Former DOE Assistant Energy Secretary and renowned blogger Joe Romm has written some authoritative pieces on the beyond 2°C question. According to Joe, the International Energy Agency (IAE), a highly respected and traditionally cautious firm, released a bombshell in their annual world energy report last November. Among other things the IAE states that today's "planned policies … will lead to irreversible and potentially catastrophic climate change.” The IAE estimates current policies will lead to 6°C warming by the end of this century, and that for every $1 we don’t invest in clean energy technologies before 2020 “an additional $4.30 would need to be spent after 2020 to compensate for the increased emissions.”  In short: today’s policies = disaster and waiting to change increases the costs of change by more than 400% next decade.

Romm says, 

“The IEA is one of the few organizations in the world with a sophisticated enough global energy model to do credible (i.e non-hand-waving) projections of the cost of different emissions pathways and the costs of delaying efforts to achieve them. Those who counsel waiting for breakthrough technologies are urging us on a path that is unsustainable, irreversible, potentially catastrophic, and economically indefensible, according to the IEA.

“The IEA’s 2008 analysis of the 2°C warming pathway demonstrated that the total shift in investment needed to stabilize at 450 ppm is only about 1.1% of GDP per year — and that is not a “cost” or hit to GDP, because much of that investment goes towards saving expensive fuel."

The IAE therefore advocates that global/industrial greenhouse emissions cannot peak any later than 2017. A graph from their November report: 

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