Climate Change 2001:
Working Group I: The Scientific Basis
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Executive Summary

CO2 concentration trends and budgets

Before the Industrial Era, circa 1750, atmospheric carbon dioxide (CO2) concentration was 280 ± 10 ppm for several thousand years. It has risen continuously since then, reaching 367 ppm in 1999.

The present atmospheric CO2 concentration has not been exceeded during the past 420,000 years, and likely not during the past 20 million years. The rate of increase over the past century is unprecedented, at least during the past 20,000 years.

The present atmospheric CO2 increase is caused by anthropogenic emissions of CO2. About three-quarters of these emissions are due to fossil fuel burning. Fossil fuel burning (plus a small contribution from cement production) released on average 5.4 ± 0.3 PgC/yr during 1980 to 1989, and 6.3 ± 0.4 PgC/yr during 1990 to 1999. Land use change is responsible for the rest of the emissions.

The rate of increase of atmospheric CO2 content was 3.3 ± 0.1 PgC/yr during 1980 to 1989 and 3.2 ± 0.1 PgC/yr during 1990 to 1999. These rates are less than the emissions, because some of the emitted CO2 dissolves in the oceans, and some is taken up by terrestrial ecosystems. Individual years show different rates of increase. For example, 1992 was low (1.9 PgC/yr), and 1998 was the highest (6.0 PgC/yr) since direct measurements began in 1957. This variability is mainly caused by variations in land and ocean uptake.

Statistically, high rates of increase in atmospheric CO2 have occurred in most El Niño years, although low rates occurred during the extended El Niño of 1991 to 1994. Surface water CO2 measurements from the equatorial Pacific show that the natural source of CO2 from this region is reduced by between 0.2 and 1.0 PgC/yr during El Niño events, counter to the atmospheric increase. It is likely that the high rates of CO2 increase during most El Niño events are explained by reductions in land uptake, caused in part by the effects of high temperatures, drought and fire on terrestrial ecosystems in the tropics.

Land and ocean uptake of CO2 can now be separated using atmospheric measurements (CO2, oxygen (O2) and 13CO2). For 1980 to 1989, the ocean-atmosphere flux is estimated as -1.9 ± 0.6 PgC/yr and the land-atmosphere flux as -0.2 ± 0.7 PgC/yr based on CO2 and O2 measurements (negative signs denote net uptake). For 1990 to 1999, the ocean-atmosphere flux is estimated as -1.7 ± 0.5 PgC/yr and the land-atmosphere flux as -1.4 ± 0.7 PgC/yr. These figures are consistent with alternative budgets based on CO2 and 13CO2 measurements, and with independent estimates based on measurements of CO2 and 13CO2 in sea water. The new 1980s estimates are also consistent with the ocean-model based carbon budget of the IPCC WGI Second Assessment Report (IPCC, 1996a) (hereafter SAR). The new 1990s estimates update the budget derived using SAR methodologies for the IPCC Special Report on Land Use, Land Use Change and Forestry (IPCC, 2000a).

The net CO2 release due to land-use change during the 1980s has been estimated as 0.6 to 2.5 PgC/yr (central estimate 1.7 PgC/yr). This net CO2 release is mainly due to deforestation in the tropics. Uncertainties about land-use changes limit the accuracy of these estimates. Comparable data for the 1990s are not yet available.

The land-atmosphere flux estimated from atmospheric observations comprises the balance of net CO2 release due to land-use changes and CO2 uptake by terrestrial systems (the "residual terrestrial sink"). The residual terrestrial sink is estimated as -1.9 PgC/yr (range -3.8 to +0.3 PgC/yr) during the 1980s. It has several likely causes, including changes in land management practices and fertilisation effects of increased atmospheric CO2 and nitrogen (N) deposition, leading to increased vegetation and soil carbon.

Modelling based on atmospheric observations (inverse modelling) enables the land-atmosphere and ocean-atmosphere fluxes to be partitioned between broad latitudinal bands. The sites of anthropogenic CO2 uptake in the ocean are not resolved by inverse modelling because of the large, natural background air-sea fluxes (outgassing in the tropics and uptake in high latitudes). Estimates of the land-atmosphere flux north of 30°N during 1980 to 1989 range from -2.3 to -0.6 PgC/yr; for the tropics, -1.0 to +1.5 PgC/yr. These results imply substantial terrestrial sinks for anthropogenic CO2 in the northern extra-tropics, and in the tropics (to balance deforestation). The pattern for the 1980s persisted into the 1990s.

Terrestrial carbon inventory data indicate carbon sinks in northern and tropical forests, consistent with the results of inverse modelling.

East-west gradients of atmospheric CO2 concentration are an order of magnitude smaller than north-south gradients. Estimates of continental-scale CO2 balance are possible in principle but are poorly constrained because there are too few well-calibrated CO2 monitoring sites, especially in the interior of continents, and insufficient data on air-sea fluxes and vertical transport in the atmosphere.

The global carbon cycle and anthropogenic CO2

The global carbon cycle operates through a variety of response and feedback mechanisms. The most relevant for decade to century time-scales are listed here.

Responses of the carbon cycle to changing CO2 concentrations

Feedbacks in the carbon cycle due to climate change

Other impacts on the carbon cycle

Modelling and projection of CO2 concentration

Process-based models of oceanic and terrestrial carbon cycling have been developed, compared and tested against in situ measurements and atmospheric measurements. The following are consistent results based on several models.

Two simplified, fast models (ISAM and Bern-CC) were used to project future CO2 concentrations under IS92a and six SRES scenarios, and to project future emissions under five CO2 stabilisation scenarios. Both models represent ocean and terrestrial climate feedbacks, in a way consistent with process-based models, and allow for uncertainties in climate sensitivity and in ocean and terrestrial responses to CO2 and climate.

Implications

CO2 emissions from fossil fuel burning are virtually certain to be the dominant factor determining CO2 concentrations during the 21st century. There is scope for land-use changes to increase or decrease CO2 concentrations on this time-scale. If all of the carbon so far released by land-use changes could
be restored to the terrestrial biosphere, CO2 at the end of the century would be 40 to 70 ppm less than it would be if no such intervention had occurred. By comparison, global deforestation would add two to four times more CO2 to the atmosphere than reforestation of all cleared areas would subtract.

There is sufficient uptake capacity in the ocean to incorporate 70 to 80% of foreseeable anthropogenic CO2 emissions to the atmosphere, this process takes centuries due to the rate of ocean mixing. As a result, even several centuries after emissions occurred, about a quarter of the increase in concentration caused by these emissions is still present in the atmosphere. CO2 stabilisation at 450, 650 or 1,000 ppm would require global anthropogenic CO2 emissions to drop below 1990 levels, within a few decades, about a century, or about two centuries respectively, and continue to steadily decrease thereafter. Stabilisation requires that net anthropogenic CO2 emissions ultimately decline to the level of persistent natural land and ocean
sinks, which are expected to be small (<0.2 PgC/yr).



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