Since I’m waiting for the rain to stop so that I can finish cutting the grass, I thought I might post some thoughts I’ve had about hockey sticks. It’s partly motivated by a discussion I was having in the comments of this Quark Soup post, and partly by a sense that there is still some confusion about their actual scientific relevance. This is just going to be a bit of a brain dump, so apologies if it isn’t coherent.
- The first thing I was going to say relates to the point that David Appell is making here. Our current understanding is that, on multi-decadal timescales, our climate responds mainly to changes in external forcings. Also, our current understanding suggests that – over the last thousand years – the largest change in forcing has been the change in anthropogenic forcing that has occured over the last hundred years or so. Therefore, we would expect the global surface temperatures of the last 1000 years, or so, to have a hockey-stick-like shape. This isn’t to say that they must have, simply that our current understanding suggests that this is the shape we would expect.
- We have an instrumental temperature record that goes back 130 years or so. It’s highly unlikely that any proxy reconstruction will overturn the instrumental temperature record. If a reconstruction does not have a blade, or if the blade is not robust, it does not bring the instrumental temperature record into question. It might bring the reconstruction into question, but that’s a different issue.
- Related to the above, the interesting thing about proxy reconstructions is not really the 20th portion, but the pre-industrial portion. What was our temperature history prior to the period for which we have instrumental temperatures? That allows us to put the 20th century into the context of the last 1000 years, or more.
- Steve McIntyre (who was involved in the discussion on David Appell’s blog) seems to be highlighting that the recent Ocean2K reconstruction does not have a blade. Well, the data appears to end in 1900 and the paper title is Robust global ocean cooling trend for the pre-industrial Common Era, so why would we expect there to be a blade [edit : What I say here isn’t strictly correct. As Sou points out the paper uses 200-year bins, so the final bin is 1800-2000, but the data point shown – at 1900 – is the average of 1800 – 2000]. It does discuss the 20th century in the Supplementary information, but only considers 21 of the 57 proxies and says treat with caution. So, this paper seems to be suggesting that there was a robust global cooling trend in pre-industrial sea surface temperatures. The instrumental temperature record tell us that sea surface temperatures have been rising since the late-1800s. Overall, a hockey-stick-like shape.
- There is little that proxy reconstructions can do to overturn the basics of anthropogenic global warming (AGW). They are, however, interesting for many reasons. For example, they can be used to try and constrain climate sensitivity (how sensitive are we to changes in external forcing) and they can be used to try and determine the magnitude of internal variability.
- What if proxy reconstructions show large variability prior to the industrial period? Could this overthrow the fundamentals of AGW? Not really. It could mean that our climate is very sensitive to small changes and that climate sensitivity is high. It could mean that we should expect large variability on top of the long-term anthropogenic trend. It doesn’t provide a reason for questioning AGW.
- Related to the above, those who like (or think) that climate sensitivity is low, often seem to use these reconstructions the wrong way around. For example, they argue that there was a Medieval Warm Period and that – hence – our current warming isn’t unusual. Well, the current position is that the Medieval Warm Period was not global in extent and probably not as warm as it is today. However, even if it was, it would be more indicative of a high sensitivity to small changes in forcing, than evidence against AGW. It’s certainly not evidence that we shouldn’t take our current warming seriously. If you want to argue for low climate sensitivity you should really be promoting reconstructions that should little variability, not those that show large variability and past warm periods.
Anyway, I think the rain has stopped, so I should go and finish cutting the grass. As I said, this was just a bit of a brain dump. I may well not have explained some things as clearly as I would have liked, and may have left some things out. Feel free to correct me, or add more, through the comments.