Take a look at any chart of global average temperature trends during the past several decades (below, for example), and chances are that you’ll notice a lot of short-term ups and downs, with an overall trend towards warmer than average conditions. Those zigs and zags from one year to the next are largely due to natural phenomena such as El Nino and La Nina, which help tip the odds in favor of warmer or cooler years. The longer-term warming trend is most likely due in large part to manmade greenhouse gas emissions, such as carbon dioxide.

The “raw” (or unadjusted) global average temperature departure from average since 1979 according to five datasets (three surface datasets, GISS, NCDC, and CRU and two satellite datasets, RSS and UAH), before natural cycles are removed. (Environmental Research Letters)

One of the major challenges facing climate researchers has been figuring out a way to tease out the signal of manmade climate change from the background “noise” of natural climate variability. A new study published in Environmental Research Letters is aimed at accomplishing this goal, and dispelling one of the key arguments made by skeptics of manmade climate change.

Researchers Grant Foster of Tempo Analytics and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research examined five widely used time series of global temperatures at the surface and in the lower atmosphere during the period 1979 to 2010. Their statistical analysis builds upon previous studies and estimates the impacts of the three top sources of natural climate variability - The El Nino-Southern Oscillation or ENSO; aerosol particles from volcanic eruptions; and fluctuations in the energy output of the sun.

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.

The “true global warming signal”. In this “adjusted” image, several natural sources of climate variability are removed from the five temperature datasets (averaged together here) shown in the top image. (Enivronmental Research Letters)

In other words, their new time series shows the climate system under the influence of mostly human activities. However, it should be noted that their methods do not completely remove all natural sources of climate variability from the temperature record.

The study presents yet more evidence contradicting the climate meme that global warming stopped in 1998, although as with most memes, this one will likely persist regardless of what the actual evidence shows.

“When the fluctuations in temperature over the last 32 years (which tend to obscure the continuation of the global warming trend) are accounted for, it becomes obvious that there has not been any cessation, or even any slowing, of global warming over the last decade (or at any time during this time span),” the study states. “All five data sets show statistically significant warming even for the time span from 2000 to the present.”

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, emphasizing the urgency of confronting the human influence on climate.”

The study finds that for all five adjusted data sets, 2009 and 2010 are the two hottest years since 1979.

The study also contains unique insights into how naturally-occurring phenomena, such as El Nino/La Nina and solar variability, affect global temperatures. It finds that El Nino/La Nina events have the largest short-term influence on global temperatures, followed by volcanic aerosols and fluctuations in the sun’s energy output.

For example, La Nina conditions have been present off and on since 2010, and this has reduce global temperatures somewhat, particularly when looked at from the perspective of lower atmospheric temperatures. But, even with a strong La Nina event, 2011 is likely to be one of the top 10 warmest years on record, as reported by the World Meteorological Organization (WMO). The WMO expects 2011 to become the warmest La Nina year on record, which may demonstrate that manmade climate change is beginning to overwhelm natural factors to some extent.

Temperature departure from average since 1950, with La Nina years shaded in blue. (World Meteorological Organization)

Interestingly, Foster and Rahmstorf find that El Nino/La Nina events, along with volcanic eruptions, tend to alter temperatures in the lower atmosphere more than they do surface temperatures. This would account for some of the differences between surface and lower atmospheric data.

In fact, after removing the top natural sources of short-term climate variability, the study shows closer agreement between the surface and lower atmospheric temperature records.