We recently passed 400 parts per million of CO2 in the atmosphere; the status quo will take us up to 1,000 ppm, raising global average temperature (from a pre-industrial baseline) between 3.2 and 5.4 degrees Celsius. That will mean, according to a 2012 World Bank report, "extreme heat-waves, declining global food stocks, loss of ecosystems and biodiversity, and life-threatening sea level rise," the effects of which will be "tilted against many of the world's poorest regions," stalling or reversing decades of development work. "A 4°C warmer world can, and must be, avoided," said the World Bank president.
But that's where we're headed. It will take enormous effort just to avoid that fate. Holding temperature down under 2°C — the widely agreed upon target — would require an utterly unprecedented level of global mobilization and coordination, sustained over decades. There's no sign of that happening, or reason to think it's plausible anytime soon. And so, awful shit it is.
Nobody wants to say that. Why not? It might seem obvious — no one wants to hear it! — but there's a bit more to it than that. We'll return to the question in a minute, but first let's look at how this unsatisfying debate plays out in public.
Are scientists keeping it real?
The latest contretemps was sparked by a comment in Nature by Oliver Geden, an analyst at the German Institute for International and Security Affairs. In it, he made a simple argument. Politicians, he says, want good news. They want to hear that it is still possible to limit temperature to 2°C. Even more, they want to hear that they can do so while avoiding aggressive emission cuts in the near-term — say, until they're out of office.
Climate scientists, Geden says, feel pressure to provide the good news. They're worried that if they don't, if they come off as "alarmist" or hectoring, they will simply be ignored, boxed out of the debate. And so they construct models showing that it is possible to hit the 2°C target. The message is always, "We're running out of time; we've only got five or 10 years to turn things around, but we can do it if we put our minds to it."
That was the message in 1990, in 2000, in 2010. How can we still have five or 10 years left? The answer, Geden says, is that scientists are baking increasingly unrealistic assumptions into their models.
Can we really suck a bunch of carbon out of the atmosphere?
Geden focuses on one such assumption: that substantial negative emissions will be possible in the latter half of the 21st century. We will be able to suck thousands of megatons of carbon out of the atmosphere, so humanity can go net negative by 2100, even if we emit a bunch more carbon in the short term.
The mechanism for negative emissions is supposed to be bioenergy — burning plant mass — coupled with carbon capture and sequestration. The combo is called BECCS, and in theory, it buries more CO2 than it emits.
(Sanchez et al., 2015)
If you work enough BECCS into your model, you can almost double humanity's "carbon budget" — the amount of carbon we can still pump in the atmosphere without passing 2°C. After all, if you can suck half the carbon out, you can afford to pump twice the carbon in.
But is large-scale BECCS plausible? There's the problem of finding a source of biomass that doesn't compete with food crops, the harvesting of which does not spur additional emissions, and which can be found in the enormous quantities required. The IPCC scenarios that come in below 2°C require BECCS to remove between 2 and 10 gigatons of CO2 a year from the atmosphere by 2050. By way of comparison, all the world's oceans combined absorb about 9 gigatons a year; all the world's terrestrial carbon sinks combined absorb about 10 gigatons a year.
These scenarios mean potentially doubling the capacity of terrestrial carbon sinks, capturing and burying — permanently, without leaks — gigatons of CO2 a year. How will it be monitored? What if it leaks or is breached?
There's no consensus on the viability of widespread BECCS, which, after all, doesn't exist yet. One 2014 commentary in the journal Nature Climate Change, co-bylined by 14 researchers, raised serious doubts about the feasibility of large-scale BECCS and the wisdom of betting the climate farm on it. They note that "deployment of large-scale bioenergy faces biophysical, technical and social challenges, and CCS is yet to be implemented widely," and that "widespread deployment [of BECCS] in climate stabilization scenarios might become a dangerous distraction."
