Yesterday, two scientists published a stern critique of the longstanding target to limit global warming to two degrees above pre-industrial levels.

Branding the target “wrong-headed” and “tenuous”, the authors argue we should ditch the two degree target in favour of a suite of “vital signs” that would let us track the Earth’s health.

The commentary, published in the journal Nature, has generated a certain  amount of interest. We asked climate scientists for their thoughts.

Setting a target

In 1992, the United Nations Framework Convention on Climate Change (UNFCCC) decided the objective of global climate policy should be to stabilise humans’ influence on the climate below the level at which it can be considered “dangerous”.

As temperatures rise, so do the risks of climate change. As the recent report on climate change impacts from the Intergovernmental Panel on Climate Change (IPCC) puts it:

“Increasing magnitudes of warming increase the likelihood of severe, pervasive, and irreversible impacts.”

With governments worldwide recognising the need to keep rising temperatures in check, it’s important to have a goal, Professor Nigel Arnell, director of the Walker Institute for Climate System Research, tells us:

“When we’re trying to work out what future climate change might do and how to reduce it, you need some form of metric or indicator on order to judge how well particular policies achieve that goal.”

A good indicator

Curbing temperature rise has been central tenet of climate policy for two decades. One of Victor and Kennel’s main criticisms in the Nature commentary is the international community’s narrow focus on temperatures at earth’s surface.

Rising ocean temperatures, sea level rise, melting glaciers and global ice loss, extreme weather and changes to the water cycle are all indicators of continuing warming, say the authors.

“Scientifically, there are better ways to measure the stress that humans are placing on the climate system than the growth of average global surface temperature”

Observable indicators of a changing global climate include snow cover, sea ice extent, ocean heat content and sea level rise. Source: IPCC AR5 Summary for Policymakers.

Nobody disagrees with the authors’ observation that there is more to climate change than global mean temperature, says Professor Thomas Stocker, co-chair of the recent IPCC report.

“I agree with Victor and Kennel that the 2C target is oversimplified â?¦ [It] does not cover all aspects and processes that are able to cause dangerous interference with the climate system and to induce unacceptable impacts.”

But as Victor and Kennel themselves note, a single indicator that encompasses all climate change risk isn’t possible. Arnell explains:

“Any single metric is going to miss things â?¦ [But] if you want to have a target, having a global surface temperature target is not an unreasonable thing to do.”

Most other ways to monitor climate change are closely linked to global average surface temperatures anyway, explains the University of Edinburgh’s Professor Gabi Hegerl:

“[W]hatever Arctic warming you have scales closely with how much global warming you have. Rainfall changes are to a high extent driven by temperature changes, for example.”

Two degrees

As well as the general complaint of using surface temperatures to gauge climate change, Victor and Kennel brand the two degrees target “wrongheaded” for having no firm basis in science.

Scientifically-speaking, there’s no definitive threshold beyond which climate change tips the balance into being “dangerous”, explains Professor Reto Knutti from the Swiss Federal Institute for Technology:

“[S]ome impacts are already a problem in some sectors today, whereas others will become a problem at 2C, some only at 4C, etc â?¦ [D]eciding what constitutes dangerous interference with the climate system requires value judgments.”

But Arnell says keeping the target is important as long as it’s understood what it does and doesn’t mean. It’s not a safety limit, he says:

“Two degrees doesn’t equal dangerous, it’s a continuum â?¦ there isn’t a magic threshold”

However, it’s almost certainly true that policymakers don’t think of the target in such simplistic ways, explains Dr Joeri Rogelj, expert in climate change uncertainty at the Swiss Federal Institute for Technology:

“In contrast to what Victor and Kennel suggest, policy makers were very much informed in 2009 and 2010 by the IPCC AR4 on what could be “dangerous” and decided that limiting warming to below 2C of global temperature increase would be an acceptable level of aggregated risk. Suggesting that policy makers are not aware of the implications of 2°C is being kind of out of sync with the policy reality”.

As well as representing a low risk pathway, the key to two degree target is its simplicity, Stocker says:

“The power of the 2C target is that it is pragmatic, simple and straightforward to understand and communicate, all important elements when science is brought to policymakers.”

And we shouldn’t underestimate the importance of having something clear-cut to aim for, Knutti adds:

“[T]he 2C target should be seen as an anchor, a focal point that people can easily understand and refer to.”

“Vital signs”

If not two degrees then what?

Victor and Kennel argue that two degree target should be replaced by a series of measures that together give a complete health check on the planet. They call these “planetary vital signs”.

They say policymakers should track ocean heat content as an indicator of long term risk, and polar temperatures as they are sensitive to shifts in climate, for example.

But the scientists we spoke to seem unconvinced.

A lot of different indicators might work well for monitoring how the climate is changing. But for policy purposes, you need a “simple, shorthand metric”, says Arnell:

“It’s difficult to come up with what a sensible alternative metric would be â?¦ I have no problems conceptually with a series of vital signs as a measure of how the climate is changing, but you can’t use all of them to set a climate policy – it gets too complicated.”

There’s also the question of how people understand such targets. Whatever is used must mean something to the public, Professor Joanna Haigh, Co-Director of the Grantham Institute for Climate Change and Environment, suggests. Having lots of separate indicators risks adding too much “noise” because of the uncertainty associated with each one.

No excuse for delay

Developing the proposed new set of indicators would take time, say the authors of the new Nature commentary, and they wouldn’t be ready for next year’s international climate talks in Paris. This is bad news for swift action, Knutti tells us:

“[T]he plan to agree next year to start discussing for the next twenty years what indicators one might pick â?¦ and how to translate them into emission reduction targets doesn’t sound like an efficient way to agree quickly on emission reduction targets â?¦ arguments to focus on a large number of different targets and indicators is the perfect recipe to delay the climate negotiations even further.”

Professor Pierre Friedlingstein also doubts a complex picture with lots of variables – not all changing at the same rate – would help speed up the mitigation effort. He tells us:

“In a recent paper we showed that at the current rate of emissions, the 2°C target would be likely crossed in about 30 years. We can replace that target by any combination of  atmospheric CO2, ocean heat, sea-ice, etc, it will not change the main message  There isn’t much time left for inaction.”

Professor Rogelj echoes this sentiment, telling us.

“One of the most important mistakes in the arguments of Victor and Kennel is that they seem to assume that by tracking “vital signs” instead of only pursuing to limit warming to below 2°C, the world would have more time to act and climate action would be less stringent. This is  wrong.”

Professor John Fasullo tells us he remains unconvinced that developing new metrics as a  replacement to the two degrees target would be any more effective. He says:

“There is a need to frame climate goals in terms that are well understood by the public and that policy makers can communicate to their constituents. The 2C goal does this and it is unclear that other efforts, though only vaguely alluded to by the authors, can be similarly effective.”

IPCC co-chair Thomas Stocker agrees. The target may be imperfect, he says, but:

“I find it irresponsible to propose to abandon the 2C target for something which is not yet well defined and which is not fully understood by the scientists.”

Related links (h/t Realclimate)

• The Guardian:  Could the 2 degree climate target be completely wrong?
• Climate Progress:  2C Or Not 2C: Why We Must Not Ditch Scientific Reality In Climate Policy
• A  more detailed analysis [pdf] of Victor and Kennel by scientists from Climate Analytics