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by Leon Clifford
Two leading climate scientists have attributed the so called pause in global warming to a combination of effects including air pollution and internal climate variability - particularly linked to La Nina Pacific Ocean cooling events.
Gavin Schmidt, Deputy Chief of NASA's Goddard Institute for Space Studies, and Thomas Karl, Director of NOAA's National Climatic Data Center (NCDC), were speaking to reporters taking part in a telephone press conference organised by NASA on Tuesday 21 January to discuss the release of global temperature data for 2013.
NOAA data shows that 2013 tied with 2003 as the fourth warmest year on record since 1880 while NASA data shows that 2013 tied with 2006 and 2009 as the seventh warmest year on record. Both scientists pointed out that the actual differences between the data from the two organisations are very small even though they result in a different ranking position.
The last decade is the warmest in the historical record, Schmidt said.
When asked about the apparent pause in global warming seen since 1998, both scientists pointed to the impact of naturally occurring phenomena.
The trend in increasing global average temperatures in the last 10 to 15 years appears to be lower compared with the trend seen in the years before and "this appears to be a function of internal variability in the system", Schmidt told reporters.
NOAA's Karl pointed out that there had been more frequent La Nina cooling events in the period since the pause had begun.
Schmidt also suggested that there has been slightly more volcanic activity during the period since the pause began and pointed to the fact that the sun has been slightly dimmer than had been anticipated. "All these things are factors," he said.
But Schmidt also highlighted a major uncertainty in the level of air pollution which contributes to aerosols in the atmosphere and to the uncertainty about the direct and indirect effects that these have on the climate - which is known as the climate forcing. Aerosols can act a reflective shields but they also interact with clouds and affect cloud formation which can in turn affect the trend in global temperatures.
"Our ability to properly quantify air pollution - aerosols - is actually not very good," said Schmidt. "We've had a problem defining aerosol forcing. This is one of biggest gaps in the data for the climate when we are looking at long terms trends".
"We have yet to see a final reckoning," said Schmidt. " "We do not know what the aerosol forcing has been over the (recent) period."
The impact of the Pacific Ocean on global temperatures was also discussed by the two scientists.
Karl pointed out that as well as El Nino Pacific Ocean warming events and La Nina Pacific Ocean cooling events there was a longer term change in the Pacific Ocean, known as the Pacific Decadal Oscillation (PDO), which had an impact on global temperatures and that this was currently in a cooling phase.
In general, years with El Ninos tend to be slightly warmer and years with La Nina tend to be slightly cooler, but the impact of El Nino and La Nina events do not hide the long term warming trend, the scientists explained.
Long term trends due to increases in greenhouse gases are apparent "regardless of whether it is an El Nino or La Nina year," said Schmidt.
The trend of temperature increases across El Nino years is the same as the trend of temperature increases in La Nina years and this is also the same as the trend of temperature increases seen in so called neutral years where there is neither an El Nino nor a La Nina, according to Schmidt. "Trends in each group are identical," he said.
The two scientists also addressed the issue of sea ice extent and in particular the differences between the sea ice extent in the Arctic, which has been declining, and the Antarctic, which has been increasing and has been at record levels during 2013.
"Arctic sea ice is considerably down on the last 10 to 15 years. In contrast, Antacrtica has a tendency to show increases. In the past year Antarctic sea ice extent is the greatest on record," said Karl.
Schmidt drew a comparison between what he called the circumpolar decline in the Arctic where sea ice loss is apparent across the whole polar region and the more "regional specificity" of sea ice gains in the Antarctic.
"In Antarctica it is a more complex pattern," he explained highlighting changes in wind circulation patterns and more complex teleconnections as contributing factors.
Schmidt added that "one number" - the level of the sea ice extent in Antarctica - "does not do justice to the complexity of the southern hemisphere sea ice situation".
See the slide deck from the NASA press conference here.
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