Plants and climate change: Positive or negative?
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.
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.