It has been another "normal" global-warming summer in the Northern Hemisphere. The United States sweltered in the hottest July on record, following the hottest spring on record. More than 60 percent of the contiguous United States 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 U.S. 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 Niño 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 Niño 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 upward 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 Niño and cold La Niña 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 Niño conditions peak, and is correspondingly colder after La Niña. La Niña 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 Niña year on record.
It is straightforward to remove the effects of the solar and El Niño 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 Niño event comes along. In fact, predictions by the U.S. National Oceanic and Atmospheric Administration suggest that El Niño 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 Niño. To expose the fallacy of that, we just need to look again at where the heat comes from: below or above.
In the case of El Niño, the warmth comes from the ocean. During El Niño events, the global ocean releases heat, whereas during La Niña events, it recharges its heat store. That is confirmed by satellite measurements of the radiation balance: During recent La Niña events our planet did not lose heat to space. On the contrary, it absorbed more than normal. That is to be expected: When the ocean exposes colder waters at its surface, as during La Niña, these soak up extra heat.
So if global warming of the past decades was due to El Niño or another mechanism involving heat from the ocean, the ocean would have lost heat. But the heat content has gone up, not down. And it is well understood why: because we created a radiation imbalance by adding greenhouse gases to our atmosphere.
The signal of global warming caused by humans is very clear, despite attempts by certain parties to drown it out with a lot of noise.
This article originally appeared in New Scientist.