CO2 is currently the most important greenhouse gas (IPCC 2007a,b). It is involved in a complex carbon cycle (which also includes CH4), which in turn is closely connected via feedbacks to the total climate system (Houghton 2007). CO2 is a trace gas on Earth and it is present in the atmosphere in a relatively low concentration. However, this concentration has varied strongly over geological time (Doney and Schimel 2007). CO2 is implicated in virtually all of the great climate shifts in the history of our planet, including the coming and going of ice ages, warm ice-free states and the collapse of the Earth into a globally frozen state some 600 million years ago, the latter event being known as snowball earth (Hoffman and Schrag 2002; Pierrehumbert 2005). It was also involved in preserving conditions favourable to life on the young Earth, when the sun was much fainter (25-30%) than it is today (Kasting and Catling 2003). Of course, there is a lot to say about CO2 in the biogeochemical carbon cycle and its many relations to climate. For further study, the overview publications compiled by Doney and Schimel (2007), Hansen et al. (2007a), and Houghton (2007) and the articles cited by those authors can be referred to.
Immediately before the beginning of the industrial era, the atmospheric CO2 concentration amounted to about 280 ppm and varied between 260 and 280 ppm in the 1,000 of years previously (Houghton 2007). Because there is relatively little CO2 in the atmosphere to begin with, modern human activity has the prospect of doubling its concentration within the twenty-first century. A large mount of the emitted CO2 is taken up by the oceans and the vegetation, but a significant part remains within the atmosphere. State-of-the-art assessments based in part on the oxygen measurement methods developed by Ralph Keeling, the son of famous Dave Keeling, allocate about 35% of the emitted CO2 to an ocean uptake and about 15% to land sinks, mainly the large forests distributed over the continents (Sabine et al. 2004; Broecker and Kunzig 2008). Due to its long atmospheric residence time, CO2 is well mixed over the hemispheres and its concentration increase over time can be assessed to first order by determining the relevant values at one location. Today, there are approximately 100 stations worldwide where weekly air samples are collected and analysed for CO2. Keeling started the first systematic monitoring of CO2 concentrations in 1958 at the Mauna Loa observatory, Hawaii, in the middle of the Pacific and far away from any large industrial sources (Keeling et al. 1976). The resulting concentration curve has become a sort of an icon of global warming in modern times, and is also known as Keeling-curve (Broecker and Kunzig 2008). In Fig. 4, the Mauna Loa CO2 concentration is plotted, beginning in 1958 and including the most recent observed data. Atmospheric CO2 rose from about 315 ppm when the measurements started to about 385 ppm by 2007, about
38% above the pre-industrial value of 280 ppm. The present concentration is the highest, which has been recorded over the last 800,000 years (Luthi et al. 2008) and is probably also the highest over the last 20 million years (Pearson and Palmer 2000). The annual fluctuations seen on the overall rising curve are due to seasonally changes in the uptake of CO2 by vegetation mainly in the Northern hemisphere; the corresponding curve (not shown here) taken at the South Pole looks rather smooth in comparison. The inlay graph in Fig. 4 shows the CO2 increase rate at Mauna Loa, with positive values throughout and a clear tendency for higher values in later years. The subtle variations in the annual increase rates indicate the response of terrestrial and marine processes to climate variability (Doney and Schimel 2007). The increase rate in the global average concentration of atmospheric CO2 between 2000 and 2006 was 1.93 ppm year-1. This rate is the highest since the beginning of continuous monitoring in 1958 and is a significant increase over the increase rates in earlier decades; the average growth rates for the 1980s and the 1990s were 1.58 and 1.49 ppm year-1, respectively (Canadell et al. 2007). The authors also estimate that 35% of the increase in the atmospheric CO2 growth rate between 1970-1999 and 2000-2006 was caused by a decrease in the efficiency of the land and ocean sinks in removing anthropogenic CO2 (18%) and by an increase in carbon intensity of the global economy (17%). The remaining 65% was due to the increase in the global economy. Due to the still strongly increasing CO2 emissions, the concentra tion of CO2 will continue to rise in the coming decades, thus further enhancing the anthropogenic greenhouse effect. In relation to these prospects and for the treatment of other greenhouse gases, Section 4 and the most recent UN climate assessment report (IPCC 2007a,b) should be referred to.
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