Degrees of change: the IPCC’s projections for future temperature rise
- 15 Apr 2014, 12:00
- Robin Webster
Many governments are trying to reduce greenhouse gas
emissions. But unless policymakers raise their ambition
significantly, temperatures are likely to rise beyond safe levels.
We examine the pathways that could take us towards a two degrees
temperature rise by the end of the century - or considerably
On Sunday, the Intergovernmental Panel on Climate
Change (IPCC) released the last in a series of three reports, which
together assess the physical evidence that climate change is
expected impacts over the course
of this century and what would
need to happen to curb the rise in greenhouse gases.
Embedded in the reports are the scientists'
projections for how high temperatures are likely to rise this
century - and what that's likely to mean for ecosystems and
societies around the world.
The IPCC bases its projections for future temperature
rise on two different techniques.
First, the IPCC has created its own storylines, or
scenarios, describing how high temperatures are likely to rise in
the future and what that might mean. The scenarios vary according to different predictions for
how societies develop and how much effort we make to reduce
greenhouse gas emissions over the course of this century.
Secondly, the IPCC assesses a large number of
scenarios from different experts. For its
third report into greenhouse gas
emissions, the IPCC assessed 1200 different pathways, created by
different modelling teams around the world.
300 of these were 'baseline' scenarios - where the
governments of the world make little or no effort to tackle climate
change, and fossil fuel emissions keep going up at the present
rate. The other 900 are 'mitigation' pathways, where policies are
introduced to limit the rise.
Avoiding two degrees
In order to minimise the risk of "dangerous" human
interference in the climate system, the international community has agreed we need to
limit temperature rise to two degrees Celsius above pre-industrial
Achieving this limit wouldn't prevent all the
effects. Some risks of climate change are "
considerable" at one or degrees Celsius
above pre-industrial levels, the IPCC says. The physical evidence shows that the world is
already experiencing adverse effects, including for example
Arctic sea ice melt, as a result of a
Temperature rise is limited to less than two degrees
by the end of the century in only one of the IPCC's storylines - a
scenario of aggressive mitigation.
As a result of its own modelling and the different
scenarios it assessed, the IPCC concludes that avoiding the two
degrees rise means reducing global emissions by at least two fifths
by 2050, and tripling or quadrupling the share of energy the world
gets from low-carbon energy by the same date. It probably also
new, untested technologies to reduce
the level of carbon in the atmosphere, the IPCC says.
Preventing three degrees
The world isn't on track to avoid a two degrees rise
at the moment. So what track is it on?
At the international climate negotiations in Cancun,
pledged to limit the rise in greenhouse
gas emissions up to 2020. We don't know what's going to happen
after that, but it gives the IPCC some idea of where we might be
If followed up, the IPCC says the Cancun pledges are
roughly consistent with scenarios preventing a three degrees rise
over the course of this century - but not preventing a two degrees
Topping four degrees
In the first decade of this century, emissions rose
faster than ever before. Many of the world's economies are growing
rapidly, and the world's burning a lot of coal to generate energy.
Without explicit efforts to reverse the increase, the rise in
emissions going to continue, the IPCC predicts.
In its '
business as usual' story line humanity
continues to burn significant amounts of coal, and greenhouse gas
emissions continue to increase at their present rapid rate. This
scenario - and others the IPCC assessed where no effort at all is
made to reduce emissions - suggest temperatures could rise by
3.7 to 4.8°C by the end of the century, the panel says.
This level of temperature rise is likely to have very
significant impacts on the world's societies and natural systems.
At four degrees of warming, global climate risks are "high to very
high", the IPCC says. It would mean:
"...severe and widespread impacts on
unique and threatened systems, substantial species extinction,
large risks to global and regional food security, and the
combination of high temperature and humidity compromising normal
We've summarised the scenarios the IPCC assesses, and
how they match up with its storylines in the following graphic:
The scenarios the IPCC created and assessed don't
predict the future. Instead they give an indication of changing
levels of risk to ecosystems and human societies over the course of
this century, according to the level of greenhouse gas emissions
released by human activities.
Many uncertainties remain. There isn't enough
evidence to know, for example, when different ecosystem tipping
points may be passed. At some point the Amazon may reach a point
starts to dry out, releasing even more
carbon into the air. But the IPCC doesn't know when that's going to
The scenarios also give an indication of how
difficult it's going to be to reduce greenhouse gas emissions. If
the world doesn't start the job of reducing greenhouse gas
emissions until 2030, it could still be possible to prevent
the worst effects of climate change. But it would be a lot more
difficult and expensive, the IPCC says - and would probably involve
the use of as yet unknown technologies to suck carbon from the
The IPCC's overall message is that the risks of
"severe, pervasive, and irreversible impacts" increase as
temperatures rise. The scenarios in its report give an indication
of how likely those impacts actually are.