The more I read about the trillionth ton (or tonne for our non-American friends), the more intrigued I am by its power to change the way we approach the threat of global warming. I wrote last week about the idea, which represents a whole new way of thinking about carbon emissions, but I’d like to take another stab at it, in hopes of spreading the meme further than my last post managed.
The “trillionth ton” refers to the amount of carbon in the atmosphere. Before the invention of the steam engine and everything that followed since the middle of the 18th century, there was a certain amount of that fundamental element in the air. What humans managed to do over between 1750 and 2000 is transfer somewhere between 500 and 600 billion tons of the stuff from its long-term hiding place underground in the form of coal, oil and natural gas into the atmosphere.
There, in the form of carbon dioxide, methane, and few other molecules, it absorbs and re-radiates heat, warming the planet as it does. Figuring out just how much carbon results in how much warming has been a major focus of climatology for more than a century. The general agreement among those who are paid to calculate such relationships is that if you double the amount of carbon in the air, you raise the global average temperature by 3 °C (almost 6 °F). There is an important caveat here, but I’ll get to that later.
Most climatologists suspect that dangerous climate change will kick at only 2 °C above pre-industrial levels and while this is somewhat arbitrary, it’s warmer than the Earth has been for millions of years, and it’s as good a guess as you’re going to find in the literature. We’ve already raised the Earth’s temperature by 0.7 or 0.8 °C. and there’s upwards of another half a degree or so in the proverbial pipeline even if we stop burning everything now.
The question according to this new meme becomes, how much carbon can we afford to pour into the air before we hit 2 degrees of total warming? We’re talking about the cumulative emissions of carbon, not a specific concentration or a rate of emissions, and we aren’t even concerned about a time deadline, just a grand total of allowable emissions.
A pair of papers just published in Nature are in rough agreement that the Earth, with all its carbon sinks and oceanic heat-redistributive powers, might be able to handle a trillion tons of the stuff and stay below the extra 2 degrees. But we’ve already emitted more than half of that trillion, so we can only afford another 400 million tons or so before our only options become geo-engineering, sucking carbon out of the air and other schemes so expensive they make current plans to scale back emissions by 90 percent in a few decades look like a bargain by comparison.
But wait. Only part of our emissions comes in the form of carbon dioxide from fossil-fuel combustion. Most of the rest come from agriculture and forestry. We’ll have to deal with those too, but it means that our allowable energy-producing carbon emissions quota is down to about 275 billion tons. Remember that our quota includes everything since the year 2000. And since then we’ve been emitting almost 10 billion tons a year, which means we’ve already used up nearly a third of that 275 billion tonnes.
Which leaves us with around 190 billion tonnes. That’s it. Just 20 years at business-as-usual rates.
Our options are limited. We can continue on the current path, as Bjorn Lomborg and his acolytes would have us do, for another decade or so on the assumption that future carbon-free ways of producing electricity and moving our cars will be dramatically cheaper than they are today, and then make the entire switch in a year or two. But I think most will see the problems with that strategy.
Or we can start bringing our emissions down immediately, as fast as possible. Here’s how a group of climatologists, including the lead authors or the “trillionth ton” papers explain the situation in a Nature Climate commentary:
A tonne of carbon is a tonne of carbon, whether released today or in 50 years time. Emitting CO2 more slowly buys time, perhaps vital time, but it will only achieve our ultimate goal in the context of a strategy for phasing out net CO2 emissions altogether.
At some point in the past few years, without any fanfare, we burned the half-trillionth tonne. Somewhere out there, in a coal seam, hydrocarbon reservoir or some as-yet-undiscovered exotic form of fossil carbon, lies the trillionth tonne. Its fate, perhaps more than any other consequence of climate-change policy, is inextricably linked to the risk of dangerous climate change. Where will it be in the twenty-second century?
But now to the caveat I mentioned. All of this trillion-ton talk is based on notion of the climate sensitivity of 3 °C for a doubling of carbon dioxide levels. Again, that’s become an accepted relationship. But it’s based only on “fast feedbacks” like the amount of water vapor in the air. (As the world warms, more water evaporates, and water vapor warms the world, too, resulting in a positive feedback.)
There are, however, long-term feedbacks, such as how much sunlight the Earth reflects back into space before it can trapped in the atmosphere. NASA’s James Hansen, using ancient records of climate change, calculates that the real climate sensitivity over the long term is more like 6 °C. If that’s true, then amount of carbon we can safely emit may be significantly lower than a trillion tons, and what remains of that quota proportionally smaller, too. We may even have already used it all up.
Here’s how Stephen Schneider, in the same issue of Nature Climate, deals with the lack of consensus on this question:
Even if there’s no inherent limitation on scientists’ ability to figure out the climate’s sensitivity, since it’s proven so hard to home in on, learning to live with the uncertainty might be the safest bet. But it’s not a reason for inaction, Schneider stresses. “Policy depends upon a generational transformation of basic energy production systems,” he says. “You can’t wait until you know. By that time it’s way too late to do anything about it. That’s not how anybody treats cancer, that’s not how anybody makes investments, that’s not how the military operates. And we are not entitled to this luxury.”