Did you notice that RC called it "Meinshausen et al"? Barbarians :-). Anyway, they liked the paper whilst I'm less sure. As far as I can tell its not really a question of science in dispute, just what you make of it. So what M et al. do is instead of the std.ipcc "force a GCM with CO2 and see how climate changes" they try to reverse this process, and see what level of CO2 produces a given temperature change. They can't do this with GCMs, of course, so are fitting the GCM stuff to a simpler model. And since there is uncertainty in the sensitivity, there is uncertainty in the result, but at heart it all seems simple enough and scientifically rather dull, though politically interesting. M et al. pretty well admit that they have spotted a gap in the literature and headed straight for it: there is still an important gap in the literature relating emission budgets for lower emission profiles to the probability of exceeding maximal warming levels; a gap that this study intends to fill (isn't there an old joke about that? Can't find it now). The other thing they find is the rather convenient result that warming depends on total CO2 emissions, rather than the CO2 trajectory. Given plausible trajectories and CO2 lifetime and climate response times this too is not exactly surprising; but its good to have it written down.
M et al. skip rather lightly over the 2 oC threshold. 2 oC is obviously somewhat arbitrary; all they say to justify it is "We focus here on 2 oC relative to pre-industrial levels, as such a warming limit has gained increasing prominence in science and policy circles as a goal to prevent dangerous climate change " and 25 is Schellnhuber in Avoiding Dangerous Climate Change. I've commented on this before (and got a lot of comments in reply, but didn't change my mind). RC seem to think that 2 oC is arbitrary, but on the high side: We feel compelled to note that even a "moderate" warming of 2Â°C stands a strong chance of provoking drought and storm responses that could challenge civilized society. My feeling is that when this stuff starts to cost real money rather than policitcal promises, and people start saying "OK, limit CO2 to keep us below 2 oC, remind me again just exactly why 2 oC is so dangerous" they are not going to find the answers very convincing. There is risk, of course, but there is risk in every course of action.
More interestingly (to me) is a comment in the Nature intro saying "Yet only a third of economically recoverable oil, gas and coal reserves can be burned before 2100 if that 2Â°C warming is to be avoided", ref'ing this paper. I've started to wonder recently if we really do have the reserves to get much beyond 2xCO2, so picked up on this - perhaps they will provide some good sources. But what M et al. say is: We show that, for the chosen class of emission scenarios, both cumulative emissions up to 2050 and emission levels in 2050 are robust indicators of the probability that twenty-first century warming will not exceed 2 oC relative to pre-industrial temperatures. Limiting cumulative CO2 emissions over 2000-50 to 1,000 Gt CO2 yields a 25% probability of warming exceeding 2 oC--and a limit of 1,440 Gt CO2 yields a
50% probability--given a representative estimate of the distribution of climate system properties. As known 2000-06 CO2 emissions3 were 234 Gt CO2, less than half the proven economically recoverable oil, gas and coal reserves can still be emitted up to 2050 to achieve such a goal. So is the half (or a third, where ever that comes from) emissions to 2050, or to 2100? If it emissions to 2050, then we're going to need to emit *all* of the economically viable reserves to keep on the "bad" trajectory to 2100.
M et al. (fig 3) says that total econ-viable reserves of CO2 are ~2500 Gt. It also has as x-axis the cumulative emissions 2000-2049, and that axis ends at 2500. A1F1 on that graph is past 2500, which presumably means that to get A1F1 to 2100 we have to burn *more* than the total econ-viable reserves. That seems rather unlikely. Indeed, from that graph, it would appear that most of the std IPCC/SRES scenarios are effectively impossible. It isn't obvious to me what is wrong with my reasonning; but if it is correct I would have expected M et al. to point out that many of the SRES trajectories are impossible, and they don't. Up to now I've been defending SRES (on wiki etc.) against those who say the trajectories just won't work; but I'm now rather less sure.
Minor point: their figure 2(e) appears slightly dodgy. They show scary-looking temperature changes of 4-6 oC with central value of 5 oC to 2100 from A1F1, and that is only the inner range. It doesn't look totally compatible with AR4 SPM.5. Partly that is because they put their zero at 1860 (which is quite defensible) and partly because they have picked the high-end A1F1... but from SPM.5 it looks to me as though A1F1 should be ~4.6 above 1900, not +5.
FWIW, I think we're going to find out what +2 oC and above is like, because I don't think we've got the will to avoid it. Studies like this one are probably politically useful less because of the new science they contain than because they push the aggregate weight of discourse in a certain direction, and give those who want to push for limits some useful ammo. But the response to the credit crunch makes it rather clear, as mt has despairingly commented, that our priority number one is a growing economy. Only once we have that will we consider other low priority matters, like, err, not growing the economy :-).
[TL points to a nice oildrum post summarising some recent literature. Which indicates that 450-600 ppm CO2 is a likely limit. It also says what I've been thinking - that very few people are studying this, compared to those doing other aspects of cl ch -W]
Serious effects for ocean marine organisms long before a 2 K increasee above a 1860--1900 reference level.
Furthermore, ocean acidification is happening even more quickly in the Arctic, as shown in Stenacher et al. (2009, April), âImminent ocean acidification in the Arctic projected with the NCAR global coupled carbon cycle-climate model,â http://www.biogeosciences.net/6/515/2009/bg-6-515-2009.pdf (open access):
We highlight that the Arctic Ocean surface becomes undersaturated with respect to aragonite at even lower CO2 concentration (than the Southern Ocean). The combination of observation-based estimates â¦ with NCAR CSM1.4-carbon model projection indicates that 10% of the surface water along the investigated Arctic transect will become undersaturated for at least one month of the year when atmospheric CO2 exceeds 409 ppm.
