Livestock farmers in KwaZulu Natal have witnessed an odd change in the landscape there which, until recently, has gone unexplained. Since the 1950s they've noticed how thickets of trees have steadily encroached on the valuable grazing land provided by traditional savannah in the area.

For years the change was thought to be the work of altered fire regimes or grazing mismanagement.

Recently the National Botanical Institute and the University of Cape Town's botany department have come up with a more likely explanation: increasing atmospheric carbon dioxide (CO2) due to the burning of fossil fuels is "fertilising" trees, causing them to encroach on the grassland.

Researchers at these institutions grew a series of indigenous trees in conditions of varied atmospheric CO2 concentrations to simulate conditions experienced by plants during the past 12 000 years.

Since the previous glacial period, ending 12 000 years ago, atmospheric CO2  has risen from an estimated 180 parts per million (ppm) to 270 ppm in the pre-industrial era, jumping to 370 ppm in just 200 years since the mass burning of fossil fuels began.

Results showed that under pre-industrial conditions, trees would have grown very slowly, without the materials needed for building their carbon-rich architecture. As CO2  levels in the experiments were increased, the trees grew faster and more robustly above and below ground.

Grape vines can be expected to respond with similar vigour to increasing CO2. Another study on the impact of global warming on wine, published in the Journal of Wine Research, speculated that increased vigour might not translate to increased quality in the grape (and hence wine). Referring to experiments done on certain strains of wheat, the study reported that wheat grown under current CO2 conditions showed reduced protein content, irregular granule size and poorer baking properties compared with wheat grown under pre-industrial CO2 levels.

Furthermore, as a study by Stanford University suggests, other environmental changes may work in conjunction with increasing atmospheric CO2 to stunt plant growth: "higher temperatures, increased precipitation or increased nitrogen deposits in the soil" from the burning of fossil fuels will combine negatively for the success of such plants.

Meanwhile another phenomenon has been recorded:  CO2 levels have risen more sharply than usual in the past two years (by 2 ppm per year instead of the usual 1.5 ppm), without a relative increase in fossil fuel burning or a natural process to explain the rise. Dr David King, chief scientist for the British government, expressed concern that this could indicate that the natural  CO2 absorption systems may have become saturated. }

Oceans and forests, for instance, absorb atmospheric  CO2 as part of the carbon cycle. If they have reached their capacity for absorption, this could spark runaway global climate change, the Guardian reported earlier this week.

A great deal of uncertainty remains, but what we do know is this: by 2050 - 46 years from now - the Western Cape can expect to experience a 550 ppm atmospheric CO2 concentration (significantly higher than the current 370 ppm), temperatures will climb by about 0.5°C to 1°C and critical winter rainfall will drop by about 25% along with a change in the seasons in which it falls.

Leonie Joubert is a freelance science journalist with a special interest in climate related issues.