Climate change: assessing the risks

Updated on 23/04/2023

by Dan Bernie, Met Office

In late 2014 AVOID 2 was asked to perform some calculations for a project on risk for the Foreign and Commonwealth Office of the UK Government. These looked at the future temperature response and the impact of additional earth system processes, such as permafrost thawing.

Earlier this year I joined a group of Chinese academics and scientists from around the world at a workshop in Beijing to present this work and to hear about the range of potential impacts of climate change. Meeting with a diverse group of experts from many disciplines was very interesting and highlighted how different problems in different research area can often be tackled with a similar if not shared approach. There were colleagues with knowledge of subjects from coastal flood defences through to disease transmission, all of whom had overlapping requirements for information on our future climate. The trip was also a chance to see some of the iconic Olympic venues around the city.

The main output of the meeting was the growing awareness that thinking of future climate change and impacts in terms of the time we have left is both a good communication tool and useful to planners and policy makers. My own work shows that following a pathway of increasing emissions might take the world above the 2ºC limit, suggested by many as a tolerable maximum level, in just a few decades time. The calculations also show that there is a chance of reaching much higher levels of warming, for instance 7ºC is possible. Going to lower emissions reduced the chance of going over unacceptable levels of warming level and even where potentially dangerous warming levels are exceeded they tends to be much later, allowing more time to prepare.

The output from the meeting will be written up and published with our colleagues at FCO and seeing the finished product will be an exciting conclusion to this work.

Dealing with uncertainty

by Simon Sharpe, Science, Innovation and Climate Department, Foreign and Commonwealth Office

Once upon a time there was a frog sitting in a pot of water.  The water seemed to be getting warmer, and the frog wondered whether this might be a problem. He asked his science adviser for advice. The science adviser took some measurements, and confirmed that the water was indeed getting warmer, although he could not be sure about how fast it would warm in future, or for how long the warming would go on. He told the frog: “in five minutes time, the water is likely to be 2°C warmer, plus or minus a degree or two”.  The frog thought: ‘That sounds ok”, and decided to stay where he was.

After a while, as the water continued to warm, the frog wondered whether there was more that the science adviser could have told him.  He called the adviser back, and asked “What’s the worst that can happen in this situation?”  The adviser replied: “Oh, that’s easy.  You could boil to death.”  The frog was surprised that this seemingly policy-relevant piece of information hadn’t been brought to light earlier.  He said “That sounds bad.  How likely is that?”  The science adviser did a few more calculations, then told him: “Well, it’s not very likely to happen in the next five minutes, but it will carry on getting more likely as time goes by, and at some point it will be more likely than not.” The frog decided it was best not to wait and see, and jumped out of the pot.

In a situation of uncertainty, there is often a big difference between the most likely outcome, and the outcome with the worst consequences. If the worst consequences could be really bad, then avoiding these is likely to be the focus of decision-making.

In the case of climate change, it is not obvious what an overall worst case scenario would be. There are many variables, and many uncertainties. But we can at least improve our assessment of the risks if for each variable, we ask first “What’s the worst that could happen?” and then “How likely is that?” That’s what we did at a meeting in Beijing last month, attended by climate scientists, government advisers, and experts in risk from the financial sector.

One example discussed at the meeting was the risk of climate change to human health, in the form of heat stress. A paper by Steven Sherwood and Matthew Huber[i] shows that if the global average temperature rose by around 7°C, then some parts of the world could experience conditions of heat and humidity that would exceed the limit of human physiological tolerance – so that even somebody resting in the shade, cooled with water, would die of heat stress. AVOID 2 programme scientists Jason Lowe and Dan Bernie showed that for a high emissions pathway (RCP8.5), the commonly-used CMIP-5 ensemble of climate models indicated that a global temperature rise of 7°C was possible (though unlikely) by the end of this century, became rapidly more likely thereafter, and became more likely than not in the latter half of next century.

Avoiding dangerous climate change will not be as easy as a frog jumping out of a pot.  But it may be crucial to our decision-making to understand what it is that we wish to avoid – and how, if we do not act sufficiently, it may be becoming more likely over time.

Sherwood, S and Huber, M, ‘An adaptability limit to climate change due to heat stress’, PNAS (2010)

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