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IPCC fourth assessment was too optimistic

Since 2000, emissions from fossil fuel combustion have grown three times faster than in the mid-late 1990s. “Emissions are now outside the whole envelope of possibilities considered in the IPCC’s fourth assessment report,” said Chris Field of Stanford University and the Carnegie Institution for Science at a press briefing at the AAAS Annual Meeting. “The emissions trajectory used was too optimistic - we didn’t think broadly enough.”

To make matters worse, nobody’s certain how effective the carbon sink currently provided by the oceans and land will be in the future, or even whether they will become a source. For example, in greenhouse studies, additional carbon dioxide increased plant growth by around 50% but Field says that, while this was considered an immature topic in the fourth assessment report, we now know that nutrient and other restrictions will stop that from happening in the field.

“Every new piece of information I see makes the scary side look scarier,” added Field. “The situation is more complicated than we thought in AR4 - we have higher emissions and a less friendly natural system. We will have to avoid more carbon emissions than we thought - either start earlier or make more aggressive cuts.”

A number of delegates were concerned that the lengthy IPCC report process could delay policymakers from taking action. “The challenge is that we can either be fast or we can be good,” said Field, who is one of the leaders of the fifth IPCC assessment report, due for publication in 2013/14. With an eye to more “policy-relevant timescales”, the IPCC will release between two and five special reports that take 12-18 months to produce before this. The first will be on renewable energy; scientists will decide at a meeting in Turkey next month whether to go ahead with a special report on climate extremes and adaptation to those extremes.

In line with the general mood at the conference, Field was optimistic about the new US administration and climate change mitigation. “There is lots of talk that we may see the US re-emerge as a leader on this important issue,” he said. “I hope it does.”
 

Moving from fourth to fifth

So how will the fifth assessment compare science-wise? During  AR4, eighteen research groups contributed mainly physical climate models with century timescales, detailed Ronald Stouffer of the Geophysical Fluid Dynamics Laboratory in Princeton, US. In contrast AR5 will see 25 groups contribute a mix of earth system models and global climate models with decadal to century timescales. Earth system models “close the carbon cycle” by looking at the effect of biological changes on climate; typically they contain details of atmospheric chemistry, ocean ecology and biogeochemistry, plant ecology and land use.

According to Stouffer, some of the modelling challenges that remained at the end of the fourth assessment report include clouds and aerosols, oceanic heat uptake, regional climate information, land ice modelling, and the carbon cycle. As well as tackling some of these challenges, the fifth assessment will also include emerging frontiers of research such as decadal prediction, and the feedback between climate and air pollution.

 

Posted by Liz Kalaugher on February 15, 2009 7:48 PM | Permalink