The Second Number: 565 Gigatons
Scientists estimate that humans can pour roughly 565 more gigatons of carbon dioxide into the atmosphere by midcentury and still have some reasonable hope of staying below two degrees. (“Reasonable,” in this case, means four chances in five, or somewhat worse odds than playing Russian roulette with a six-shooter.)
This idea of a global “carbon budget” emerged about a decade ago, as scientists began to calculate how much oil, coal and gas could still safely be burned. Since we’ve increased the Earth’s temperature by 0.8 degrees so far, we’re currently less than halfway to the target. But, in fact, computer models calculate that even if we stopped increasing CO2 now, the temperature would likely still rise another 0.8 degrees, as previously released carbon continues to overheat the atmosphere. That means we’re already three-quarters of the way to the two-degree target.
How good are these numbers? No one is insisting that they’re exact, but few dispute that they’re generally right. The 565-gigaton figure was derived from one of the most sophisticated computer-simulation models that have been built by climate scientists around the world over the past few decades. And the number is being further confirmed by the latest climate-simulation models currently being finalized in advance of the next report by the Intergovernmental Panel on Climate Change. “Looking at them as they come in, they hardly differ at all,” says Tom Wigley, an Australian climatologist at the National Center for Atmospheric Research. “There’s maybe 40 models in the data set now, compared with 20 before. But so far the numbers are pretty much the same. We’re just fine-tuning things. I don’t think much has changed over the last decade.” William Collins, a senior climate scientist at the Lawrence Berkeley National Laboratory, agrees. “I think the results of this round of simulations will be quite similar,” he says. “We’re not getting any free lunch from additional understanding of the climate system.”
We’re not getting any free lunch from the world’s economies, either. With only a single year’s lull in 2009 at the height of the financial crisis, we’ve continued to pour record amounts of carbon into the atmosphere, year after year. In late May, the International Energy Agency published its latest figures – CO2 emissions last year rose to 31.6 gigatons, up 3.2 percent from the year before. America had a warm winter and converted more coal-fired power plants to natural gas, so its emissions fell slightly; China kept booming, so its carbon output (which recently surpassed the U.S.) rose 9.3 percent; the Japanese shut down their fleet of nukes post-Fukushima, so their emissions edged up 2.4 percent. “There have been efforts to use more renewable energy and improve energy efficiency,” said Corinne Le Quéré, who runs England’s Tyndall Centre for Climate Change Research. “But what this shows is that so far the effects have been marginal.” In fact, study after study predicts that carbon emissions will keep growing by roughly three percent a year – and at that rate, we’ll blow through our 565-gigaton allowance in 16 years, around the time today’s preschoolers will be graduating from high school. “The new data provide further evidence that the door to a two-degree trajectory is about to close,” said Fatih Birol, the IEA’s chief economist. In fact, he continued, “When I look at this data, the trend is perfectly in line with a temperature increase of about six degrees.” That’s almost 11 degrees Fahrenheit, which would create a planet straight out of science fiction.
So, new data in hand, everyone at the Rio conference renewed their ritual calls for serious international action to move us back to a two-degree trajectory. The charade will continue in November, when the next Conference of the Parties (COP) of the U.N. Framework Convention on Climate Change convenes in Qatar. This will be COP 18 – COP 1 was held in Berlin in 1995, and since then the process has accomplished essentially nothing. Even scientists, who are notoriously reluctant to speak out, are slowly overcoming their natural preference to simply provide data. “The message has been consistent for close to 30 years now,” Collins says with a wry laugh, “and we have the instrumentation and the computer power required to present the evidence in detail. If we choose to continue on our present course of action, it should be done with a full evaluation of the evidence the scientific community has presented.” He pauses, suddenly conscious of being on the record. “I should say, a fuller evaluation of the evidence.”
So far, though, such calls have had little effect. We’re in the same position we’ve been in for a quarter-century: scientific warning followed by political inaction. Among scientists speaking off the record, disgusted candor is the rule. One senior scientist told me, “You know those new cigarette packs, where governments make them put a picture of someone with a hole in their throats? Gas pumps should have something like that.”