How much CO2 can nature digest ?

Note from EFN :

For natural CO2 sequestration, the key figure (see below) is not what ecosystems currently sequester every year, rather it is the amount that is mixed each year between the surface layer of oceans and the deep oceans. The upper ocean layer (in CO2 equilibrium with the atmosphere) can mix about 1.7 Gtonnes ±0.5 Gtonnes of Carbon per year with the deep ocean (IPCC 2001).

This is the 'rate limiting step'.

Atmosphere <-> Surface Layer of Oceans <-> Deep Oceans -> Sediment

Fast            (Small Vol)      1.7GtC/y (Large Vol)        0.2GtC/y

In the short term the average amount per inhabitant that the planet - ocean and vegetation - can digest without harming the climate is about 12 billion tons of CO2 i.e. the amount absorbed by the ecosystems every year. If shared equally by 6 billion inhabitants this gives 2 tons per inhabitant, about half of which is for collective use and the other half for individual use, therefore, in the short term, emissions should be limited to an average of 1 ton per inhabitant per year (1000 kgs of CO2).

In the longer term the key factor is the amount of carbon that can be sequestrated in the bottom of the oceans (see below), the figure is then smaller at 1.7 billion tons of CO2 (IPCC 2001, or Socolow & Pascala), i.e. 280 kgs of CO2 maximum per person and per year (only half of which is for individual use and the other half for collective/community use). The limitation is then 140 kgs of CO2 for individual emissions (per year).

In the very long term (when it comes to seeing how much sediments are formed in the bottom of the oceans), this figure (for individual emissions) comes down to only 10 kgs per capita/year (20 kilos per person taking in consideration the community/collective use).

See details in text below. See also : IPCC 2001, or Socolow & Pascala.

Reviewing the Impact of Increased Atmospheric CO2 on Oceanic pH and the Marine Ecosystem

C. Turley, J.C. Blackford, S. Widdicombe, D. Lowe, P.D. Nightingale and A.P. Rees
Plymouth Marine Laboratory, Prospect Place, Plymouth



There has been an increase in atmospheric carbon dioxide from 280 ppm in AD1800 to 380 ppm at the present day. This increase is due to a supply of anthropogenic CO2 to the atmosphere which is currently estimated at 7 GtC /yr [4]. The observed annual increase in atmospheric CO2 represents 3.2 GtC /yr, the balance being removed from the atmosphere and taken up by the oceans and land. There is now generally good agreement that the ocean absorbs 1.7 ± 0.5 GtC /yr [4]. Note that the rate-limiting step in the long-term oceanic uptake of anthropogenic CO2 is not air-sea gas exchange, but the mixing of the surface waters with the deep ocean [7]. Whilst the ocean can theoretically absorb 70-80% of the projected production of anthropogenic CO2, it would take many centuries to do so [8].

There is also a large natural annual flux of CO2 between the ocean and the atmosphere of almost 90 GtC /yr that, pre-1800, was believed to be almost in balance. This huge influx and efflux is due to a combination of marine productivity and particle sinking (the biological pump) and ocean circulation and mixing (the solubility pump). Phytoplankton growth consumes dissolved inorganic carbon (DIC) in the surface seawater causing an undersaturation of dissolved CO2 and uptake from the atmosphere.

The re-equilibration time for CO2 is slow (typically several months) due to the dissociation of CO2 in seawater (see below). Ocean circulation also results in air-sea exchange of CO2 as the solubility of CO2 is temperature dependent. Warming decreases the solubility of CO2 and promotes a net transfer of CO2 to the atmosphere, whereas cooling results in a flux from the atmosphere to the ocean. Anthropogenic CO2 modifies the flux from the solubility pump as CO2 availability does not normally limit biological productivity in the world's oceans.

However, the observation that the net oceanic uptake of anthropogenic CO2 is only about 2% of the total CO2 cycled annually across the air-sea interface ought to be of major concern. The significant perturbations arising from this small change in flux imply that the system is extremely sensitive. Any resulting changes in the biogeochemistry of the mixed layer could have a major impact on the magnitude (or even sign) of the total CO2 flux and hence on the Earth's climate [9].