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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


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