What does the next IPCC report say about climate change?
- 18 Dec 2012, 16:30
- Roz Pidcock
Last week saw a draft of the IPCC's upcoming report on the
science of climate change leaked onto the internet. Although it is
not the final version, some
news outlets have reported the contents of the draft, which
provides indicators of how the science of climate change has
changed and developed since the last IPCC report was published in
2007.
Every five or six years, the Intergovernmental Panel on Climate
Change (IPCC) produces an
in-depth assessment of the state of research in all areas of
climate science, from precipitation patterns to polar ice.
The next one, known as the Fifth Assessment Report (AR5), is due to be released next year. But a
blogger
leaked an early version online on Friday. So what do the main
differences appear to be between this report and the last one?
Human vs natural causes
According to the new report, there is now "incontrovertible
evidence" that atmospheric concentrations of greenhouse gases have
increased in the last 200 years, causing the average temperature of
earth's atmosphere and oceans to rise.
So much evidence now exists that the draft says scientists are
"virtually certain" human activity is the main driver of climate
change , which means they are at least 99 per cent sure. In AR4,
the figure was lower -
90 per cent - so scientific certainty has increased.
There is "very high confidence" - which means that there is lots
of evidence in agreement - that natural forcing affects the climate
much less than human activity. Solar activity has had a cooling
effect since 1980, the draft report suggests. (Bottom blue bar,
below.) Compare this to the total warming effect from human
activity, shown by the orange criss-crossed bar.
Source: Second Order Draft of the IPCC 5th Assessment Report
(AR5).
Global temperature and uncertainty
Each IPCC report uses a new generation of climate models. The
new models represent more of the processes that make up the global
climate in
greater detail.
In AR5, the IPCC uses four new scenarios to estimate how the
climate could change in the future, known as Representative
Concentration Pathways (RCPs).
These work differently from the scenarios used in the last version
of the report, because they specify concentrations of greenhouse
gases and work out how the climate will respond, rather than trying
to predict how human activity will change to affect emissions.
There is still uncertainty about how the climate will evolve,
and so the IPCC provides a range of possible temperature rises for
each scenario rather than a single prediction. For example, the
most pessimistic scenario (RCP8.5) predicts temperatures will rise
between about three and five degrees by 2100, shown in red in the
right hand graph below.
Source:
Knutti & Sedlacek (2012).
The new RCPs (right) capture a greater range in possible
temperatures than in AR4 (left). But the uncertainty around the
projections is also bigger. That may seem like a step backwards
but better understanding doesn't necessarily mean reducing
uncertainty - as a
recent study by Knutti and Sedlacek puts
it:
"More research uncovers a picture that
is more complicated; thus, uncertainty can grow with time...but
these should not prevent those working on climate impacts,
mitigation and adaptation from making decisions".
Rising sea levels
Projections of sea level rise by 2100 are significantly higher
in the AR5 draft than they were in AR4. The new report says sea
level is likely to rise by between 29 and 82 centimeters by the end
of the century, compared to 18-59 centimeters in the 2007
report.
The reason is that previous projections were limited by
scientists' understanding of how quickly the Greenland and
Antarctic ice sheets are melting. At the time of AR4, there was
even uncertainty over whether Antarctica was
gaining or losing ice overall.
Newer
research confirms that both ice sheets are melting rapidly and
that the pace is accelerating. The IPCC include a contribution to
sea level rise by ice sheets of 0.11 metres by 2100, which is the
main reason why predictions of sea level rise are higher this time
round.
Extreme events
Scientific understanding of extreme events like droughts and
tropical cyclones has increased significantly since AR4. For that
reason, the IPCC released a Special Report on Extreme Events (SREX) earlier this year.
In general, new evidence since 2007 suggests the picture is more
complicated for many types of extreme event than previously
thought, which means that the future projections have changed a bit
too.
For example, in AR4 scientists predicted the area of the world
experiencing drought was "likely" to increase, but now there is
only "medium confidence" in that prediction. Scientists also
refined their 2007 prediction of a "likely" increase in global
cyclone activity to project an increase in cyclone intensity, but
not frequency.
What's still being debated?
The main reason for uncertainty surrounding temperature
projections is that it's hard to pin down how sensitive the climate
is to a doubling of carbon dioxide - known as the Equilibrium
Climate Sensitivity (ECS). In AR4, scientists estimated for the
first time a likely range for ECS of between two and 4.5 degrees.
Research is ongoing to see if any of the uncertainty can be
reduced.
Changes in how much sunlight the earth's surface reflects are
also important. Clouds can increase reflectivity and cool the
planet, or they can have a warming effect. But processes that
affect cloud formation are hard to measure accurately.
For example, aerosols are tiny particles in the atmosphere that
can stimulate clouds to form. Scientists are confident that
aerosols offset a substantial portion of anthropogenic warming, but
new research since AR4 suggests the contribution is less than
previously thought.
A climatic
tipping point is a threshold beyond which the climate undergoes
an irreversible shift from one physical state to another. According
to AR5, although some models show evidence of global or
regional tipping points, particularly related to
Arctic sea ice, many aspects are still
under debate.
Bring on 2013
From what we can see, the five years of additional data since
2007 serves to strengthen the main conclusions from AR4,
particularly the role of human activity, and fine-tunes previous
temperature and sea level rise projections. But as we said before,
the leaked report is a draft and not the final version. So it's
important to be aware that some things may still change and new
evidence can be included between now and its official release in
September next year.