Four degrees and beyond: the potential for a global ... - Amper

8 M. New et al.
In the final plenary at a scientific conference on climate change in Copenhagen
in March 2009, a discussion with the Prime Minister of Denmark, Anders
Rasmussen, produced an interchange that demonstrates the tensions between
evolving scientific knowledge and policy decisions. When told by a scientific panel
that even a 2 ◦ C target might allow too much warming, with serious damages
and possible tipping points occurring below 2 ◦ C, the Prime Minister expressed
frustration: ‘It was a hard battle to get agreement on two degrees, a real challenge,
and now you tell me it’s not enough and we need less than two!’. 1
2. Feasibility of 2 ◦ C
At the same time that science was suggesting that 2◦C might not be as safe
a guardrail as previously thought, there was growing evidence suggesting that
dramatic emission cuts were required to have any reasonable chance of staying
below the 2◦C target. For example, Rogelj et al. [8] argued that having a 50 : 50
chance of constraining warming to 2◦C would require developed countries to cut
emissions by up to 80 per cent below 1990 levels by 2050, but that even the best
case commitments prior to Copenhagen only resulted in a4percentcutby2020
and a 63 per cent cut by 2050. They concluded that there was ‘virtually no chance
of limiting warming to 2◦C above preindustrial temperatures’.
The challenges involved in keeping below 2◦C have if anything increased since
the 2009 Copenhagen meeting. While the introduction of the 2◦C and 1.5◦C targets in the Copenhagen Accord is significant, it has not been underpinned by
adequate action. Parties were invited to communicate voluntary targets or actions
for 2020 and their base year to the Secretariat by 31 January 2010. But this did
not entice any stronger reduction pledges, bringing into question the feasibility
and will of large emitters to seriously aim for the 2◦C target, let alone 1.5◦C. At
the UNFCCC negotiations in Bonn in June 2010, the first formal negotiations
after Copenhagen, expectations were expressed that a legally binding agreement
would not be reached before 2012 at COP17 in South Africa. Yvo de Boer, in his
final remarks as head of the UNFCCC before stepping down, said that it could
take up to 10 years for negotiations to deliver a robust and effective agreement [9].
In this issue, Bowerman et al. [10] build on earlier work [11] to illustrate clearly
the importance of cumulative carbon emissions budgets as a determinant of peak
global temperature. They show that a total of 1 trillion tonnes of carbon (TtC)
from 1750 to 2500 will produce a ‘most likely’ peak warming of 2◦C. Importantly,
the uncertainties in carbon cycling and climate system response to atmospheric
CO2 mean that there is considerable likelihood of exceeding the most likely figure
of 2◦C. Bowerman et al. [10] also demonstrate that the relationship between
cumulative emissions and peak temperature is largely insensitive to the emissions
pathway; therefore, a continued steep rise of emissions after 2010, with a high
peak and steep post-peak decline, can produce the same peak temperature as a
flatter emissions profile, provided they both keep to a 1 TtC cumulative total.
Bowerman et al. [10] extend their earlier work by showing the likelihood range for
peak temperatures under cumulative carbon budgets arising from less aggressive
emissions reduction policies: 2 and 3 TtC result in most likely peak temperatures
of 3◦C and 4◦C over preindustrial.
1http://climatecongress.ku.dk. Phil. Trans. R. Soc. A (2011)
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