A couple of weeks ago, Telegraph blogger and climate skeptic James Delingpole and Friends of the Earth head of campaigns Andrew Pendleton went head to head on the BBC's Daily Politics show. Now, they're debating how global surface temperature has changed over the last couple of decades in a post on Daily Politics presenter Andrew Neil's blog.
What to make of this? Last year's BBC Trust review of the corporation's science
coverage concluded that as there is an agreed fact-base about climate science,
this should be reflected in reporting. BBC programmes sometimes give minority
skeptic opinions "undue place", creating "false balance",
the report said, with examples of programmes "giving equal coverage to the
views of a determined but deluded minority and to those of a united but less
Andrew Neil is clearly sympathetic to climate skeptic arguments - although he stops short of declaring his own views. He presents the blog post as a discussion of one of the "great claims" of climate skeptics about climate science - that global temperatures stopped rising in 1995.
But in doing so, he frames acceptance of the existence of man-made global warming as a matter of belief - "Those who believe in man-made global warming, like FoE," - and presents the matter as a tussle over two competing theories - that temperature rise has stopped, or that it hasn't.
But scientists don't believe that global temperatures have 'stopped rising'. So is this another case of false balance? We take a close look at what both Pendleton and Delingpole have to say on the issue.
Pendleton: Global warming has not stopped
Pendleton starts off with a graph showing the global surface temperature trend over the last 130 years. We provide a more up to date version (including 2011 temperatures), that also extends a little further back in time:
"The data in the graph is important for three reasons. First: it shows a long term warming trend - and quite a dramatic one - beginning in the early 20th century and, evidently, still underway. Second: it shows how four separate data sets reach broadly the same conclusion. And third: it shows how much fluctuation there is from year-to-year and, in fact, from decade-decade."
These are fair points. The average global surface temperature has increased by around 0.8 degrees Celsius since the start of the 20th century, and there is considerable year to year variability in global surface temperature, caused by natural variations in things like the ENSO cycle and solar activity along with aerosol production from natural processes and human activity.
Pendleton then quotes UCL climate science Professor Chris Rapley:
"No climate scientist ever stated or expected the global average temperature to rise as a smooth curve. If you look back over the data for the last 100 years rather than just cherry-picking a short [in climate terms] period, the fluctuations are very clearly evident. But so is the upward trend, especially of the last 40 years."
Again, this is true, and is echoed by other scientists, including Met Office scientist Professor Richard Betts:
"No matter how many times we say that "global warming" means a rise of average temperature across the world, decade by decade, and not every year being consistently warmer than the last in every place on Earth, there are still those that get this mixed up."
Pendleton then moves on to the stats, pointing out that Bob Ward and his team at the Grantham Research Institute on Climate Change and the Environment have a useful blog explaining that the current hiatus in surface temperature warming is not unusual.
This is because there are natural variations in the climate, so to pick out the long-term temperature increase arising from greenhouse gas emissions we have to examine longer periods of time. The following table (from the blog) shows that there have been many other 15 year periods over the last four decades with no statistically significant warming.
Researchers in the US looking at every possible linear temperature trend (longer than two years) for the available surface temperature record found:
"Since 1945, all periods longer than 22 years indicate warming [...] In contrast, changes shorter than a few decades can be either positive or negative. The recent cooling trend is evident in the global record beginning in 2001. Such changes, however, are not statistically significant and are in fact relatively common in the historical record."
Modelling research suggests that we will probably see similar periods that don't show a significant warming trend in the future, although as the warming trend increases over the coming century, such cooling periods will probably occur less frequently.
Pendleton goes on:
"A further complicating factor is that while the long-term trend is clear, part of the explanation for the fluctuations over the short term is that the energy from the sun is stored in different places - the land, the oceans and the atmosphere."
True. Global surface temperature is actually quite a small component of the climate system. Global warming results from an energy imbalance in the climate system, where more heat energy is entering than escaping the atmosphere. The oceans account for over 90 per cent of the warming to the climate system since 1955, with the atmosphere, the continents and melting ice accounting for the rest.
