Introduction
This is another post, possibly the last, in my series looking at the pause
that this been claimed happened in the early part of the 21st century.
Last time I looked at a post from Watt's Up With That, by Dr David Whitehouse, NEW STUDY CONFIRMS: THE WARMING 'PAUSE' IS REAL AND REVEALING . (Archived version here, as the original is currently missing graphics.) I looked at the specific claim of global warming stopping in 1998, made by Bob Carter, and pointed out that apart from being a very short period, it required a huge increase in the overall rate of warming.
In this post I'll look at yet another claim of a pause made by Dr Whitehouse in the article. This pause starts in in 2001 and lasting 13 years.
Meet the New Pause
Dr Whitehouse's pause is introduced when he talks about the Nature report
saying there are multiple definitions of the pause
:
What the authors miss, with their three definitions of the pause, is a simple fact we have often pointed out. Look at HadCRUT4 from 2001 (after the 1999-2000 El Nino/La Nina event) until 2014 (before the start of the recent El Nino event) and you will see the temperature is flat. Apart from the recent El Nino there has been no global increase since 2001, even though there have been El Ninos and La Ninas in that period. Now that's what I call a pause.
Dr David Whitehouse
Here's the graph he uses:
So why does Whitehouse start his pause in 2001, when Carter was convinced warming had stopped in 1998? The answer is that Whitehouse wants to insist that all the warming over the last couple of years was the result of a strong El Niño, and should not be included in any warming trend as it is not caused by humans. As he says,
The pause ended not because of gradual global warming but because of a natural weather event whose temporary increased rate of global warming was far too large to be anthropogenic.
There's some truth in that; it would be completely wrong to look at the past 7 years and claim it shows accelerated warming when you are ending on a couple of unusually hot years. If you did, you could claim that the world has been warming at a rate of over 5 °C / century since 2011, 3 times the rate since 1970. Of course, no-one would claim a 7 year trend as having any meaning.
Whitehouse doesn't want to allow a Strong El Niño to be counted as part of a trend, but it would then be a touch hypocritical to point to Carter's Pause - which started on a strong El Niño - as evidence of no warming.
By shifting the start of the pause to 2001, Whitehouse is trying to show that the pause doesn't depend on the 1998 El Niño. His pause doesn't start with an El Niño, and is just an arbitrary period of 13 years that contains both El Niños and La Niñas.
The problem is he's still being very selective in his starting point. Why not look at the trend from 1999 if all you wanted to do was discount the 1998 El Niño? Because then he would have a modest amount of warming (0.87° / century). Or why not calculate the trend including both El Niños? Then the trend would be 1.33 °C / century. The period 2001 to 2013 is selected precisely because it gives a 13 year period of near zero warming. It is a cherry-pick.
Before and After the Pause
What Whitehouse's pause does illustrate is that whilst 1998 has a big impact on the rate of warming, it isn't the only factor. To illustrate the problem let's play my usual game of asking what impact these 13 years of no-warming had on the overall rate of warming. I'll follow Bob Carter's definition of the warming period and look at the trend starting in 1970.
The trend before the 1998 El Niño, from 1970-1997 was 1.56°C / century.
By the start of the Whitehouse Pause the 1970-2000 trend had risen to 1.66.
Whitehouse could argue that this rise was only due to the natural 1998 El Niño and the trend up to 1997 was the true rate of global warming, but the difference is minor and I'll assume that the figure of 1.66 is the underlying rate of warming before the pause. So what would you expect to happen to the trend by the end of the 13 year Whitehouse Pause?
Amazingly, the trend up to the end of the Whitehouse Pause is identical to the trend to the start of the pause. This is purely a coincidence - choosing different start years would have produced different results. But it does illustrate the point that all these pauses have had no significant effect on the overall trend. This is not what you would expect if global warming had really stopped, or slowed down.
Annotating the Pause
To explain how it's possible that Whitehouse can show 13 years of zero warming, yet it not have any effect on the underlying trend I'll take a painfully long look at the HadCRUT temperatures, year-by-year. The following graph shows annual anomalies since 1970 to 2000, just before the start of the Whitehouse Pause.
The trend line is the trend from 1970-2000, projected up to the current day. If, at the end of 2000, you felt that this trend was a reasonable model of how warming would continue - that is you were not expecting a pause, you would expect future years to be close to the trend line, rising slowly, but with roughly as many years below the line as above it.
In all the following graphs this trend line is unchanged, that is it represents the trend as it would have been predicted from 2000. Years belonging to the Whitehouse Pause are marked in red. Let's just look at the first year of the pause, 2001.
2001 was hot, remarkably so considering it was not an El Niño year. It was the second warmest year on record, marginally warmer than 1997 and much warmer than any year before 1997. It's also warmer than the trend, which starts to explain why it is the first year of the Whitehouse Pause. Now jump forward to 2007, half way through the Whitehouse Pause.
This is where it gets interesting. 2001 was warm, but 2002 was warmer, 2003 even warmer, and 2005 warmer still. If it hadn't been for 1998, HadCRUT would be showing 4 record breaking years in a 5 year period. In fact 2005 is a new record, narrowly beating 1998.
More importantly, each of the first 7 years, is above the trend line. That's an unusually long run of warm years, and it's no coincidence that this is the first half of the Whitehouse Pause.
Now at this point, if you were an observer at the end of 2007, you might be wondering if something had happened to the climate - had the rate of warming accelerated, had we moved into a new phase of warming where temperatures were going to be warmer even than the post '98 El Niño suggested? Or was it just a chance that there had been 7 hot years in a row? If it was chance and the trend was a good estimate of future warming there would be a good possibility of cooler years appearing later.
And that's exactly what we see over the next 6 years.
During the next 6 years, 4 were below the trend, only 2 above it. The two coldest years, both La Niña years, were well below the trend, though there was also one record breaking El Niño year in 2010.
I find it difficult see any evidence that HadCRUT temperatures between 2001 and 2013 were not in keeping with the trend up to 2001. Compared with the projected trend, there were more warmer than cooler years, but not significantly more. The only reason Whitehouse can claim any sort of pause is that more of the warmer years happened at the start of the period, and more of the colder years happened in the later half.
Here's the up to date graph.
The last two years have been very warm, and yes this is in part because of the recent El Niño. But, they are not so far above the trend line as 1998. If you accept warming has continued unabated since 2000, there is nothing too surprising here - the absolute warmth of the last two years are a result of the strong El Niño, on top of the underlying trend. But, if you believe that global warming stopped in 2000, then the El Niño was exceptionally warm, which raises the question of why it was so extreme.
The above is what the 20th century looked like compared with an unchanged rate of warming. The following shows what it looked like compared with a trend that stops in 2000, as Whitehouse is implying.
Every year of his pause was warmer than what would be expected if the pause was real. Of course, this isn't the pause Dr Whitehouse shows. He shows the actual trend through the 13 years without the context of the previous years, giving the impression that warming simply stopped in 2000. Here's what the actual pause trend line looks like.
As with other pauses, the flat line requires a big rise in temperatures at its start.
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