Can we estimate the tipping point into irreversible climate change? We assess the One Hundred Months campaign
- 04 Oct 2012, 12:00
- Roz Pidcock
Is it possible to forecast the date at which we can no longer
reverse the effects of manmade climate change? This tipping point
is the basis of the New Economics Foundation (NEF)'s One Hundred
Months campaign, now at its halfway point. But how solid is the
science behind the 100-month countdown?
An
open letter, signed by 50 politicians, thinktank
representatives and non-governmental organisations appeared in
Monday's Guardian. The letter stated that in 50 months from now,
human activity will have released enough greenhouse gases into the
atmosphere to push the earth past a critical threshold into
uncontrollable climate change.
The letter marks the halfway stage in the One Hundred Months
campaign, which
NEF launched in August 2008 to galvanise efforts to combat climate
change.
NEF predicts that by December 2016, the concentration of
greenhouse gases in the atmosphere will be high enough to trigger a
new phase of global climate change from which the chances of
recovery are slim. The campaign website frames this prediction
rather alarmingly, featuring a digital countdown and the
statement:
"[W]hen the clock stops ticking, we
could be beyond our climate's tipping point, to the point of no
return".
We take a closer look at the predictions by "
global warming experts" that underpin the campaign. Does the
100-month deadline hold any water?
Tipping points
The thinking behind the 100-month deadline is that it represents a
critical point, known as a tipping point, beyond which the earth's
climate system undergoes an irreversible shift from one physical
state to another. The concept of tipping points is well established
in climate science and some examples include the melting of the
Greenland ice sheet, the destruction of the Amazon rainforest, the
thawing of Arctic permafrost and the slowdown of the global ocean
conveyor belt that transports heat around the globe - called
thermohaline circulation.
But while scientists are confident that tipping points exist in
theory, predicting when they might occur is far more difficult. The
One Hundred Months campaigners base their calculations on a 2006
analysis in which scientists estimated that once the concentration
of greenhouse gases in the atmosphere surpasses 400 parts per
million (ppm), it's unlikely we'll be able to stabilise global mean
surface temperature at two degrees Celsius above pre-industrial
levels.
Two degrees is the maximum acceptable figure adopted by the
European Union as a manageable level of warming, based on IPCC
findings, beyond which it becomes unlikely that serious negative
effects can be avoided. In calculations like these, greenhouse gas
concentration is expressed as carbon dioxide equivalent, which is a
way of including all greenhouse gases, not just carbon dioxide. In
an IPCC context, the term "likely" refers specifically to a
probability of 66-90 per cent.
Forecasting the future
Scientists have used
many different techniques, including looking at historical
climate change - known as palaeoclimatology - and forecast models,
to estimate when the earth will reach one or more tipping points.
And recent research
has suggested that not all tipping points can be ruled out below
two degrees of warming.
In any case, atmospheric concentrations of carbon dioxide as of
August 2012 were already 392 ppm - well
on the way to the 400 ppm mark for greenhouse gases.
Research suggests that if we end up following the IPCC's
highest emission scenario, which is still plausible, we could be on
course for a global temperature rise of four degrees above
pre-industrial levels by 2060.
To estimate the length of time until the 400 ppm threshold would
be reached, the campaigners
combined atmospheric concentrations of carbon dioxide
equivalent in 2007 with an estimated annual growth rate in
emissions of of 3.3 per cent. They also assumed no change in other
natural and manmade factors that affect the climate system, such as
the amount of radiation reaching earth from the sun and tiny
particles in the atmosphere (aerosols), which could have a warming
or cooling effect.
Positive feedbacks
A complicating factor is that as the climate warms, the warming
could trigger changes in the climate system that further reinforce
global warming, accelerating progress towards climatic tipping
points. One example is that melting ice reduces the ability of the
earth's surface to reflect incoming radiation, known as albedo,
allowing more heat to be absorbed. Despite their negative
consequences, such self-reinforcing events are known in science
terms as positive feedbacks.
To account for the emissions that these positive feedbacks would
cause on top of direct emissions to the atmosphere, the One Hundred
Months campaign included in their calculation an additional
concentration of carbon dioxide of 19 ppm by the end of the
century. This is at the lower end of the range put forward by 11
studies using different climate models, which suggested the
figure could be anywhere between 19 and 200 ppm. The campaign's
estimate is therefore the most optimistic in terms of emissions
until the end of the century.
Calculating 100 months
Drawing together atmospheric concentrations of carbon dioxide
equivalent, predictions of the annual growth rate and the
contribution of positive feedbacks, the campaigners arrived at a
figure of 100 months from the start date until atmospheric
greenhouse gas concentrations reached 400 ppm, giving them a
countdown to the end of December 2016.
Amid a large amount of scientific uncertainty, the idea of
translating a narrow selection of research on potential tipping
points into an countdown to irreversible climate change is unlikely
to sit comfortably with many climate scientists. As Professor
Timothy Lenton, a climate change scientist at the University of
Exeter, explains in a
recent paper :
"There is currently a huge gulf between
natural scientists' understanding of climate tipping points and
economists' representations of climate catastrophes".
Dr Joe Smith, a senior lecturer in environmental change at the
Open University and a co-signatory on the Guardian open letter,
seems to share these concerns. He says:
"For sure media and politics love a
clear claim and a clear date... [but] I don't think the science of
climate change allows us to put a number on when a critical climate
threshold has been passed."
100 months: a scientific or a political
statement?
But given that the One Hundred Months campaign makes a prediction
where scientists are yet to find agreement and that we are already
approaching the 400 ppm target at the halfway point of the
campaign, does it still have any relevance?
Supporters say that highlighting the need for politicians and
society to think differently about the effect of their actions on
the climate is the the One Hundred Months campaign's main aim.
Smith says:
"I see any definition of 'dangerous
climate change' as a political act not a scientific fact. That
doesn't make it any less urgent. Indeed politics is the right place
for urgency once the science has provided a pretty robust risk
analysis (it has)".
As the One Hundred Months campaign passes its halfway point, it's
worth reflecting on the science that lies behind it. Although the
100-month deadline is more of a media tool than a scientific one,
it's clear that the evidence base has moved on in the 50 months
since the campaign started.
Update 04/10/2012, 10.00: This article has
been updated to clarify in which circumstances greenhouse gas
concentration is expressed as carbon dioxide equivalent. Thanks to
Andy Wiltshire for pointing this out.