3 The Carbon Cycle and Atmospheric Carbon Dioxide Co-ordinating Lead Author I.C. Prentice Lead Authors G.D. Farquhar, M.J.R. Fasham, M.L. Goulden, M. Heimann, V.J. Jaramillo, H.S. Kheshgi, C. Le Quéré, R.J. Scholes, D.W.R. Wallace Contributing Authors D. Archer, M.R. Ashmore, O. Aumont, D. Baker, M. Battle, M. Bender, L.P. Bopp, P. Bousquet, K. Caldeira, P. Ciais, P.M. Cox, W. Cramer, F. Dentener, I.G. Enting, C.B. Field, P. Friedlingstein, E.A. Holland, R.A. Houghton, J.I. House, A. Ishida, A.K. Jain, I.A. Janssens, F. Joos, T. Kaminski, C.D. Keeling, R.F. Keeling, D.W. Kicklighter, K.E. Kohfeld, W. Knorr, R. Law, T. Lenton, K. Lindsay, E. Maier-Reimer, A.C. Manning, R.J. Matear, A.D. McGuire, J.M. Melillo, R. Meyer, M. Mund, J.C. Orr, S. Piper, K. Plattner, P.J. Rayner, S. Sitch, R. Slater, S. Taguchi, P.P. Tans, H.Q. Tian, M.F. Weirig, T. Whorf, A. Yool Review Editors L. Pitelka, A. Ramirez Rojas Contents Executive Summary 185 3.5.2 Interannual Variability in the Rate of Atmospheric CO2 Increase 208 3.1 Introduction 187 3.5.3 Inverse Modelling of Carbon Sources and Sinks 210 3.2 Terrestrial and Ocean Biogeochemistry: 3.5.4 Terrestrial Biomass Inventories 212 Update on Processes 191 3.2.1 Overview of the Carbon Cycle 191 3.6 Carbon Cycle Model Evaluation 213 3.2.2 Terrestrial Carbon Processes 191 3.6.1 Terrestrial and Ocean Biogeochemistry 3.2.2.1 Background 191 Models 213 3.2.2.2 Effects of changes in land use and 3.6.2 Evaluation of Terrestrial Models 214 land management 193 3.6.2.1 Natural carbon cycling on land 214 3.2.2.3 Effects of climate 194 3.6.2.2 Uptake and release of 3.2.2.4 Effects of increasing atmospheric anthropogenic CO2 by the land 215 CO2 195 3.6.3 Evaluation of Ocean Models 216 3.2.2.5 Effects of anthropogenic nitrogen 3.6.3.1 Natural carbon cycling in the deposition 196 ocean 216 3.2.2.6 Additional impacts of changing 3.6.3.2 Uptake of anthropogenic CO2 by atmospheric chemistry 197 the ocean 216 3.2.2.7 Additional constraints on terrestrial CO2 uptake 197 3.7 Projections of CO2 Concentration and their 3.2.3 Ocean Carbon Processes 197 Implications 219 3.2.3.1 Background 197 3.7.1 Terrestrial Carbon Model Responses to 3.2.3.2 Uptake of anthropogenic CO2 199 Scenarios of Change in CO2 and Climate 219 3.2.3.3 Future changes in ocean CO2 3.7.2 Ocean Carbon Model Responses to Scenarios uptake 199 of Change in CO2 and Climate 219 3.7.3 Coupled Model Responses and Implications 3.3 Palaeo CO2 and Natural Changes in the Carbon for Future CO2 Concentrations 221 Cycle 201 3.7.3.1 Methods for assessing the response 3.3.1 Geological History of Atmospheric CO2 201 of atmospheric CO2 to different 3.3.2 Variations in Atmospheric CO2 during emissions pathways and model Glacial/inter-glacial Cycles 202 sensitivities 221 3.3.3 Variations in Atmospheric CO2 during the 3.7.3.2 Concentration projections based on Past 11,000 Years 203 IS92a, for comparison with previous 3.3.4 Implications 203 studies 222 3.7.3.3 SRES scenarios and their implications 3.4 Anthropogenic Sources of CO2 204 for future CO2 concentration 223 3.4.1 Emissions from Fossil Fuel Burning and 3.7.3.4 Stabilisation scenarios and their Cement Production 204 implications for future CO2 3.4.2 Consequences of Land-use Change 204 emissions 224 3.7.4 Conclusions 224 3.5 Observations, Trends and Budgets 205 3.5.1 Atmospheric Measurements and Global References 225 CO2 Budgets 205 The Carbon Cycle and Atmospheric Carbon Dioxide 185 Executive Summary accuracy of these estimates. Comparable data for the 1990s are not yet available. The land-atmosphere flux estimated from atmospheric CO2 concentration trends and budgets observations comprises the balance of net CO2 release due to Before the Industrial Era, circa 1750, atmospheric carbon dioxide land-use changes and CO2 uptake by terrestrial systems (the ± (CO2) concentration was 280 10 ppm for several thousand years. “residual terrestrial sink”). The residual terrestrial sink is It has risen continuously since then, reaching 367 ppm in 1999. estimated as −1.9 PgC/yr (range −3.8 to +0.3 PgC/yr) during the The present atmospheric CO2 concentration has not been 1980s. It has several likely causes, including changes in land exceeded during the past 420,000 years, and likely not during the management practices and fertilisation effects of increased past 20 million years. The rate of increase over the past century atmospheric CO2 and nitrogen (N) deposition, leading to is unprecedented, at least during the past 20,000 years. increased vegetation and soil carbon. The present