Increased Atmospheric CO2 Levels

Year ESTIMATED CHANGE IN THE GLOBAL MEAN SURFACE AIR TEMPERATURE (°C) 

Before closing this section on the consequences of a potential change in atmospheric CO2 and trace gas concentrations, a word of caution is necessary. The release of CO2 to the atmosphere by human activities is indeed a great Geophysical Experiment (Revelle and Suess, 1957), and as such the consequences 

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Tropical forests and savannas are the primary source of C emissions that originate from biomass burning (73, 75). However, temperate forests are also sources of atmospheric carbon. Harmon et al. (77) reported that conversion of primary temperate forests to younger, second-growth forests lead to increases in atmospheric CO2 levels, due to losses in long-term carbon storage within these forests. They ascertained that timber exploitation of 5 million hectares of primaiy forests in the Pacific Northwest of North America during the past century has resulted in the addition of 1,500 Tg of C to the atmosphere. 

Based on the results of this modeling effort, the global coastal ocean appears to have acted as a net source of CO2 to the atmosphere over the past 300 y due to the combined production of CO2 from biocalcification and an overall state of net heterotrophy. This role is expected to switch to that of a net CO2 sink in response to rising atmospheric CO2 levels and increasing nutrient runoff from land. The latter is stimulating new production and causing an overall state of net autotrophy. Recent observational... 

Figure 8-18. Dependence of net CO2 uptake on external C02 level for leaves of representative C3 and C4 plants. C3 plants require a higher Noo2at tlie 02 compensation point (Jco2= 0) and for C02 saturation than C4 plants. We note that because photosynthesis for C4 plants is already nearly saturated at current atmospheric CO2 levels, higher C02 levels generally will not substantially enhance their photosynthetic rates, whereas the increasing atmospheric C02 levels will progressively increase net C02 uptake for C3 plants.

As is now well known, the significance of increased atmospheric CO2 concentration is that CO2 is a key absorber of thermal infrared radiation in the atmosphere, and a large increase in global CO2 levels could result in global temperature rise (Woodwell, 1978 Niehaus, 1979). 

A perusal of the literature suggests that carbon sequestration by iron fertilization is not a panacea for the anthropogenic carbon emissions that increased atmospheric CO2 from 280 p,atm at the start of the industrial revolution to 369 p,atm by December 2000 (Keeling and Whorf, 2001). Ocean models suggest that enhanced ocean uptake of carbon with iron fertilization of the Antarctic Ocean will at best draw down atmospheric CO2 by 70 xatm if carried out continuously for a century (Peng and Broecker, 1991a,b) and damp the annual anthropogenic CO2 input to the atmosphere by less than 30% of current annual emission levels (Joos et al, 1991). 

Berendse F., van Breeman N., Rydin H., Buttler A., Heijmans M., Hoosbeek M. R., Lee J. A., Mitchell E., Saarinen T., Vasander H., and Wallen B. (2001) Raised atmospheric CO2 levels and increased N deposition cause shifts in plant species composition and production in Sphagnum bogs. Global Change Biol. 7, 591—598. 

Since all these gas emissions are byproducts of organic matter decomposition they are natural sources, but the magnitude of these fluxes may be altered by human intervention both directly, via destruction of intertidal sediments, and also indirectly, via increases in atmospheric CO2 levels (Dacey, Drake Klug, 1994) or changes in nitrate fluxes (Malcolm, this volume). Thus while intertidal sediments are very sensitive to climate change, they are not wholly passive since they can themselves influence... 

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