After a peaceful couple of weeks, I managed to write a post yesterday about Curry vs Mann that caused another minor Twitter storm. It was the normal infantile tweets from the usual suspect, and I didn’t get to see them all as I’ve blocked most. To be fair, I don’t really blame them. The dialogue is pretty poor overall, but that doesn’t mean I’m going to suddenly take what they say all that seriously. Just to be clear, I know that Judith Curry is a serious climate scientist and I’m just an anonymous blogger. Feel free, therefore, to take what I say with as big or as small a pinch of salt as you’d like. Also, I didn’t moderate any of the comments on yesterday’s post, so if you wish to make a comment, feel free to do so (ideally taking the comment and moderation policies into account).
I was also criticised by some for saying I hadn’t read Judith’s testimony. Firstly, the post yesterday was about the challenge to Michael Mann – not specifically about her testimony – and, secondly, I didn’t say that; I said I hadn’t read it thoroughly. However, given that criticism, I have read it more thoroughly and thought I would add some comments. The testimony is long, so I won’t comment on everything. Also, as usual, if anyone thinks I’ve got something wrong or mis-represented something, feel free to point that out in the comments.
In an overall sense, my issue is not that what Judith has presented is factually incorrect, my issue is with what she doesn’t mention and how she interprets these facts. Judith has five main topics that I thought I would briefly (well, probably not briefly) comment on below
The IPCC AR5 WGI Report – a weaker case for anthropogenic global warming
Judith includes the attribution statements from the AR4 and AR5 documents
– AR4 (2007): “Most of the observed increase in global average temperatures since the mid-20th century is very likely (>90% confidence) due to the observed increase in anthropogenic greenhouse gases.” (SPM AR4)
– AR5 (2013): “It is extremely likely (>95% confidence) that human influence has been the dominant cause of the observed warming since the mid-20th century.” (SPM AR5)
but somehow concludes that the case is weaker. I know some have interpreted the change from anthropogenic greenhouse gases to human influence as somehow significant, and as somehow weakening the latter statement. I believe, however, that the change in wording is because there are other anthropogenic influences (aerosols, for example) that contribute and, since 2007, we’ve improved our understanding of these influences.
I discussed the attribution statement in an earlier post. Since 1950 the evidence really does suggest that anthropogenic influences have dominated and we really are very confident that most (if not all) of the warming since 1950 is anthropogenic.
Recent hiatus in surface warming and discrepancies with climate models
This is the part where I think Judith avoids mentioning things that would be regarded by most as relevant. She fails to really discuss the oceans and fails to mention the recent Cowtan & Way paper that indicates that considering the warming in the Arctic (which isn’t covered by some datasets) likely increases the rate of surface warming from around 0.05oC per decade to, possibly, 0.1oC per decade. Also, most of the excess energy goes into the oceans, with only a few percent associated with surface warming. Hence, as Andrew Dessler’s testimony indicated, we expect the surface temperature to show variability (a small change in the rate of ocean warming can have a big impact on the rate of surface warming).
Judith also says,
The observed global temperature record, particularly since 2005, are on the low end of the model envelope that contains 90% of the climate model simulations, and observations in 2011-2012 are below the 5-95% envelope of the CMIP5 simulations.
As Victor points out in this comment, we expect this to happen sometimes. The range of model estimates is an illustration of our uncertainty, hence we would expect the surface temperatures to fall outside the 90% range about 10% of the time. If it didn’t, then that would suggest that we are able to model the surface temperatures more accurately than our model range indicates. We’d eliminate the models that do badly and constrain some of the parameters so as to reduce the uncertainty range. Once we’d done this, we’d then expect the surface temperatures to fall outside the 5-95% range, again, about 10% of the time. If the observations never fall outside the 5-95% range, then I think it would no longer be a 5-95% range.
What’s important, therefore, is the range of possible warming suggested by the model and how often the surface temperatures fall outside various confidence intervals. It’s my understanding that the variation in surface temperature is consistent with the model ranges, and so the fact that it’s falling outside the 5-95% range is not – at this stage – particularly significant. In fact, I think Doug MacNeall was trying to make this case to some on Twitter a while ago but I don’t think those he was talking to quite understood what he was trying to illustrate and he seemed to avoid pressing the point (understandable, given who he was talking to).
