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Surface warming slowdown doesn’t affect climate sensitivity, study says

  • 21 May 2013, 12:00
  • Roz Pidcock

Sourced under creative commons

A group of scientists has just published a new estimate of how sensitive earth is to rising greenhouse gases. Their value for 'climate sensitivity' is at the low end of what scientists have previously suggested - but there's still a lot of uncertainty.

Importantly, the study shows slower surface temperature rise over the past decade doesn't change scientists' projections of how much warming we can expect in the long term.

What scientists call 'equilibrium climate sensitivity' is the warming we can expect from a doubling of carbon dioxide in the atmosphere, compared with pre-industrial levels. It's a number scientists are still trying to pin down - and it's important, because the higher climate sensitivity is, the more warming there will be.

In its last report, published in 2007, the Intergovernmental Panel on Climate Change (IPCC) estimated a likely range of climate sensitivity of between two and 4.5 degrees Celsius, with a best estimate of three degrees. But it didn't rule out lower or higher values and since then, scientists have continued to try to narrow the range of uncertainty surrounding the number.

Energy inventory

The short new study, published as a letter in Nature Geoscience, is based on measurements of how much heat the land, ocean, ice and atmosphere have absorbed between 1970 and 2009.

The new calculations produced a best estimate for climate sensitivity of 1.9 degrees Celsius, which is similar to a couple of other studies taking a similar approach that have attracted media attention recently.

A value of 1.9 degrees Celsius sits just below the IPCC's likely range. But the new estimate also comes with a large uncertainty range - of 0.9 to five degrees. One of the co-authors on the new study, Professor Piers Forster, tells us:

"The message is that the uncertainties are really too large to offer tight constraints on climate sensitivity - a wide range is still possible."

Hence, the paper isn't a conclusive argument for a lower value of climate sensitivity. But lead author Alexander Otto tells us it might support lowering the bottom boundary of the IPCC's likely range a bit:

"[T]he energy budget allows lower values than two degrees, but it is also consistent with values up to four or five degrees."

Other scientists argue that these new results make the upper end of the IPCC range for climate sensitivity look unlikely. Professor James Annan says on his blog:

"I hope [the new estimate] reflects a change in thinking from the IPCC authors involved … [The result] implies a marked lowering of the IPCC 'likely' range."

Sensitivity and the surface warming slowdown

Despite surface temperatures rising relatively slowly in the last decade and a half, the new study finds temperature data from the last decade suggests a best estimate of climate sensitivity that's consistent with the previous three decades - around two degrees Celsius. The uncertainty range is a bit narrower though at 1.2 to 3.9 degrees.

The similarity between decades doesn't support some media suggestions that the recent surface warming slowdown might be explained by a lower value of climate sensitivity. As Otto explains:

"[T]he important message on equilibrium climate sensitivity is that there isn't [a message] … [T]here isn't any obvious inconsistency between the energy budget of the past decade and the conventional range."

In other words, looking at the whole climate system, the earth warmed by a similar amount during the last decade as it did in previous ones. That's because estimating climate sensitivity the way the authors did takes into account all elements of the climate system - not just the atmosphere. While surface temperatures may have risen relatively slowly recently, the study suggests the slower pace of warming is being compensated by faster temperature rise elsewhere.

The authors' top line conclusion is that although their best estimate of climate sensitivity for the last decade is relatively low, the range accompanying the data is consistent with the range projected by current climate models. The overlap for the whole period - 1970 to 2009 - with climate models is even stronger, say the authors, which means there's no reason to revise down scientists' estimates of the warming we can expect in the long term.

Transient response

Scientists say the most likely reason for the shift from more to less atmospheric warming in the last decade is that natural climate variability is causing more heat to enter the oceans.

Climate sensitivity calculated for surface temperatures alone is called the Transient Climate Response (TCR). TCR is easier to estimate than equilibrium climate sensitivity because it doesn't consider long term climate feedbacks, like the delay in atmospheric warming caused by heat entering the oceans. But for that same reason, it only gives part of the picture.

The study calculates TCR based on the last decade, giving an estimate of 1.3 degrees with an uncertainty range of 0.9 to 2.0 degrees. The new TCR range is slightly lower than some IPCC models project. Otto tells us:

"[H]ence we conclude that the more extreme versions of these models might be inconsistent with the data and less likely … [but] we need to be careful about interpreting just one decade, and the 1970 to 2009 period as a whole suggests 0.7 to 2.5 degrees."

It's this result that much of the media coverage of the paper has picked up on. It's arguable that TCR gives a useful indication of the amount of warming we'll directly experience in the short term. But for a complete impression of the warming we can expect in the long term, equilibrium climate sensitivity remains the relevant number.

Sensitivity is not the same as eventual warming

The overall scientific view of the likely range of equilibrium climate sensitivity won't be altered on the back of one study. It seems fair to say scientists' projections of long term temperature rise remain largely unchanged - for now at least.

Finally, it's worth stressing that equilibrium climate sensitivity is not equivalent to total warming, it's the eventual warming expected per doubling of carbon dioxide after all feedbacks have been accounted for.

The IPCC's high emissions scenario (A1F1 ) suggests we could see a doubling of carbon dioxide above pre-industrial levels by 2050. That means that even with a climate sensitivity sitting at or just below the lower end of the IPCC range, by mid century the world would be committed to two degrees of warming above pre-industrial levels.

Two degrees is the target set by the United Nations Framework Convention on Climate Change (UNFCCC), which is often suggested as a threshold to avoid dangerous climate change. And after 2050, unless emissions had fallen to zero, further warming would occur.

As Forster puts it:

"[The new study] has an effect but not a massive effect on projections. We still need to mitigate carbon dioxide emissions very significantly to keep below two degrees [of warming above pre-industrial levels]."

Meanwhile, if climate sensitivity is at the upper end of the IPCC's likely range, reports suggesting a four degree temperature rise above pre-industrial levels by the end of the century look optimistic given current emission trajectories.

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