Thanks to Larryâs comment 424 on the RealClimate thread âHit The Breaks Hardâ.
"Up to now I've been defending SRES (on wiki etc.) against those who say the trajectories just won't work; but I'm now rather less sure." - why did You defend? Should read TheOilDrum.com more...
I agree, to be less sure is more reasonable. The truth is, he have probably passed the peak oil. Now the question is, which technologies will win.
If coal-to-liquids, gas-to-liquid and non-conventionals, CCS, or hydrogen cars, thats bad.
If wind, solar, nuclear, energy efficiency, reforestation, and reduced overall consumption, we have a chance... :-)
BTW, not 2Â°C is as much important, as stopping population and economic growth. But we need to stop *both* - stopping economic growth without population growth is prescription for conflict...
TOD has some summary on subject "peak fossil fuels v. climate change scenarios" at http://europe.theoildrum.com/node/5084 .
Hansen no longer agrees with the upper limit:
"There may have been times in the Earthâs history when CO2 was as high as 4000 ppm without causing a runaway greenhouse effect. But the solar irradiance was less at that time.
What is different about the human-made forcing is the rapidity at which we are increasing it, on the time scale of a century or a few centuries. It does not provide enough time for negative feedbacks, such as changes in the weathering rate, to be a major factor.
There is also a danger that humans could cause the release of methane hydrates, perhaps more rapidly than in some of the cases in the geologic record.
In my opinion, if we burn all the coal, there is a good chance that we will initiate the runaway greenhouse effect. If we also burn the tar sands and tar shale (a.k.a. oil shale), I think it is a dead certainty."
http://www.columbia.edu/~jeh1/ (Bjerknes Lecture 12.20.08)
Hmmmm... Forgive me if this is obvious, but isn't it the rate of warming, rather than the total warming over per-industrial levels, that we should be concerned about? 2Â°C over 50 years is pretty worrying, 2Â°C over 5000 years much less so.
[I think that is bundled in... implicitly, this is 2 oC over 100 years, or to 2100, or whatever. If you follow back some of the why-is-this-dangerous stuff, then the rate of change comes in -W]
I have a certain sympathy for peak oilers, for peak oil will happen in our lifetime, and both the timing isn't clear, and being prepared a few years early would far better than the alternative. But peak coal, in this century, is nutty.
Let's start off with one coal deposit, and what Wikipedia said:
"he Powder River Basin is the largest coal mining region in the United States, but most of the coal is buried too deeply to be economically accessible."
Yes, read the references. At the current mine site price of coal, US$10.47/ton, about 6% of this deposit is producible. This 6% will provide ~40% of the USA's coal for decades. I edited the page, we shall see how long it takes the peak coal nutters will revert it back.
"Recoverable coal, which is the portion of available coal remaining after subtracting mining and processing losses, was determined for a stripping ratio of 10:1 or less. After mining and processing losses were subtracted, a total of 77 billion short tons of coal were calculated (48 percent of the original coal resource)."
At the current rate of production, that's about 170 years worth of coal that can be strip mined. Powder River coal is low sulphur, so is a premium product, there are extensive deposits elsewhere in the USA of higher sulphur coal that are being mined at a slower rate. For that matter, why stop with surface mining? And why not then use in-situ coal gasification?
As for cost, page 105 figure 66.
A paper I'm reading by Toby Tyrrell (on ocean acidification) cites the following paper
Marland, G., Boden, T. A. and Andres, R. J. (2005) Global, Regional, and National CO2 Emissions. In: Trends: A Compendium of Data on Global Change. Carbon Dioxide Information Analysis Center, Oak Ridge National Laboratory, U.S. Department of Energy, Oak Ridge, TN, USA
in support of this statement: "we have burnt slightly less than one tenth of the easily recoverable fossil fuels, ~300 Gt C out of about 4000 Gt C (Marland et al., 2005)."
I don't have the Marland et al paper.
Economically recoverable reserves today and economically recoverable reserves in 2100 are completely different beasts. If you allow for the development of technology to, say, mine sea-bed clathrates, the sky is the limit on potential hydrocarbon emissions.
As someone who is somewhat familiar with petroleum, at least, I can say that most of the economically viable / run-out-of-fossil-fuels / etc. estimates are sketchy at best IMO. Exploration and extraction science and technology will be developed for the demand, and no one is really sure what people are willing to pay for fossil fuel resources down the road. Especially if the momentum of the status quo infrastructure stays fossil fuel based, the mere cost of converting should keep us extracting oil, coal, and gas for a long time. I personally think that there is definitely more economically viable fossil fuel than is necessary to double CO2.
[I'm a little bit concerned by the word "pay" in this context. Pay in... money? Well obviously we have infinite amounts of that. But pay in terms of fuel to extract the fuel? -W]
I meant "pay" in terms of money, to take a shot at what is "economically viable". As far as I see it, if fuel costs more, then it will cost more to extract it as well, and the consumer price of fuel will represent that. Does that help clarity?
My feeling is that fossil fuels will be used until we want to stop, either because there is a large moral/environmental/catastrophe-worried response to stop (likely a vocal minority) or because today's alternative energy becomes cheaper than the fossil fuels (likely what pushes the real change).