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
- Uptake of anthropogenic CO2 by the ocean is primarily governed
by ocean circulation and carbonate chemistry. So long as atmospheric CO2
concentration is increasing there is net uptake of carbon by the ocean, driven
by the atmosphere-ocean difference in partial pressure of CO2.
The fraction of anthropogenic CO2 that is taken up by the ocean
declines with increasing CO2 concentration, due to reduced buffer
capacity of the carbonate system. The fraction taken up by the ocean also
declines with the rate of increase of atmospheric CO2, because
the rate of mixing between deep water and surface water limits CO2
- Increasing atmospheric CO2 has no significant fertilisation effect
on marine biological productivity, but it decreases pH. Over a century, changes
in marine biology brought about by changes in calcification at low pH could
increase the ocean uptake of CO2 by a few percentage points.
- Terrestrial uptake of CO2 is governed by net biome production
(NBP), which is the balance of net primary production (NPP) and carbon losses
due to heterotrophic respiration (decomposition and herbivory) and fire, including
the fate of harvested biomass. NPP increases when atmospheric CO2
concentration is increased above present levels (the “fertilisation” effect
occurs directly through enhanced photo-synthesis, and indirectly through effects
such as increased water use efficiency). At high CO2 concentration
(800 to 1,000 ppm) any further direct CO2 fertilisation effect
is likely to be small. The effectiveness of terrestrial uptake as a carbon
sink depends on the transfer of carbon to forms with long residence times
(wood or modified soil organic matter). Management practices can enhance the
carbon sink because of the inertia of these “slow” carbon pools.
Feedbacks in the carbon cycle due to climate change
- Warming reduces the solubility of CO2 and therefore reduces uptake
of CO2 by the ocean.
- Increased vertical stratification in the ocean is likely to accompany increasing
global temperature. The likely consequences include reduced outgassing of
upwelled CO2, reduced transport of excess carbon to the deep ocean,
and changes in biological productivity.
- On short time-scales, warming increases the rate of heterotrophic respiration
on land, but the extent to which this effect can alter land-atmosphere fluxes
over longer time-scales is not yet clear. Warming, and regional changes in
precipitation patterns and cloudiness, are also likely to bring about changes
in terrestrial ecosystem structure, geographic distribution and primary production.
The net effect of climate on NBP depends on regional patterns of climate change.
Other impacts on the carbon cycle
- Changes in management practices are very likely to have significant effects
on the terrestrial carbon cycle. In addition to deforestation and afforestation/reforestation,
more subtle management effects can be important. For example, fire suppression
(e.g., in savannas) reduces CO2 emissions from burning, and encourages
woody plant biomass to increase. On agricultural lands, some of the soil carbon
lost when land was cleared and tilled can be regained through adoption of
- Anthropogenic N deposition is increasing terrestrial NPP in some regions;
excess tropospheric ozone (O3) is likely to be reducing NPP.
- Anthropogenic inputs of nutrients to the oceans by rivers and atmospheric
dust may influence marine biological productivity, although such effects are
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.
- Modelled ocean-atmosphere flux during 1980 to 1989 was in the range -1.5
to -2.2 PgC/yr for the 1980s, consistent with earlier model estimates and
consistent with the atmospheric budget.
- Modelled land-atmosphere flux during 1980 to 1989 was in the range -0.3
to -1.5 PgC/yr, consistent with or slightly more negative than the land-atmosphere
flux as indicated by the atmospheric budget. CO2 fertilisation
and anthropogenic N deposition effects contributed significantly: their combined
effect was estimated as -1.5 to -3.1 PgC/yr. Effects of climate change during
the 1980s were small, and of uncertain sign.
- In future projections with ocean models, driven by CO2 concentrations
derived from the IS92a scenario (for illustration and comparison with earlier
work), ocean uptake becomes progressively larger towards the end of the century,
but represents a smaller fraction of emissions than today. When climate change
feedbacks are included, ocean uptake becomes less in all models, when compared
with the situation without climate feedbacks.
- In analogous projections with terrestrial models, the rate of uptake by
the land due to CO2 fertilisation increases until mid-century,
but the models project smaller increases, or no increase, after that time.
When climate change feedbacks are included, land uptake becomes less in all
models, when compared with the situation without climate feedbacks. Some models
have shown a rapid decline in carbon uptake after the mid-century.
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.
- The reference case projections (which include climate feedbacks) of both
models under IS92a are, by coincidence, close to those made in the SAR (which
- The SRES scenarios lead to divergent CO2 concentration trajectories.
Among the six emissions scenarios considered, the projected range of CO2
concentrations at the end of the century is 550 to 970 ppm (ISAM model) or
540 to 960 ppm (Bern-CC model).
- Variations in climate sensitivity and ocean and terrestrial model responses
add at least -10 to +30% uncertainty to these values, and to the emissions
implied by the stabilisation scenarios.
- The net effect of land and ocean climate feedbacks is always to increase
projected atmospheric CO2 concentrations. This is equivalent to
reducing the allowable emissions for stabilisation at any one CO2
- New studies with general circulation models including interactive land and
ocean carbon cycle components also indicate that climate feedbacks have the
potential to increase atmospheric CO2 but with large uncertainty
about the magnitude of the terrestrial biosphere feedback.
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
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).