Sometimes we need to think the unthinkable, particularly when dealing with a problem as dangerous as climate change - there is no room for dogma when considering the future habitability of our planet. It was in this spirit that I and a panel of other specialists in climate, economics and policy-making met under the aegis of the Stockholm Network thinktank to map out future scenarios for how international policy might evolve - and what the eventual impact might be on the earth's climate.
We came up with three alternative visions of the future, and asked experts at the Met Office Hadley Centre to run them through its climate models to give each a projected temperature rise. The results were both surprising, and profoundly disturbing. We gave each scenario a name. The most pessimistic was labelled "agree and ignore" - a world where governments meet to make commitments on climate change, but then backtrack or fail to comply with them. Sound familiar? It should: this scenario most closely resembles the past 10 years, and it projects emissions on an upward trend until 2045.
A more optimistic scenario was termed "Kyoto plus": here governments make a strong agreement in Copenhagen in 2009, binding industrialised countries into a new round of Kyoto-style targets, with developing countries joining successively as they achieve "first world" status. This scenario represents the best outcome that can plausibly result from the current process - but ominously, it still sees emissions rising until 2030.
The third scenario - called "step change" - is worth a closer look. Here we envisaged massive climate disasters around the world in 2010 and 2011 causing a sudden increase in the sense of urgency surrounding global warming. Energised, world leaders ditch Kyoto, abandoning efforts to regulate emissions at a national level. Instead, they focus on the companies that produce fossil fuels in the first place - from oil and gas wells and coal mines - with the UN setting a global "upstream" production cap and auctioning tradable permits to carbon producers.
Instead of all the complexity of regulating squabbling nations and billions of people, the price mechanism does the work: companies simply pass on their increased costs to consumers, and demand for carbon-intensive products begins to fall. The auctioning of permits raises trillions of dollars to be spent smoothing the transition to a low-carbon economy and offsetting the impact of price rises on the poor. A clear long-term framework puts a price on carbon, giving business a strong incentive to shift investment into renewable energy and low-carbon manufacturing. Most importantly, a strong carbon cap means that global emissions peak as early as 2017.
This "upstream cap" approach is not a new idea, and our approach draws in particular on a forthcoming book by the environmental writer Oliver Tickell. However, conventional wisdom from governments and environmental groups alike insists that "Kyoto is the only game in town", and that proposing any alternative is dangerous heresy.
But let's look at the modelled temperature increases associated with each scenario. "Agree and ignore" sees temperatures rise by 4.85C by 2100 (with a 90% probability); for "Kyoto plus", it's 3.31C; and "step change" 2.89C. This is the depressing bit: no politically plausible scenario we could envisage will now keep the world below the danger threshold of two degrees, the official target of both the EU and UK.
This means that all scenarios see the total disappearance of Arctic sea ice; spreading deserts and water stress in the sub-tropics; extreme weather and floods; and melting glaciers in the Andes and Himalayas. Hence the need to focus far more on adaptation: these are impacts that humanity is going to have to deal with whatever now happens at the policy level.
But the other great lesson is that sticking with current policy is actually a very risky option, rather than a safe bet. Betting on Kyoto could mean triggering the collapse of the West Antarctic ice sheet and crossing thresholds that involve massive methane release from melting Siberian permafrost. If current policy continues to fail - along the lines of the "agree and ignore" scenario - then 50% to 80% of all species on earth could be driven to extinction by the magnitude and rapidity of warming, and much of the planet's surface left uninhabitable to humans.
Billions, not millions, of people would be displaced.
So which way will it go? Ultimately the difference between the scenarios is one of political will: the question now is whether humanity can summon up the courage and foresight to save itself, or whether business as usual - on climate policy as much as economics - will condemn us all to climatic oblivion.
James Nash is a climate scientist with Greatest Planet (www.greatestplanet.org). Greatest Planet is a non-profit environmental organization specialising in carbon offset investments.
James Nash is solely responsible for the contents of this article.