A hellish vision of a world warmed by 4C within a lifetime has been set
out by an international team of scientists, who say the agonisingly slow
progress of the global climate change talks that restart in Mexico today
makes the so-called safe limit of 2C impossible to keep. A 4C rise in the
planet's temperature would see severe droughts across the world and
millions of migrants seeking refuge as their food supplies collapse.

"There is now little to no chance of maintaining the rise in global
surface temperature at below 2C, despite repeated high-level statements
to the contrary," said Kevin Anderson, from the University of Manchester,
who with colleague Alice Bows contributed research to a special
collection
of Royal Society journal papers published tomorrow. "Moreover, the
impacts associated with 2C have been revised upwards so that 2C now
represents the threshold [of] extremely dangerous climate change."

The new analysis by Anderson and Bows takes account of the non-binding
pledges made by countries in the Copenhagen Accord, the compromise
document that emerged from the last major UN climate summit, and the
slight dip in greenhouse gas emissions caused by the economic recession.
The scientists' modelling is based on actual tonnes of emissions, not
percentage reductions, and separates the predicted emissions of rich and
fast-industrialising nations such as China. "2010 represents a political
tipping point," said Anderson, but added in the report: "This paper is
not intended as a message of futility, but rather a bare and perhaps
brutal assessment of where our 'rose-tinted' and well-intentioned
approach
to climate change has brought us. Real hope and opportunity, if it is to
arise at all, will do so from a raw and dispassionate assessment of the
scale of the challenge faced by the global community."

A rise of 4C could be seen as soon as 2060 in a worst case scenario,
according to research in the same journal, led by the Met Office's
Richard Betts and first revealed in the Guardian last year. Betts accepts
the scenario is extreme but argues it is also plausible given the rapidly
rising trend in emissions.

Rachel Warren, at the University of East Anglia, described a 4C world in
her research paper: "Drought and desertification would be widespread ...
There would be a need to shift agricultural cropping to new areas,
impinging on [wild] ecosystems. Large-scale adaptation to sea-level rise
would be necessary. Human and natural systems would be subject to
increasing levels of agricultural pests and diseases, and increases in
the frequency and intensity of extreme weather events."

Warren added: "This world would also rapidly be losing its ecosystem
services, owing to large losses in biodiversity, forests, coastal
wetlands, mangroves and saltmarshes [and] an acidified and potentially
dysfunctional marine ecosystem. In such a 4C world, the limits for human
adaptation are likely to be exceeded in many parts of the world."

Another Met Office study analyses how a 4C rise would differ from a 2C
rise, concluding that threats to water supplies are far worse, in
particular in southern Europe and north Africa, where regional
temperatures would rise 6-8C. The 4C world would also see enhanced
warming
over most of the US, Canada and northern Asia.

In sub-Saharan Africa, "the prognosis for agriculture and food security
in a 4C world is bleak", according Philip Thornton, of Kenya's
International Livestock Research Institute, who led another research
team. He notes there will be an extra billion people populating the
continent by 2050.

"Croppers and livestock keepers in sub-Saharan Africa have in the past
shown themselves to be highly adaptable to short- and long-term
variations
in climate. But the kind of changes that would occur in a 4C+ world would
be way beyond anything experienced in recent times. It is not difficult
to envisage a situation where the adaptive capacity and resilience of
hundreds of millions of people could simply be overwhelmed by events,"
Thornton's team concludes.

Another team tackled another complex question: what would happen to humid
tropical forests in a 4C world? There was the risk of forest die-off in
the Amazon, central America and parts of Africa, but some regions, eg
around the Congo basin, would have the potential for forest expansion.
The scientist noted, however, that in practice deforestation might be the
most important factor.

The speed – as well as the size – of the temperature rise is crucial too,
warned scientists from Oxford University, as faster rates of global
warming could outpace the ability of human civilisation and the natural
world to adapt. "Dangerous climate change depends on how fast the planet
is warming up, not just how hot it gets," said Myles Allen of Oxford
University's department of Pphysics. "It's not just how much we emit, but
how fast we do so."

http://www.guardian.co.uk/environment/2010/nov/29/climate-change-
scientists-4c-temperature