You Are In The Old Site Archive - Click Here To See The Main Site With Current Stories

Global Warming Pause Due To Pacific Says Trenberth

06.12.2013 19:00 Age: 5 yrs

Leading climate scientist Kevin Trenberth tells that the pause in global warming may be caused by the Pacific Ocean. And in a new paper he argues that the pause does not mean climate change has stopped but that it is simply "manifested in different ways".

Trenberth: “The centre of action is the Pacific Ocean but the main places where heat goes deep into the ocean are the Atlantic and Southern Oceans rather than the Pacific," he told Courtesy: NCAR

Click to enlarge. Figure from the paper shows how winds interact with the sea and contribute to piling up water in the western Pacific.

Click to enlarge. Figure from the paper shows the relationship between the long term global warming trend and the Pacific Ocean oscillations.


by Leon Clifford


Leading climate scientist Kevin Trenberth has told that he believes the pause in global warming may be caused by long term changes in the Pacific Ocean.

Trenberth and colleague John Fasullo argue in a new scientific paper that the massive El Nino Pacific Ocean warming event that occurred in 1997 and 1998 triggered the pause. They say that the El Nino caused a large loss of heat from the deep ocean to the sea surface that resulted in a cooling of the oceans. Since then the deep ocean has been absorbing heat back from the upper ocean and so cooling the atmosphere.

The implication is that the heat being absorbed from the atmosphere by the oceans has offset the underlying and ongoing warming of the atmosphere due to green house gases. As the deep ocean waters have slowly warmed they have taken heat from the upper ocean which has then cooled the atmosphere. This is the cause of the apparent hiatus in global warming that has manifested itself as a halt in the rise in global mean atmospheric temperatures seen in the second half of the 20th century.



Trenberth and Fasullo, from the US National Center for Atmospheric Research in Boulder Colorado, suggest that long term oscillations in the Pacific Ocean, known as Pacific Decadal Oscillations (or PDOs) drive alternate 20-plus year cycles of upper ocean warming and cooling which also involve heat being exchanged with the atmosphere. The implication of this is that when the Pacific is in a negative phase the upper ocean loses heat and so cools the atmosphere, and that when it is in a positive phase the upper ocean warms and so heats the atmosphere.









Trenberth points to three lines of evidence that support this idea: measurable and recorded changes in the wind strength, satellite altimeter radar measurements of sea level and an analysis of ocean heat data.




Each phase of these PDO cycles last between 20 and 30 years, according to the historical record. Positive phases of the PDO took place from 1923 to 1942 and from 1976 to 1998, and negative phases from 1943 to 1976 and after 1999, according to data. Trenberth believes that it is possible that another significant El Nino event will mark the transition from the current negative PDO to a positive PDO. This may then result in a return to rising atmospheric surface temperatures.





Global warming first became evident beyond the bounds of natural variability in the 1970s, but increases in global mean surface temperatures have stalled in the 2000s. Increases in atmospheric greenhouse gases, notably carbon dioxide, create an energy imbalance at the top-of-atmosphere (TOA) even as the planet warms to adjust to this imbalance, which is estimated to be 0.5 W m−2over the 2000s. Annual global fluctuations in TOA energy of up to 0.2 W m−2