November 23^rd, 1999

There is strong evidence that the terrestrial biosphere has acted as a net carbon sink over the last two decades, and more specifically, that the carbon sink mostly resides in the northern hemisphere between 30 and 70 degrees latitude. This view is supported by several independent lines of evidence that includes measurements of [CO₂] in air and ocean surface waters, CO₂-isotopic composition (¹³C/¹²C, ¹⁸O/¹⁶O), ratio of oxygen to nitrogen, and forest productivity along with historical land cover data.

As CO₂ emissions continue to increase over the next decades, the strength of the ocean and terrestrial sinks are expected to increase proportionally to the atmospheric CO₂ growth rate. That is to say, the current proportion of carbon being locked in land and oceans will remain constant in the future: about 1/3 of the total carbon emissions from human activities will accumulate in the atmosphere, 1/3 will go into terrestrial biosphere, and 1/3 into the oceans. If the expected increase in sink strength was not realized, atmospheric CO₂ growth will be much faster than currently predicted and policies to stabilize CO₂ concentrations will fall short in meeting their targets.

There is now increasing evidence that the terrestrial carbon sink may "saturate" within the next century, and before we are able to stabilize CO₂ concentration at any given level. Some results also suggest that after saturation of the terrestrial sink takes place, the sink will decline subsequently. The processes involved in this saturation response are various, and include both physiological and structural processes at the scales of ecosystems and landscapes. Below I describe the various lines of evidence that support this view.

CO₂ fertilization effect

Increasing atmospheric CO₂ concentrations will enhance the terrestrial biosphere sink in the near future, the so called CO₂ fertilization effect. Doubling atmospheric CO₂ has been shown to stimulate photosynthesis quite consistently between 40% and 60% above that of current CO₂ concentrations. However, Net Primary Production (NPP) responses to doubling atmospheric CO₂ are much smaller than previously thought. A recent synthesis of experimental results done by the Global Change and Terrestrial Ecosystems (GCTE) Project showed a mean biomass accumulation at 700 ppm of only 14% higher than that for current CO₂ (Mooney et al. 1999). Previous estimates from pot and greenhouse experiments showed a 30-40% increase, but we now know that this is an overestimate of the potential of CO₂ fertilization. We have also learned that highly nitrogen- and phosphorus-limited systems will show smaller CO₂ fertilization effects, and that temperature-limited systems such as boreal forest and tundra will show little response, if any, to further CO₂ increase.

Another important finding from ecosystem experiments is that net CO₂ gain by terrestrial ecosystems (carbon uptake by photosynthesis minus carbon lost in respiration) may saturate at around double pre-industrial CO₂ concentrations (550-650 ppm) because other limiting factors will prevent from responding at higher CO₂ concentrations. Recent experiments show that plants exposed to multiple levels of CO₂ are more responsive within the range of 280 ppm (pre-industrial) to 650 ppm, and that a saturation response and subsequent decline takes place at higher CO₂ concentrations (Fig. 1).

Recent model analyses have also suggested that the so called "missing carbon" (i.e., the terrestrial carbon sink) could be explained by the CO₂ fertilization effect (Thompson et al. 1996). The suggestion is that the C sink is the result of the disequilibrium between the increasing primary production (as atmospheric CO₂ increases) and respiration of the increasingly larger carbon pools (litter and soil organic matter) that lags behind by some years to decades. The net balance remains positive as long as the atmospheric CO₂ continues growing. As we slow down the CO₂ growth, and particularly when CO₂ levels reach stabilization, the terrestrial carbon sink will decrease because soil respiration (carbon emissions) will catch up with photosynthesis (carbon uptake).

Temperature effect on microbial respiration

Increased temperature will enhance both photosynthesis, following a saturation curve, and soil respiration, following an exponential curve (Fig. 2). The relative importance of these two processes will depend on the climate and CO₂ sensitivity of the various ecosystems and C stocks. However, ecosystems will tend to show smaller carbon gains at higher CO₂ concentrations – when the CO₂ fertilization effect may decline -, and at higher temperatures because the increase carbon emissions from microbial decomposition. It is conceivable that respiration losses will be larger than carbon uptake by photosynthesis some time in the next century. Ecosystems with a high soil organic matter content are more likely to show this behavior as found in a study of the Canadian boreal forest (Goulden et al. 1998). A synthesis of a further twenty-three field studies also showed consistent increased belowground respiration when soils were experimentally heated up to 5°C (GCTE-NEWS, in preparation). However, an equally consistent response was the increase of N mineralization which is likely to enhance plant growth.

Nitrogen fertilization effects

Increasing nitrogen deposition due to fossil fuel burning and use of fertilizers has been shown to enhance the terrestrial carbon sink in N-limited ecosystems. However, recent findings show that a large portion of this nitrogen deposited on land remains in the soil, contributing little to carbon sequestration, - which occurs more efficiently when nitrogen is incorporated in vegetation (larger C:N ratio) (Nadelhoffer et al. 1999). Future nitrogen deposition may have little effect on carbon sequestration because much of the new deposition will occur in already N-saturated soils such as in Western European forests. Likewise, large amounts of nitrogen deposition are also expected to take place in agricultural lands, largely in the tropics, whose capacity to sequester C is intrinsically small and where soils are mostly P-limited (Hall and Matson 1999).

Land use change

In addition to the above physiological considerations, land use change plays a major role in the long-term dynamics of carbon sources and sinks. Since the beginning of the industrial revolution, land use change has contributed about one third of the total carbon released to the atmosphere by human activities, and 124 Pg C (Pg = 10¹⁵ g C) during the period between 1850 and 1990 (Houghton 1999).

