See this <a href="http://www.realclimate.org/index.php?p=115#more-115"> article</a> on RealClimate.org.

Gavin Schmidt and Stefan Rahmstorf argue that ice age climate should be the major constraint on model skill and that the cpdn climate sensitivity range is not realistic.

There are several things in the article that I'm not sure about, but will think a bit more before posting.

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____________________________<br>
<a href="http://www.boincforum.info/boinc/">boinc forum</a> and <a href="http://www.uk4cp.co.uk/">United Kindom</a> team, my climate change <a href="http://www.livejournal.com/users/mike_atkinson/">blog</a>.

One thing I'd be concerned abourt with using results from this slab model is; Are there any positive feedback effects in the <i>warming</i> direction..? I've seen many models that cooled in control phase 2 but then were relatively stable after warming in phase 3. So, <i>is</i> there any effect that would cause 'runaway feedback' warming in certain areas of the world during phase 3 that wouldn't show up at all in control phase 2..?

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<a href="http://www.nmvs.dsl.pipex.com/"><img src="http://boinc.mundayweb.com/cpdn/stats.php?userID=6&amp;team=off&amp;trans=off"></a>

<a href="http://www.nmvs.dsl.pipex.com/">Distributed Mania</a>

&gt; See this <a href="http://www.realclimate.org/index.php?p=115#more-115">
&gt; article</a> on RealClimate.org.
&gt;
&gt; Gavin Schmidt and Stefan Rahmstorf argue that ice age climate should be the
&gt; major constraint on model skill and that the cpdn climate sensitivity range is
&gt; not realistic.

It's an interesting article and an interesting point. It's true that it would be really good to have some good paleo constraints on the models, and that's something we are intending to do in conjunction with Paul Valdes's paleo group in Bristol. I had a brief chat with Myles about Stefan &amp; Gavin's article this morning - we'll post something as a reply (soon-ish, I hope). We're reasonably relaxed about it - it's the kind of thing you often hear from people when you talk about constraining climate sensitivity (volcanic constraints (Pinatubo) is another one, but I think that's a weaker case). I think the real story is that we've found - within the parameters of the experiment (and that's a really important and quite strong qualification) - a wider range of physically plausible model** behaviours than have previously been identified. In particular we've shown that the phase space of the model is quite non-linear.

**Subject to the constraints we have so far applied (we're working on more). The article itself is quite cautious in this regard, even if the front page of London's Metro last Thursday wasn't... The long-term goal for all of us is obviously to quantify the likelihood of these models, given the observational constraints we have. We aren't there yet, and this is really just an interesting way-point.

Dave

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I think the arguement in Stefan &amp; Gavin's article about the ice age climate constraining a 2xCO2 doubling Climate Sensitivity is dependent on the assumption that the sensitivity is linear in the entire range of CO2 values from ice age levels (much below present) to 2x preindustrial values.

They also seem to be apportioning ice age climate sensitivity among albedo, CO2 and atmospheric dust (ignoring other forcings) in a way that the errors in CO2 forcing and temperature change are correlated and then assuming that they are uncorrelated. I may be wrong about this, I'm still learning, and they may be justified in treating them as uncorrelated even when they are correlated.

At first sight it seems that a paleo climate model ensemble would work well within the constraints of the cpdn experiments. It would be interesting to see if parameter sets that have skill with recent climate also perform well during an ice age? Has this type of experiment been done before?

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____________________________<br>
<a href="http://www.boincforum.info/boinc/">boinc forum</a> and <a href="http://www.uk4cp.co.uk/">United Kindom</a> team, my climate change <a href="http://www.livejournal.com/users/mike_atkinson/">blog</a>.

&gt; I think the arguement in Stefan &amp; Gavin's article about the ice age
&gt; climate constraining a 2xCO2 doubling Climate Sensitivity is dependent on the
&gt; assumption that the sensitivity is linear in the entire range of CO2 values
&gt; from ice age levels (much below present) to 2x preindustrial values.

This is true, I think.

&gt; At first sight it seems that a paleo climate model ensemble would work well
&gt; within the constraints of the cpdn experiments. It would be interesting to see
&gt; if parameter sets that have skill with recent climate also perform well during
&gt; an ice age? Has this type of experiment been done before?

We have plans for such an ensemble, working with Paul Valdes from the University of Bristol. It's been submitted and is currently waiting to be funded, or not. We think it's a great idea, but then we were pretty damn keen on climateprediction.eu, too, and that fell through.

I've posted a fairly chatty, fairly general reply to Gavin and Stefan's article to the realclimate.org site. They (sensibly enough) review all submissions, so it won't be appearing immediately.

Dave

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