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Plants and climate change: Positive or negative?

  • 09 Mar 2011, 17:40
  • Verity

New results from the University of Utrecht suggest that plants will lose less water as carbon dioxide levels rise, and the warming effect of plants as temperatures go up may be smaller than expected.

But looking at the wider scientific picture shows that this result doesn't mean vegetation won't be contributing to extra temperature rise. 'Carbon dioxide is good for plants' is a well-worn climate sceptic argument. But as is often the case, the situation is not that simple, and the scientific literature shows that there's still uncertainty and fierce debate within the scientific community about how plants will respond to global warming.

This is handily illuminated by two new papers from the same research group in Utrecht, covering the response of plants to high levels of CO2. The behaviour of plants as carbon dioxide goes up creates a climate 'feedback' - depending on how the plants respond, they can either limit warming (a negative feedback), or cause more (a positive feedback).

The first Utrecht paper investigated the speed at which certain plants 'transpire' or lose water through their pores at different carbon dioxide levels. The second paper then used this data to model likely changes in the water cycle based on a doubling of atmospheric CO2. The results suggest the amount of water lost through transpiration decreases as CO2 rises, which also means that heat transferred to the atmosphere by the plants will also decrease.

Wattts -utrecht

Wattsupwiththat incorrectly headlined the press release as showing that a 'negative' feedback had been found.

Some climate sceptics have taken these papers as evidence that plants produce a negative warming feedback - i.e. that this effect will balance or reduce rising temperatures caused by CO2. But this isn't right - while the study shows that the feedback from transpiration gets smaller as carbon dioxide increases, that feedback remains positive - i.e. it still provides additional warming.

As is often the case though, looking at only bits of scientific evidence doesn't tell the whole story, because transpiration is not the only thing that affects the climate feedbacks caused by plants, and the response of vegetation to climate change is complicated.

For example, photosynthesis - the conversion of carbon dioxide into plant food - transfers water and energy into the atmosphere over short timescales, causing an immediate short-term warming effect. But over longer timescales, plants store carbon dioxide - a negative CO2 feedback. So depending on the timescale you're considering, plants have a different feedback effect.

The feedback effect also varies depending on the physical scale you're considering. On a larger scale, water and carbon cycles are influenced by the type of plants that dominate the landscape, as the response of different types of plants to rising CO2 varies.

Changes in vegetation cover can also affect the climate by changing the landscape. If vegetation spreads into previously snowy regions, the surface becomes less reflective and absorbs more heat, leading to warming. Vegetation can also alter surface roughness, changing convection patterns in the atmosphere, and changing the hydrological cycle. Finally, there is also uncertainty in how soils - rich in organic matter- will respond to rising CO2 under different conditions. Changes in soil could affect the carbon cycle and other nutrients cycles.

The 2007 IPCC report acknowledges that the scientific understanding of many of these effects is low, and attempts to assess the combined response of all these feedbacks provides no definitive answer. The Utrecht papers are part of a process of refining our understanding of how the natural world will respond to rising temperatures and CO2 levels.

Preliminary attempts (behind paywall) to model the overall response of land plants to CO2-driven climate warming suggested a positive (i.e. warming) feedback. Subsequently, other models indicated that there was an overall negative (i.e. cooling) feedback. Now, the most recent models indicate a positive feedback - so at the moment it looks like we're in for more warming as plants respond to climate change.

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