The most striking recent development to emerge from UN climate negotiations is the growing consensus that within a generation the whole world will have to stop spewing carbon dioxide into the air from energy use.
This means that within the lifetimes of today's toddlers we would entirely eliminate CO2 emissions, unless they are offset by subtractions. (Carbon dioxide could be subtracted naturally, for example, by growing more trees.)
In shorthand, the new goal is "net zero"—and the sooner the better. Climate hawks say it should be met as early as 2050. Others see a few more decades of wiggle room, but they too emphasize the need for rapid action.
Are these ambitious targets at all plausible?
The answer depends in part on the definitions of what constitutes a safe amount of climate change. Two degrees Celsius of warming is the widely accepted target, but some say the goal should be 1.5 degrees. At present, the world seems to be aimed at 4 degrees of warming, at least.
It depends also on how certain we want to be about achieving the chosen target. To have a "likely" chance (a 66 percent probability) means scoring deeper, earlier reductions than what's needed for a mere 50-50 chance of success. To have a 95 percent chance at success, cuts would have to be radically deeper and astonishingly fast.
A new briefing note from Climate Analytics describes the timetables in great detail.
For a 66 percent likelihood of avoiding 2 degrees of warming at the end of this century, global greenhouse gas emissions by 2050 would have to be 40-70 percent below 2010 levels; CO2 emissions alone from energy and industry would have to drop 35-80 percent. Overall emissions of all greenhouse gases would need to reach zero some time between 2080 and 2100, but energy and industry emissions of CO2 would have to reach zero sooner, by 2060 to 2075.
Whatever the timetable, it is plainly the view of nations involved in shaping a climate treaty in Paris by year-end—from the leading industrial states of Europe to the smallest island states—that the time has come to move in the direction of zero emissions, and as rapidly as possible.
Idea Takes Hold
The net-zero goal emerged last year out of the fifth assessment report of the Intergovernmental Panel on Climate Change (IPCC). The panel detailed more thoroughly than ever the concept of a global carbon budget—the amount of cumulative emissions that scientists say cannot be exceeded if the world is to keep global warming within safe limits.
Scientists and energy policy experts translated that work into an ambition of breathtaking simplicity: to phase out carbon emissions entirely.
At talks in Lima in Peru, the idea took hold more strongly than ever, and in a treaty-drafting session in Geneva this month, it was widely embraced.
"The text mentions net-zero, near zero, climate neutrality or full decarbonization 15 times," wrote Simon Evans in an excellent treatment of the subject at Carbon Brief. "Some include deadlines for reaching net zero, others do not."
Net-zero advocates say getting there by 2050 would be none too soon.
That extraordinarily ambitious timeframe is what a group of prominent business leaders, who call themselves the B Team, recommended on Feb. 5. Among their leaders are Paul Polman, the chief executive of Unilever, and Richard Branson, the founder of Virgin Atlantic.
They say the goal is reasonable, and spreading.
"Momentum is already growing in finance, business and political circles for a net zero goal," wrote Mary Robinson, a former president of Ireland who is the UN Secretary General's special envoy on climate change.
"But we need more. Adopting the net zero goal is not something to be put off until later," she went on in the foreword to a report published by Track0.org called "The Business Case for Adopting the Long-Term Goal for Net Zero Emissions."
The big oil and gas companies such as Shell, Exxon and BP call it an unrealistic idea. So do some big petro-states like Saudi Arabia.
The 2050 timeline is "not really practical," said David Hone, the climate change adviser for Shell.
"The Paris agreement should certainly be geared around an end-goal of net-zero emissions, but the realistic, albeit still aggressive, time span for this is 80+ years, not 35 years," he wrote on his blog.
Hone said he does understand the imperative.
"There is no doubt that to quickly and decisively solve the climate issue and have a better than even chance of keeping the surface temperature rise below 2 degrees C that we need to do this," he wrote, "but that doesn't mean we can."
Certainly the momentum under current policies is still taking the world in the wrong direction, despite pledges to act by many major polluting countries.
BP, in its annual forecast this week, said that it expects emissions of carbon dioxide to rise another 25 percent by the year 2035. That would certainly make it all but impossible to reach zero by 2050, and probably less than likely to get there by the end of the century—unless something changes.
One particularly powerful change, the big oil companies often say, would be the worldwide use of carbon pricing schemes, either through taxes or cap-and-trade regimes. Another would be a breakthrough allowing widespread use of carbon sequestration—capturing emissions and burying them in the ground.
In any event, there are credible reviews that show that it is technically possible for society to choose realistic pathways to deep decarbonization—even to eliminate emissions from fossil fuels altogether.
A newly updated study called "Pathways to Deep Decarbonization in the United States" conducted by economic consultants and two U.S. national labs using standard energy and economy models found that "it is technically feasible to achieve an 80 percent greenhouse gas reduction below 1990 levels by 2050 in the United States, and that multiple alternative pathways exist to achieve these reductions using existing commercial or near-commercial technologies."
The cost, it said, "does not appear prohibitive." But it would "constitute an ambitious transformation of the energy system."
The report is part of a comprehensive effort organized by the Sustainable Development Solutions Network and the Institute for Sustainable Development and International Relations, which last year brought together experts from around the world to advise the UN on deep decarbonization pathways.
Elsewhere, there's even a new on-line calculator you can use to try out various pathways toward decarbonization, answering the question posed by its sponsors: "Is it physically possible to meet our climate targets and ensure everyone has good living standards by 2050?" (Spoiler alert: the sponsors themselves generated four "plausible pathways" to a 2-degree-warmer world.)
Optimists frequently say that shifting investment from fossil fuels to alternatives will prove economically rewarding, not punitive, especially when the benefits from reducing health impacts from pollution are taken into account.
"Many still seem to be talking and acting as if change is too difficult and costly and as if delay is not a problem," wrote Nicholas Stern, a preeminent climate and development scholar, in a December 2014 policy paper. "Yet we can now see that the transition to a low-carbon economy can be highly attractive, embodying strong and high quality growth, driven by strong investment and innovation."
For all the scenarios describing pathways to zero emissions, there are many doubters who say societies just don't change that fast. One critical article questioned the feasibility of achieving "historically unprecedented improvements in energy intensity." It kicked off a spirited debate recently among climate bloggers, including David Roberts of Grist, Joe Romm of Climate Progress and Andrew Revkin of Dot Earth.
Whatever the uncertainties, the scientific consensus is clearly warning against business as usual. A new summary of mainstream thinking by Australia's National Academy of Science hits all the highlights. The current rate of emissions is leading to temperatures not seen on the earth for millions of years, and the risks of that are high. At the current rate of emissions, the 2-degree limit would be violated within just a few more decades. What's needed long term are reductions of 5.5 percent to 8 percent per year.
Short of geoengineering, which has risks and costs all its own, just about "the only way to stop human-induced climate change is to reduce greenhouse gas emissions to net-zero levels," the Australian academy said. The longer this takes to achieve, and the more greenhouse gases that are emitted in the meantime, the larger the scale of future climate change."