Research shows that the world's oceans have warmed over the last 50 years, and probably even since the late 19th or early 20th century. About two-thirds of the heat going into the oceans is taken down and stored in the deep ocean, below 700 metres depth.
"In a nutshell. the simple answer to the question posed on the programme and to the challenge by Andrew Neil is 'no, global warming has not stopped.'"
So while surface warming has been slower in recent years, heat has still been going into the system - the world is still warming.
Delingpole: No warming since 1995 is "grossly misleading"
Delingpole sets up an interesting contradiction in his submission to the blog. He begins by saying that arguing there's been "no warming since 1995/1998" is "quite useful" - it's certainly an argument he's used frequently in the past.
But he then argues that you can't trust the temperature data anyway:
"It presupposes that the temperature data sets maintained and quoted by the climate alarmist establishment are reliable and trustworthy. But they're not: they have been subject to "adjustments" by scientists who, as we saw in the Climategate emails, have political motivations and a financial vested interest in exaggerating the extent of 'global warming'."
In fact the adjustments made to global temperature records are to remove any artificial bias in local temperature records from phenomena such as the urban heat island effect - where built-up, urban areas tend to be warmer than the surrounding countryside.
Aside from the scientists at the University of East Anglia, three other main groups produce independent records of surface temperature. There are also two satellite records of lower atmospheric temperature, which show a very similar global temperature trend. Finally, a group of scientists reanalysed temperature data in the BEST study, and came to similar conclusions to all of the other research groups.
Delingpole also argues that temperature differences over recent years are too small to really assess:
"The differences in temperature are often by as little as 1/100th of a degree. [...] We are in the realm, here, of angels dancing on the head of a pin."
We can rephrase this: temperatures over the last decade are all roughly as warm as each other. But this doesn't mean that warming has stopped, as Betts explains:
"The last decade was the warmest on record, followed by the 1990s and then the 1980s, so the world is definitely warming up."
He goes on:
"Funny, isn't it, how most of the significant "anomalous" warming "detected" by NASA in recent years (notably 2010) were in those parts of the world where there are no weather stations?"
Some of the biggest warming trends in recent years has been in the Arctic - which is warming at about twice the rate of the rest of the globe as loss of sea ice means the Arctic ocean absorbs more heat energy. There are fewer weather stations in the Arctic than in more populated areas of the globe, but it isn't true that there aren't any.
"Since 1850 the world has been emerging from the Little Ice Age. Of course decades at the top of that gentle warming trend (of about 0.8 degrees C) are going to be, on average, warmer than those at the bottom."
The suggested causes of the Little Ice Age - low solar activity and/or pronounced volcanic activity - are not currently happening. Solar activity has been declining since the mid 20th century whilst the world's average surface temperature has risen. So the current warming is not likely to be a recovery from the Little Ice Age.
Delingpole finishes with:
"What we CAN say is this. AGW theory is predicated on the idea that there is a strong correlation between man-made CO2 output and global warming. But while in the last decade - thanks largely to China - CO2 levels have continued to rise dramatically, world temperatures most definitely have not. This suggests that there are more, many more, things responsible for climate than CO2 levels. Such as - duh - the activity of the sun."
The problem with this argument is that if you remove the effects of short-term climate drivers, such as solar activity, aerosols, and ENSO, from the global surface temperature record, you are left with a pretty steady warming trend over the last thirty years, as demonstrated in research from last year:
In summary, as you might expect, Delingpole's account bears little relation to what the scientific community says, while Pendleton is basically repeating things that climate scientists have concluded.
Clearly neither Pendleton nor Delingpole are climate scientists. Perhaps appropriately for appearing on the Daily Politics, they both have very different views on what climate policy should be. As the BBC Trust review points out, on climate "Where policy is concerned, the argument is far from resolved. Science can inform the debate, but policy implications of global warming remain a legitimate part of the news agenda."
But this is a discussion of the science of climate change, not politics or policies. So it leaves the question - if the Daily Politics wants to discuss the science of climate change, why doesn't it turn to climate scientists?
This piece is cross-posted with the kind permission of The Carbon Brief.