Answering my question

[I posted an email to an Engineers and Scientists for Sustainability group that i was briefly involved with at the University of WA, about the question of CO2 vs CO2e and stabilisation targets. This is very good and clarifying response i recieved from a friend of mine.]

On Wed, Jan 16, 2008 at 12:16:58PM +0900, Trent Hawkins wrote:
> Great article, but im confused. The articles talks about 450ppm CO2 (only)
> as being the oldskool theoretical benchmark, whereas i thought it was 450ppm

Yes, this point is confusing and being an op-ed I'll wait for a paper or further statement to clarify.

Now, apart from being a "total GHG" v CO2 issue, it's also an issue whether they're talking about stabilisation concentration or not.

> CO2e. See the confusion is that we are already somewhere close to 450ppm
> CO2e, round about 430-440 at the moment. Given the lag in the system, this

There is a lot of confusion about this point in general. What you say is both true and false. This is actually best described in terms of forcings, not gas concentrations, but I'll try to use the latter.

The current CO2 concentration is about 383ppm. There have been also increases in other greenhouse gasses, like CH4, NOx, CFCs, etc, which add approximatelly 70ppm CO2 equivalent worth of forcing on top of that.

However, there are other additional manmade forcings, such as SO2, which effectively cancel out the non-CO2 gases in the atmosphere.

So the nett effect _currently_ is about 375ppm CO2e.

Now some vocal people like Monbiot, Tim Flannery have made statement along the lines of what you're saying. There's an article about this at realclimate, here:
http://www.realclimate.org/index.php/archives/2007/10/co2-equivalents/

(Monbiot is now calling for reductions of 110%, see: http://www.beyondzeroemissions.org/node/251)

However, there's an additional uncertainty here, because some of the negative forcing agents have much shorter lifetimes than the long-lived greenhouse gasses and so may precipitate out quickly if they stop being emitted.

To be honest, I haven't found a clear explanation (by a climatologist) which address this issue adequatelly. I think one could expect that if all emissions ceased today, the effective forcing would go up, but probably not up to 430-440ppm CO2 equivalent levels.

> would mean that we pretty much need to be doing what the article suggests
> and stabilise carbon emissions ASAP. Obviously Hanson's research says we
> need to aim for 350ppm CO2 (only) and we are currently at 380ppm +, then we
> need to basically have negative GHG emissions, but is this really anything
> overwhelmingly different to what previous studies have shown?

So, is this saying anything overwhelming different? It seems to be, but I can't be sure, because the op-ed isn't clear. Hansen has previously advocated keeping CO2 levels below 450-475ppm (see attached letter and paper), resulting in an additional temperature increase of ~1degC (~1.8degC above pre-industrial).

Now, it _seems_ he's advocating 350ppm or somewhere closer to 1.5degC above pre-industrial. With last year's record arctic ice-melt, it's plausible his position has changed, but we'll have to wait for more information to be sure.

Alternatively, it is possible that he was just talking about 350ppm _stabilisation_, which is broadly consistent with not passing 450ppm at all (and the AR4), as well as 450ppm CO2e stabilisation.

As far as negative GHG emissions are concerned, then no, we wouldn't have to extract GHGs out of the atmosphere if we suddenly stopped emitting today (to reach 350ppm CO2 stabilisation). All we'd need to do is reduce our emissions by more than the airborne fraction.

The airborne fraction is the proportion of our emissions that ends up in the atmosphere and has historically been around 58% (though is likely to increase, and probably is already beginning to). The other 42% has been absorbed by the ocean and biosphere.

Now, the nett CO2 flux between the ocean and atmosphere is mostly dependant upon the CO2 concentration in the two systems and the temperature. (Think of the CO2 flux in a shaken beer bottle) i.e. additional emissions only have a slight effect on it in the short term.

So that if we bring emissions down by about 60% globally, the atmospheric concentration should stabilise, for a while. If we decrease by more then it would decline.

There's a good piece which touches on this issue here:
http://www.realclimate.org/index.php/archives/2006/11/how-much-co2-emission-is-too-much/

Of course the reality of the situation is that we won't suddenly stop emitting, hence we should be cutting emissions as fast as possible.