Vegetation response may be as important as climate sensitivity

"This is a big result as to date, efforts to understand and reduce uncertainties in future climate projections have focused mainly on atmospheric feedbacks, especially in clouds," Ben Booth of the Met Office Hadley Centre, UK, told environmentalresearchweb. "This work suggests that better understanding land carbon feedbacks is equally important."

Booth and colleagues from the University of Exeter, University of Leeds, Centre for Ecology and Hydrology, all in the UK, and James Cook University, Australia, used a single emissions scenario (A1B) and a fully coupled climate-carbon cycle model (HadCM3C). They changed the levels of six parameters – the sensitivities of plant photosynthesis and soil respiration to temperature, stomatal conductance, soil water availability and surface evaporation, and plant competition.

Altering these parameters resulted in a difference in projected atmospheric CO₂ concentrations of 461 ppm by 2100; the results ranged from 669 to 1130 ppm, equivalent to a temperature range of 3.3–5.7 °C.

"The experiments enabled us to identify which component of the land carbon cycle represents the dominant control on future change," said Booth. "The results point to the temperature controls on photosynthesis, and highlight the current limited nature of data on plant photosynthetic responses at high temperatures."

Many models indicate that land will become a net source, rather than a sink, of CO₂ by the end of the century. If plants can adapt to higher temperatures, this will affect the amount of CO₂ they can store.

"There is a growing interest within the plant physiological community on processes via which plants may acclimate to changing climate," said Booth. "These results, which place the temperature-photosynthesis response at the heart of the magnitude of the future carbon-cycle response, highlight the need to further understand and start including these acclimation processes within future land carbon-cycle frameworks."

The land carbon-cycle component of the team's model considers how plants and soils respond to changes in temperature, moisture and CO₂. Modelling dynamical changes in vegetation in this way enables exploration of how factors such as CO₂ fertilisation will mitigate against higher temperatures and water stress, say the researchers.

Booth and colleagues reported their work in Environmental Research Letters (ERL).