In this scenario the nations do not collaborate effectively but the temperature increase is kept below two degrees nonetheless. Here’s how this might happen.
Copenhagen showed that the USA and China were unwilling to make the changes necessary to achieve the lifeboat scenario whilst China wasn’t even willing for other countries to make the necessary commitments. In this situation most major GHG emitters will give climate change a low priority and the pace of climate change will accelerate in line with the IPCC’s business-as-usual scenario.
By about 2020 the political leaderships of China, India and USA will have recognised that the threat of climate change is serious and urgent but they will remain locked into existing attitudes and policies.
There will then be a serious climate crisis. It might be a storm, flood, drought or fire. Its immediate consequences may be very severe – thousands of deaths and $Bs lost in property damage. However its largest impact may come from symbolic damage, eg the collapse of the Statue of Liberty in a major storm-surge.
This will lead one major nation, let’s call it Maverick, to a realistic re-examination of the climate change threat. It will conclude that it is already too late for the orderly conservation-based approach described in the Lifeboat scenario. As a result, Maverick will take unilateral action in the form of one or more major geo-engineering programmes. Maverick will also introduce strong domestic emission-reduction policies and launch a major campaign for international collaboration. These programmes will restrain the temperature growth within ten years but will probably have a variety of adverse effects on other nations.
At least some of these nations will oppose these geo-engineering programmes but Maverick will use its diplomatic, cultural, financial and commercial muscle to neutralise this opposition. It’s not clear whether war can be completely avoided in this scenario but I’m assuming that any military action against Maverick will not stop its geo-engineering efforts. Maverick will also use its leverage to prevent other powers from benefiting disproportionately from its expenditure on geo-engineering.
The initial hostility to Maverick’s unilateralism will, eventually, be followed by acceptance of its inevitability and even desirability. This scenario is unstable and could degenerate into either of the high temperature scenarios. However, Maverick’s unilateralism may buy enough time for the creation of a consensus between the main powers. This consensus could allow this scenario to evolve into Lifeboat. It will not be sustainable if it doesn’t.