The Island of Doubt

"If you flipped on one of the news channels that covers the financial news ... and there was a number that was updating once every five years, the commentators would have a hard time finding something to talk about," Kevin Parker, the global head of Deutsche's asset management team, told reporters. "The minute you convert that to a real-time number, it can serve as a backdrop for lots of conversations."

Those conversations would much more interesting and useful if the clock, instead of tallying up how much CO2 we're pumping out (800 tonnes a second), gave us a count down of how much carbon we can still safely emit before we commit the planet to what Jim Hansen calls "dangerous anthropogenic interference."

In effect such a clock would be doing the math for the rest of us, who wouldn't need to perform a serious of functions on the existing clock's readout as they do now. I mean, so what if there's 3.6 trillion tonnes of CO2 up there? Without also knowing how much was there before we started burning coal and petroleum products, and at what point those gases will cause the Earth to warm more than 2 °C above pre-industrial levels, it's not a very useful number.

If, instead, the clock's algorithms were to start with the observation that, in order to keep the world's average temperature below that point we can only emit another 250 billion tonnes of carbon dioxide, and count down from there, at 800 tonnes a second, the resulting readout would be several orders of magnitude more dramatic and provocative.

To be sure, there's no hard number that everyone agrees on of just how much more CO2 (or carbon) the Earth's climate can handle before we trigger tipping points. The figure offered above is based on a couple of recent papers in Nature, and it may yet prove to be less than accurate, in either direction. But again, simply keeping track of cumulative emissions tells us nothing useful unless we do apply some additional numerical guesswork.