The sun is seen behind smoke billowing from a chimney of a
heating plant in Taiyuan, Shanxi province. A coal-dependent
manufacturing base has made China the world's biggest
contributor to climate change. REUTERS
Global average temperatures will rise at least 4C by 2100
and potentially more than 8C by 2200 if carbon dioxide
emissions are not reduced, according to a major climate change
study published today.
The Australian study, which is in the journal Nature, found
global climate is more sensitive to carbon dioxide than most
The research also appears to solve one of the great unknowns
of climate sensitivity, the role of cloud formation and
whether this will have a positive or negative effect on
In New Zealand, four degrees of warming would mean extreme
rainfall events would increase by more than 30 per cent,
marked drying everywhere in summer, and varying and
widespread impacts on agriculture.
Under the 2200 scenario, the climate of Auckland would become
equal to that of Tonga, glacier cover would vanish, and sea
levels up to 3m higher would pose serious issues for our
"Our research has shown climate models indicating a low
temperature response to a doubling of carbon dioxide from
pre-industrial times are not reproducing the correct
processes that lead to cloud formation," said Professor
Steven Sherwood, the lead author from the University of New
South Wales' Centre of Excellence for Climate System Science.
"When the processes are correct in the climate models the
level of climate sensitivity is far higher."
Previously, estimates of the sensitivity of global
temperature to a doubling of carbon dioxide ranged from 1.5C
to 5C, he said.
"This new research takes away the lower end of climate
sensitivity estimates, meaning that global average
temperatures will increase by 3C to 5C with a doubling of
The key to this narrower but much higher estimate could be
found in the real world observations around the role of water
vapour in cloud formation.
Observations showed that when water vapour was taken up by
the atmosphere through evaporation, the updraughts could
either rise to 15km to form clouds that produce heavy rains
or rise just a few kilometres before returning to the surface
without forming rain clouds.
When updraughts rise only a few kilometres they reduce total
cloud cover because they pull more vapour away from the
higher cloud-forming regions.
However water vapour was not pulled away from cloud-forming
regions when only deep 15km updraughts were present.
The researchers found climate models that show a low global
temperature response to carbon dioxide do not include enough
of this lower-level water vapour process.
Instead they simulate nearly all updraughts as rising to 15km
and forming clouds.
When only the deeper updraughts are present in climate
models, more clouds form and there is an increased reflection
of sunlight. Consequently, the global climate in these models
becomes less sensitive in its response to atmospheric carbon
However, real world observations showed this behaviour is
When the processes in climate models are corrected to match
the observations in the real world, the models produce cycles
that take water vapour to a wider range of heights in the
atmosphere, causing fewer clouds to form as the climate
This increased the amount of sunlight and heat entering the
atmosphere and, as a result, increases the sensitivity of our
climate to carbon dioxide or any other perturbation.
The result is that when water vapour processes are correctly
represented, the sensitivity of the climate to a doubling of
carbon dioxide - which will occur in the next 50 years -
means we can expect a temperature increase of at least 4C by
"Climate sceptics like to criticise climate models for
getting things wrong, and we are the first to admit they are
not perfect, but what we are finding is that the mistakes are
being made by those models which predict less warming, not
those that predict more," Professor Sherwood said.
Auckland climate scientist Dr Jim Salinger, author of the new
book Living in a Warming World, said the findings were
"This is a very significant paper, because it shows the
climate sensitivity to doubling carbon dioxide is now
significantly higher than formerly believed."
- Jamie Morton