The pitfalls of carbon offsets

Planting trees to absorb carbon dioxide seems like an ideal solution for a firm like ST Microelectronics striving to reduce its contribution to global warming. But biological carbon sequestration is an uncertain science and fraught with problems.

Reforestation brings its own troubles: it can lower water tables; planting inappropriate or exotic species can lead to pest infestations; and forests need to be managed indefinitely. More fundamentally, there are large gaps in current knowledge of how effective plants are at sequestering CO2 in the medium- to long-term, and quantifying and verifying carbon with accuracy is still a long way off.

A recent review by the Royal Society was distinctly guarded about the idea of land-based biological sequestration.1 It estimates that, taken together, terrestrial vegetation and soils contain around 2,300 gigatonnes of carbon - three times as much carbon as the atmosphere - and that they are currently absorbing around 3.2Gt, give or take 1.6Gt, of CO2 annually. This equates to some 40% of man-made emissions.

The review concludes that there is potential for this figure to be increased by a maximum of 2Gt per year by 2050, but warns that the resulting 100Gt is just 25% of the reductions in CO2 emissions required by then to avoid destabilisingly large global warming. What is more, there is "little potential for increasing the land carbon sink thereafter."

  • Temporary solution: Nearly all carbon taken up by biomass is eventually returned to the atmosphere through respiration or combustion. But the report notes that the period of storage can be maximised by selection of crops and management regimes - trees can store carbon for decades, and soils for centuries.

    Uptake of carbon is affected by external factors including temperature, nutrient availability and CO2 concentrations. Ironically, the recent rise in atmospheric CO2 combined with improved nitrogen availability from combustion emissions and fertiliser use has boosted tree growth and carbon uptake, particularly in temperate latitudes. But as temperatures rise carbon uptake is reduced, offsetting this trend.

    In fact, the report points out that some models predict that older forests could become net emitters of CO2 as the ratio of decomposing wood to new growth becomes unfavourable. This leads it to conclude that "the change in forest age structure is a more important factor than expansion in total forest area."

  • One step forward, 310 steps back: Another uncertainty surrounds the role of trace gases such as nitrous oxide (N2O), and methane which are, respectively, 310 and 21 times more potent greenhouse gases than CO2 over 100 years. Biological sequestration could have an effect on emissions of these two gases - in particular, increased planting of nitrogen-fixing plants or using nitrogenous fertilisers could lead to higher emissions of N2O. Similarly, while disturbing soils can considerably reduce their effectiveness at storing carbon, low tillage land management options could also increase N2O levels.

  • Quantification and verification: The biggest bugbear about biological carbon sequestration is the uncertainty involved. Techniques for measuring carbon uptake are primitive and not accurate. Furthermore, inventories of carbon in forests are incomplete, particularly in the developing world.

    Another area of uncertainty is the permanence of the solution - the length of time that forests or soils can keep carbon out of the atmosphere is key to their viability as storage options, yet this is still the subject of much debate.

    A better option, says the Royal Society, is to switch to renewable energy sources such as solar, wind, and biofuels.

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