News that doesn't receive the necessary attention.

Tuesday, November 25, 2014

Global warming pause that began in 1998 could last until 2033 per Science Magazine peer reviewed study-BBC

"Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years." Meaning ongoing pause could continue until 2033 (1998 + 35= 2033).
 
8/21/14, "Global warming slowdown 'could last another decade'," BBC, Matt McGrath

"The hiatus in the rise in global temperatures could last for another 10 years, according to new research.

Scientists have struggled to explain the so-called pause that began in 1999, despite ever increasing levels of CO2 in the atmosphere.
 
The latest theory says that a naturally occurring 30-year cycle in the Atlantic Ocean is behind the slowdown. The researchers says this slow-moving current could continue to divert heat into the deep seas for another decade. 
.. 
However, they caution that global temperatures are likely to increase rapidly when the cycle flips to a warmer phase.

According to the Intergovernmental Panel on Climate Change (IPCC), global average temperatures have increased by around 0.05C per decade in the period between 1998 and 2012. This compares with a decadal average of 0.12 between 1951 and 2012.

More than a dozen theories have been put forward on the cause of this pause in temperature growth that occurred while emissions of carbon dioxide were at record highs.

These ideas include the impact of pollution such as soot particles that have reflected back some of the Sun's heat into space.

Increased volcanic activity since 2000 has also been blamed, as have variations in solar activity. 

The most recent perspectives have looked to the oceans as the locations of the missing heat.

Last year a study suggested that a periodic upwelling of cooler waters in the Pacific was limiting the rise.

However this latest work, published in the journal Science, shifts the focus from the Pacific to the Atlantic and Southern oceans.

The team, lead by Prof Ka-Kit Tung from the University of Washington, US, says there is now evidence that a 30-year current alternately warms and cools the world by sinking large amounts of heat beneath these deep waters.

They've used observations from a network of devices called Argo floats that sample the oceans down to 2,000 metres.

Ice age fears 

The researchers say that there was another hiatus between 1945 and 1975 due to this current taking down the heat, that led to fears of a new ice age

From 1976 though, the cycle flipped and contributed to the warming of the world, as more heat stayed on the surface.

But since the year 2000, the heat has been going deeper, and the world's overall temperatures haven't risen beyond the record set in 1998

"The floats have been very revealing to us," said Prof Tung.

"I think the consensus at this point is that below 700 metres in the Atlantic and Southern oceans [they are] storing heat and not the Pacific."

A key element in this new understanding is the saltiness of the water. The waters in the Atlantic current coming up from the tropics are saltier because of evaporation. This sinks more quickly and takes the heat down with it.

Eventually though, the salty water melts enough ice in Arctic waters to lower the saline level, slowing down the current and keeping the heat near the surface. "Before 2006 the saltiness was increasing, this indicated that the current was speeding up," said Prof Tung.

"After 2006, this saltiness is diminishing but it's still above the long-term average. Now it is slowly slowing down. "Once it gets below the long-term average, then it is the next period of rapid warming."

As well as the data from the Argo floats, Prof Tung has also examined the Central England Temperature record, that dates back over 350 years. He believes that this confirms the

regular 70-year cycles of warm and cold spells

This historic pattern, he says, could extend the current period of pause. 

"We probably may have another 10 years, maybe shorter as global warming itself is melting more ice and ice could flood the North Atlantic, but historically we are in the middle of the cycle."

Several other researchers in this field acknowledge the Tung analysis is part of a growing body of evidence that suggests the Atlantic has a role in the pause. 

Prof Reto Knutti from the ETH Zurich has recently published a review of all the current theories on the hiatus.

"I see the studies as complementary, and they both highlight that natural variability in ocean and atmosphere is important in modifying long term anthropogenic trends," he said.

"A better understanding of those modes of variability is critical to understand past changes (including differences between models and observations during the hiatus period) as well as predicting the future, in particular in the near term and regionally, where variability dominates the forced changes from greenhouses gases."

Other scientists say that the Atlantic hypothesis is interesting but a much longer range of observations is needed.

"We really don't have a lot of data," said Dr Jonathan Robson from the University of Reading, UK.

"So if there is this 60-year oscillation in the ocean, we haven't observed it all, basically we've observed the impact of it. We may have to wait 15-20 years to know what's going on."

Prof Tung believes that whatever the cause and the length of the pause, we are on a "rising staircase" when it comes to global temperatures that will become apparent when the Atlantic current switches again.

