Folks who question the reality or seriousness of climate change are making a lot of noise about how the planet's warming has slowed down or even stopped.
Because Earth's atmosphere has warmed less over the past 15 years than most climate models anticipated, these people declare that the models are worthless. They go on to claim that the models' failure shows how the climate is far more complex, uncertain and unpredictable than scientists admit. And if we can't predict the future climate, then we shouldn't take any action now to prevent warming that may not happen.
In short: The slowdown is evidence of great scientific uncertainty about climate change, and this uncertainty justifies climate-policy inaction. This argument may sound reasonable, but it's actually a noxious mixture of non sequiturs and selective use of scientific facts.
First, let's get the facts straight. Warming of Earth's atmosphere has indeed slowed over the past 15 years. According to the scientific-technical assessment released by the United Nations Intergovernmental Panel on Climate Change in late September, "the observed global-mean surface temperature has shown a much smaller increasing linear trend over the past 15 years than over the past 30 to 60 years."
Depending on which data are used, the 1998-2012 warming trend is about one-third to one-half of the trend over the entire 1951-2012 period. Despite this slowdown, 2010 was the warmest year, 2005 was the second warmest year, and the 2000s were the warmest decade since the beginning of the instrumental record.
Almost all of the 100-odd model runs analyzed by the IPCC projected a higher rate of warming over the past 15 years. The report's authors attribute the gap between observations and model predictions mainly to two factors.
First, events like El Nino and La Nina oscillations in the waters and winds of the equatorial Pacific can cause internal variations in the climate. Lately, we've had more La Nina events, which cool the climate, than the models anticipated. And second, the models likely overestimated the sun's brightness and underestimated the effect of recent volcanic eruptions that spewed sunlight-reflecting particles into the atmosphere.
The IPCC notes that internal climate variation is likely to even out over time and that the sun's brightness will likely increase from its recent lows. Meanwhile, humanity's emissions of greenhouse gases are rising. So, "barring a major volcanic eruption," warming is likely to accelerate in coming years.
Climate scientists are confident that rapid warming will resume, because satellite measurements show the Earth in radiative imbalance – with more energy coming from space than going back out – since at least 1970. An enormous amount of extra energy stays within Earth's climate system, roughly equivalent to the energy of 400,000 Hiroshima bombs a day. About 93 per cent of this energy goes into the ocean, some melts ice on land and sea, some warms the continents, and the remaining 1 per cent warms the atmosphere.
Because the atmosphere retains so little of this extra heat, its warming can slow even while the rest of the climate system, including the oceans, warms more quickly. Warming of the oceans causes them to expand and sea levels to rise. Sure enough, in recent years, sea levels have continued to rise about as fast as predicted.
People making a fuss about the warming slowdown tend not to mention Earth's energy imbalance, the relatively small amount of energy retained in the atmosphere or the continued sea-level rise. They also rarely mention that climate scientists have long anticipated that the atmosphere's warming would sometimes pause as a result of internal variation.
And while it's true that the models' failure to predict this particular pause shows that climate science still confronts uncertainty, scientists have always been frank about this challenge. The latest IPCC report, in fact, includes a list of more than 30 key uncertainties.
In any case, uncertainty doesn't justify inaction. Among other things, uncertainty means that outcomes could turn out worse than predicted, not just better.
Also, modern societies rely all the time on a balance of scientific evidence – in the face of great uncertainty – to make important decisions. They began regulating tobacco even when scientists were still very uncertain about the exact link between smoking and cancer. And they regularly make similar sorts of decisions about which drugs to deem safe, when to limit the use of toxic chemicals and how to design critical infrastructure, such as electricity grids.
People who argue for inaction on climate change are betting that the vast majority of climate scientists are wrong. The balance of evidence is decisively against them.
Thomas Homer-Dixon is CIGI chair of global systems at the Balsillie School, University of Waterloo. Andrew Weaver is a Lansdowne Professor in the School of Earth and Ocean Sciences, University of Victoria, and a Green Party MLA.