Why global temperatures held steady for 10 years

Global warming temporarily ground to a halt over the last 10 years, thanks to increased pollution from China, the El Niño system in the Pacific, and a slight drop in the energy Earth gets from the sun.
"Global warming stopped in 1998" is one of the most common reasons people offer for not believing in 
Scientific studies on climate helped establish...Image via Wikipediaclimate change. It certainly looks like a problem for anyone claiming that humanity's greenhouse gas emissions are warming the planet: after all, we kept on pumping out carbon dioxide faster than ever, yet nothing happened to the temperature.

But according to the new analysis, various short-term factors can account for the slowdown. Most of those variables are going to change direction soon. So the halt in warming may be only temporary.
To find out if the slowdown could be explained, Robert Kaufmann of Boston University in Massachusetts and colleagues used a statistical model of the climate.
They took data collected between 1998 and 2008 on several factors that can affect the climate, including greenhouse gas emissions, incoming radiation from the sun, and sulphur pollution from burning coal and other industrial activities.
Then they plugged the information into their model, ran it for the 1998-2008 period, and asked: does it replicate what global temperatures actually did?
The short answer is yes. In the model, global temperatures held steady, showing no significant rise over the study period.
A major reason for this is the rise in coal use in China. This produces a lot of sulphur particles, which cool the global climate. This more-or-less cancelled out the warming effect of the greenhouse gas emissions.
That shouldn't come as a surprise. It's well-established that aerosol particles can have a major impact on the climate. In south-east Asia, particularly China and India, there is often a "brown haze" of pollution that has an overall cooling effect on the planet.
With the two human-produced effects cancelling each other out, natural variation in the climate took hold. As it happened, two of the natural trends were towards cooling.
The first was the El Niño Southern Oscillation (ENSO), a cyclic change in the behaviour of the Pacific Ocean. 1998 saw the system in an extreme state, so the Pacific dumped a lot of heat into the atmosphere and surface temperatures spiked as a result. Since then ENSO has gone in the other direction, so the Pacific has taken heat from the atmosphere.
And the second shift came from the sun, which goes through a regular 11-year cycle of changing activity. From a peak in 2000, solar activity fell steadily to a low in 2007, so it sent less radiation our way.
It's possible - though by no means certain - that the sun will stay quiet for the next few decades. Unfortunately any cooling effect is likely to be small, and would stop as soon as the sun perked up again.
But while the sun might keep putting the brakes on global warming - slightly - the other variables won't be so obliging. ENSO will swing back in the other direction and warm the surface again. And China is planning to cut the pollution from its coal power plants, because it is so harmful to human health.
So there are two key messages we can take from the research. The first is that the brief halt in global warming doesn't necessarily mean there's a problem with climate science: known factors can account for it. And the second is that the reprieve may be only temporary.
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