Are temperature targets fit for purpose?
- 03 Jul 2013, 18:15
- Freya Roberts
A new study challenges the idea that temperatures
targets, such as the widely known 2 degrees Celsius limit, are fit
for purpose when it comes to preventing dangerous interference with
the climate system.
research suggests that if other changes in the system, like sea
level rise, were factored into climate targets, the amount of
carbon we could emit and stay safe would be much lower.
It's a more holistic approach to assessing climate risk - could
such an approach benefit other bits of climate research too?
The most well known climate change target suggests global
temperature rise should be limited to two degrees Celsius above
pre-industrial times. Whether or not two degrees is the right
number, it's been widely touted as a sort of 'safe
limit' for well over a decade now and sits at the heart of
international climate politics.
Scientists have worked hard to work out exactly
what keeping below two degrees would mean for our emissions. In
particular, some studies focus on what are called 'cumulative
for example calculating that if humans
release no more than 650 more billion tonnes (gigatonnes) of carbon
dioxide by mid-century we will have an 80 per cent chance of
staying below two degrees.
But while these studies focus on how much more carbon
is acceptable from a temperature perspective, the
new study in the journal Nature suggests safe levels of
emissions would be much lower if we factored in other parts of the
Changes in the system
The study by Steinacher et al suggests that climate targets need
to be based on a broader assessment of climate risks, rather than
looking just at temperature change. Their modelling shows that
emissions consistent with 'safe' temperature rise would produce
unacceptable results in other aspects of the climate.
Steinacher and his team took a number of variables related to
the climate system - temperature rise, sea level rise, ocean
acidification, change in plant productivity, and loss of carbon
from soils - and defined what they called acceptable limits of
risk. They then used computer models to work out how much carbon
dioxide could be released before these limits were reached.
They found that to stay below all of the limits they set,
greenhouse gas emissions would have to be much lower than if they
were only trying to keep temperature rise below two degrees.
To stand a 66 per cent chance of keeping below two degrees,
emissions of carbon would need to be limited to about 570
gigatonnes by the end of the century. But staying below the
safe limits on all six variables, including temperature, would mean
limiting emissions to between 290 and 350 gigatonnes of carbon, the
There are some big uncertainties here, and these figures aren't
exact, but the point is fairly clear. When you consider the impact
of emissions on other parts of the climate system, cumulative
emissions need to be considerably lower.
A holistic approach
The research, published this evening, comes just a few
days after a
study in the journal Proceedings of the National Academy of
Sciences suggested that the combined influence of climate impacts
can be much greater than their individual effects.
An international team of scientists modelled how
the impacts of climate change on water availability, the spread of
disease, agricultural production and ecosystems could overlap in
different regions of the world.
Southern Europe, the Ethiopian highlands and
northern India emerge as worst-off regions. While individual
impacts might affect other countries more severely, the combined
effect of multiple impacts make these locations
This is preliminary work with some significant
uncertainties - in fact it's the first time multiple impacts have
been modelled together, the researchers say. But it gives a similar
sort of message: looking at changes in one part of the climate
system, or one specific impact of climate change, is useful. But
looking at the sum of many changes is a much better way to truly
gauge what future emissions will mean for climate change, and what
the overall impact of those changes will be.
Steinacher et al. (2013). Allowable carbon
emissions lowered by multiple climate targets. Nature. DOI:
Piontek et al. (2013). Multisectoral climate
impact hotspots in a warming world. Proceedings of the National
Academy of Sciences. DOI: 10.1073/pnas.1222471110