AR5_Many Indicators

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

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.

, published in the journal Nature,

. 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

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

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

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 … [But] if you want to have a target, having a
global surface temperature target is not an unreasonable thing to

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

“[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 2°C, some only at 4°C, etc … [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 … there isn’t a magic

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:

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 2°C 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

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

“The power of the 2°C
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 2°C 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

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

“It’s difficult to come
up with what a sensible alternative metric would be … 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 … and how to translate them into
emission reduction targets doesn’t sound like an efficient way to
agree quickly on emission reduction targets … 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

“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

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

“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 2°C target for something
which is not yet well defined and which is not fully understood by
the scientists.”