According to one of its lead authors, and the latest draft [of the forthcoming IPCC AR5 report] seen by New Scientist, the report will say: “CO2-induced warming is projected to remain approximately constant for many centuries following a complete cessation of emission. A large fraction of climate change is thus irreversible on a human timescale, except if net anthropogenic CO2 emissions were strongly negative over a sustained period.”
In other words, even if all the world ran on carbon-free energy and deforestation ceased, the only way of lowering temperatures would be to devise a scheme for sucking hundreds of billions of tonnes of carbon dioxide out of the atmosphere.
As a rough guideline, 1ppm is equivalent to about 7Gt CO2. If we’re currently at 400ppm, getting down to 350ppm (a number I would suspect everyone here is familiar with), would require us to suck 350Gt CO2 from the atmosphere and sequester it. And that’s 350Gt in excess of our ongoing emissions, of course, since it has to be a net reduction.
But wait, since roughly half of our emissions wind up in the ocean — remember, that whole ocean acidification thing, a.k.a. global warming’s evil twin — as a back-of-the-envelope calculation we have to double that to 700Gt. As we draw down the atmospheric CO2 level, the ocean will give back the CO2 we forced into it, you see.
I will leave as an exercise for you, Dear Readers, how many Empire State Buildings per year (or month or day or hour) that task works out to, at 365,000 tons per ESB and whatever length of time you care to speculate it would take to accomplish that imposing task. Oh, and while you’re at it, feel free to work up some economic numbers. The usual ballpark figures I’ve seen for DAC (direct air capture) on a massive scale is somewhere around $200 per ton. Assume some technowizardry happens and we can cut that price in half. That whittles the cost to a mere $70 trillion. There’s also the question of where the heck we’d put — permanently, mind you — all that CO2. 700Gt of anything is really freaking big.
Oh, and did I mention that cutting all CO2 emissions would necessarily cut cooling aerosol emissions from burning fossil fuels? According to the copy of the AR5 report that was leaked in December, man-made aerosols have a cooling effect of nearly half of current net man-made climate forcing. That puts a very interesting spin on the question of how quickly you want to assume we could suck all that CO2 out of the air. We might find ourselves in a position of delaying fossil fuel cuts while we ramp up a DAC operation large enough to do the job, and then taper off CO2 emissions.
All of which is to say that we’ve put ourselves in a very deep and nasty hole with our past emissions. Can we get out? Of course. But anyone who tries to tell you that the situation isn’t urgent, or that it will be cheap or easy to save ourselves from almost unimaginable climate impacts, is either shockingly deluded or a liar of the first order.