atmospheric CO2 increase is caused by anthro- Modelling based on atmospheric observations (inverse pogenic emissions of CO2. About three-quarters of these modelling) enables the land-atmosphere and ocean-atmosphere emissions are due to fossil fuel burning. Fossil fuel burning (plus fluxes to be partitioned between broad latitudinal bands. The sites a small contribution from cement production) released on of anthropogenic CO2 uptake in the ocean are not resolved by average 5.4 ± 0.3 PgC/yr during 1980 to 1989, and 6.3 ± 0.4 inverse modelling because of the large, natural background air- PgC/yr during 1990 to 1999. Land use change is responsible for sea fluxes (outgassing in the tropics and uptake in high latitudes). the rest of the emissions. Estimates of the land-atmosphere flux north of 30°N during 1980 ± The rate of increase of atmospheric CO2 content was 3.3 to 1989 range from −2.3 to −0.6 PgC/yr; for the tropics, −1.0 to 0.1 PgC/yr during 1980 to 1989 and 3.2 ± 0.1 PgC/yr during 1990 +1.5 PgC/yr. These results imply substantial terrestrial sinks for to 1999. These rates are less than the emissions, because some of anthropogenic CO2 in the northern extra-tropics, and in the the emitted CO2 dissolves in the oceans, and some is taken up by tropics (to balance deforestation). The pattern for the 1980s terrestrial ecosystems. Individual years show different rates of persisted into the 1990s. increase. For example, 1992 was low (1.9 PgC/yr), and 1998 was Terrestrial carbon inventory data indicate carbon sinks in the highest (6.0 PgC/yr) since direct measurements began in northern and tropical forests, consistent with the results of inverse 1957. This variability is mainly caused by variations in land and modelling. ocean uptake. East-west gradients of atmospheric CO2 concentration are an Statistically, high rates of increase in atmospheric CO2 have order of magnitude smaller than north-south gradients. Estimates occurred in most El Niño years, although low rates occurred of continental-scale CO2 balance are possible in principle but are during the extended El Niño of 1991 to 1994. Surface water CO2 poorly constrained because there are too few well-calibrated CO2 measurements from the equatorial Pacific show that the natural monitoring sites, especially in the interior of continents, and source of CO2 from this region is reduced by between 0.2 and 1.0 insufficient data on air-sea fluxes and vertical transport in the PgC/yr during El Niño events, counter to the atmospheric atmosphere. increase. It is likely that the high rates of CO2 increase during most El Niño events are explained by reductions in land uptake, The global carbon cycle and anthropogenic CO caused in part by the effects of high temperatures, drought and 2 fire on terrestrial ecosystems in the tropics. The global carbon cycle operates through a variety of response Land and ocean uptake of CO2 can now be separated using and feedback mechanisms. The most relevant for decade to 13 atmospheric measurements (CO2, oxygen (O2) and CO2). For century time-scales are listed here. 1980 to 1989, the ocean-atmosphere flux is estimated as −1.9 ± − ± 0.6 PgC/yr and the land-atmosphere flux as 0.2 0.7 PgC/yr Responses of the carbon cycle to changing CO2 concentrations based on CO2 and O2 measurements (negative signs denote net • Uptake of anthropogenic CO2 by the ocean is primarily uptake). For 1990 to 1999, the ocean-atmosphere flux is governed by ocean circulation and carbonate chemistry. So − ± estimated as 1.7 0.5 PgC/yr and the land-atmosphere flux as long as atmospheric CO2 concentration is increasing there is net −1.4 ± 0.7 PgC/yr. These figures are consistent with alternative uptake of carbon by the ocean, driven by the atmosphere-ocean 13 budgets based on CO2 and CO2 measurements, and with difference in partial pressure of CO2. The fraction of anthro- 13 independent estimates based on measurements of CO2 and CO2 pogenic CO2 that is taken up by the ocean declines with in sea water. The new 1980s estimates are also consistent with the increasing CO2 concentration, due to reduced buffer capacity of ocean-model based carbon budget of the IPCC WGI Second the carbonate system. The fraction taken up by the ocean also Assessment Report (IPCC, 1996a) (hereafter SAR). The new declines with the rate of increase of atmospheric CO2, because 1990s estimates update the budget derived using SAR method- the rate of mixing between deep water and surface water limits ologies for the IPCC Special Report on Land Use, Land Use CO2 uptake.