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There are, we are told, only two options.
Either we stop burning fossil fuels before our carbon dioxide emissions drive the planet's average temperature up a full 2degC, in which case we will push the world into the biggest-ever recession. Or we continue to burn fossil fuels and push the planet into runaway warming, with lethal consequences for a large part of the human race.
The 2008 bank crash that triggered the recent recession was caused mainly by reckless investment that created a ''bubble'' in house prices. When the bubble burst, hundreds of billions of dollars' worth of investments suddenly became worthless. The losses were so great that they nearly brought the whole banking system down.
This time, the problem is a ''carbon bubble''. The market valuation of the world's 200 biggest oil, gas and coal companies is about $4 trillion, a figure based on the assumed value of their confirmed reserves that are still in the ground. Or, more precisely, a figure based on the assumption that they will eventually be able to sell all of those reserves to customers who want to burn them.
On the strength of that assumption, the fossil fuel companies have been able to take on $1.5 trillion of debt, and last year alone they spent $647 billion in the search for even more oil, gas and coal reserves. But what if they will never be able to sell all of their reserves? What if the need to avoid runaway warming forces governments to curb the burning of fossil fuels, so that much of those reserves has to stay underground forever?
This is the focus of a new report titled ''Unburnable Carbon 2013''. The report's authors, the Grantham Research Institute at the London School of Economics and the Carbon Tracker Initiative, have the support of organisations like the HSBC and Citibank, the Standard and Poor's rating agency, and the International Energy Agency.
Their conclusion is that if we are to have a 50% chance of stopping the warming before +2degC, then at least two-thirds of the fossil fuel reserves listed at present will have to stay in the ground permanently. If they cannot be burned, then they have no economic value. Therefore, the market valuation of the fossil fuel companies is three times higher than it should be.
The report assumes rationality will prevail, and at some point a limit will be imposed on the burning of fossil fuels. In this new reality, the debt burden of the fossil fuel companies becomes unsustainable and there is a financial meltdown that dwarfs 2008. Global warming is held to +2degC but at the cost of the Mother of All Recessions.
The other option is that no controls are imposed on burning fossil fuels, and the carbon bubble does not burst until the warming breaks through the 2degC limit and triggers the natural feedbacks that will carry us inexorably up to +6degC.
That implies mass death and possibly civilisational collapse by the end of the century, but the fossil fuel reserves will retain their assumed value for the meantime and there will be no financial crash.
This is the scenario the market is betting on and at the moment most of the evidence supports that wager. The ideological and commercial interests that oppose action on climate change have triumphed in the United States and Canada, and without the US, decisive action is hard to imagine.
The denial campaign has not explicitly defeated science elsewhere but four years of recession in Europe have had much the same effect, sapping the will of governments to spend money fighting climate change. Last week, for example, the European Parliament refused to fund a scheme to rescue the carbon emissions trading scheme, once the centrepiece of the EU's climate strategy.
In big, rapidly developing countries like China and India, the race for growth takes priority over cutting carbon emissions. And just when you think things couldn't get worse, along comes shale gas to expand the fossil fuel reserves even further.
It's a grim choice: either financial meltdown if we act decisively to halt climate change, or physical meltdown if we don't. But there is, unfortunately, a third alternative. In fact, it's the likeliest outcome by far.
First, we go on growing our emissions at the current rate (3% per year) for the next couple of decades, and the fossil fuel industry thrives. Then, when it's already too late and we have crossed the +2degC limit, the actual warming (which always lags the growth in emissions by a decade or more) frightens us into taking action at last.
So we lurch into a crash programme to cut fossil-fuel use - and suddenly the market wakes up to the fact that a lot of those reserves will have to stay in the ground forever. If you liked the sub-prime mortgage fiasco in 2008, you'll positively love this one.
It's not either Disaster A or Disaster B. It's first one and then the other, interlocking and mutually reinforcing. And Disaster B will mean there's no money left to do anything about Disaster A.
- Gwynne Dyer is an independent London journalist.