Executive Summary3.1 Introduction3.2 Terrestrial and Ocean Biogeochemistry: Update on Processes3.3 Palaeo CO2 and Natural Changes in the Carbon Cycle3.4 Anthropogenic Sources of CO23.5 Observations, Trends and Budgets3.6 Carbon Cycle Model Evaluation3.7 Projections of CO2 Concentration and their ImplicationsReferencesThe Carbon Cycle and Atmospheric Carbon DioxideCo-ordinating Lead AuthorI.C. PrenticeLead AuthorsG.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. WallaceContributing AuthorsD. 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. YoolReview EditorsL. Pitelka, A. Ramirez Rojas3ContentsExecutive Summary 1853.1 Introduction 1873.2 Terrestrial and Ocean Biogeochemistry: Update on Processes 1913.2.1 Overview of the Carbon Cycle 1913.2.2 Terrestrial Carbon Processes 1913.2.2.1 Background 1913.2.2.2 Effects of changes in land use and land management 1933.2.2.3 Effects of climate 1943.2.2.4 Effects of increasing atmospheric CO21953.2.2.5 Effects of anthropogenic nitrogen deposition 1963.2.2.6 Additional impacts of changingatmospheric chemistry 1973.2.2.7 Additional constraints on terrestrial CO2uptake 1973.2.3 Ocean Carbon Processes 1973.2.3.1 Background 1973.2.3.2 Uptake of anthropogenic CO21993.2.3.3 Future changes in ocean CO2uptake 1993.3 Palaeo CO2and Natural Changes in the CarbonCycle 2013.3.1 Geological History of Atmospheric CO22013.3.2 Variations in Atmospheric CO2during Glacial/inter-glacial Cycles 2023.3.3 Variations in Atmospheric CO2during the Past 11,000 Years 2033.3.4 Implications 2033.4 Anthropogenic Sources of CO22043.4.1 Emissions from Fossil Fuel Burning and Cement Production 2043.4.2 Consequences of Land-use Change 2043.5 Observations, Trends and Budgets 2053.5.1 Atmospheric Measurements and Global CO2Budgets 2053.5.2 Interannual Variability in the Rate of Atmospheric CO2Increase 2083.5.3 Inverse Modelling of Carbon Sources and Sinks 2103.5.4 Terrestrial Biomass Inventories 2123.6 Carbon Cycle Model Evaluation 2133.6.1 Terrestrial and Ocean Biogeochemistry Models 2133.6.2 Evaluation of Terrestrial Models 2143.6.2.1 Natural carbon cycling on land 2143.6.2.2 Uptake and release of anthropogenic CO2by the land 2153.6.3 Evaluation of Ocean Models 2163.6.3.1 Natural carbon cycling in the ocean 2163.6.3.2 Uptake of anthropogenic CO2by the ocean 2163.7 Projections of CO2Concentration and theirImplications 2193.7.1 Terrestrial Carbon Model Responses to Scenarios of Change in CO2and Climate 2193.7.2 Ocean Carbon Model Responses to Scenarios of Change in CO2and Climate 2193.7.3 Coupled Model Responses and Implications for Future CO2Concentrations 2213.7.3.1 Methods for assessing the response of atmospheric CO2to different emissions pathways and model sensitivities 2213.7.3.2 Concentration projections based on IS92a, for comparison with previous studies 2223.7.3.3 SRES scenarios and their implications for future CO2concentration 2233.7.3.4 Stabilisation scenarios and their implications for future CO2emissions 2243.7.4 Conclusions 224References 225185The Carbon Cycle and Atmospheric Carbon DioxideExecutive SummaryCO2concentration trends and budgetsBefore the Industrial Era, circa 1750, atmospheric carbon dioxide(CO2) concentration was 280 ± 10 ppm for several thousand years.It has risen continuously since then, reaching 367 ppm in 1999. The present atmospheric CO2concentration has not beenexceeded during the past 420,000 years, and likely not during thepast 20 million years. The rate of increase over the past centuryis unprecedented, at least during the past 20,000 years. The present atmospheric CO2increase is caused by anthro-pogenic emissions of CO2. About three-quarters of theseemissions are due to fossil fuel burning. Fossil fuel burning (plusa small contribution from cement production) released onaverage 5.4 ± 0.3 PgC/yr during 1980 to 1989, and 6.3 ± 0.4PgC/yr during 1990 to 1999. Land use change is responsible forthe rest of the emissions.The rate of increase of atmospheric CO2content was 3.3 ±0.1 PgC/yr during 1980 to 1989 and 3.2 ± 0.1 PgC/yr during 1990to 1999. These rates are less than the emissions, because some ofthe emitted CO2dissolves in the oceans, and some is taken up byterrestrial ecosystems. Individual years show different rates ofincrease. For example, 1992 was low (1.9 PgC/yr), and 1998 wasthe highest (6.0 PgC/yr) since direct measurements began in1957. This variability is mainly caused by variations in land andocean uptake.Statistically, high rates of increase in atmospheric CO2haveoccurred in most El Niño years, although low rates occurredduring the extended El Niño of 1991 to 1994. Surface water CO2measurements from the equatorial Pacific show that the naturalsource of CO2from this region is reduced by between 0.2 and 1.0PgC/yr during El Niño events, counter to the atmosphericincrease. It is likely that the high rates of CO2increase duringmost El Niño events are explained by reductions in land uptake,caused in part by the effects of high temperatures, drought andfire on terrestrial ecosystems in the tropics. Land and ocean uptake of CO2can now be separated usingatmospheric measurements (CO2, oxygen (O2) and 13CO2). For1980 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/yrbased on CO2and O2measurements (negative signs denote netuptake). For 1990 to 1999, the ocean-atmosphere flux isestimated as −1.7 ± 0.5 PgC/yr and the land-atmosphere flux as−1.4 ± 0.7 PgC/yr. These figures are consistent with alternativebudgets based on CO2and 13CO2measurements, and withindependent estimates based on measurements of CO2and 13CO2in sea water. The new 1980s estimates are also consistent with theocean-model based carbon budget of the IPCC WGI SecondAssessment Report (IPCC, 1996a) (hereafter SAR). The new1990s estimates update the budget derived using SAR