Geochemical cycles slipping into reverse

Water, nitrogen, carbon and phosphorus cycling has changed more rapidly in the second half of the 20th century than at any time in recorded human history, according to the authors of a UN report assessing the consequences of ecosystem change for human wellbeing.

The Millennium Ecosystem Assessment was commissioned by UN secretary-general Kofi Annan in 2000. Academics and environmentalists worldwide have now delivered what Annan claims is the first comprehensive global evaluation of the world’s major ecosystems.

Increased yields of crop production systems have reduced the pressure to convert natural ecosystems into cropland. But intensification has increased pressure on inland water ecosystems, reduced biodiversity in agricultural landscapes, and called on higher energy inputs in the form of mechanisation and the production of chemical fertilisers, notes the report put together by over 1000 experts.

’The capacity of ecosystems to provide services [such as water, fuel, biochemicals and pharmaceuticals] derives directly from the operation of natural biogeochemical cycles that in some cases have been significantly modified,’ write the authors.

The effect of changes in terrestrial ecosystems on the carbon cycle has reversed over the past 50 years: terrestrial ecosystems were a net source of CO₂ in the 19th century and became a net sink around the middle of the 20th century. The atmospheric concentration of CO₂ increased by about 34 per cent in 250 years.

The role of ecosystems in carbon sequestration has grown thanks to reforestation and forest management, as well as changed agriculture practices and the fertilising effects of nitrogen deposition and increasing atmospheric CO₂.

The total amount of reactive, anthropogenic nitrogen increased ninefold between 1890 and 1990 as a result of increased fertiliser use. Of all the synthetic nitrogen produced, since its first production in 1913, more than half was used in the past 20 years.

The use of phosphorus fertilisers and the rate of phosphorus accumulation in agricultural soils increased nearly threefold between 1960 and 1990.

’Once an ecosystem has undergone a nonlinear change, recovery to the original state may take decades or centuries and may sometimes be impossible,’ fear the report’s authors.

Bea Perks