A double dose of climate science from the BBC's Today Programme

  • 17 May 2013, 14:30
  • Roz Pidcock

The BBC's Today programme has seven million listeners, so how it covers climate science is quite important. This morning's programme saw a report on climate change and recent temperature rise, followed by an interview with the well-known climate scientist Dr James Hansen.

Broadly speaking, the programme did a good job of navigating what has become an entangled web of scientific issues, although it perhaps inevitably lacked clarity on a few points.

BBC environment correspondent Roger Harrabin posed the question "what kind of risk are we taking with the climate?" With greenhouse gases in the atmosphere rising, earth's surface temperature - that's the air over the land and ocean - has risen more slowly over the past decade and a half than in previous decades.

The Today programme report explored why this might be, while Dr Hansen was on hand to explain why despite the recent slow pace of surface warming, the science of climate change isn't really in doubt.

Warming still happening

Harrabin suggested that after being "largely ignored by mainstream science for a while" climate skeptic voices questioning the pace of global warming are now being heard. He suggested that "the scientific establishment agrees that global warming appears to have stalled", and that while "scientists thought temperatures would be driven steadily upward", surface temperature hasn't risen appreciably since 1998.

But this is a simplification, as Harrabin unpacked throughout his report. The word "stalled" suggests surface temperature rise has stopped - and might also imply it's expected to remain static. In the second segment, Dr James Hansen took this point head on, telling the programme that's not what scientists think at all. In response to Sarah Montague's opening statement that "global average will have been roughly the same for two decades", Hansen said:

"Well, I should correct what you just said. It's not true that temperature has not changed in the last two decades. In the last decade, it's warmed only by a tenth of a degree as opposed to two tenths of a degree in the preceding decade. But that's just natural variability - there's no reason to be surprised by that at all."

Scientists have told us they don't expect a steady year-on-year or decade-on-decade increase in warming. Natural climate cycles, like the El Nino that caused 1998 to be exceptionally warm, causes global temperatures to jump around from year to year. The long term trend is still one of warming.

Incidentally, Hansen's tenth of a degree figure refers to the five-year running average surface temperature over the decade 2000 to 2010. Some reaction to his interview online has suggested a different figure - of 0.01 degrees. That's the trend in monthly temperature between 2003 to 2013, and this really just illustrates the difficulties of measuring temperature over such short timescales. As scientists have repeatedly stressed, these are effectively temperature snapshots that need to be looked at in the context of previous decades.

On the Today programme Sir John Houghton, former chief scientist of the Intergovernmental Panel on Climate Change (IPCC), explained this point:

"If you look at the average, [the temperature] is still going up. You see a bump at 1998 and beyond, but the steady rise of about 0.1 degree per 10 years is still there."

Ocean warming

The Today report explained that rising greenhouse gas emissions must result in more warming - "That carbon dioxide warms the climate is not in doubt."

So if there's more carbon dioxide, and the warming isn't staying in the atmosphere, where is it going?

Surface temperature is only a small (but important) part of the climate system. In fact, most of the extra heat the planet absorbs goes into the oceans, climate scientist at the University of Reading Dr Richard Allan tells us:

"The vast ocean has a huge capacity to store heat … To balance the books you need to be measuring the energy coming in at the top of the atmosphere and all the places where it's going to really understand how the climate is heating."

Where Is Global Warming Going _infographic

Over the last decade or so when surface warming has slowed, there's some evidence that the amount of heat going into the deep ocean has increased. Sir Brian Hoskins, head of the Grantham Institute for Climate Change at Imperial College, explained to Roger Harrabin:

"You can have a decade or so where the ocean is storing up heat, but the atmosphere is not warming up much."

Scientists think the most likely reason for more heat entering the oceans rather than staying in the atmosphere is due to natural climate cycles that fluctuate slowly over timescales of a few decades.

Allan explains to us why this makes looking at temperature changes over short timescales a bad idea:

"[I]f you're measuring for a period of ten, 20 or even 30 years, you might just be measuring part of that natural cycle. You need to have a longer term perspective so you can put these bumps and troughs in context of longer timescale changes."

Importantly, the oceans absorbing more heat doesn't mean that the planet is "adapting" to extra warming, as Hansen's interviewer Sarah Montague seemed to imply at one point.

Decades in which atmospheric temperature rise slows down only to speed up again afterwards have occurred throughout the temperature record. With plenty of carbon dioxide in the atmosphere trapping heat, what we're seeing now is a temporary situation - atmospheric temperature will continue to rise when natural variability cycles turn in the other direction.

Climate sensitivity

The scientists in Harrabin's piece discuss natural variability as the most likely driver of the surface warming slowdown. Nevertheless, he examined an alternative idea - is the surface warming slowdown happening because the earth is less sensitive to greenhouse gases than previously thought?

The quick answer is no. We've written more about this - but here's a brief summary.

Scientists define climate sensitivity as how much the planet will warm with a doubling of carbon dioxide. The IPCC suggest that climate sensitivity is most likely between two and 4.5 degrees Celsius.

Harrabin referred to some recent research based on observational estimates of climate sensitivity that support of lower values. But as we've highlighted before, other estimates of climate sensitivity using climate models still support the higher end of the IPCC's likely range.

The Today programme actually did a pretty good job of unpacking this complicated area:

"[Mainstream scientists] have concluded that the top extreme [above 4.5 degrees Celsius] of the temperature range forecast for a doubling of carbon dioxide is less likely than ever. But the main projection remains about the same - something between two and 4.5 degrees Celsius."

Allan tells us the surface warming slowdown "doesn't add much physical insight" into scientists' estimates of climate sensitivity, adding that it tells us more about variability than long term warming. Hansen echoed this view extremely strongly on the Today programme, saying that a decade of surface warming slowdown provides "no change at all in our understanding of climate sensitivity and where the climate is headed."

Other scientists have told us basically the same thing. Professor Reto Knutti from the Institute for Atmospheric and Climate Science in Zurich told us recently:

"My personal view is that our overall assessment [of the value of climate sensitivity] hasn't changed much."

Key messages

While the recent surface warming slowdown doesn't challenge the fundamentals of climate science, there are some interesting questions about what's driving it. And the Today Programme gave a pretty good rundown of what is a complicated area.

It's also good that Hansen did a particularly good job of explaining why - although these are interesting questions - they are "details" that "shouldn't distract from the main message." Hansen ended his interview with a strongly worded statement on those who use this period of slower surface temperature rise to cast doubt on climate science:

"Deniers want the public to be confused, they raise these minor issues which are something between experts and then we forget about what is the main story: The main story is carbon dioxide is going up, it's going to produce a climate which is going to have dramatic changes if we don't begin to reduce our emissions."

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