The Future of Climate Projections
- 13 Jun 2012, 17:00
- Freya Roberts
Predicting the future is hard.
Projecting future climates is an inexact science. But changes to
the way scientists make their predictions could put us on a path to
greater certainty.
Predicting future climate
The Intergovernmental Panel on
Climate Change is the most comprehensive assessment of the
science of climate change in existance. Its job is consider the
field of climate change science, synthesise conclusions about how
the climate is behaving from the scientific literature, and then
try and determine what the effects will be on our planet. Finally,
the IPCC process tries to assess what the impacts of these changes
will be, and suggests how we can adapt to them.
It's a big, all-encompassing project to try and 'sum-up' climate
change, but the process can be broken down into some fundamental
bits:
The current approach for projecting climate change. Source:
Moss et al., 2010
As a first step, scientists work out what human factors are
driving emissions, and how they are likely to change in the future.
With this information, they can estimate how emissions might
increase.
This is the idea behind 'emissions scenarios', but the challenge
is that elements of uncertainty at this early stage can have a
knock-on effect on the accuracy of the overall projections.
Making emissions scenarios
An emissions scenario is designed to give plausible broad-brush
picture of what greenhouse gas emissions might do in the future. To
form them, scientists make assumptions about the main factors
driving emissions - things like changes in population, advances in
technology, and social and economic development. They make informed
guesses about how these drivers will change over time, and by
mapping all these changes together, they calculate total emissions
at a stage in the future.
But human behaviour is the main thing which will affect future
emissions - and since it's uncertain how humans will act in the
future, it's very difficult to know how emissions will
change.
So the IPCC
produce a number of scenarios , varying the assumed changes
slightly in each, to give a number of plausible future outcomes.
None of these scenarios is expected to be 'the one' - the aim is to
demonstrate a range.
But there are problems with doing it like
this
This way of projecting climate change is useful, but limited by a
number of factors.
One of the biggest limitations lies at the heart of projecting
future emissions. For example, one tool the IPCC use is a pretty
simple equation called the 'Kaya Identity', which goes like
this:
The Kaya Identity , one of the framing tools used by the IPCC
see page 105 of SRES )
This equation can be widely and easily applied, but it assumes
each driver has a similar effect on greenhouse gas emissions, when
in practice, some things make more of a difference than
others.
Population is one example.
A new review in Nature Climate Change demonstrates this well:
for a long time researchers have proposed that a growing population
brings greater environmental stress (aka more emissions). The idea
holds that emissions rise in tandem with population.
But actually, that doesn't appear to be the case. The emissions of
a growing population increase at a faster rate than the number of
people, according to anempirical
study. It sampled a number of countries over the period of 1960
- 2005 and found that at national level, an increase in
population size of 10% made greenhouse gas emissions rise by
between 12.7% and 18.6%
The practical upshot of this finding is that simple equations like
the Kaya Identity probably underestimate the effect of population
growth on emissions, and such errors are then passed on into the
calculated emissions totals, and the projected climate impacts that
are based on them.
The whole process is also very slow - from start to finish it
takes about 10 years to create scenarios and map them through
to impacts studies. And the uncertainty increases with each
step.
Doing something new
These issues are probably why, in advance of its Fifth Assessment
Report (AR5), the
IPCC has been reevaluating the way it projects future
emissions.
Instead of starting with a socio-economic story about the future
and calculating the amount of greenhouse emissions it gives rise
to, it will start by specifying some likely concentrations of
greenhouse gases in the atmosphere over the near future, and then
working out how the climate will respond. By taking humans out of
equation, the idea goes, the uncertainties of working out physical
changes to the climate system will be reduced.
These emissions concentration levels will be called
'representative concentration pathways' (RCPs), and they can be
used to look forwards, using earth-system models to work out what
physical changes in the climate system may result from these
greenhouse gas concentrations, and also to look backwards: using
socio-economic models to see how our actions might lead to certain
concentrations of greenhouse gases.
Essentially, rather than making assumptions about how driving
forces will change, the new process will make assumptions about how
greenhouse gas concentrations will change. The greenhouse gases
will now be the starting point, rather than a middle stage. In
theory this will make climate projections more reliable and more
useful for policy makers when they are trying to decide the best
measures for adaptation.