7 Couplings Between Changes in the Climate System and Biogeochemistry Coordinating Lead Authors: Kenneth L. Denman (Canada), Guy Brasseur (USA, Germany) Lead Authors: Amnat Chidthaisong (Thailand), Philippe Ciais (France), Peter M. Cox (UK), Robert E. Dickinson (USA), Didier Hauglustaine (France), Christoph Heinze (Norway, Germany), Elisabeth Holland (USA), Daniel Jacob (USA, France), Ulrike Lohmann (Switzerland), Srikanthan Ramachandran (India), Pedro Leite da Silva Dias (Brazil), Steven C. Wofsy (USA), Xiaoye Zhang (China) Contributing Authors: D. Archer (USA), V. Arora (Canada), J. Austin (USA), D. Baker (USA), J.A. Berry (USA), R. Betts (UK), G. Bonan (USA), P. Bousquet (France), J. Canadell (Australia), J. Christian (Canada), D.A. Clark (USA), M. Dameris (Germany), F. Dentener (EU), D. Easterling (USA), V. Eyring (Germany), J. Feichter (Germany), P. Friedlingstein (France, Belgium), I. Fung (USA), S. Fuzzi (Italy), S. Gong (Canada), N. Gruber (USA, Switzerland), A. Guenther (USA), K. Gurney (USA), A. Henderson-Sellers (Switzerland), J. House (UK), A. Jones (UK), C. Jones (UK), B. Kärcher (Germany), M. Kawamiya (Japan), K. Lassey (New Zealand), C. Le Quéré (UK, France, Canada), C. Leck (Sweden), J. Lee-Taylor (USA, UK), Y. Malhi (UK), K. Masarie (USA), G. McFiggans (UK), S. Menon (USA), J.B. Miller (USA), P. Peylin (France), A. Pitman (Australia), J. Quaas (Germany), M. Raupach (Australia), P. Rayner (France), G. Rehder (Germany), U. Riebesell (Germany), C. Rödenbeck (Germany), L. Rotstayn (Australia), N. Roulet (Canada), C. Sabine (USA), M.G. Schultz (Germany), M. Schulz (France, Germany), S.E. Schwartz (USA), W. Steffen (Australia), D. Stevenson (UK), Y. Tian (USA, China), K.E. Trenberth (USA), T. Van Noije (Netherlands), O. Wild (Japan, UK), T. Zhang (USA, China), L. Zhou (USA, China) Review Editors: Kansri Boonpragob (Thailand), Martin Heimann (Germany, Switzerland), Mario Molina (USA, Mexico) This chapter should be cited as: Denman, K.L., G. Brasseur, A. Chidthaisong, P. Ciais, P.M. Cox, R.E. Dickinson, D. Hauglustaine, C. Heinze, E. Holland, D. Jacob, U. Lohmann, S Ramachandran, P.L. da Silva Dias, S.C. Wofsy and X. Zhang, 2007: Couplings Between Changes in the Climate System and Biogeochemistry. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M.Tignor and H.L. Miller (eds.)]. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Couplings Between Changes in the Climate System and Biogeochemistry Chapter 7 Table of Contents Executive Summary .................................................... 501 7.5 Aerosol Particles and the Climate System .................................................................... 555 7.1 Introduction ......................................................... 503 7.5.1 Aerosol Emissions and Burdens Affected 7.1.1 Terrestrial Ecosystems and Climate .................... 503 by Climatic Factors .............................................. 555 7.1.2 Ocean Ecosystems and Climate ......................... 503 7.5.2 Indirect Effects of Aerosols on Clouds 7.1.3 Atmospheric Chemistry and Climate ...................504 and Precipitation.................................................. 559 7.1.4 Aerosol Particles and Climate ............................. 504 7.5.3 Effects of Aerosols and Clouds on Solar Radiation at the Earth’s Surface ..........................563 7.1.5 Coupling the Biogeochemical Cycles with the Climate System ...................................................504 7.5.4 Effects of Aerosols on Circulation Patterns .........564 7.2 The Changing Land Climate System .......... 504 7.6 Concluding Remarks ......................................... 566 7.2.1 Introduction to Land Climate ...............................504 Frequently Asked Question 7.2.2 Dependence of Land Processes and FAQ 7.1: Are the Increases in Atmospheric Carbon Climate on Scale ................................................. 505 Dioxide and Other Greenhouse Gases During the Box 7.1: Surface Energy and Water Balance ...................... 505 Industrial Era Caused by Human Activities? ........... 512 Box 7.2: Urban Effects on Climate .................................... 506 References ........................................................................ 568 7.2.3 Observational Basis for the Effects of Land Surface on Climate .....................................507 7.2.4 Modelling the Coupling of Vegetation, Moisture Availability, Precipitation and Surface Temperature ......................................................... 509 7.2.5 Evaluation of Models Through Intercomparison ................................................... 