An important candidate mechanism for explaining the Northern Hemisphere carbon sink is forest regrowth in abandoned agricultural lands. If this mechanism is confirmed as the cause of the observed sink, then the nature of this sink will be only temporary and will largely disappear when forests reach maturity within the next few decades.

Projections of global land-use change for the next 100 years suggest extensive land conversion word-wide, and particularly in Central Africa and parts of Asia in order to meet future food demands of a growing population (Cramer et al. 1999). This increase in land clearing will further increase carbon emissions from the terrestrial biosphere (GCTE Synthesis - Walker et al. 1999). Other land management practices, however, may strengthen C sinks like changes in timber harvesting, wildfire suppression, and expansion of "Kyoto forests" (IGBP 1998), although it is unlikely that these practices will have any significant effect in preventing further atmospheric CO₂ growth.

Landscape Disturbances

Disturbances such as fire and forest pests are key components of the net carbon balance of terrestrial ecosystems. Changes in the frequency of fire and other disturbances lead to large, short-term carbon losses followed by a long, slow recovery of carbon stocks. For instance, the net carbon balance of Canadian forests has changed from being a strong carbon sink during the first part of this century to a weak sink over the last few decades due to increased fire frequency and logging practices (Kurz and Apps 1999). Increasing evaporation demands due to warmer temperatures and the increasing frequency and intensity of El Niño Southern Oscillation (ENSO) events, as predicted for a warmer future, will undoubtedly result in an increased fire frequency in many regions of the world, particularly in boreal and tropical forests.

Summary

It is expected that the current terrestrial sink will increase in the near future due to increased CO₂, temperature and nitrogen deposition. However, as further increase of CO₂ and temperature takes place, terrestrial sinks are likely to show a saturation response within the next 50 years.

GCTE has further analyzed the saturation response in a recent model inter-comparison of six dynamic global vegetation models that account for interactions of ecosystem carbon and water exchange with vegetation dynamics (GCTE-DGVM activity 1999, in preparation). The analyses show that the current terrestrial carbon sink of the order of 2 Pg C/year will rise to about 4.5 Pg C/year, and will level off by about 2030. Four of the six models showed a further climate-induced decline of the carbon sink after 2050.

Current IPCC scenarios on future growth of atmospheric CO₂ are not taking into account the possible saturation response of the terrestrial carbon sink. If this response does occur, the accumulation of atmospheric CO₂ will take place at a much faster rate than currently thought, and greenhouse policies developed under a business-as-usual scenarios will fall short of meeting their targets.

References and Notes

Cramer W, Shugart HH, Noble IR, Woodward FI, Bugmann H, Bondeau A, Foley JA, Gardner RH, Lauenroth WK, Pitelka LF, Sutherst RW (1999) Ecosystem composition and structure. In: Walker BH, Steffen WL, Canadell J, Ingram JSI, Editors. The Terrestrial Biosphere and Global Change. Implications for Natural and Managed Ecosystems. Cambridge University Press, London.

GCTE-NEWS (Network of Experimental Warming Studies) (1999) A cross-biome synthesis of the warming effects on ecosystem functioning (in preparation).

GCTE-DGVM Activity (Focus 2- Cramer W.) (1999) Dynamic responses of global terrestrial ecosystems to changes in CO₂ and climate (in preparation).

Goulden ML, Wofsy SC, Harden JW, Trumbore SE, Crill PM, Gower ST, FriesT, Daube BC, Fan SM, Sutton DJ, Bazzaz A, Munger JW (1998) Sensitivity of Boreal forest carbon balance to soil thaw. Science 279: 214-217.

IGBP Terrestrial Carbon Working Group: Steffen, W, Noble, I, Canadell, J, Apps, M, Schulze, E-D, Jarvis, PG, Baldocchi, D, Ciais, P, Cramer, W, Ehleringer, J, Farquhar, G, Field, CB, Ghazi, A, Gifford, R, Heimann, M, Houghton, R, Kabat, P, Körner, C, Lambin, E, Linder, S, Mooney, HA, Murdiyarso, D, Post, WM, Prentice, IC, Raupach, MR, Schimel, DS, Shvidenko, A and Valentini, R (1998) The terrestrial carbon cycle: Implications for the Kyoto protocol. Science 280: 1393-1394.

Hall SJ, Matson PA (1999) Nitrogen oxide emissions after nitrogen additions in tropical forests. Nature 400: 152-155.

Houghton RA. (1999) The annual net flux of carbon to the atmosphere from changes in land use 1850-1900. Tellus Ser. B 51B: 298-313.

Kurz WA, Apps MJ. 1999. A 70-yr retrospective analysis of carbon fluxes in the Canadian forest sector. Ecological Applications 9: 526-547.

Mooney H, Canadell J, Chapin FS, Ehleringer J, Körner Ch, McMurtrie R, Parton W, Pitelka L, Schulze D-E (1999) Ecosystem Physiology Responses to Global Change. Pages 141-189 in BH Walker, WL Steffen, J Canadell, JSI Ingram, editors. The Terrestrial Biosphere and Global Change: Implications for Natural and Managed Ecosystems. Cambridge University Press, London.

Nadelhoffer KJ, Emmett BA, Gundersen P, Kjonaas OJ, Koopmans CJ, Schleppi P, Tietema A, Wright RF (1999) Nitrogen deposition makes a minor contribution to carbon sequestration in temperate forests. Nature 398: 145-148.

Thompson MV, Randerson JT, Malmstrom CM, Field CB (1996) Change in net primary production and heterotrophic respiration: How much is necessary to sustain the terrestrial carbon sink? Global Biogeochemical Cycles 10: 711-726.

Walker BH, Steffen WL, Canadell J, Ingram JSI, Editors (1999) The Terrestrial Biosphere and Global Change. Implications for Natural and Managed Ecosystems. Cambridge University Press, London.