The chart above shows various emissions scenarios for anthropogenic CO2 emissions out to 2100 (expressed in Gigatonnes of C/yr). The black curve shows actual data according to BP from 1965 to 2011. The next four curves (green, yellow, orange, and red) are from the four representative concentration path (RCP) scenarios that are being used in the 5th IPCC report. These replace the old A1FI, B1, etc. You can find the data for them here (they ask for your contact details). The blue dashed curve we will come to shortly.
I would like to critique the RCP curves which I find implausible in various ways. Let's start by characterizing them. The highest emissions scenario is the red curve labeled RCP-8.5 (the curves are labeled by the total increase in radiative forcing by 2100, so RCP-8.5 corresponds to 8.5W/m2 of additional forcing by the end of the century). This is clearly the "business as usual" scenario that assumes we grow emissions at pretty high rates (they grew 2.8%/yr on average from 2001 to 2011) and continue to do so well out into the second half of the century. Emissions (not concentration in the air) only start to stabilize in the last decade or two of the century.
At the other extreme, the green curve (RCP-2.6) assumes that the emissions curve starts to bend immediately right now, peaks in the next ten years, and then heads down over the course of several decades until it turns negative after 2080 (we start reabsorbing carbon from the atmosphere).
The yellow (RCP-4.5) and orange (RCP-6.0) scenarios are intermediate ones with rather strange details - eg RCP-6.0 involves stabilizing emissions for the next couple of decades but then allowing them to go up markedly again after 2030.
Ok. I'd like to make the following fairly simple common-sense assumptions about the overall shape of the problem:
- People have a strong tendency to be short-sighted and greedy and ignore distant and theoretical-seeming problems.
- When faced with an imminent serious threat that is graphically clear to them, people pull together and take strong action in response to the threat.
- Right now, humanity en-masse is pretty much ignoring climate change and growing emissions at a rapid pace as the developing world moves into the fossil-fuel powered industrial age (especially China in the last decade).
- Climate change is a very serious problem with a longish lag between action and consequence. However, well before the end of this century, business-as-usual will result in intolerable conditions over much of the land area currently used by civilization.
This shows the anticipated number of hundred degree days in the United States in the recent past and by the 2080s in a high emissions scenario (along the lines of RCP-8.5). The point is that conditions that presently only occur in southwestern deserts like the Mojave will obtain throughout much of the country, with obviously disastrous consequences for agriculture, forests and all other forms of ecosystem, as well as human quality of life:
I argue that while humans are short-sighted and greedy, we aren't so stupid that we won't call the fire brigade once the house is clearly on fire. Long before we get to the 2080s, we will be going all out to resolve the problem, to stop emitting carbon and to start reabsorbing it. We will be installing solar panels and wind turbines everywhere, making every building and vehicle as efficient as possible, installing grid capacity and storage all over to deal with the inevitable intermittency of renewable power, capturing carbon from the air and stashing it in buildings and underground and so forth.
So in terms of scenario building, I want to see something that doesn't sharply kink at 2010 - there seems no reason to think that right now is the moment when the entire human race is going to get with the program. Instead, we should assume emissions will continue to rapidly grow for another decade, perhaps two, hard to say exactly. But then, as the consequences become clearer and clearer - the fires, the droughts, the floods, the storms, the food price spikes, people will start to panic. Then the political will to really tackle the problem will emerge - sooner in some countries (Europe has already made a start), later in others. Since the problem is not technically impossible, we will go about solving it. However, given the long life of infrastructure, this is likely to take several decades.
Thus the blue dashed curve (SSWAG):
Here I start at the RCP-8.5 level in 2010, grow at recent rates for another decade, then start to slow, with emissions peaking around 2030. After that, I assume we are in full-scale panic mode with dreadful (and steadily worsening) weather and frantic full-on attempts to convert all infrastructure to carbon neutral, and then go carbon negative in an attempt to stabilize the climate. Hence the curve drops rapidly in mid-century and then goes negative by the 2070s. Of course, I'm not going to claim that I know the exact shape, date of the peak, etc with much accuracy. But it has to look qualitatively something like that blue dashed line.
Note that the concentration in the air will start to decline some time after emissions peak but before they hit zero as the natural world is absorbing quite a lot of our emissions (though this could change in future if natural ecosystems start to degrade rapidly enough under the influence of the changing weather - particularly the droughts).
Obviously, we are going to experience a lot of climate change and if we fail to stabilize the climate in some zone reasonably tolerable for civilization, then the latter will collapse (and not leave much basis for human sustenance behind it either). I do think the blue path is quite risky. However, we won't experience a climate-change driven collapse without the system first making an all-out effort to solve the problem. Let's hope we succeed.