Bayswater _Power _Station _with _coal

Hopes of keeping global warming below the
long-established target of two degrees above pre-industrial levels
are rapidly eroding, according to a collection of papers in two
Nature journals today.

That might sound like a gloomy backdrop to this
week’s climate summit, convened by UN director-general Ban Ki Moon
to refocus world leaders’ attention on climate action.

But chalking up the two degrees target as a
political failure is a “naive” way to look at climate ambition and
could even obstruct future negotiations, the authors

Origin of two degrees

In 1992, the United Nations Framework Convention
on Climate Change (UNFCCC)
said 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”.

Scientifically-speaking, there’s no definitive
threshold beyond which climate change tips the balance into being
“dangerous”. But as temperatures rise, so do the

Curbing temperature rise to
two degrees
above pre-industrial levels has
become the most widely accepted point beyond which climate change
risks are considered unacceptably high.

As the
recent report
on climate change impacts from the
Intergovernmental Panel on Climate Change (IPCC) puts

“Increasing magnitudes
of warming increase the likelihood of severe, pervasive, and
irreversible impacts. Some risks of climate change are considerable
at 1 or 2°C above preindustrial levels … The precise levels of
climate change sufficient to trigger tipping points… remain
uncertain, but the risk … increases with rising temperature”.

A two degree limit has been a symbolic focus of
climate ambition for the past two decades – it is the limit
recommended by the UK’s
Committee on Climate Change
, for

Blowing the budget

Carbon dioxide – the biggest contributor to global
warming – has a predictable relationship with temperature. That
means scientists can calculate how much carbon we can emit and
still have a reasonable chance of staying below two degrees. This
is known as a
carbon budget

Global emissions from fossil fuel-burning and
cement production grew by 2.5 per cent per year on average between
2004 and 2013. Source: Friedlingstein et al., (2014)

To have a two thirds chance of staying below two
degrees, total emissions from the beginning of the industrial
revolution to the time we stop burning carbon would need to stay
below 3,670 billion tonnes of carbon dioxide, one
in Nature Geoscience notes.

When you allow for emissions of methane, CFCs,
ozone, nitrous oxide and soot – which also warm the atmosphere –
the carbon dioxide budget is an even tighter 3,200 billion tonnes,
note Pierre Friedlingstein and colleagues in the paper

However, global carbon dioxide emissions from fossil
fuels and cement production have continued to grow by
2.5 per cent per year
, on average, in the past decade.

That means two thirds of the carbon dioxide
budget to stay below two degrees has already been spent. And if
countries continue to emit at current rates, the budget is likely
to be exhausted in 30 years time, the paper

Top down vs bottom up

The two degree target, and a finite carbon
budget to accompany it, seems like an appealing way to visualise
the scale of climate action needed. At least, from a scientific

So why haven’t we seen
emissions drop in response?

Coming to a global agreement for limiting global
warming to two degrees means finding a way to divvy up the
remaining carbon budget between countries. And that’s a tough ask,
a separate
in Nature Climate Change

The Australian National University’s Michael
Raupach and colleagues consider
some potential options, noting that the challenge of
resource-sharing between countries cuts across the thorny issues of
fairness, governance, institutions and ethics.

This ‘top down’ approach of quota sharing has
made little progress in the last two decades of climate
negotiations, the authors note.

Bayswater _Power _Station _with _coal

Over 90 per cent of global emissions come from
burning fossil fuels and cement production.

An obstacle to progress

The two degree target may even be more of a
hindrance than a driver of progress, argues a Nature
Geoscience commentary
by David Frame and colleagues, including the University of
Oxford’s Myles Allen.

The authors suggest it focuses too much
attention on short term fixes at the expense of a long term vision
for meaningful change. The paper

“So far, international
focus on near term Kyoto style targets has allowed post 2020 (or
2030) emissions to remain largely undiscussed, even though these
determine the success or otherwise of climate

A separate comment
by David Victor in Nature Climate Change has
some strong words on the topic, suggesting a “top down” limit on
warming is an “abstract”, “naive” and “fictitious”

A more realistic perspective of the world,
Victor says
, is a ‘bottom up’ approach to effort sharing, whereby
nations decide for themselves what they can reasonably achieve in
terms of carbon cuts by a certain date.

As part of the so-called
Durban Platform
, an agreement adopted in
South Africa in 2011 to forge a new treaty which will come into
force in 2020, many countries have already
to limit or reduce emissions by
2020, the paper
by Friedlingstein notes.

All UN countries are expected to submit pledges
by March 2015.

Spurring action

As another long season of climate diplomacy gets
underway, hopes for success rest on fresh ideas,
momentum and a re-energised sense of collective effort sharing,
today’s collection of papers concludes.

The focus for international climate policy
should be on policies, architecture and commitment, not specific
goals and outcomes, they suggest.

A reality check about the two degree goal is
necessary to ensure next week’s UN summit and efforts to negotiate
a new climate deal in Paris in 2015 aren’t dismissed in a “fizzle
of disagreement and dashed expectations”, Victor warns

Failing to limit warming to two degrees above
pre-industrial levels is no doubt a missed opportunity. And as for
whether this week’s negotiations will garner a renewed sense of
optimism, all eyes turn to New York to see what countries bring to
the table.