Introduction

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Carbon dioxide (CO2) is without doubt the best-known anthropogenic greenhouse gas. As long ago as 1895, the Swedish Nobel laureate Svante Arrhenius saw that the increased emissions of CO2 resulting from a rapid rise in fossil fuel burning had the potential to affect global temperatures. In his landmark paper 'On the Influence of Carbonic Acid (Carbon Dioxide) in the Air upon the Temperature of the Ground' he considered the radiative effects of CO2 and water vapour on the surface temperature of the earth. Arrhenius calculated that if the concentrations of CO2 increased by 250300% compared to 1895 levels, temperatures in the Arctic could rise by 8-9°C. At the time his paper was published, such an increase in atmospheric CO2 concentrations remained theoretical. Even if global CO2 concentrations were increasing, there was no way to reliably measure such increases.

In 1958 the late Charles Keeling and his colleagues began measurements of CO2 at Mauna Loa in Hawaii. Mauna Loa is a huge mid-ocean volcano, rising to over 4000 m above sea level. The height and location of Mauna Loa make it an ideal place to monitor changes in the composition of the atmosphere. It is away from any major local sources of CO2, and its great height means

©CAB International 2007. Greenhouse Gas Sinks (eds D.S. Reay, C.N. Hewitt, K.A. Smith and J. Grace)

that CO2 concentrations measured there are representative of those across much of the northern hemisphere.

The classic data-set of climate changes obtained at Mauna Loa - the 'Keeling record' (Fig. 1.1) - has shown a rapid increase in the mixing ratio of CO2 in the atmosphere, rising from 315 ppm in 1960 to over 380 ppm in 2005. The Mauna Loa record is not just notable for the rapid increase in CO2 that it shows, but also for the sink signal it contains. Rather than increasing as a steady rate, the line zigzags up and down over each year, being highest in the northern hemisphere winter and lowest in the summer. This is the atmospheric signal of a sink - the massive uptake of CO2 by terrestrial and oceanic plants during summer in the northern hemisphere.

This zigzag provides clear evidence of the importance of such sinks in controlling the concentrations of greenhouse gases in our atmosphere, but it also shows that this control is limited. The zigzag may be obvious, but even more so is the underlying upward trend - the sinks are no longer balancing the sources.

Since the industrial revolution, concentrations of CO2 in our atmosphere have increased at an ever-faster rate, and are now 30% greater. The rapid increase in CO2 emissions observed during the last 250 years is expected to continue for several decades to

Mauna Loa Observatory, Hawaii

Mauna Loa Observatory, Hawaii

Year 21-July-04

Fig. 1.1. Monthly averaged carbon dioxide (CO2) concentration measured at Mauna Loa from 1958 to 2004. (From Keeling, C.D. and Whorf, T.P. (2004) Atmospheric CO2 concentrations derived from flask air samples at sites in the SIO network. In: Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, Tennessee.)

Year 21-July-04

Fig. 1.1. Monthly averaged carbon dioxide (CO2) concentration measured at Mauna Loa from 1958 to 2004. (From Keeling, C.D. and Whorf, T.P. (2004) Atmospheric CO2 concentrations derived from flask air samples at sites in the SIO network. In: Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, US Department of Energy, Oak Ridge, Tennessee.)

come. Various scenarios have been examined, depending on factors like fossil fuel use and efficiency. Even the best case scenario predicts further increases in CO2 emissions until at least 2040.

Many of the scenarios indicate that by the mid-21st century emissions of CO2 should at least start to level off, though some predict increases in emissions throughout this century. Although the different scenarios predict a wide range of trends in emissions, the predicted net effect on atmospheric CO2 concentrations in the future is fairly consistent. All predict a further increase in CO2 concentrations by the end of this century, with some of the scenarios predicting a doubling or even trebling of current levels. If the predicted increases in CO2 and other greenhouse gas concentrations are translated into temperature changes, a global temperature increase of 1.4-5.8°C is predicted for 2010. This compares to an increase of ~0.6°C over the last 100 years.

The large variation between predictions of the different scenarios underlines the complexity involved in making such predictions and the great uncertainty inherent in climate change models. Key to these predictions is a host of assumptions about what effect the varied feedbacks to the global carbon cycle will have. For instance, there is the possibility of a reversal from CO2 sink to source in Amazonia, due to increased drying caused by climate change. There is also the so-called CO2 fertilization effect, where increased concentrations of CO2 in the atmosphere may promote faster plant growth and result in CO2 removal from the atmosphere (see Hymus and Valentini, Chapter 2, this volume). In soils there is the positive feedback of increased temperatures resulting in higher decomposition and respiration rates, leading to greater CO2 emissions (Smith and Ineson, Chapter 4, this volume). In particular, the thawing of permafrost soils may result in greatly increased emissions of both CO2 and methane.

Even in the oceans, where the buffering capacity for increased global temperatures is much larger than on land, increased stratification arising from warming of surface waters may reduce rates of CO2 uptake by phytoplankton due to nutrient limitation (Sabine and Feely, Chapter 3, this volume).

In the following chapters we will see how the various carbon sinks and sources interact, and how the myriad of positive and negative feedbacks within the global climate system affect them now, and could affect them in the future.

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