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Roz Pidcock

Roz Pidcock

04.10.2012 | 12:00pm
ScienceCan we estimate the tipping point into irreversible climate change? We assess the One Hundred Months campaign
SCIENCE | October 4. 2012. 12:00
Can we estimate the tipping point into irreversible climate change? We assess the One Hundred Months campaign

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.

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