Letter: Balance upset

From David Powlson, Institute of Arable Crops Research

In his letter, Alan Herod points out important facts about the global carbon
cycle and climate change—but draws dangerously erroneous conclusions
(28 April, p 53).

In the absence of large-scale human interference or natural environmental
change, movements of carbon between the atmosphere and land surface are roughly
in balance. Roughly 120 gigatonnes of carbon per year are fixed by vegetation
through photosynthesis, about half of which is rapidly returned from plants back
to the atmosphere and half enters the soil. In the long term, again in the
absence of external interference, soil moves towards an equilibrium carbon
content, with inputs from plants and outputs as CO₂ from the activity
of soil organisms being approximately equal at 60 gigatonnes of carbon per
year.

Unfortunately, two factors have disrupted this happy
equilibrium—deforestation and the insertion into the atmosphere of fossil
carbon from coal, oil and natural gas that has been outside the biological cycle
for millions of years. In addition, there has in the past been some release of
carbon from soil caused by cultivating grasslands such as the North American
prairies.

The logical approach is to tackle the factors causing the disequlibrium, by
slowing deforestation and aiming for maximum replacement of fossil fuels by
renewables. In addition, it makes sense to adopt practices that increase the
amount of carbon held in soils and vegetation. The Intergovernmental Panel on
Climate Change estimates that there is potential for increasing land sinks by
about 100 gigatonnes in the next 50 years. This could be achieved by a
combination of reforestation, slowing deforestation and changes in agricultural
management.

All of these approaches are valuable because, given the political will and
financial incentives, they can start immediately and will deliver a range of
additional environmental benefits. But they are not alternatives to cutting
fossil fuel emissions. Soil and vegetation sinks will reach a new equilibrium
value after 50 to 100 years and then cause no further net removal of
CO₂ from the atmosphere.

Harpenden, Hertfordshire