At least three-quarters of global temperature rise since the 1950s caused by humans
- 05 Dec 2011, 09:30
- Verity Payne
At least three-quarters of the rise in average global
temperature since the 1950s is due to human activity, according to
new research published in the journal Nature Geoscience.
Researchers using a new technique determined that since the middle
of the last century greenhouse gas emissions would have resulted in
a temperature rise of around 0.85 °C, although that potential
temperature rise that has been partially counterbalanced by
aerosols cooling the Earth, offsetting about half that
amount.
Overall, temperatures have risen by
around 0.75 °C since the start of the 20th century.
The Earth's climate fluctuates all the time, changed by natural
processes like volcanoes and solar activity, as well as
human-caused greenhouse gas and aerosol emissions. These things
which affect the climate, whether natural or man-made, are
collectively called 'forcings'.
To work out how much any one forcing - greenhouse gases, say -
contributes to temperature change, scientists generally use a
technique known as 'optimal fingerprinting'. This involves using
computer models to examine the effect of each forcing in
turn.
Modellers can do the same for each of the other forcings in turn,
and then perform statistical analysis on the results, which enables
them to determine how much each forcing influences temperature
compared to the others. (This technique does assume that
temperature patterns are accurately simulated by the climate
models.)
Based on optimal fingerprinting, the last report from the
Intergovernmental Panel on Climate Change, (AR4, published 2007)
concluded that:
"Most of the observed increase in global
average temperatures since the mid-twentieth century is very likely
due to the observed increase in anthropogenic greenhouse gas
concentrations... It is extremely unlikely that global climate
change of the past 50 years can be explained without external
forcing, and very likely that it is not due to known natural causes
alone"
The new paper doesn't use optimal fingerprinting, but
essentially comes to the same result. The new technique is less
complicated than optimal fingerprinting, and uses a simpler climate
model.
We spoke with the paper's lead author Prof. Reto Knutti, Institute
for Atmospheric and Climate Science, ETH Zürich, who explained what
the team had done:
"Our method is based on a much simpler
model that is run many thousands of times with different parameter
combinations. Then we compare with the observed warming and ocean
heat uptake for every simulation, and keep those that are
consistent with the observations (note that deciding what
'consistent' means is not trivial). But that gives us a large set
of parameter combinations constrained by observations. We can then
run the model with each of those parameter sets with each forcing
individually, e.g. only CO2. That gives us a best estimate and
uncertainty for the CO2 contribution. Same with the other
forcings."
This alternative technique can be used alongside the earlier
methods, he said:
"The fact that our results are entirely
consistent with the optimal fingerprinting, but based on a
different method, model and type of observation, is an argument for
even higher confidence in human induced climate change."
Other scientists working in the field welcomed the results as
providing another scientific methodology which came to similar
conclusions to previous work, increasing confidence in the results.
Prof Piers Forster, Professor of Climate Change at the University
of Leeds, said:
"It's pretty convincing stuff:
observations and the physical law of energy conversation have been
used to show greenhouse gases are responsible for global
warming and that alternative scenarios violate this law of nature.
Previous proofs have relied on complex climate models, but
this proof doesn't need such models - just careful observations of
the land, ocean and atmospheric gases."
Dr Peter Stott, Head of Climate Monitoring and Attribution at
the Met Office Hadley Centre, said:
"This study confirms that, even when
using different approaches, the observed warming seen over the last
50 years is dominated by man-made emissions of greenhouse gases.
With similar results from the Met Office Hadley Centre and
other researchers elsewhere this is further proof that the observed
temperature rise we have seen can be attributed to human
activity."
