Scientists 'expect climate tipping point' by 2200

The global climate is more than likely to slip into an unpredictable state with unknown consequences for human societies if carbon dioxide emissions continue on their present course, a survey of leading climate scientists has found.

Almost all of the leading researchers who took part in a detailed analysis of their expert opinion believe that high levels of greenhouse gases will cause a fundamental shift in the global climate system – a tipping point – with potentially far-reaching consequences.

The 14 scientists, all experts in their fields of climate research, were asked about the probability of a tipping point being reached some time before 2200 if global warming continued on the course of the worst-case scenarios predicted by the Intergovernmental Panel on Climate Change (IPCC).

Nine of the fourteen scientists said that the chances of a tipping point for the high scenario were greater than 90 per cent, with only one saying that the chances were less than 50:50. At current rates of CO2 emissions, the world is on course for following the higher trajectory on global warming suggested by the IPCC.

The survey, published in the journal Proceedings of the National Academy of Sciences, was carried out by a team led by Granger Morgan of Carnegie Mellon University in Pittsburgh to try to assess the level of consensus among climate scientists over some of the uncertainties about future predictions.

They asked the 14 researchers, who included such climate luminaries as Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research in Germany and Tom Wigley of the US National Centre for Atmospheric Research in Boulder, Colorado, to complete an email survey, which was followed-up by face-to-face interviews.

One question focussed on the possibility of a “basic state change” to the climate system “with global consequences persisting over several decades”. The scientists were asked whether they thought such a tipping point was likely within the next 200 years based on three different climate change scenarios – low, medium and high.

The authors of the report found that for the high trajectory, 13 of the 14 experts said that the probability of reaching a tipping point was greater than 50 per cent, and 10 said that the chances were 75 per cent or more. Mr Granger and his colleagues pointed out that the high temperature scenario was still within the range of plausible scenarios offered by the IPCC.

Myles Allen, a climate researcher at the University of Oxford, who was one of the 14 interviewees, said that a basic change to the state of the climate system marks uncharted territory with potentially unpredictable consequences.

“This kind of study is helpful for people to understand that there is a clearly a range of views among scientists and that is inevitable but the level of consensus is pretty high. People broadly agree about what’s happening and what is likely under different scenarios,” Dr Allen said.

All of the experts agree that in the coming century we are likely to witness an increase in global average temperatures not experienced in the past 10,000 years. However, a tipping point is a more qualitative change to the way the climate system behaves, Dr Allen said.

“Part of the point here is to try to quantify what it is we don’t know to quantify the chances of events and the things we can’t really anticipate. The key thing we’re talking about is a transition to a climate that is fundamentally different to the one we’ve experienced,” Dr Allen said.

“It’s about a transition to a terra incognita of climate. It’s about moving into a territory for the climate system that is fundamentally unexplored. The main concern is that we can’t really predict how ecosystems and human society will respond to it,” he said.

The increase in carbon dioxide emissions over the coming few decades will be crucial in determining the sort of climate that the world will live in by the year 2200. Current CO2 levels, running at about 380 parts per million (ppm), are likely to rise to 1,000 ppm if nothing is done to curb emissions from the burning of fossil fuels such as oil and coal.

“We are certainly capable of committing ourselves to an emissions trajectory that make 1,000 ppm in 2200 almost inevitable if we make the wrong decisions over the next 20 years,” Dr Allen said.

“You are actually talking about emissions most of which actually occur in the century we are in. The emissions that commit you to 1,000 ppm in the year 2200 actually occur mostly over the next 50 years. The emissions decisions we make over the next 50 years commit us to the climate we’re going to have to deal with 150 years time – that’s the point,” he said.