If 2013 breaks heat record, how will deniers respond?

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With an El Ni?o on the way, 2013 could be the warmest year on record. But the climate-denial machine will keep on churning

IT HAS been another "normal" global-warming summer in the northern hemisphere. The US sweltered in the hottest July on record, following the hottest spring on record. More than 60 per cent of the contiguous US is suffering from drought, as are parts of eastern Europe and India. In the Arctic, sea ice cover is at a record low and the Greenland ice sheet shows what the US National Snow and Ice Data Center calls "extraordinary high melting". Global land temperatures for May and June were the hottest since records began in the 19th century.

Meanwhile, El Ni?o conditions are forecast to develop in the tropical Pacific Ocean, warming up ocean surface temperatures. Some observers have predicted that this will lead to record-breaking global temperatures next year.

If El Ni?o does arrive and temperature records are broken, there will inevitably be much discussion of the causes of the warming. So now is a good time to sort signal from noise in the global temperature records.

For the past 30 years, global temperature has shown a linear warming trend of 0.16 ?C per decade (Environmental Research Letters, vol 6, p 044022). When looking for the cause of this warming, a physicist will look for the heat source. One possibility is that the oceans are releasing heat. But measurements show the opposite: the oceans are soaking up heat. The other possibility is that the heat is coming from above, and indeed it is: more radiation is entering the top of the atmosphere than leaving it. This is because increasing amounts of greenhouse gases in the atmosphere hamper the loss of heat into space.

Superimposed on this global-warming signal is short-term natural variability, which makes some years hotter and some colder. Some, notably 2005 and 2010, stick out above the trend line, whereas others, like 2008 and 2011, stay below it. But overall, temperatures are creeping upwards within a corridor of plus or minus 0.2 ?C around the trend line. Climate deniers use this variability to claim there is a slowdown in global warming, by cherry-picking time intervals that happen to start in the upper part of the corridor and end in the lower. They mix up signal and noise.

Three known factors explain much of the natural variation. The first is volcanic eruptions - the eruption of Mount Pinatubo in the Philippines in 1991 was followed by three cold years, for example. Then there is the sun's variability, mostly in the form of the 11-year sunspot cycle. Finally, there is the irregular oscillation between warm El Ni?o and cold La Ni?a conditions in the Pacific.

We have independent measurements describing all three that we can easily correlate to global temperature changes. This shows, for example, that during a solar maximum the globe is about 0.1??C warmer than during a solar minimum, but also that solar activity has contributed nothing to the warming trend of the past 30 years. In fact, it has acted to reduce it, but the effect is so small that the hottest year on record, 2010, was near the end of the deepest solar minimum since satellite measurements began in the 1970s.

The analysis further shows that global temperature typically reaches a maximum about four months after El Ni?o conditions peak, and is correspondingly colder after La Ni?a. La Ni?a episodes in 2008 and 2011 have cooled the past few years, masking the warming trend. But while 2011 was cool in the context of the previous 10 years, it was the hottest La Ni?a year on record.

It is straightforward to remove the effects of the solar and El Ni?o cycles from the data, just as unemployment figures routinely have seasonal effects removed. Once this is done, and regardless of the global temperature dataset used, the result is always a steady warming trend that has been no slower in the past decade than it was in the previous two - and which, incidentally, agrees with what is predicted by the Intergovernmental Panel on Climate Change.

Now solar activity is on the way back up and it is only a matter of time before the next El Ni?o event comes along. In fact, predictions by the US National Oceanic and Atmospheric Administration suggest that El Ni?o conditions are likely to arrive any time now. These two factors, combined with the ongoing warming trend, make it likely that a global temperature record will be set next year - unless a major volcano erupts.

Perhaps a record year will silence those unscientific voices who claim that global warming has come to an end. But the denial industry has already come up with a plan B: to claim that global warming is completely down to El Ni?o. To expose the fallacy of that, we just need to look again at where the heat comes from: below or above.

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