There’s been talk of a “pause” or “slow-down” in global warming. People who dispute that climate change is a problem shout that “There hasn't been any warming for at least a decade and a half”, and claim that “hysterical” commentators and duplicitous scientists are ignoring the data.
I think it’s helpful to actually look at the data, indeed to look at lots of data, and not just cherry-pick a 15-year fragment of a particular surface-temperature data-set.
“How the surface temperature is changing” does not answer “is the globe warming?” The globe is not just a surface — the globe consists of land, atmosphere, ice (especially ice in glaciers) and oceans. The surface temperature is only an indicator of how much heat is being taken up by the land, the atmosphere, and just the top few tens of metres of the oceans; the heat capacity of the oceans is much bigger than the heat capacity of the land and the atmosphere). So, to establish “is the globe warming?”, we really must pay attention to the oceans, and ask what is happening to the total heat content of the oceans, the ice, the land, and the atmosphere.
The Intergovernmental Panel on Climate Change (IPCC) have helpfully provided their best estimate of this total heat content in figure 1. Has global warming “stopped” since 1997? Not at all: the heat content of the globe has increased by about 110 zetajoules from 1997 to 2010. (That corresponds to an average in-flow of power of 0.5 watts per square metre of the earth’s surface, consistent with the imbalance in radiation created by the last century’s increases in greenhouse gases.)
So where is this “pause”? The time-series in which people perceive a “pause” is the surface temperature history;
Looking at these graphs, I struggle to perceive any striking “warming pause” in the last decade. Yes, if you cherry-pick the start year and fit a straight line to ten years’ data, you can notice a slow-down in warming – but such slow-downs happen often: as you can see in figure 3, you can find five back-to-back periods of “no surface warming” in the last forty years!
Why is the surface temperature now on the low side of the range of predictions made by climate models ten years ago? First, let’s be clear: no climate scientist ever predicted a steady, monotonic warming of the surface temperature. There are credible mechanistic explanations for the observed wobbles, involving the Pacific Ocean, volcanoes, and the solar cycle.
The global climate is a complex system which fluctuates naturally on a wide range of time-scales. Some of these variations in the climate system are induced by external forces such as solar or volcanic activity, and some are spontaneously generated by the chaos of the climate system itself.
One of the most important of the spontaneous natural variations is called the El Niño Southern Oscillation. Typically the Trade winds blow over the equatorial Pacific from east to west and drive warm surface waters to the western Pacific, with cold water being sucked up from depth to the surface in the eastern tropical Pacific. In “El Niño” conditions, the Trade winds weaken and warm water spreads into the central Pacific, and the sucking up of cold water in the east is suppressed; in “La Niña” conditions, the Trade winds are strengthened, the warm water is confined further to the west, and more cold water is sucked up in the east alongside South America. El Niño conditions tend to be associated with slightly warmer global surface temperatures than average, and La Niña conditions (thanks to the sucking up of cold deep ocean water and the burying of warmer surface water) tend to be associated with colder surface temperatures than average.
The record-breaking warm year, 1998, was an extremely strong El Niño year; the past few years have seen a dominance of La Niña conditions in 2008 and especially 2010-11. La Niña sucked heat from the surface and buried it in the ocean, slightly reducing surface temperatures.
Superposed on this spontaneous fluctuation, which accounts for much of the surface-temperature “slow down”, there have been two coincident effects, both contributing a further slight cooling effect: the solar cycle happened to go from a solar maximum in 2001-2 to a minimum in 2009, and the level of aerosols from volcanoes (which reflect sunlight) increased two-fold from 2000 to 2010.2 These three effects — solar activity, volcanic activity, and spontaneous natural variability in the oceans — can explain most of the “slow-down”.3
What do other data tell us about climate change? The way some climate inactivists talk about climate science, you might think that it’s all about just one time-series, the global average surface temperature. But there are many, many things that can be measured, and scientific predictions have been made for decades about, for example: the spatial and temporal pattern of changes in temperature; changes in ice volumes; increases in humidity; and sea level rise. In 1975, climate scientists predicted the spatial fingerprint of increases in carbon dioxide: they predicted warming in the troposphere (that’s the bottom 10 km or so of the atmosphere) and cooling in the stratosphere (the top bit of the atmosphere, from roughly 20 km up); and they predicted that there would be significantly more warming in the Arctic than in latitudes nearer the equator.
What do the data show? The predictions about the spatial fingerprint of greenhouse gases are being fulfilled. The Arctic has warmed more than the world average. The stratosphere has cooled, as predicted. Global sea level is estimated to have risen (4th panel on the left in Figure 4) at a rate much faster than at any time in the last 2000 years. Glaciers have been shrinking and losing ice world-wide, with very few exceptions (5th panel on the right in Figure 4), and the rate of loss of ice seems to have got bigger in the last two decades.4 Humidity has increased (which means the probability of extreme intense rainfall has increased).
Perhaps the most compelling data to pay attention to are those relating to Arctic warming. Figure 4 (5th panel on the left) and figure 5 show the Arctic summer sea-ice extent and September sea-ice extent. Arctic sea-ice has been declining faster than many models predicted. Perhaps predictably, climate inactivists persist in misrepresenting these data, dressing up occasional up-ticks in Arctic sea-ice as “a definite recovery trend” (David Whitehouse, quoted in the Telegraph in 2010).5
The bottom line? The climate is changing, the globe is warming, and we should expect global surface temperatures to continue to wobble around the long-term warming trend predicted by the physics.
2 See IPCC AR5 Chapter 8 figures 8.12, 8.13.
3 For further reading on statistical modelling of short-term effects of the El Niño Southern Oscillation (ENSO), solar variability, and volcanic aerosols, see the paper by Foster and Rahmstorf referenced here: http://www.skepticalscience.com/graphics.php?g=52
4 IPCC AR5 WG1, Summary for policy makers.