, we dis- Because many aspects of the terrestrial C sink are ame- cuss the dynamics of the future terrestrial C balance nable to purposeful management, its basis and dynam- based on results from stand-alone and fully coupled car- ics need to be well understood. bon-climate models, and their accuracy in representing There is a sparse understanding of the multiple pro- various physiological and ecological sink/source pro- cesses responsible for the terrestrial C sink, the relative cesses. Recommendations for future research are made importance of each of them, and their future dynamics. at the end of the chapter. Such information is needed to predict the future strength and distribution of biospheric terrestrial sinks, and there- fore critical for the design of future pathways of decar- 6.2 Location of the Current Terrestrial Carbon Sinks bonization. Terrestrial biospheric models and fully carbon- The dual constraint of the measured global atmospheric coupled General Circulation Models (GCMs) are pro- CO2 concentration growth rate and the relatively weak gressively incorporating more C processes thought to north-south CO2 concentration gradient indicate a net drive significant carbon-climate feedbacks. However, sink of about 2 Pg C yr–1 in the northern extratropics being limited by poor understanding and the slow pro- (Tans et al. 1990). These observations are corroborated δ 13 cess of building ever more complex models, they still by additional information from C and O2/N2, as well represent a partial set of processes. This limits the abil- as increasingly sophisticated inverse modeling with aug- ity to produce robust projections of the biosphere’s role mented flask data (Rayner et al. 1999; Kaminiski et al. in the future atmospheric CO2 growth and its impacts 1999; Gurney et al. 2002). on climate change. Despite the robustness of the global and latitudinal In this paper we demonstrate that the underlying ecol- average estimates, inverse modeling calculations are ogy of terrestrial biospheric CO2 sinks suggests that, de- poorly constrained at the continental scale, particularly spite having the potential for increased C sink owing to with regard to the longitudinal partition of the sink. atmospheric and climate change over the next decades, Model intercomparisons suggest that the Northern Hemi- In: Canadell JG, Pataki D, Pitelka L (eds) (2007) Terrestrial Ecosystems in a Changing World. The IGBP Series, Springer-Verlag, Berlin Heidelberg 60 CHAPTER 6 · Saturation of the Terrestrial Carbon Sink sphere terrestrial sink is relatively evenly distributed processes are soil respiration in response to warming, across North America, Europe and Asia, with boreal permafrost thawing and subsequent decomposition of North America being C neutral (Gurney et al. 2002; Yuen organic matter, fires, deforestation, and peatland drain- et al. 2005). Large uncertainties still remain for north- age. Sink saturation occurs when the increase in efflux ern Africa, tropical America, temperate Asia and boreal becomes equal to the increase in uptake for a given pe- Asia. Interpretations of global distributions of atmo- riod of time producing no increase in net uptake. spheric O2/N2 ratio (Keeling et al. 1996) and inverse When assessing whether and when C sinks might analyses (Rayner et al. 1999; Gurney et al. 2002) conclude saturate in the future, there are two types of dynamics that tropical ecosystems are not a strong net sink, and to be considered: (i) Saturation of C uptake processes most likely they are C neutral. Given that tropical forests which refers to the diminishing sensitivity of a sink pro- are well documented as current large CO2 emitters via cess to an external forcing or stimulus (e.g., CO2 fertili- deforestation, this implies that there must be a strong zation effect on photosynthesis as atmospheric CO2 in- balancing tropical sink of equivalent magnitude to that creases); and (ii) increasing C emission driven by exter- of emissions from tropical deforestation. During the nal forcing (e.g., warming effects on heterotrophic res- 1980s–1990s this was estimated to be 0.6 to 2.2 Pg C yr–1 piration). (DeFries et al. 2002; Archard et al. 2002; Houghton 2003). We refer to C pool saturation when the annual emis- Atmospheric data provides limited information about sion itself becomes equal to the annual uptake which in- the processes driving terrestrial sinks. Such information volves two types of processes: (i) the “fast response” requires the addition of ground-based observations, ex- C uptake (e.g., net photosynthesis) is matched by the “fast periments, and modeling (Canadell et al. 2000; Global response” C emission (e.g., heterotrophic respiration), or Carbon Project 2003). Processes responsible for the sinks (ii) long timescale, “slow response” processes cause pools in the tropical, mid-, and high-latitudes need to be equally to reach maximum size (e.g., tree death rate matches new understood since the saturation of any one can have large tree growth rate or, for soil, the capacity for organic mat- impacts on the growth rate of atmospheric CO2. ter stabilization by clay surfaces is exhausted). 6.3 Dynamics of Processes that Contribute 6.4 Processes Contributing to Terrestrial Carbon to Carbon Sink Saturation Sink Saturation A decrease in the long term terrestrial net CO2 sink can Our current understanding of the global net C sinks re- arise from a decrease in the C uptake component or in- quires invoking three types of processes (Table 6.1): crease in the emission component (see Box 6.1). Key sink (i) processes driven by changes in atmospheric composi- processes are CO2 fertilization of photosynthesis, N fertili- tion (e.g., CO2 fertilization on photosynthesis, nitrogen (N) zation of net primary production, woody vegetation thick- deposition fertilization, pollution damage); (ii) processes ening and encroachment, forest regrowth in abandoned driven by climate change (e.g., temperature and precipi- cropland, and afforestation/resforestation. Key C emission tation effects on gross primary production and het- erotrophic respiration); and (iii) processes driven by land-use change or land management (e.g., deforestation, Box 6.1. Description of GPP, NPP, NEP and NBP forest regrowth, woody encroachment, forest thickening Plants take up carbon dioxide via photosynthesis. The sum of due to fire suppression, afforestation and reforestation, photosynthesis over a year is termed Gross Primary Produc- changes in soil C pools under cultivation and grazing). tivity (GPP). The actual C fixed into plants, the Net Primary Productivity (NPP) of an ecosystem or growth of an individual The relative strengths of each of these processes contrib- plant, is the balance between GPP and the C lost through plant uting to the current sink and its spatial distribution re- respiration (i.e., the construction and maintenance cost). It is main largely to be evaluated (Houghton 2002; Smith 2005). not until C losses by microbial respiration (heterotrophic res- piration, Rh) in litter and soil, and by herbivory are accounted for that we obtain the net C balance of an ecosystem. This net balance is termed Net Ecosystem Productivity (NEP). How- 6.4.1 Processes Driven by Atmospheric ever, when considering long periods of time and large regions (or the whole terrestrial biosphere for that matter) we need to Composition Change include other processes that contribute to loss of C such as, fires, harvest, erosion, and export of C in river flow. The eco- 6.4.1.1 CO2 Fertilization Effect system C balance measured over the full disturbance/recov- ery cycle is often termed the Net Biome Productivity (NBP) which for a system in equilibrium is zero. Continuous enrich- Short term responses of leaf net photosynthetic rate to ment of the resource base on which the biome depends by CO2 CO2 concentration saturate at around 800–1 000 ppm.