7

Couplings Between Changes in theClimate 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. Krcher (Germany), M. Kawamiya (Japan), K. Lassey (New Zealand), C. Le Qur (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. Rdenbeck (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.

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Couplings Between Changes in the Climate System and Biogeochemistry Chapter 7

Table of Contents

Executive Summary .................................................... 501

7.1 Introduction ......................................................... 5037.1.1 Terrestrial Ecosystems and Climate .................... 503

7.1.2 Ocean Ecosystems and Climate ......................... 503

7.1.3 Atmospheric Chemistry and Climate ................... 504

7.1.4 Aerosol Particles and Climate ............................. 504

7.1.5 Coupling the Biogeochemical Cycles with the Climate System ................................................... 504

7.2 The Changing Land Climate System .......... 5047.2.1 Introduction to Land Climate ............................... 504

7.2.2 Dependence of Land Processes and Climate on Scale ................................................. 505

Box 7.1: Surface Energy and Water Balance ...................... 505

Box 7.2: Urban Effects on Climate .................................... 506

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

7.5 Aerosol Particles and the Climate System .................................................................... 555

7.5.1 Aerosol Emissions and Burdens Affected by Climatic Factors .............................................. 555

7.5.2 Indirect Effects of Aerosols on Clouds and Precipitation.................................................. 559

7.5.3 Effects of Aerosols and Clouds on Solar Radiation at the Earths Surface .......................... 563

7.5.4 Effects of Aerosols on Circulation Patterns ......... 564

7.6 Concluding Remarks ......................................... 566

Frequently Asked QuestionFAQ 7.1: Are the Increases in Atmospheric Carbon Dioxide and Other Greenhouse Gases During the Industrial Era Caused by Human Activities? ........... 512

References ........................................................................ 568

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Chapter 7 Couplings Between Changes in the Climate System and Biogeochemistry

Executive Summary

Emissions of carbon dioxide, methane, nitrous oxide and of reactive gases such as sulphur dioxide, nitrogen oxides, carbon monoxide and hydrocarbons, which lead to the formation of secondary pollutants including aerosol particles and tropospheric ozone, have increased substantially in response to human activities. As a result, biogeochemical cycles have been perturbed signifi cantly. Nonlinear interactions between the climate and biogeochemical systems could amplify (positive feedbacks) or attenuate (negative feedbacks) the disturbances produced by human activities.

The Land Surface and Climate

Changes in the land surface (vegetation, soils, water) resulting from human activities can affect regional climate through shifts in radiation, cloudiness and surface temperature.

Changes in vegetation cover affect surface energy and water balances at the regional scale, from boreal to tropical forests. Models indicate increased boreal forest reduces the effects of snow albedo and causes regional warming. Observations and models of tropical forests also show effects of changing surface energy and water balance.

The impact of land use change on the energy and water balance may be very signifi cant for climate at regional scales over time periods of decades or longer.

The Carbon Cycle and Climate

Atmospheric carbon dioxide (CO2) concentration has continued to increase and is now almost 100 ppm above its pre-industrial level. The annual mean CO2 growth rate was signifi cantly higher for the period from 2000 to 2005 (4.1 0.1 GtC yr1) than it was in the 1990s (3.2 0.1 GtC yr1). Annual emissions of CO2 from fossil fuel burning and cement production increased from a mean of 6.4 0.4 GtC yr1 in the 1990s to 7.2 0.3 GtC yr1 for 2000 to 2005.1

Carbon dioxide cycles between the atmosphere, oceans and land biosphere. Its removal from the atmosphere involves a range of processes with different time scales. About 50% of a CO2 increase will be removed from the atmosphere within 30 years, and a further 30% will be removed within a few centuries. The remaining 20% may stay in the atmosphere for many thousands of years.

Improved estimates of ocean uptake of CO2 suggest little change in the ocean carbon sink of 2.2 0.5 GtC yr1

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 decline if atmospheric CO2 continues to increase.

Interannual and inter-decadal variability in the growth rate of atmospheric CO2 is dominated by the response of the land biosphere to climate variations. Evidence of decadal changes is observed in the net land carbon sink, with estimates of 0.3 0.9, 1.0 0.6, and 0.9 0.6 GtC yr1 for the 1980s, 1990s and 2000 to 2005 time periods, respectively.

A combination of techniques gives an estimate of the fl ux of CO2 to the atmosphere from land use change of 1.6 (0.5 to 2.7) GtC yr1 for the 1990s. A revision of the Third Assessment Report (TAR) estimate for the 1980s downwards to 1.4 (0.4 to 2.3) GtC yr1 suggests little change between the 1980s and 1990s, and continui