I keep hearing that "most" of the observed global warming is man-made, but I think that is just short hand to avoid confusing the most casual audience, and it perhaps causes more misunderstanding than it avoids.
More than 100% of the warming is due to anthropogenic warming forcings. That is balanced by a hard-to-constrain anthropogenic cooling.
Whether the residual is even attributable to natural variability is not obvious to me; the residual may be dominated by adjustment transients and nonlinear couplings.
But is the residual a warming or a cooling? I think there is no evidence that we would be in a warming period had humans gone suddenly extinct in 1700 or so.
Natural forcing is basically volcanic and solar, right? Is there any evidence that these have been weighing on the warming side over the past century?
Skeptical science basically says no, by the way.
What you are missing is natural variability, which in principle could have caused some of the warming. Also, that "most" is a statement at the "very likely" level. The real best estimate is, as you say, "roughly all" of the warming, or even "more than all".
ReplyDeleteI think James means "unforced" or "internal" by "natural", above, which I used in a more inclusive sense.
ReplyDeleteYes, this is what I think as well.
The whole awkward concept of "more than all" seems to arise from the misframing of the core public issue as "warming" in the first place.
> "...the misframing of the core public issue as "warming" in the first place. "
ReplyDeleteHow could you frame it differently, such that a) the consequences are obvious to a harried listener and b) the cause is "human actions"?
(seems to me, all the other consequences *besides* the warming are harder to attribute to humans)
Also: what already-published paper explicitly tackles this (% warming that's attributable to humans), and if none, why the heck not?
p.s. apologies if these have already been addressed, as they likely have; the memory I have for Qs seems to surpass the one for answers.
Also: by "variability", James does not mean "cycles", correct?
ReplyDelete(at least, no cycles we're currently aware of)
IIRC Milankovitch cycles are in a (slight) cooling phase, so that would seem to be the residual given enough time to let roporockany shorter-term volcanic and solar influences even out.
ReplyDeleteI think there was an RC post on this that concluded with just plain "all of it." IMHO most of the problem is that just a few years there was a whole lot of uncertainty. People get trained to describe things in a certain way and it becomes hard to untrain them.
Trenberth & Schmidt quotes, on how much of the warming has been human-caused
ReplyDeleteAt least in the Arctic, there is evidence of a natural cooling trend in the previous 1900 years being reversed by anthropogenic warming:
ReplyDeleteHighest Arctic Temperatures in 2000 Years
http://capitalclimate.blogspot.com/2009/09/highest-arctic-temperatures-in-2000.html
I think James means "unforced" or "internal" by "natural", above
ReplyDeleteYes, having chosen my words carefully I then went and typed the wrong thing...
Anna, the formulation is basically agnostic as to (internal) "cycles" versus "noise", although in both cases one might expect the direction of recent internal variability to be opposed in future (unless it's truly a random walk). My own hunch is that there has been a little internal positive variability to augment the total forcing over the late C20th, but not a lot (eg Tsonis and Swanson without the hype).
If you use the IPCC transient sensitivity range (1 to 3°C for 2xCO2, most likely 2°C), the CO2 increase alone has caused 0.5 to 1.5°C warming, most likely 1°C. Add in other GHGs, and it rises to 0.7 to 2.1°C, most likely 1.5°C. But of course man-made aerosols complicate the whole thing.
ReplyDelete