Clear and present danger

The IPCC's findings that we've already emitted more than half the total CO2 we can while halting global warming at 2C is a stark wake up call, argues Paul Suff

The science is unequivocal and responsibility attributed: the world is warming and human activity is the cause. That is the conclusion of the fifth assessment of climate science by the Intergovernmental Panel on Climate Change (IPCC).

Global temperatures are likely to rise by between 0.3°C and 4.8°C on 1985–2003 levels by the end of the century and scientists are 95% certain that humans are to blame for the unprecedented warming of the Earth over the past few decades.

Although an 0.3°C rise in global temperatures does not seem too dramatic, it’s extremely unlikely that greenhouse-gas (GHG) emissions will be reduced sufficiently over the next 87 years to keep warming to this level in 2100, unless cuts are substantial and sustained. Currently, we are not even on a trajectory that will keep warming below the critical 2°C threshold.

Advances in climate modelling since the last assessment in 2007 mean scientists can now calculate a global carbon budget – the amount of CO₂ we can emit and keep warming below 2°C. Worryingly, the latest report reveals that we’ve already burned more than half the total budget. And, according to the World Resources Institute (WRI), we may have as little as 30 years left until the budget is spent completely.

The IPCC puts the budget at around 1,000 gigatonnes of carbon emissions (GtC) from the start of the industrial revolution, and calculates that by 2011 531 GtC had already been emitted.

That means that more than half the budget was used up over 250 years, and the WRI suggests that, under a carbon-intensive trajectory, the remaining 469 GtC will be exhausted by 2044. However, the 1,000 GtC budget is just for CO₂; when non-carbon dioxide emissions are factored in, the budget drops to 800 GtC, leaving just 269 GtC left to spend.

Despite the overwhelming consensus among climate scientists – the IPCC’s assessment is based on the findings of 9,200 peer-reviewed papers, two-thirds of which have been published since 2007 – sceptics maintain that the climate models are flawed, having failed, for example, to predict the recent “pause” in surface temperatures.

The IPCC acknowledges that short periods of slower surface warming will occur. Between 1998 and 2012, for example, the surface of the Earth warmed at a rate of 0.05°C per decade – slower than the 0.12°C per decade trend since 1951. This is due to natural variability, says the IPCC, adding that trends based on short records are very sensitive to the beginning and end dates, and do not in general reflect long-term climate trends.

The El Niño-Southern Oscillation is one such naturally occurring phenomenon, and 1998 was the second strongest El Niño year in the 20th century, helping to make it one of the warmest on record. Since then, there has been a series of volcanic eruptions and strong La Niña episodes, which both have a cooling effect.

It’s also worth pointing out that more than 90% of the heat trapped by GHGs since the 1970s has gone into the ocean, in effect “hiding” the heat from the surface but raising sea levels.

Sceptics may also ask why scientists are only 95% certain human activity is responsible for climate change? Well, 95% is pretty much the gold standard in certainty for scientists.

Complete certainty is rare because there are so many factors they do not fully understand or over which they cannot exercise control – and that degree of caution does not apply only to climate science.

As the IPCC unveiled its findings, EU climate commissioner Connie Hedegaard tweeted the right question: “Whose side are you on? Those who want to act on 95% certainty or those who gamble on the last 5%?”