CRC for Greenhouse Accounting

The Carbon Cycle

Our basic scientific understanding of the 'greenhouse effect' suggests that human activities are implicated in the observed changes in the Earth's climate. Observed changes are primarily due to human intervention in the global carbon cycle, so it is necessary to examine this cycle in some detail.

The main carbon pool is in rocks with transfers to and from the atmosphere via processes such as CO2 and CO vented from volcanoes and CO2 laid down in limestone etc, varying only on geological time scales. Fossil fuel deposits are another major carbon pool of about 16,000 Gt (Gigatonnes, i.e. 109 tonnes). These have been built up over millions of years and are currently being released through human actions at the rate of about 6 Gt C/yr. The main pool of non-geological carbon is in the ocean (39,000 Gt C), which is about 50 times more than the 750 Gt C in the atmosphere. Most of the oceanic pool is in the deep ocean and has very slow turnover rates. Another large pool is in soil organic matter (2,000 Gt C) with another 500 Gt C in terrestrial biota.

Fluxes between the atmosphere and terrestrial ecosystems and the ocean are substantial - about 60 Gt C per year for the land biota and 90 Gt C per year for the ocean. In pre-industrial times the balance between uptake by terrestrial biota and emissions back to the atmosphere through respiration, decay and burning were approximately in balance. The ocean fluxes were similarly in balance.

However, since about 1,750 small additional fluxes from the use of fossil fuels and from large-scale land clearance have added extra carbon to the atmosphere and have disrupted that balance. Some of the carbon has remained in the atmosphere while some has been taken up by terrestrial biota and the oceans. Each year approximately a third of the excess carbon from fossil use and land-clearing remains in the atmosphere; a third is taken up by the oceans and a third is retained in terrestrial ecosystems. The current best estimates from IPCC are shown in Table 1.

Inputs	Gt C per year	Sinks	Gt C per year

Fossil Fuels	6.3	Remains in atmosphere	3.3

Land Use	2.0	Oceans activities	2.3

		Terrestrial ecosystems	2.7

	8.3		8.3

(Table 1. The global carbon budget in the 1990s)

It is critically important that terrestrial fluxes are included in monitoring the carbon cycle. They contribute 25% of the input of carbon to the atmosphere and 30% of eventual removals of carbon and thus play a major role in the carbon cycle. They represent opportunities for reducing greenhouse gas emissions by modifying land use activities and for enhancing uptake of carbon from the atmosphere.

Even if we were to decide not to allow sinks a role in compliance with the Kyoto Protocol, failure to monitor these fluxes will leave significant unknowns in our efforts to reduce greenhouse gases in the atmosphere. These unmeasured fluxes are far larger than the targets in greenhouse gas reduction set out in the Kyoto Protocol and variations in them may exceed our efforts towards compliance through other means.

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