Cherry Slopes: Picking trends in temperature records
Posted by softestpawn on July 22, 2009
Drawing straight lines through datasets is a popular and easy way of showing a trend.
We can say that I am more tipsy now than when I came into the pub, and conclude that the trend is increased blethering, and that I’m likely to continue that way all else being equal. But we know we cannot really predict trends from it: the bar may close, I may run out of money, I may drink faster or slower depending on who is in and what music is on.
There’s a similar approach that says that, as it’s warmer now than a hundred years ago and we industrialised since then, we can conclude Stuff about trends in temperature. However with any complex system, it’s highly likely that it would be different to what it was a hundred years ago, so we can’t conclude anything directly from the fact that there is a difference.
So rather than looking at a straight line drawn through a dataset, we can look at features in that record and compare them with the proposed causes, and for that we need to have an idea about what we would expect to be features and what are natural variations.
For example, unless you’re familiar with, it might be worth reading this on Random Walks. [Updated: this was inline here but is really a separate subject]
‘Cherry’ Picking Periods for Trends
‘Cherry picking’ is deliberately choosing a range of data to demonstrate your point, and deliberately discarding data that does not.
Recently, as temperature measurements have plateaued or dropped, arguments rage over what a valid period is for establishing trends from the record. There is the IPCC showing how trends were increasing in the run up to their 2007 report (Working Group 1, chapter 3):
And the ‘deniers’ showing how trends are flat or decreasing:
Now they are for different periods, but the argument used is the same: show recent trend and imply future values. And those trends are obviously very susceptible to the range of values picked to draw a line through.
“Open Mind” quite rightly and rudely castigates people who cherry pick the last decade to show a plateau in temperature trend, and then in a remarkable example of ‘cognitive dissonance’ picks the last forty years to show global warming. (In fact shorter periods – such as two decades – can apparently be used to show warming). Similarly the ‘modern global warming era (1975-present)’ is used on the site to establish modern global warming, because it’s the period that warmed, a kind of self-harvesting cherry.
The UK Met Office simply say quite correctly that there’s not enough data to say that it’s stopped, but don’t go down the sticky avenue to say what would be enough to do so.
If we choose two endpoints rather than run to the present, we can do even more, especially because we have the big lump in 1998 with the troughs either side. Even using just one endpoint, we can go from a 0.2K/decade drop ‘trend’ (as above), to a plateau, to a 0.04K/decade rising ‘trend’ by including it or not:
These guys claim [with an unsupported optimistic “It’s not ideal but it’s not too bad”] that we can remove the effects of the Ninos by modelling them, to reveal the underlying ‘trend’. But then they also reckon that maybe the 1998 El Nino was more than just ‘weather’, and that it has had longer lasting effects and overheated the system which is just cooling slowly back to the ‘trend’.
In fact they draw a nice graph to show this, using a trend based on 1979-1997 records, which I’ve redrawn here using the same Hadley dataset, and it shows the current temperature trends returning to the underlying trend:
And they assure us that they haven’t been cherry picking by using 1979-1997, and that anything other range would be similar as long as you don’t include post 1998, and presumably that you do include up to 1997 (Otherwise I could carefully pick 1944 to 1976 and show a flat ‘trend’).
So they are cherry picking. In fact their own choice of range looks really quite dark and red and shiny and tasty: 1979-1997 removes the cooler 1978 and warmer 1998 years that are usually included to show how dangerously drastic the temperature increase is. By finding the slower rise shown above, the temperature record looks like it is ‘approaching’ the long term trend. Whereas if we include 1978 and 1998 to calculate the trend, the record is ‘departing’ from the trend:
We could take the full dataset that we have, which for Hadley runs 1850-2009. Using that to get our trend we get even longer to allow current temperatures to ‘fall’ to return to the trend (straight blue line) before continuing it’s ‘inexorable’ rise:
But claiming that the recent plateau is just a return to a longer trend gets in the way of the claim that the longer trend is dangerous. The 1850-2009 linear trend is about 0.05K/decade; by 2100 we would expect half a degree rise. Even realclimate’s 1979-1998 trend above is roughly 0.1K/decade; by 2100 we would expect a single degree rise. Few people claim any more that temperatures are “running away”.
It’s not true to say that you can prove anything with statistics; but you can imply very different things using different methods. When you’re given some numbers or a graph, have a good look at the surrounding data and see if it perhaps gives a different story.
Snide Political Postscript
If earlier predictions are failing then putting off the Signs is a good political move. By claiming that there might be more cooling for another 10 or 20 years, then much faster rising, you can maintain the threat of doom while giving you plenty of time to drift into another job, or make your fame as a political activitist.
If in the meantime temperatures rises, well then your earlier predictions are vindicated. You can’t lose.