Bottom line - their is no way the MWP [Medieval Warming Period] (whenever it was) was as warm globally as the
last 20 years. There is also no way a whole decade in the LIA [Little Ice Age] period was more than 1 deg C
on a global basis cooler than the 1961-90 mean. This is all gut feeling, no science, but
years of experience of dealing with global scales and varaibility.
The 50 or 60 climate scientists were convinced that human activities had made a profound difference to temperature/climate, and were likely to make a greater difference in the future. They needed to develop a response to the obvious observation that spectacular changes in climate had happened in the past with no human intervention at all. As a step in this direction, they came up with the "hockey stick"--temps flat for hundreds of years until 1900, then upward at a steep and steady rate through the 20th century. The MWP wasn't very warm, if it existed at all, and the Little Ice Age wasn't very cool--natural variability was less than the dramatic changes now being caused by human beings. At best this wouldn't answer questions about 5000 years ago or more, but it would be effective rhetorically. Defenders of the CRU point out that they don't sound, in e-mails that they expect to remain private, like they are hatching a plot, deliberatelyh falsifying evidence, laughing at the rubes, etc. It's more that they are convinced that the data will fall their way sooner or later. As soon as it does, they stop being skeptical about it. Once they have one graph on their side, they are ruthless in their criticism of any attempt to question it.
There has been a lot of talk about passages from Kevin Trenberth. This one from October 2009:
The fact is that we can’t account for the lack of warming at the moment and it is a travesty that we can’t. The CERES data published in the August BAMS 09 supplement on 2008 shows there should be even more warming: but the data are surely wrong. Our observing system is inadequate.
He would rather believe every thermometer in the world is broken than give up on the models.
Then a further exchange:
How come you do not agree with a statement that says we are no where close to knowing where energy is going or whether clouds are changing to make the planet brighter. We are not close to balancing the energy budget. The fact that we can not account for what is happening in the climate system makes any consideration of geoengineering quite hopeless as we will never be able to tell if it is successful or not! It is a travesty!
Coming from Trenberth, this amounts to saying: he's pretty confident he can identify the major factors affecting temperature/climate; but he doesn't know how they interact, or whether warming or cooling is likely to be prevalent in a specific period. A very simple view is that increased CO2 causes increased temps, probably modest increases at first; there is a closed energy system, so there is nowhere for added heat to go, and so there is a forcing that causes a more dramatic temperature increase. This is the Gore view, but Trenberth seems to say he has no idea whether it is true, and he thinks this is true of all the peer-reviewed climatologists.
There's also a line that cooling by SO2 may be canceling warming by CO2--he still believes the latter, but he does not know the extent of the former.
UPDATE: Not Trenberth: MacCracken indicates that man-made SO2 has a cooling effect, and could cancel out the effects of man-made CO2 for all anyone knows:
From: Mike MacCracken [mailto:email@example.com]
> > > Sent: 03 January 2009 16:44
> > > To: Phil Jones; Folland, Chris
> > > Cc: John Holdren; Rosina Bierbaum
> > > Subject: Temperatures in 2009
> > >
> > > Dear Phil and Chris--
> > >
> > > Your prediction for 2009 is very interesting
> > (see note below for notice that went around to email list for a lot
> > of US Congressional staff)--and I would expect the analysis you have
> > done is correct. But, I have one nagging question, and that is how
> > much SO2/sulfate is being generated by the rising emissions from
> > China and India (I know that at least some plants are using
> > desulfurization--but that antidotes are not an inventory). I worry
> > that what the western nations did in the mid 20th century is going
> > to be what the eastern nations do in the next few decades--go to
> > tall stacks so that, for the near-term, "dilution is the solution to
> > pollution". While I understand there are efforts to get much better
> > inventories of CO2 emissions from these nations, when I asked a US
> > EPA representative if their efforts were going to also inventory
> > SO2 emissions (amount and height of emission), I was told they were
> > not. So, it seems, the scientific uncertainty generated by not
> > having good data from the mid-20th century is going to be repeated
> > in the early 21st century (satellites may help on optical depth, but
> > it would really help to know what is being emitted).
> > >
> > > That there is a large potential for a cooling
> > influence is sort of evident in the IPCC figure about the present
> > sulfate distribution--most is right over China, for example,
> > suggesting that the emissions are near the surface--something also
> > that is, so to speak, 'clear' from the very poor visibility and air
> > quality in China and India. So, the quick, fast, cheap fix is to put
> > the SO2 out through tall stacks. The cooling potential also seems
> > quite large as the plume would go out over the ocean with its low
> > albedo--and right where a lot of water vapor is evaporated, so maybe
> > one pulls down the water vapor feedback a little and this amplifies
> > the sulfate cooling influence.
> > >
> > > Now, I am not at all sure that having more
> > tropospheric sulfate would be a bad idea as it would limit
> > warming--I even have started suggesting that the least expensive and
> > quickest geoengineering approach to limit global warming would be to
> > enhance the sulfate loading--or at the very least we need to
> > maintain the current sulfate cooling offset while we reduce CO2
> > emissions (and presumably therefore, SO2 emissions, unless we manage
> > things) or we will get an extra bump of warming. Sure, a bit more
> > acid deposition, but it is not harmful over the ocean (so we
> > only/mainly emit for trajectories heading out over the ocean) and
> > the impacts of deposition may well be less that for global warming
> > (will be a tough comparison, but likely worth looking at). Indeed,
> > rather than go to stratospheric sulfate injections, I am leaning
> > toward tropospheric, but only during periods when trajectories are
> > heading over ocean and material won't get rained out for 10 days or so.
> > > Would be an interesting issue to do research on--see what could be done.
> > >
> > > In any case, if the sulfate hypothesis is
> > right, then your prediction of warming might end up being wrong. I
> > think we have been too readily explaining the slow changes over past
> > decade as a result of variability--that explanation is wearing thin.
> > I would just suggest, as a backup to your prediction, that you also
> > do some checking on the sulfate issue, just so you might have a
> > quantified explanation in case the prediction is wrong. Otherwise,
> > the Skeptics will be all over us--the world is really cooling, the
> > models are no good, etc.
> > And all this just as the US is about ready to get serious on the issue.
> > >
> > > We all, and you all in particular, need to be prepared.
> > >
> > > Best, Mike MacCracken