Historically plants limited CO2 sinks

Weathering, or natural degradation, of silicon-containing rocks should have produced a significant carbon sink over the past 24 million years. A new study suggests that plants have reduced this sink and kept CO2 levels at a higher level.

Natural sinks of CO2 play an important role in adaptation and mitigation policies for climate change. In order to predict future climate change, it is necessary to understand global carbon cycles, their past changes and their limits. Different natural processes can either increase or decrease atmospheric CO2 concentration (known as positive and negative feedback). Weighing up the balance of these feedback mechanisms makes predicting climate change very complex. This research sheds new light on one type of feedback.

Records based on analyses of prehistoric 'alkenones' (compounds produced by a type of plankton which provide important clues to past climates) indicate that atmospheric CO2 concentrations ranged from 1000 to 1500 parts per million (ppm) from about 45 to 34 million years ago. CO2 concentrations then dropped, approaching modern levels of 270 ppm before industrial era / 385 ppm today by about 24 million years ago. This historic drop in CO2 concentrations is believed to be partly driven by the weathering of silicate rocks such as basalt. These rocks consume CO2 through chemical reactions that vary with temperature and moisture. High weathering rates are observed in wet warm regions that have experienced tectonic activity. As such it is thought that global warming and increased low-latitude tectonic activity during the past 24 million years should have created a significant sink in these rocks. However, atmospheric CO2 concentrations did not decline to the levels that would be expected.

The study proposes that CO2 levels were stabilised because a negative feedback, or counteractive mechanism, involving land plants, reduced the weathering of silicate rocks during the early to late Miocene period (5 to 23 million years ago). Land plants usually accelerate the rates of weathering through a variety of means. For example, roots can break up the minerals and increase the surface area for the chemical reactions to occur. However, land plants need CO2 to survive so low levels of CO2 can inhibit plant growth and therefore reduce weathering.

The study used a model (the Sheffield Dynamic Global Vegetation Model1) to simulate the sensitivity of vegetation to atmospheric CO2 at various CO2 concentrations ranging from 50 to 500 ppm. Simulations were performed in upland regions where silicate rocks are more susceptible to weathering.

The results indicate that the global and tropical forest production of new organic matter and the amount of roots decline abruptly at CO2 concentrations of about 200 ppm. Both organic matter and roots are properties of ecosystems that are known to enhance weathering. Transpiration - or the release of water vapour - from the canopy is also reduced when CO2 concentrations drop to 200 ppm. This suggests that, although the weathering of silicate rocks reduces CO2, a threshold level of CO2 is reached below which plant activity no longer contributes to weathering. When this threshold of about 200 ppm is approached, the potential of silicate rocks to act as a CO2 sink is reduced.

The researchers recognise that their modelling results are simplifications and do not include the full range of geological and biological feedback systems. For example, the demise of the land plants could affect erosion rates and soil stability. More detailed models are required to include the different systems and analyse the role of this negative feedback mechanism more thoroughly.

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