There’s a mantra circulating at the moment claiming that global warming “stopped 17 years ago.” It is of course being pushed by the pseudosceptics in the climate denial echo chamber. However, even people who should know better have been heard to repeat something like that.
Rodney Hide, a former New Zealand ultra conservative politician has assured us “The world stopped getting warmer 17 years ago. That’s incontrovertible” (see my post “Incontrovertible” is it, Rodney? for my take on that). And one of the commenters on my blog at SciBlog seems willing to treat Rodney’s assurance as a simple fact. Of course the pseudosceptics proudly and loudly reassert similar claims.
But many of those repeating this mantra are attributing the claim to authoritative sources, like the the Intergovernmental Panel on Climate Change (IPPC) and leading climate scientists and institutions.
So what’s the truth. Has global warming “stopped?” Are climate scientists saying it has stopped?”
Short answer is actually no. Slightly longer answer is along the lines that the current rate of global temperature increase seems to have slowed, global temperatures may even have plateaued, but that doesn’t support a claim that global warming has “stopped!” Or stopped 17 years ago.
IPCC Chairman misrepresented
Firstly – lets deal with the use of Rajendra Pachauri, Chairman of the IPPC, as a source for this mantra. This appears to go back to a report in the Australian which claimed he “acknowledged a 17-year pause in global temperature rises.”
Trouble is, there is no record to back up the claim and the IPCC communications office said it does not accurately represent Pachauri’s thoughts on the subject.
The only statement the Australian article actually attributed to Pachauri on this subject is that “global average temperatures had plateaued at record levels and that the halt did not disprove global warming.” And that is paraphrasing Pachauri and not quoting him directly.
As the blog Skeptical Science pointed out (see Did Murdoch’s The Australian Misrepresent IPCC Chair Pachauri on Global Warming?) if he “had he said that global surface air temperatures have plateaued and that this doesn’t disprove global warming, he would be 100% correct.” And that is what a number of well-known climate scientists also have said. Usually no mention of 17 years and certainly no claim that global warming had “stopped” 17 years ago.
To help clarify I repeat below two figures from my recent post “Incontrovertible” is it, Rodney? These show global air temperatures for the last 17 years and for the long-term – since 1880.
Line plot of global mean land-ocean temperature index, 1880 to present, with the base period 1951-1980. The dotted black line is the annual mean and the solid red line is the five-year mean. The green bars show uncertainty estimates. [This is an update of Fig. 1A in Hansen et al. (2006).]
As I said about the first figure in my recent post:
“There’s a lot of noise so all we can say from that data is the warming rate is in the range of -0.02 and 0.17 °C/decade (95% confidence level). That’s the problem with such short time periods.”
Putting short-term trends in context of long-term record
The data in the first figure must be put into the context of the longer term changes. And as the 2nd figure shows a number of short periods over the longer term which had a similar pattern to that in the first figure. It would be silly, especially with hindsight, to claim that global warming “stopped” in 1990, or 1985, or 1975, and so on. Yet this is what some people are doing.
It’s easy to find short time periods where the global temperature trend is not significantly different to zero – that’s the nature of a record with this sort of variability or noise. A record which also results from a number of factors and is therefore not a simple correlation with one cause.
So it is silly to cherry pick a short period and then make an absolute claim (global warming has stopped) – and especially to claim that somehow something happened in 1975 so that “global warming stopped 17 years ago. Think about it. Take that first figure a just select the last 10 years. The trend will also not be significantly different to zero – are we then going to claim something happened in 2002 to “stop” global warming?
No, of course not. The only reason 17 years is mentioned is that one can’t go back further than that without the trend being significantly different from zero. It’s a cherry-picked date – cherry picked to produce a non-significant trend.
Have IPCC models been disproved
Another common claim is that the very recent plateau, or decrease in the rate of global warming proves the scientific climate models are wrong. More specifically I have often heard the claim that since this plateau has occurred while atmospheric CO2 levels continue to increase this proves that CO2 is not driving global warming. Even the claim that the plateau has somehow shown the scientific understanding of the fundamental properties of greenhouse gases is wrong.
The naivety of the last claim is to think that climate scientists consider CO2 to be the only factor influencing the climate – they just don’t. Consequently one should not expect to see a simple correlation between global temperature and atmospheric CO2. Any attempt to understand or model climate change must include many more inputs than CO2.
As for models in general here is a couple of factors:
- All models are inaccurate. That’s just the nature of the attempt to understand complex systems – we can’t expect to get things perfect. And when anomalies occur this may actually help us improve the models by incorporating other factors or more realistic physical parameters. Despite this models have important uses as long as we understand their limitations.
- Models require inputs – inputs which may change, often unpredictably, over time. Therefore it is silly to expect model projections to always be correct or accurate further down the track.
For example, there could be weather conditions increasing heat inputs into the deep ocean which could not have been incorporated several years ago. Or there could have been an increase of particulates from increased coal use which had not been predicted. Political changes can produce economic changes which influence inputs. These are some of the ideas that have been suggested to help explain the current plateau or reduced rate of global temperature increase.
So the real test of the model is not to use inputs based on predictions made several years before, but to update inputs so that the model more correctly represents current situations.
But, more basically, it’s important to recognise that the global climate is complex. Simple mechanisms are not going to explain the details in the global temperature record. So be careful of people who advance simple explanations to discredit the science.