Reiner Grundmann, who is a Professor of Science and Technology Studies at the University of Nottingham, yesterday tweeted the following statement and question.
I think climate scientists should make up their mind: either there is a ‘pause’ in temp rise or there isn’t. Is it result of cherrypicking?
— Reiner Grundmann (@ReinerGrundmann) October 13, 2013
I do find the question, in some sense, a little odd. It’s a bit like asking someone a very complex question and then insisting on a yes or no answer. Although I’m not an actual climate scientist, I responded to Reiner with some of my thoughts on the topic. I will say that I found the exchange quite intriguing (which shouldn’t necessarily be perceived as positive). I partly think I expected better from an academic. Of course, I have had exchanges with Richard Tol and Paul Matthews (what is it with the University of Nottingham, by the way), so should probably have realised that such an expectation was naive. I just didn’t expect Reiner to follow the “let’s misrepresent what the other person is saying” style of dialogue. Also being accused of being a “true believer” by a Professor of Science and Technology Studies – when pointing out that anthropogenic global warming is about energy, not just about surface temperatures – was a bit of a surprise.
Anyway, although Reiner may not actually be interested in my thoughts (or in an actual answer for that matter), I thought I might express my views here on whether or not there has actually been a “pause” and the significance of this apparent “pause”. Bear in mind, though, that I’m not actually a climate scientist and you might learn more from the comments than from the post itself.
- To answer the basic question of whether or not there has been a “pause” in the rise of global surface temperatures, the answer is probably not. Even if you start your trend calculation in 1998 (a very hot year) the Skeptical Science Trend Calculator will tell you that – with the exception of the RSS dataset – the likely trend is around 0.05oC per decade. Even the IPCC SPM says
the rate of warming over the past 15 years (1998–2012; 0.05 [–0.05 to +0.15] °C per decade), which begins with a strong El Niño, is smaller than the rate calculated since 1951 (1951–2012; 0.12 [0.08 to 0.14] °C per decade).
The numbers in square brackets are the 90% confidence interval and so there is a chance that the actual trend could be negative. Given this confidence interval, however, there is only a 20% chance of a cooling trend. I suspect that it is actually less likely than this (Victor Venema could probably clarify this though) as the scatter in the data is primarily real natural variations, rather than measurement errors, and so we can probably have more confidence in the mean trend than the basic statistics would indicate (although, I will accept that this isn’t a particularly statistically robust argument).
Some do interpret the uncertainties being larger than the trend as implying that there’s been no warming (i.e., that the warming is statistically insignificant). That view is incorrect. This view would only be correct if you could show that no warming was the expected result. The uncertainties actually tell us something about the likelihood of different possible trends and tell us that the most likely trend is the mean trend of 0.05oC per decade. Furthermore, such large uncertainties are perfectly normal for such short time intervals so should not, in themselves, be seen as anything particularly significant.
So, the surface temperature datasets (with the exception of the RSS dataset) are themselves not consistent with a “pause” in surface temperature rises since 1998. A slowdown, maybe, but not a pause. One should also bear in mind that 1998 was a very hot year and, so, the subsequent trend is likely to be lower than if a slightly earlier year was chosen (the trend since 1995 is close to 0.1oC per decade). One can also consider the land only datasets, which show much more warming than the global datasets, although do have larger uncertainties.
Another point is that the “pause” only refers to surface temperature. The ocean heat content continues to rise and Arctic sea ice extent (and volume) continues to decline. The rate at which the total energy in the climate system is rising (1022J per year) is also consistent with what would be expected and so implies that the fundamentals of anthropogenic global warming (AGW) are robust. The so-called “pause” doesn’t, in any way, suggest that there are problems with the theory of global warming.
Some seem to think that AGW theory implies that atmospheric greenhouse gases simply act to increase surface temperatures. This isn’t fundamentally correct. Atmospheric greenhouse gases act to trap outgoing longwave radiation, causing the total energy in the climate system to rise. Some of this energy goes into the oceans, some melts polar ice, and some heats the land and atmosphere. So, yes AGW theory does predict that global surface temperatures have to rise (and, in fact, they need to rise so as to reduce the energy imbalance) but that is not the be-all and end-all of AGW theory. Fundamentally it’s about increasing the energy in the climate system, some of which acts to increase surface temperatures.
Something Reiner implied was that switching from surface temperatures to energy was an attempt to mislead (or implied that the IPCC had been misleading people if I implied that they had done so). No, talking about energy – rather than surface temperatures – is simply because energy is the fundamental quantity. That’s why there are a number of figures in the IPCC documents that show how radiative forcings (measured in energy per square metre per second) have changed since 1750. This illustrates by how much the energy in the climate system must have increased since that time. However, these figures typically only indicate anthropogenic forcings and so do not include other, indirect, forcings and feedback that will likely increase the net change in total energy.
- There’s also the issue of variability. I don’t believe any climate scientist has ever indicated that surface temperatures should rise smoothly and monotonically. It’s clear that there is an element of internal/natural variability. Tamino has a recent post showing that although the previous 15 years have seen slower surface warming than expected, you can easily find a different, recent, 15 year period when the surface warming was much faster than expected.
Also, only a small fraction of the excess energy associated with AGW heats the surface. Therefore small changes in the energy entering other portions of the climate system can have a big effect on surface warming. There have been a number of recent papers trying to understand the so-called “pause” (Kosaka & Xie 2013 for example) and none, as far as I’m aware, suggests that the recent slowdown is anything other than simply a consequence of some kind of internal/natural variability (ocean cycles, aerosols).
- Another issue is whether or not the “pause” could continue for a significant period (20 more years for example). Currently, we have a top-of-the-atmosphere energy imbalance of about 0.5 Wm-2. If the so-called “pause” were to continue for another 20 years, the surface temperatures (assuming the trend is 0.05oC per year) would rise by 0.1oC. This means the surface flux would increase by 0.5 Wm-2. Atmospheric CO2 concentrations would also, however, rise by 40ppm which means that the adjusted radiative forcing would also increase by 0.5 Wm2. This, however, ignores other indirect forcings and feedbacks and so the actual rise will likely be higher. This means that the total energy in the climate system will be accruing faster than it is today (and this will be even greater if people assume “pause” means no rise in global surface temperatures). If the “pause” is to continue, then that would suggest an ever increasing fraction of the excess energy will have to be entering the oceans.
So, the only way the “pause” could exist for an extended period is if an ever increasing fraction of the energy excess “hides” in the oceans and also if we managed to avoid another 1998-like El Niño event – which may be surprising given the amount of energy accruing in the oceans.
So, there you go. My thoughts on whether or not there’s been a “pause” and the significance of the so-called “pause”. Basically, there’s been no “pause” in overall warming and no actual “pause” in surface warming. Should climate scientists clarify this in some simple way? I think some have been trying to do so (by actually pointing out that there hasn’t been a pause), but it isn’t some simple yes there has, no there hasn’t type of situation. Surface temperatures clearly are rising (since 1998) slower than expected, but this is most likely because 1998 was a particularly hot year and because internal variability can influence surface warming even if it doesn’t, significantly, influence overall warming.
There’s also certainly some indication that the whole discussion of the “pause” did not originate from climate scientists themselves, but from those who are openly “skeptical” of climate science. The reason it hasn’t been addressed, by climate scientists and the IPCC, in a manner satisfactory to some is simply because it is not appropriate (scientifically at least) to do so. It’s a pity that a Professor of Science and Technology Studies doesn’t at least recognise this as a possibility.