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Unformatted text preview: Balancing the Global Carbon Budget R.A. Houghton The Woods Hole Research Center, Falmouth, Massachusetts 02540; email: [email protected] Annu. Rev. Earth Planet. Sci. 2007. 35:313–47 First published online as a Review in Advance on January 16, 2007 The Annual Review of Earth and Planetary Sciences is online at earth.annualreviews.org This article’s doi: 10.1146/annurev.earth.35.031306.140057 Copyright c ° 2007 by Annual Reviews. All rights reserved 0084-6597/07/0530-0313$20.00 Key Words carbon sinks, climate feedbacks, CO 2 , fossil fuels, oceans, terrestrial ecosystems Abstract The global carbon budget is, of course, balanced. The conservation of carbon and the first law of thermodynamics are intact. “Balancing the carbon budget” refers to the state of the science in evaluating the terms of the global carbon equation. The annual increases in the amount of carbon in the atmosphere, oceans, and land should balance the emissions of carbon from fossil fuels and deforestation. Balanc- ing the carbon budget is not the real issue, however. The real issue is understanding the processes responsible for net sources and sinks of carbon. Such understanding should lead to more accurate predic- tions of future concentrations of CO 2 and more accurate predictions of the rate and extent of climatic change. The recent past may be in- sufficient for prediction, however. Oceanic and terrestrial sinks that have lessened the rate of growth in atmospheric CO 2 until now may diminish as feedbacks between the carbon cycle and climate become more prominent. 313 A n n u . R e v . E a r t h P l a n e t . S c i . 2 7 . 3 5 : 3 1 3- 3 4 7 . D o w n l o a d e d f r o m a r j o u r n a l s . a n n u a l r e v i e w s . o r g b y U n i v e r s i t y o f C a l i f o r n i a- B e r k e l e y o n 4 / 3 / 7 . F o r p e r s o n a l u s e o n l y . 1. INTRODUCTION As of June 1, 2006, 189 nations, including the United States, adopted the United Nations Framework Convention on Climate Change (UNFCCC), which has as its objective “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” The global carbon cycle is critical to this objective because its processes define how emissions of carbon dioxide (CO 2 ) from anthropogenic activity translate into con- centrations of CO 2 in the atmosphere. How much of the carbon emitted to the atmosphere remains there, and how much of the emissions are taken up by terrestrial ecosystems and by the world’s oceans? Answers to these questions will provide at least a part of the scientific understanding necessary for establishing the amount and rate of CO 2 emissions that would meet a “safe” concentration....
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