510 7.2.6 Linking Biophysical to Biogeochemical and Ecohydrological Components .............................511 7.3 The Carbon Cycle and the Climate System ................................................... 511 7.3.1 Overview of the Global Carbon Cycle ................. 511 7.3.2 The Contemporary Carbon Budget .....................517 7.3.3 Terrestrial Carbon Cycle Processes and Feedbacks to Climate.......................................... 526 7.3.4 Ocean Carbon Cycle Processes and Feedbacks to Climate.......................................... 528 Box 7.3: Marine Carbon Chemistry and Ocean Acidifi cation ............................................ 529 7.3.5 Coupling Between the Carbon Cycle and Climate .........................................................533 7.4 Reactive Gases and the Climate System ..................................................................... 539 7.4.1 Methane ............................................................... 539 Box 7.4: Effects of Climate Change on Air Quality ............... 540 7.4.2 Nitrogen Compounds ..........................................544 7.4.3. Molecular Hydrogen ............................................547 7.4.4 Global Tropospheric Ozone .................................547 7.4.5. The Hydroxyl Radical........................................... 550 7.4.6 Stratospheric Ozone and Climate ....................... 553 500 Chapter 7 Couplings Between Changes in the Climate System and Biogeochemistry Executive Summary between the 1990s and the fi rst fi ve years of the 21st century. Models indicate that the fraction of fossil fuel and cement emissions of CO2 taken up by the ocean will Emissions of carbon dioxide, methane, nitrous oxide and of decline if atmospheric CO2 continues to increase. reactive gases such as sulphur dioxide, nitrogen oxides, carbon monoxide and hydrocarbons, which lead to the formation • Interannual and inter-decadal variability in the growth of secondary pollutants including aerosol particles and rate of atmospheric CO2 is dominated by the response tropospheric ozone, have increased substantially in response to of the land biosphere to climate variations. Evidence human activities. As a result, biogeochemical cycles have been of decadal changes is observed in the net land carbon perturbed signifi cantly. Nonlinear interactions between the sink, with estimates of 0.3 ± 0.9, 1.0 ± 0.6, and 0.9 ± climate and biogeochemical systems could amplify (positive 0.6 GtC yr–1 for the 1980s, 1990s and 2000 to 2005 time feedbacks) or attenuate (negative feedbacks) the disturbances periods, respectively. produced by human activities. • A combination of techniques gives an estimate of the The Land Surface and Climate fl ux of CO2 to the atmosphere from land use change of 1.6 (0.5 to 2.7) GtC yr–1 for the 1990s. A revision of the • Changes in the land surface (vegetation, soils, water) Third Assessment Report (TAR) estimate for the 1980s resulting from human activities can affect regional downwards to 1.4 (0.4 to 2.3) GtC yr–1 suggests little change climate through shifts in radiation, cloudiness and surface between the 1980s and 1990s, and continuing uncertainty temperature. in the net CO2 emissions due to land use change. • Changes in vegetation cover affect surface energy and • Fires, from natural causes and human activities, release to water balances at the regional scale, from boreal to tropical the atmosphere considerable amounts of radiatively and forests. Models indicate increased boreal forest reduces photochemically active trace gases and aerosols. If fi re the effects of snow albedo and causes regional warming. frequency and extent increase with a changing climate, a Observations and models of tropical forests also show net increase in CO2 emissions is expected during this fi re effects of changing surface energy and water balance. regime shift. • The impact of land use change on the energy and water • There is yet no statistically signifi cant trend in the CO2 balance may be very signifi cant for climate at regional growth rate as a fraction of fossil fuel plus cement emissions scales over time periods of decades or longer. since routine atmospheric CO2 measurements began in 1958. This ‘airborne fraction’ has shown little variation The Carbon Cycle and Climate over this period. • Atmospheric carbon dioxide (CO2) concentration has • Ocean CO2 uptake has lowered the average ocean pH continued to increase and is now almost 100 ppm above (increased acidity) by approximately 0.1 since 1750. its pre-industrial level. The annual mean CO2 growth Consequences for marine ecosystems may include reduced rate was signifi cantly higher for the period from 2000 to calcifi cation by shell-forming organisms, and in the longer