The Net Carbon Balance and Anthropogenic Emissions

The importance of CO2 balance assessment is related to the proven capacity of this gas to cause global warming and climate change. Carbon dioxide is the greenhouse gas most responsible for global warming by virtue of its atmospheric residence time of 50-200 years.

Although natural transfers of carbon dioxide are approximately 20 times greater than those due to human activity, natural transfers balance, the magnitude of carbon sources closely matching that of carbon sinks. Before the Industrial Era, atmospheric carbon dioxide concentration was stable at 280 ppm for several thousand years. It has since risen continuously, reaching 367 ppm in 1999. The present atmospheric concentration of CO2 is the highest for 420 000 years, and probably for 20 million years. Its rate of increase in the last 100 years is unprecedented, at least in the last 20 000 years. This is known from many well-replicated measurements of the composition of air bubbles trapped in Antartic ice.

The present increase is caused by anthropogenic emissions of CO2. About three-quarters of these emissions are due to the use of fossil fuels. Fossil fuel combustion plus a small contribution from cement production released an average of 5.4Pgyr_1 (Petagrams = 1015g) of carbon between 1980 and 1989, and 6.3Pgyr_1 from 1990 to 1999.

Several additional lines of evidence confirm that the recent continuing increase in atmospheric CO2 content is caused by anthropogenic CO2 emissions. In the IPCC report 'Climate Change 2001: A Scientific Basis', three explanations are expounded. First, atmospheric O2 is declining at a rate that matches fossil fuel emissions of CO2 (combustion consumes O2). Second, the characteristic isotopic signatures of fossil fuel (its lack of 14C, and depleted 13C content) leave their mark in the atmosphere. Third, the increase in observed CO2 concentration has been faster in the Northern Hemisphere, where most fossil fuel is burned. However, atmospheric CO2 is only increasing at about half the rate of fossil fuel emissions. The increase in atmospheric CO2 content was 3.3 Pgyr~ between 1980 and 1989 and 3.2 Pgyr-1 between 1990 and 1999. The rest of the CO2 emitted either dissolves in seawater and mixes into the deep ocean, or is taken up by terrestrial ecosystems. Uptake by terrestrial ecosystems is due to an excess of primary production

(photosynthesis) over respiration and other oxidative processes (decomposition or combustion oforganic material). Terrestrial systems are also an anthropogenic source of CO2 when changes in land use (particularly deforestation) lead to loss of carbon from plants and soils. Nonetheless, the global balance in terrestrial systems is currently a net uptake of CO2.

The human influence on the fluxes ofcarbon between the three 'reservoirs' (atmosphere, ocean, and terrestrial biosphere) represents a small but significant perturbation of a huge global cycle (Figure 1).

Changes in climate induced by CO2 rise have a significant effect on carbon cycle feedback mechanisms. Increases in atmospheric CO2 concentration increase plant photosynthesis and the amount of carbon stored in vegetation. However, increases in temperature also lead to increases in plant and soil respiration rates, which tend to reduce the size of the terrestrial carbon store. In some regions, the climate changes (such as decreased rainfall) can also reduce plant photosynthesis and reduce the ability of vegetation to sequester carbon. The fraction of anthropogenic CO2 taken up by the ocean declines with increasing CO2 concentration, due to reduced buffer capacity of the carbonate system. Furthermore, warming reduces the solubility of CO2 and therefore reduces uptake of CO2 by the oceans.

In general, carbon cycle can be defined as an open cycle because nature can exclude some carbon from the cycle by fossilizing it. Human activity reintroduces carbon in the cycle and, for this reason, a net flow of almost 3Pgyr_1 occurs in the atmosphere. The accumulation is due to the fact that the natural sinks (oceans and land) are not able to absorb all the carbon emitted by men, even if they absorb more and more carbon every year. This leads to ecological implications and changes in terms ofphoto-synthesis, respiration, primary production (biological processes) and carbonate solution, precipitation, and deposition (abiotic processes) whose ultimate effects are uncertain.

Figure 1 Carbon exchange among three reservoirs (atmosphere, land, and ocean): present condition (data expressed in Pgyr-1). Future perspectives are uncertain due to uncertainties in assessing the real contribution of sources and sinks and predicting climate forcing. Atmospheric carbon flow estimations may vary by ±6%; land-use change by 50%; ocean uptake by 30%.

Figure 1 Carbon exchange among three reservoirs (atmosphere, land, and ocean): present condition (data expressed in Pgyr-1). Future perspectives are uncertain due to uncertainties in assessing the real contribution of sources and sinks and predicting climate forcing. Atmospheric carbon flow estimations may vary by ±6%; land-use change by 50%; ocean uptake by 30%.

Different instruments and ecological indicators are used in order to both examine the trends in carbon balance in the past and hypothesize future scenarios. One of the main instruments adopted today is the analysis of satellite images (remote sensing analysis) which is used in order to monitor, compare, and predict land use, deforestation, and forest migrations and the correspondent carbon emission/absorption. Many other elements are monitored as indicators of the dynamics of carbon balance on Earth and, consequently, of climate change: net primary production, vegetation patterns, pollen data, phytoplankton concentration, carbonate dynamics, as well as animal behavior, snowmelt and ice formation, lightning activity, because these are all phenomena affected by changing environmental conditions mostly induced by modern carbon (un)balance.

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment