by Mark Lynas

The UK is in denial about its real carbon emissions, suggests a report from the Stockholm Environment Institute. The academics conclude that if "outsourced" emissions produced in countries like China on goods which are imported into the UK are included in our total carbon footprint, this country's total greenhouse gas emissions are 49% higher than currently reported. So we should think twice when blaming the Chinese for emitting the CO2 that is required in the manufacture of our fridges and televisions.

The report illustrates once again - as if we had forgotten - that global warming is an, er, global issue. A tonne of CO2 is a tonne of CO2, wherever it is emitted. How you do the counting is more a matter of politics than mathematics. A much greater concern is that all the politics is in danger of obscuring the increasingly drastic nature of the climate change threat. According to Andrew Simms of the New Economics Foundation, the world has only got 100 months left if we are to have a reasonably high chance of staving off runaway global warming.

This is a pretty dramatic claim, and the associated onehundredmonths.org website has an equally dramatic ticking clock counting down until runaway warming begins. "When the clock stops ticking," it states ominously, "we'll be beyond the climate's tipping point, the point of no return." Yikes. So how valid is this claim? Luckily, NEF's website provides a 100 Months technical note (pdf) explaining the calculations behind the new campaign. The first thing I noticed is that there isn't any new modelling work underlying the claim: it is based on existing science, in particular on an analysis by a researcher called Malte Meinshausen which was published in 2006.

Meinshausen was the first scientist to quantify with percentage figures the probability of exceeding certain climatic thresholds: in his 2006 paper he concluded that only by stabilising greenhouse gas concentrations in the atmosphere at 400 part per million (ppm) would it be "likely" (defined as 66-90% chance) that the world would stay below an eventual warming of two degrees. The NEF analysis has performed a fairly simple calculation, simply counting the time left before this 400ppm level is reached. The deadline, it turns out, is 1 December 2016.

There are several complicating factors, however. The 400ppm figure in question is not for CO2 only, but for a basket of atmosphere-altering gases - some of which have a positive "forcing" effect (like CO2 itself) whereas others have a negative (cooling) effect, like sulphate aerosols released by industry. Add the sum of these forcings together and you can arrive at a "CO2-equivalence" figure, which is the one that both NEF and Meinshausen use. The timescales need to be borne in mind, however: CO2 resides in the atmosphere for a century on average, whereas aerosols are washed out by rain in just a week or so.

There are other caveats too. Meinshausen is not saying that two degrees of warming will be reached with certainty when we cross the 400ppm threshold, but that the risk of seeing two degrees increases steadily thereafter. (Even at 400ppm there is still a risk of overshooting 2C, of somewhere between 2% and 57%.) At 450ppm the risk of crossing the 2C line rises to between 26 and 78%, whereas at 550ppm the risk of overshooting is between 68 and 99%. Indeed, for 550ppm the risk of overshooting even 3C ranges from 21% to 69%.

So what do all these numbers mean? Reading the small print, sceptics might complain about the false precision implied by the 100 months clock, which seems to suggest that the minute, indeed the second, we pass 400ppm we are certain to see two degrees of warming. The truth is that no one knows where any of the relevant climatic tipping points - from the disappearance of the Arctic ice cap to the release of methane from melting permafrost - actually lie. There are uncertainties regarding both what level of carbon emissions equals what temperature rise, and what temperature rise equals which climatic impacts. All we can say with near-certainty is that the warmer it gets, the further into dangerous territory we stray.

And again, there is the question of timescales. Meinshausen's two degrees calculations referred to two degrees of warming, not the minute the 400ppm line is crossed in December 2016, but when the atmosphere reaches "equilibrium" - in other words when all the warming processes have had a chance to feed through the system. Like a boiling kettle, the planet has a substantial thermal timelag - it takes a long time for ice sheets to rebalance themselves and for warmer waters to penetrate to the bottom of the deepest oceans. So even at this "tipping point" we still wouldn't see the expected two degrees of warming until the end of the century at least, if today's climate models are to be believed.

Reassuring, perhaps - but no cause for complacency. The earth's thermal timelag also means that today's emissions will keep on causing warming for decades to come, and that decisions made today on emissions cuts are essential if we are to rebalance the climate in the second half of the century.

The great danger of climate change is that it is a long-term systemic process. Self-evidently urgent threats - like wars or economic collapse - are easy to put at the top of our list of priorities. But climate change is a very slow process (note the current sceptic line of decrying the lack of year-on-year warming as hoped-for proof that it's all been a big mistake), and one where cause and effect (CO2=climate disasters) are not at all obvious at any intuitive level, hence the continuing predominance of wishful thinking, conspiracy-theorising and outright denial. Climate change clearly does not engage our natural psychological self-defence mechanisms.

This is the value of the 100 months campaign, which injects a sense of urgency into what is in reality a very slow process of cooking ourselves. We need to frame this issue as an urgent one to generate anything like an appropriate response, and indeed NEF explicitly uses the wartime analogy. But the drawback is also clear: in January 2017, after the deadline passes, people might either become fatalistic ("we've passed the tipping point, so let's give up") or might turn increasingly sceptical ("things don't look any different - I thought you said the world was going to end?"). In reality, this is a matter of risk analysis: how much risk of destroying our planetary habitat are we prepared to bear in order to keep on burning fossil fuels? Quite a lot, it would seem.

Mark Lynas is one of the leading voices in the climate change debate in the UK. He has worked full-time on the issue since 2000 .
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