"At the end we will be on the rising part of the staircase, and the rate of warming there will be very fast, just as fast as the last three decades of the 20th Century, plus we are starting off at a higher plateau. The temperatures and the effects will be more severe.""

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Science Magazine study on "slowdown" in global warming linked in above BBC article:

8/22/14, "Varying planetary heat sink led to global-warming slowdown and acceleration," sciencemag.org
. Xianyao Chen1,2, Ka-Kit Tung2,* Author Affiliations
.
1Key Laboratory of Physical Oceanography, Ocean University of China, Qingdao, China. 2Department of Applied Mathematics, University of Washington, Seattle, WA, USA.

"A vacillating global heat sink at intermediate ocean depths is associated with different climate regimes of surface warming under anthropogenic forcing: The latter part of the 20th century saw rapid global warming as more heat stayed near the surface. In the 21st century, surface warming slowed as more heat moved into deeper oceans. In situ and reanalyzed data are used to trace the pathways of ocean heat uptake. In addition to the shallow La Niña–like patterns in the Pacific that were the previous focus, we found that the slowdown is mainly caused by heat transported to deeper layers in the Atlantic and the Southern oceans, initiated by a recurrent salinity anomaly in the subpolar North Atlantic. Cooling periods associated with the latter deeper heat-sequestration mechanism historically lasted 20 to 35 years."...

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2013 Nature published study on "the hiatus" in global warming:

"Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Nina-like decadel cooling."...Nature.com

8/28/13, "Recent global-warming hiatus tied to equatorial Pacific surface cooling," Nature.com,
Yu Kosaka and Shang-Ping Xie
    "Despite the continued increase in atmospheric greenhouse gas concentrations, the annual-mean global temperature has not risen in the twenty-first century1, 2, challenging the prevailing view that anthropogenic forcing causes climate warming. Various mechanisms have been proposed for this hiatus in global warming3, 4, 5, 6, but their relative importance has not been quantified, hampering observational estimates of climate sensitivity. Here we show that accounting for recent cooling in the eastern equatorial Pacific reconciles climate simulations and observations. We present a novel method of uncovering mechanisms for global temperature change by prescribing, in addition to radiative forcing, the observed history of sea surface temperature over the central to eastern tropical Pacific in a climate model. Although the surface temperature prescription is limited to only 8.2% of the global surface, our model reproduces the annual-mean global temperature remarkably well with correlation coefficient r = 0.97 for 1970–2012 (which includes the current hiatus and a period of accelerated global warming). Moreover, our simulation captures major seasonal and regional characteristics of the hiatus, including the intensified Walker circulation, the winter cooling in northwestern North America and the prolonged drought in the southern USA. Our results show that the current hiatus is part of natural climate variability, tied specifically to a La-Niña-like decadal cooling. Although similar decadal hiatus events may occur in the future, the multi-decadal warming trend is very likely to continue with greenhouse gas increase." 

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    Climate models didn't forecast pause in global warming:

    2009, BBC: "For the last 11 years we have not observed any increase in global temperatures. And our climate models did not forecast it."...

    10/9/2009, "What happened to global warming?" Paul Hudson, BBC

    "This headline may come as a bit of a surprise, so too might that fact that the warmest year recorded globally was not in 2008 or 2007, but in 1998.

    But it is true. "For the last 11 years we have not observed any increase in global temperatures.

    And our climate models did not forecast it, even though man-made carbon dioxide, the gas thought to be responsible for warming our planet, has continued to rise."...  


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    2011 PNAS study finds "hiatus" in global warming 1998-2008:

    7/5/11, "Reconciling anthropogenic climate change with observed temperature 1998–2008," PNAS.org

     "Robert K. Kaufmanna,1 ,Heikki Kauppib, Michael L. Manna, and James H. Stockc"... 

    "Abstract"

    "Given the widely noted increase in the warming effects of rising greenhouse gas concentrations, it has been unclear why global surface temperatures did not rise between 1998 and 2008. We find that this hiatus in warming coincides with a period of little increase in the sum of anthropogenic and natural forcings....

    Data for global surface temperature indicate little warming between 1998 and 2008 (1). Furthermore, global surface temperature declines 0.2 °C between 2005 and 2008. Although temperature increases in 2009 and 2010, the lack of a clear increase in global surface temperature between 1998 and 2008 (1), combined with rising concentrations of atmospheric CO2 and other greenhouse gases, prompts some popular commentators (2, 3) to doubt the existing understanding of the relationship among radiative forcing, internal variability, and global surface temperature."...


     

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