Leading climate scientist Kevin Trenberth has told reportingclimatescience.com 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.
“It is not so much that the atmosphere warms up rather that the upper levels of sea get warmer and these interact more directly with the atmosphere,” Trenberth said. So a warmer sea surface leads to a warmer atmosphere. “More heat penetrates to the deep ocean in the negative phase and that is not the case in the positive phase,” he explained.
“We can speculate that the huge 1997–1998 El Niño event was a trigger for the change in the PDO; certainly, it led to a large loss of heat in the Pacific… that has taken years to recover from, if the recovery is even complete. Past behavior of the PDO… suggests that regimes can last for 25 years,” Trenberth and Fasullo write in their paper.
“The picture emerging is one where the positive phase of the PDO from 1976 to 1998 enhanced the surface warming somewhat by reducing the amount of heat sequestered by the deep ocean, while the negative phase of the PDO is one where more heat gets deposited at greater depths, contributing to the overall warming of the oceans but cooling the surface somewhat. The Pacific Ocean appears to account for the majority of the decadal variability… Nevertheless, the events in the Pacific undoubtedly also affect the Atlantic, Indian, and Southern Oceans as the system acts collectively to equilibrate to these changes in the flow of energy,” they write.
The paper, entitled “An apparent hiatus in global warming?”, appears the new scientific journal Earth Futures.
“There are really deep teleconnections between the Pacific and the Atlantic and Southern Oceans,” Trenberth explained. “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.”
There is also a very strong relationship with winds and sea level, according to Trenberth. Water is piling up in the western Pacific Ocean at a rate of around 10mm per year which is three times the global average. This has led to a difference in sea level, measured by satellite radars, between the western and eastern Pacific. “The sea level is 20cm higher in the western Pacific and the only way to keep it there is for strong winds to pile up the water. It is these changes in the winds that change the ocean currents and affect where the heat is going,” he explained. “But this can’t keep going for ever. The ocean wants to slop back to the east.”
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.
Trenberth said that the negative phase of the PDO is associated with wetter, cooler weather in the tropics and with colder winters in Europe. The implication is that when the phase turns positive we will see more droughts and extreme heat events and wild fires in the tropics and countries such as Australia, he said. “The biggest changes in surface temperature are on land,” he explained.
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.
“Deniers of climate change often cherry-pick points on time series and seize on the El Nino warm year of 1998 as the start of the hiatus in global mean temperature rise,” Trenberth and Fasullo comment in their paper. “This turns out, arguably, to have been the transition time from a positive to a negative phase of the PDO.”
“Global warming has not stopped; it is merely manifested in different way,” the authors state.
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 0.5–1 W m−2over the 2000s. Annual global fluctuations in TOA energy of up to 0.2 W m−2 occur from natural variations in clouds, aerosols, and changes in the Sun. At times of major volcanic eruptions the effects can be much larger.
Yet global mean surface temperatures fluctuate much more than these can account for. An energy imbalance is manifested not just as surface atmospheric or ground warming but also as melting sea and land ice, and heating of the oceans. More than 90% of the heat goes into the oceans and, with melting land ice, causes sea level to rise.
For the past decade, more than 30% of the heat has apparently penetrated below 700 m depth that is traceable to changes in surface winds mainly over the Pacific in association with a switch to a negative phase of the Pacific Decadal Oscillation (PDO) in 1999. Surface warming was much more in evidence during the 1976–1998 positive phase of the PDO, suggesting that natural decadal variability modulates the rate of change of global surface temperatures while sea-level rise is more relentless. Global warming has not stopped; it is merely manifested in different ways.