The increase in global surface mean temperatures stalled in the 2000's. However, last decade was still the warmest decade on record going back to 1880. Has global warming stopped?
Greater than 90 percent of the heat that reaches the earth's surface goes into the oceans.
Over the last decade, more than 30 percent of that heat has reached below the 700 meter depth in the oceans, which is traceable to changes in surface winds mainly over the Pacific Ocean, which has been associated with a switch to the negative phase of the Pacific Decadal Oscillation (PDO) in 1999.
Surface warming was much more evident during the positive phase of the PDO during the 1976-1998 period as less heat was absorbed by the deeper part of the oceans.
A new research paper shows that natural variability modulates the rate of surface temperature change, while sea level continues a steady increase.
There is no pause in global warming, it is just manifested in different ways, according to Kevin Trenberth who is a co-author of the study.
According to the study, the biggest fluctuations in global mean surface temperature have been identified with El Nino Southern Oscillation (ENSO).
The major El Nino event back in 1997-98 was likely a trigger for the change in the PDO, as it led to a large loss of heat in the deep Pacific.
The PDO can lock into one phase for as much as 25-30 years.
Key excerpts from the study titled "An apparent hiatus in global warming"
Deniers of climate change often cherry-pick points on time series and seize on the El Niño warm year of 1998 as the start of the hiatus in global mean temperature rise (Figure 6). This turns out, arguably, to have been the transition time from a positive to a negative phase of the PDO. The monthly time series (Figure 8) readily reveals the multidecadal regimes of the PDO (given by the black line) with positive phases from 1923 to 1942 and 1976 to 1998, and negative phases from 1943 to 1976 and after 1999. While naturally emphasizing the North Pacific, the pattern covers the entire Pacific with a somewhat ENSO-like pattern but one that is broader in the tropics [Chen et al., 2008].
If we now examine the hiatus period of 1999-2012 and compare it to the time when global warming really took off from 1976 to 1998 (Figure 9), the negative PDO pattern emerges very strongly throughout the Pacific although warming prevails in the Atlantic and Indian Oceans and on land. In other words, it is the central and eastern Pacific more than anywhere else that has not warmed in the past decade or so. In spite of some cold European winters, Europe does not standout in Figure 9 and instead is a warm region. The AMO is positive (Figure 7) and is revealed in Figure 9 to be part of a wider warming.
Variations in natural external forcings are not an explanation of the hiatus, but rather internal variations within the climate system are keys, according to the researchers.
The PDO is essentially a natural mode of variability, although there are questions about how it is affected by the warming climate, and so the plateau in warming is not because global warming has ceased. The evidence supports continued heating of the climate system as manifested by melting of Arctic sea ice and glaciers, as well as Greenland, but most of the heat is going into the oceans and increasingly into the deep ocean, and thus contributes to sea-level rise. The analysis in this article does not suggest that global warming has disappeared; on the contrary, it is very much alive but being manifested in somewhat different ways than a simple increase in global mean surface temperature.
The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or AccuWeather.com
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