Bishop Hill Bo Christiansen on hockey sticks
May 30, 2010 Climate: MWP Bo Christiansen has a guest post at Klimazwiebel looking at the way that traditional temperature reconstruction methods like RegEm and CPS underestimate past climate variability. He demonstrates a new method he has developed with Anders Moberg and shows that it is much better at capturing low-frequency variation - i.e. climate trends, although with a concurrent worsening of the high-frequency situation.
When he uses the new method on a previously used set of proxies he gets much higher variability, with a much more pronounced little ice age. THe proxy selection doesn't extend back to the medieval warm period, so we'll have to wait for further results to see what it looks like there.
As has been noted before, the advent of a more variable climate is not entirely good news for the sceptic cause. While one would expect the medieval warm period to be more pronounced, higher variability implies higher climate sensitivity and thus we would expect climate models to show much scarier projections of warming.
So we're sunk then?
Not exactly, because I think there's a sting in the tail. If projected warming is going to be even greater, then the IPCC forecasts that Lucia so carefully monitors for performance against reality, and which are on the cusp of being falsified already, will surely be shown to be unarguably wrong.
Reader Comments (5)
Your comment "While one would expect the medieval warm period to be more pronounced, higher variability implies higher climate sensitivity and thus we would expect climate models to show much scarier projections of warming" may not the bad news you anticipate. While it does show that there may be higher sensitivity, the question then should be "sensitivity to what?"
"higher variability implies higher climate sensitivity"
Yes, this argument or a variant of it has been used on Tamino by the egregious dhogaza. Jantar's point is exactly right. If we do not know the cause of the MWP, then how big or small it was does not tell us anything about climate sensitivity.
However, if the MWP was at least as warm as today, it does raise some real issues for the AGW movement.
One is that it undermines the argument that the only thing that can explain modern warming is CO2 forcing. If previous warmings occurred without CO2 forcing, obviously CO2 is not the only possible explanation.
Second, the RWP and MWP were followed by cooling and then by subsequent warming. If we are looking at an historical pattern in which over two millenia we have two warming episodes of at least the same size as todays warming, and both were followed by cooling, it must be probable that today's warming will be followed by cooling also. That undermines the argument for runaway warming.
You fall back in the end on the argument, which is much less plausible, that this time it is different. Previous warmings were due to unknown causes, of unknown dimensions. Present day warming results from a different cause (how would we prove this, then?) of a known size.
Therefore (and its hard to see where this 'therefore' comes from) this warming is different and more dangerous than previous ones and will not be followed by cooling.
It is reasonable to question whether today's warming is like the other ones. It is reasonable to wonder whether the cooling pattern will repeat. The only way to get clearer about this is to get to a better understanding of what caused the earlier warmings and coolings, and see if the factors apply still to the present one.
Or, it may just be random fluctuations with mean reversion. This is probably the null hypothesis to account for previous and present warming.
Jantar, I think our host is making the same point as you are. A warmer MWP implies either (1) a higher climate sensitivity, or (2) the presence of a significant external driving term we have missed, or (3) the presence of significant internal variability which we have missed. As Lucia has shown current climate behaviour is only just consistent with the currently assumed climate sensitivity, and is certainly inconsistent with a significantly higher sensitivity. Thus a warmer MWP would indicate that option 2 or (more plausibly) 3 is correct.
Who, pray, is Lucia?
Could you give me a URL?
Or an email address?
Generalfeldmarschall --
Lucia is the blogger who runs "The Blackboard." She has a statistical background and bent, and sometimes looks at GCM outputs as hypotheses that might be "rejected" or not, depending on the output's relationship with the record, and on the test being applied.
Here is one post from 26 March 2010 that gives a sense of her interests, and those of the folks who comment at her blog.
Carrot Eater’s Challenge: Rate of Rejections when applied to simulations pt. 2.. Check the Data Comparisons category for others.