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.

IPCC_AR5_radiative Forcing
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.

Knutti _and _Sedlacek _SRES _vesus _RCPs

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. 

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