Expert Voices: Mark Watts, Director, Arup

Last week’s Intergovernmental Panel on Climate Change (IPCC) ‘Physical Science’ report should end debate that human activity is causing climate change and that the risks are potentially catastrophic.

As George Monbiot, the British environmental journalist, put it, the IPCC's report is "perhaps the biggest and most rigorous process of peer review conducted in any scientific field, at any point in human history". Not only does it represent the consensus among hundreds of the world's leading scientists, but the IPCC process means it has been signed off by the governments of pretty much every country.

The Panel's conclusions are unequivocal: while we need to constrain temperature rises below two degrees above the pre-industrial average to avoid catastrophic climate change, there is a 50/50 chance that we will go as high as four degrees this century if carbon emissions are not curbed. In the scientific language it is "extremely likely" (i.e. odds of 95/100) that human activity is responsible (the same certainty that doctors have that smoking causes cancer).

Ipcc Report Cover

Moreover, the IPCC is explicit about the global 'carbon budget' - the total quantity of greenhouse gases we can risk putting into the atmosphere and preserve a better than two out of three chance of avoiding runaway global warming. The number is 470 gigatons which, if we continue burning fossil fuels at our current rate, we will reach in just 15 years*.

Yet despite the clarity and importance of the IPCC’s key findings much of the media coverage instead focused on the data showing that air temperatures have not increased as rapidly in the last fifteen years as the previous decade. Note, it is not that global warming has 'stopped', indeed the IPCC states that each of the last three decades “has been successively warmer at the Earth's surface than any preceding decade since 1850". It is simply that surface temperatures are not rising in a linear fashion, largely because 93 percent of the additional heat is being absorbed into the ocean, causing acidification and damage to marine life, but temporarily holding down the rate of ambient global warming.
Many climate scientists had predicted this 'pause', even if standard models were not sophisticated enough to identify such relatively small fluctuations in a long-term trend. Yet proponents of action to save humanity from climate catastrophe have nevertheless had to spend the last few days fending off increasingly ridiculous assertions that somehow the IPCC report is an admission that the threat of climate change is not so serious after all.

Nick Stern, author of the definitive report on the economic consequences of climate change, dealt with this admirably when asked by the BBC Newsnight's reliably curmudgeonly interrogator, Jeremy Paxman, if he regretted his earlier prescriptions. Yes, he replied, we weren't hard enough because the latest science shows that we underestimated the risk and the urgency.

The reality is that given a balanced understanding of the climate facts, most people would conclude that the risks posed by climate change are so high that we have to take immediate action to cut emissions and improve resilience. As Connie Hedegaard, the European Union Commissioner for Climate Action, argued: "What would you do if your doctor was 95 percent sure you had a serious illness?". The answer for the vast majority of us, of course, would be to seek urgent treatment.

The prescription for the world to avert catastrophic climate change is going to be complex and long term, but the IPCC's report at least provides a clear diagnosis. C40 Mayors’ role is more important than ever – to demonstrate that the necessary course of treatment – weaning ourselves off fossil fuels, improving energy efficiency, and investing in renewable energy – will not only fend off climate catastrophe but also deliver stronger economies and better places to live.

* According to the International Energy Agency (IEA), 31.6 gigtons of greenhouse gases were pumped into the atmosphere in 2011, up 3.2% from the previous year.