Behind the present pause in global warming - Washington Post (blog)

The temperature departure from the 1951-1980 average over the last 12 years. Trend is about flat (slightly up). (Data from NASA) Has global warming stopped since the turn of the millennium? Yes - if you examine the data in a vacuum. But has the warming pressure on the atmosphere from man-made greenhouse gases stopped? No - not at all. A closer look at the data bear that out.

It’s easy to glance at a temperature chart of the last 12 years (to the right) and draw the conclusion warming has slowed or stopped. That’s what renowned environmental scientist James Lovelock did and he’s now told MSNBC he’s backed away from his dire concerns about global warming.

“We were supposed to be halfway toward a frying world now,” he said. “The world has not warmed up very much since the millennium.”

(Nevermind Lovelock’s statement that the world should be “frying” was and is out of step with the mainstream scientific community. While scientists have long held the view global warming poses certain risks to humans and the environment, they have not stated the Earth was going to fry or that global warming could mark the end of civilization as Lovelock did.)

But Lovelock failed to appreciate that examining such a small sample of years is misleading. The atmosphere goes through natural cycles that can intensify or conceal warming from manmade greenhouse gases over short time spans.

Such a lapse in warming over a short time period is nothing new. Scientific studies have found similar pauses or slow downs embedded within the Earth’s long-term warming trend in the recent past.

A 2009 NOAA study (Easterling and Wehner) stated:

If we fit a trend line to the same annual global land-ocean temperatures for the 1977-1985 period or the 1981â€"1989 period we also get no trend, even though these periods are embedded in the 1975â€"2008 period showing a substantial overall warming.

Another NOAA study (Knight et al, 2008) found climate models predict such pauses can and should happen from time to time as a result of the various natural cycles. For example, volcanoes and solar activity can influence short-term temperature trends.

The two most prominent cycles acting on year-to-year global temperatures are El Nino and La Nina. El Ninos - associated with a warming of the tropical Pacific - add to greenhouse warming. Conversely, La Ninas - cyclical cooling of the tropical Pacific - offset greenhouse warming.

The prevalence of La Ninas over the last 12 years provide the best explanation for the recent pause. Texas state climatologist Jon Nielsen-Gammon demonstrated this exceptionally well on his blog at the Houston Chronicle.

Consider the time series he created below.

Via the Houston Chronicle: GISTemp global temperatures, with trends for El Niño, neutral, and La Niña years computed separately. (Jon Nielsen-Gammon)

If you disregard the trend lines and stare at the data points over just the last decade or so (or just look at the time series at the top of this post), you see little to no warming. But when the data points are grouped according to whether they’re La Nina years, El Nino years, or neither (neutral years) and trend lines are drawn through them, you see La Nina, El Nino, and neutral years are all inescapably trending warmer. 2011, for example, was the warmest La Nina year on record.

Here’s an excerpt of Nielsen-Gammon’s commentary and conclusions (bold text indicates added emphasis):

[W]e see a couple of recent La Niñas have caused the recent global temperature trend to level off. But be honest: doesn’t it seem likely that, barring another major volcanic eruption, the next El Niño will cause global temperatures to break their previous record? Doesn’t it appear that whatever has caused global temperatures to rise over the past four decades is still going strong?

So about that lack of warming: Yes, it’s real. You can thank La Niña.

As for whether this means that Tyndall [greenhouse] gases are no longer having an impact: Nice try.

Nielsen-Gammon isn’t the first scientist to reach this conclsion. A study published in the peer reviewed literature just last year (Foster and Rahmstorf), which CWG’s Andrew Freedman wrote about, said pretty much the same thing. As Freedman wrote:

After estimating the influence of these natural factors on global temperatures, Foster and Rahmstorf remove them from the temperature chart, leaving an adjusted time series of global temperatures from 1979 through 2010 that shows what they call “the true global warming signal” - which shows unabated warming through 2010.

...

Foster and Rahmstorf state that the 32-year “unabated increase” in average temperatures at the surface and in the lower atmosphere “is powerful evidence that we can expect further temperature increase in the next few decades...”

Other studies have proposed additional theories explaining the cause of the warming pause/slowdown. They may have also played some role in dampening the warming:

* Scientist Susan Solomon published a study in Science in 2010 stating: “Stratospheric water vapor concentrations decreased by about 10% after the year 2000. Here we show that this acted to slow the rate of increase in global surface temperature over 2000â€"2009 by about 25% compared to that which would have occurred due only to carbon dioxide and other greenhouse gases.”

* A 2011 study, also discussed by CWG’s Freedman, found reflective particles ejected into the atmosphere from coal burning in China may have offset part of the warming.

* Solar scientist Judith Lean at the Naval Research Laboratory points to the recent solar minimum. As reported in Greenwire: “According to Lean, the combination of multiple La Niñas and the solar minimum, bottoming out for an unusually extended time in 2008 from its peak in 2001, are all that’s needed to cancel out the increased warming from rising greenhouse gases. Now that the sun has begun to gain in activity again, Lean suspects that temperatures will rise in parallel as the sun peaks around 2014.”

So there are plenty of compelling reasons that together do a good job of explaining the lack of warming in recent years.

Irrespective of your level of concern about global warming, recent temperature data don’t provide much ammunition to change your view. When you break the data down and explore the causes, temperatures are doing little out of line with what scientists who have predicted manmade warming expect. Only if the temperature remains flat or decreases over the next 10-20 years, barring a compelling explanation for it (such as a massive volcano or unforeseen major decrease in energy from the sun), might there be legitimate cause for skepticism about the degree of man’s role in the course of Earth’s temperature.