Global average temperatures will rise at
least 4°C by 2100 and potentially more than 8°C by 2200 if carbon dioxide
emissions are not reduced according to new research published in Nature.
Scientists found global climate is more sensitive to carbon dioxide than most
Role of cloud formation
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 global warming.
“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 lead author from the University of New South Wales’
Centre of Excellence for Climate System Science Prof Steven Sherwood.
“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 1.5°C to 5°C.
This new research takes away the lower end
of climate sensitivity estimates, meaning that global average temperatures will
increase by 3°C to 5°C with a doubling of carbon dioxide."
The key to this narrower but much higher
estimate can be found in the real world observations around the role of water
vapour in cloud formation.
Observations show when water vapour is
taken up by the atmosphere through evaporation, the updraughts can either rise
to 15 km 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 is not pulled away
from cloud forming regions when only deep 15km updraughts are 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 15 km 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 dioxide.
However, real world observations show this
behaviour is wrong. 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 warms.
This increases 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.
of carbon dioxide
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 4°C by 2100.
“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,” said Prof.
“Rises in global average temperatures of
this magnitude will have profound impacts on the world and the economies of
many countries if we don’t urgently start to curb our emissions.
clouds bad for health