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What's causing the surface warming slowdown? Scientists tell us what they think

  • 08 May 2013, 14:15
  • Roz Pidcock

Despite greenhouse gas emissions continuing to rise fairly steadily, earth's surface - that's the land and top of the oceans - has warmed relatively slowly over the past fifteen years or so.

We asked climate scientists to give us their thoughts about what's causing the recent slower pace of surface warming. Here's what they told us.

Recent slower warming isn't unusual

To draw conclusions about climate change, climate scientists tend to look at long time periods - temperatures measured over decades, or whole centuries. The last 15 years or so is a relatively short amount of time to measure temperatures over.

There are good reasons for taking a longer view. As professor Gabriele Hegerl from the University of Edinburgh tells us, surface temperatures bounce around from one year to the next because of natural fluctuations in the climate:

"Climate change becomes more visible against [natural fluctuations when] the longer periods are considered ... To see the long-term change, long periods need to be considered, such as multiple decades."

Periods of slow surface temperature rise aren't particularly unusual. In the twentieth century there were several decades in which surface temperature rise slowed, despite greenhouse gases in the atmosphere continuing at fairly steady rates.

Professor Michael Mann from Pennsylvania State University tells us:

"[W]hen we look at periods as short as a decade over the past century, its easy to find time intervals where natural factors ... obscure the warming trend."

Looking at temperatures over the course of the whole century, the overall trend is one of warming.

While the scientists we spoke to didn't think a period of slower surface temperature rise was particularly surprising, they still want to understand the mechanics of it in more detail. Hegerl tells us:

"[T]he hiatus certainly has lasted a bit now, and raises interesting questions."

So what's causing it?

Natural variability in the climate system is the most likely reason

Natural changes in the sun's energy, volcanic eruptions and ocean cycles such as the El Niño Southern Oscillation (ENSO) influence global temperature from year to year.

Most of the scientists we spoke told us the main reason for a slowdown in surface warming is that natural variability is currently obscuring the full extent of greenhouse gas warming.

One team of scientists made an effort to strip out the known effects of natural variation from the temperature record. When they did so, the warming trend due to human activity in the past few decades was pretty clear - as the video below from Skeptical Science shows.

          

 Professor Mann explains:

"[W]hen we account for the effects of natural factors like El Niño and La Niña, volcanic eruptions, etc. we find that the warming trend is proceeding steadily at about 0.15 degrees Celsius warming per decade."

So why does natural variability cause slower surface warming? The scientists we spoke to explained the likely reason is that natural variability is redistributing the sun's heat into different parts of the climate system, notably the oceans.

The oceans are warming more, which might explain why there's less surface warming

Surface temperature is, relatively-speaking, a small part of climate change. The oceans absorb more than 90 per cent of the sun's energy that hits earth.

Temperature measurements show the deep oceans have warmed substantially in the last half century - and this warming has probably accelerated in the last decade.

        Balmaseda _et Al ._ocean _heat _content

The ocean has experienced accelerated warming since 2000 at a rate unprecedented in the last 50 years - shown in purple. Source: Balmaseda et al. ( 2013)

If more heat is being diverted to the deep oceans, we'd expect to see less surface warming.

Professor Reto Knutti from the Institute for Atmospheric and Climate Science in Zurich tells us that less warming in one part of the climate system is balanced out by more warming in another.

He says:

"[T]he Earth is warming and taking up heat, sea level is rising, ice is melting, it's just that much of the heat goes into the ocean at the moment."

Recent research suggests that other decades in the 20th century with little surface warming also saw ocean warming increase.

So what drives such shifts between the atmosphere and ocean? Professor Kevin Trenberth from the US National Centre for Atmospheric Research tells us:

"A key pattern [of natural variability] is the one called the Pacific Decadal Oscillation, which relates to the magnitude and frequency of El Niño [and La Niña] events ... One can argue that surface temperature rise was increased from about 1975 to 2000 or so but now has gone in the other direction owing to these changes."

Other signs of climate change aren't slowing

It's not just about the atmosphere and the oceans - there are many other indicators of the changing climate which show little sign of pausing.

Arctic sea ice is diminishing more rapidly than scientists expected, while ice loss from the Greenland and Antarctic ice sheets is accelerating.

Dr Richard Allan, climate scientist at the University of Reading, points out that taken as a whole, there's little sign climate change is slowing:

"Some aspects are changing more quickly than predicted by climate simulations (e.g. Arctic ice) and others are slower than the projections (e.g. surface temperature over last 15 years)."

            Shepherd _icesheet _mass _balance _total

Satellite data show that from 1992 to 2011, both ice sheets lost mass. In Greenland, the rate of loss is now five times higher than it was in the 1990s. Loss has been slower in Antarctica, still showing a doubling since the 1990s. Source: Shepherd et al., (2012)

It's understandable that there's a focus on what surface temperatures are doing - it's what we experience most directly. But surface temperatures are just one part of the climate system.

Recent surface temperatures don't change our best estimate of climate sensitivity

It's been proposed one reason surface temperatures haven't risen as much as expected recently could be because the climate isn't as sensitive to greenhouse gases as scientists previously thought. So is it evidence that "climate sensitivity" - how much the planet will warm as we put more carbon dioxide into the atmosphere - is lower than thought?

The current best estimate of climate sensitivity comes from the 2007 Intergovernmental Panel on Climate Change (IPCC) report, which put forward a "likely" range for climate sensitivity of between two and 4.5 degrees Celsius. But the organisation didn't rule out values up to six or seven degrees, although they were seen as less likely.

Parts of the media have cited some recent papers supporting low values of climate sensitivity to suggest scientists are considering whether the IPCC's "likely" range should be revised downwards.

But as far as we can see, scientists are only discussing whether the so-called " long tail" of climate sensitivity - values above 4.5 degrees - can be ruled out.

Climate sensitivity estimates based on climate models still support the higher end of the IPCC's likely scale - which means it won't be revised any time soon.

This is all quite complex stuff. But as discussions move into the media, some of this scientific detail is being lost. Professor Knutti tells us:

"Some [studies] are pointing [to the possibility that some models overestimate climate sensitivity] but I don't think this is the main factor ... But even if it were, that does not mean that climate change is not happening. It does not change the fact that the earth has warmed, the fact that the changes are attributed to human influence, nor the fact that it will continue to change"

The impression from the scientists we spoke to is that it has climate sensitivity probably isn't that relevant in explaining the recent period of slower surface warming. Refining the IPCC's likely range, if and when that happens, will depend on more than just the last decade and a half of surface temperatures.

Knutti continues:

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

So from the scientists we spoke with, the view is that slower warming is most likely down to natural variability redistributing heat between the atmosphere and the oceans. Such natural variation is by its nature short-term, and scientists expect the long term warming trend to continue.

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