Sensitivity of climate to doubled CO2 concentrations
Judith includes the AR5 statement about Equilibrium Climate Sensitivity (ECS), which is
The IPCC AR5 conclusion on climate sensitivity is stated as:
Equilibrium climate sensitivity is likely in the range 1.5°C to 4.5°C (high confidence), extremely unlikely less than 1°C (high confidence), and very unlikely greater than 6°C (medium confidence) (AR5 SPM)
and makes quite a big deal out of the lower end of the range being reduced (relative to AR4) from 2oC to 1.5oC. She also comments that in AR5, there’s no best estimate. Well, my understanding is that there are a couple of recent papers (Lewis 2013; Otto et al 2013 – for example) that produce lower estimates for the ECS. Hence, the lower end of the range has been reduced. It’s also not really possible to produce a best estimates because the new estimates are inconsistent with other estimates. Certainly interesting, but we shouldn’t automatically assume that these new papers have more credibility than the others.
Also, since AR5, refined estimates of surface warming (Cowtan & Way 2013) and some new estimates of the energy imbalance (Trenberth & Fasullo 2013) have increased the ECS values determined using energy budget estimates. These energy budget estimates also don’t capture likely non-linearities (since they don’t the full time interval over which CO2 doubles). This isn’t definitive, but there are indications that some of these lower estimates will rise as the method gets refined. Perfectly interesting scientifically, but not really sufficient – I would argue – to suggest that lower values of the ECS are more likely than the higher values suggested by AR4.
I’m not that familiar with sea level rise, so I will say very little about this. Judith concludes this section with
Global sea level has been rising for the past several thousand years. The key issue is whether the rate of sea level rise is accelerating owing to anthropogenic global warming. It is seen that the rate of rise during 1930-1950 was comparable to, if not larger than, the value in recent years. Hence the data does not seem to support the IPCC’s conclusion of a substantial contribution from anthropogenic forcings to the global mean sea level rise since the 1970s.
and includes the following figure
I had not realised that the rate of sea level rise in the 1930s was as high as it is today. I had thought sea level rise was one of the things we were more confident about and it was my understanding that some thought that the IPCC AR5 had been somewhat conservative in its sea level projections (see Aslak Grinsted’s post). So, maybe Judith has a point about sea level rise but maybe someone will clarify things in the comments. I’ll leave it at that for now.
This was another section where I thought Judith failed to mention something relevant. She discusses Arctic and Antarctic sea ice and says [t]he increase in Antarctic sea ice is not understood and is not simulated correctly by climate models. This – I believe – is true, but the rate at which Arctic sea ice extent is reducing is about 3 times greater than the rate at which Antarctic sea ice extent is increasing. Also, if one consider volume/mass the Arctic sea ice is trend is even more significant.
Judith also doesn’t mention that both Antarctic and Greenland ice sheets are losing mass. I wrote about some of this in an earlier post and, in that post, there are links to a paper that suggests that the increased Antarctic sea ice extent is because of the cold water coming from the continental ice sheets. There is also another that suggests that it is simply natural variability. Again, this isn’t conclusive, but does mean that there are some explanations for the increased Antarctic sea ice extent. It also doesn’t mean that natural variability hasn’t contributed to the decline in Arctic sea ice, but Judith herself includes the IPCC statement that Anthropogenic forcings are very likely to have contributed to Arctic sea ice loss since 1979.
So, these are just some of my thoughts. I haven’t tried to cover it all. From a physics perspective, global warming is about an increase in energy in the climate system. The rise in ocean heat content tells us that it continues despite the slowdown in surface warming, which we’d expect to show variability as it is only associated with a few percent of the excess energy. We also really can’t explain this warming without including anthropogenic influences which have almost certainly dominated since 1950. The sea levels are rising, but I don’t really know what to make of the observation that the rate in the 1930s was similar to today. The Antarctic sea ice extent is indeed increasing, but more slowly than the Arctic sea ice is reducing and, as expected given the continued accrual of energy, the amount of polar ice (sea and ice sheet) is reducing. There you go – feel free to include any interesting, insightful and constructive thoughts or corrections through the comments.