The carbon cycle

Over millennia, the amount of carbon dioxide in the atmosphere is determined by the temperature and chemical properties of the ocean. But for shorter periods of decades to centuries, equilibration between the atmosphere and the oceans can be incomplete as re-adjustment to changes in one reservoir or the other is slow. For example, each year photosynthesis by the world’s living plants absorbs from the atmosphere around 50,000 million tonnes (50 Gt) of carbon⁵ , replacing most of it by respiration during that same year. This causes seasonal variations in the amount of carbon dioxide in the atmosphere observed in records of atmospheric carbon dioxide, particularly in the Northern Hemisphere. From year to year, climate fluctuations, such as those caused by the familiar El Nino phenomenon, cause slight imbalances that lead to variations in the amount of carbon dioxide in the atmosphere. During the last ice age, as a result of changed biological conditions (half the Earth was covered with ice, sea levels were 80 metres lower than at present and ecosystems significantly different), cooler ocean temperatures and changed ocean circulation, carbon dioxide concentrations were about 40% lower than they were pre-industrially.

But over the last century or so humans have been adding carbon dioxide to the atmosphere, today at a rate of around 8 Gt of carbon per year, primarily as a result of the combustion of fossil fuels, coal, oil and natural gas. The oceans are currently only capable of removing about 2 Gt of carbon per year from the atmosphere into the deep ocean. Thus, as a result, concentrations in the atmosphere are rising steadily. Since industrialisation concentrations have increased by about 32% and each year are increasing at a little less than 1%.

Small imbalances in the seasonal, year-to-year and decade-long exchange of carbon dioxide from the combustion of fossil fuels, activities of the global biosphere, and dissolution and transport into the deep oceans leads to both temporal and spatial variations of atmospheric carbon dioxide concentration. These variations are small, but observations at several high-precision observatories around the world are used to identify the behaviour of the carbon cycle. One such observatory is at Cape Grim, North West Tasmania (see Figure 3).

Combined with measurements of the isotopic composition of the carbon and atmospheric oxygen these observations enable us to complete a picture of the past (Figure 4) and current (Table 3) global carbon budget. One surprising finding has been that, despite deforestation around the world, which must release additional carbon dioxide into the atmosphere, the global biosphere has actually grown over the past 20 years or so, partially offsetting the release of carbon from the burning of fuels and thus slowing the rate of concentrations increase from what it might otherwise have been.

Figure 3: Annual mean carbon dioxide concentration measured at Cape Grim, Tasmania , (http://www.bom.gov.au/inside/cgbaps/ (Paul Fraser, Personal Communication).

Figure 4: Global carbon emissions (left) and uptakes (right) since 1800 (in thousand million tonnes, Gt, of carbon).

Table 3: Global budget of carbon dioxide showing the rate of accumulation in the atmosphere and the main annual exchanges between the atmosphere, oceans and biosphere in thousand million tonnes of carbon per year (Gt yr-1). (After IPCC 2001).

The IPCC Fourth Assessment Report (IPCC 2007a) concluded that over the coming decades, the capacity of both the oceans and the biosphere to absorb carbon dioxide from the atmosphere is likely to decline. It was somewhat of a surprise to some scientists that this year, observational evidence was found to show that oceanic uptake has been declining (as a proportion of emitted fossil carbon dioxide) through the past 2 decades (see Canadell et al. 2007: Le Quere et al. 2007).

The combustion of carbon-based fuels that release carbon dioxide into the atmosphere is the way humans around the world generate energy to underpin their economic and social wellbeing. It includes the energy required for the heating and cooling of the houses, businesses and industry; the energy required for transportation for ourselves and products that we trade; and the energy required to power our production and manufacturing industries. The relative level of each of these activities is different for each economy around the world. Figure 5 shows the breakdown of energy production, in terms of greenhouse gases emitted, for the Australian economy.

Each greenhouse gas has a different greenhouse effect, first because its molecules absorb more or less heat and second because they stay in the atmosphere for longer or shorter periods (eg. carbon dioxide for about 80 years and methane for about 10 years). Thus, when adding up the effects of all gases, the minor gases are added according to their impacts as an equivalent amount of carbon dioxide.

Figure 5 shows that while stationary energy (that produced by the major power stations around the country) is the dominant source of emissions, other sources are important. In total Australians emit 550 million tonnes of carbon dioxide equivalent each year to produce their energy needs. This is the average for all Australians, and converts to each Australian annually emitting the equivalent of about 28 tonnes of carbon dioxide (or about 7 tonnes of carbon); one of the highest, if not the highest, rates in the world.

Figure 5: Mega tonnes of carbon dioxide equivalents6 emitted by Australia based on the Australian Greenhouse gas inventory. (see http://www.greenhouse.gov.au/inventory/2003/pubs/inventory2003.pdf).