Joe Romm: Leading Climate Scientists: ‘We Have A Global Emergency,’ Must Slash CO2 ASAP!

by Joe Romm, Climate Progress, March 22, 2016

James Hansen and 18 leading climate experts have published a peer-reviewed version of their 2015 discussion paper on the dangers posed by unrestricted carbon pollution. The study adds to the growing body of evidence that the current global target or defense line embraced by the world — 2 °C (3.6 °F) total global warming — “could be dangerous” to humanity.

That 2 °C warming should be avoided at all costs is not news to people who pay attention to climate science, though it may be news to people who only follow the popular media. The warning is, after all, very similar to the one found in an embarrassingly under-reported report last year from 70 leading climate experts, who had been asked by the world’s leading nations to review the adequacy of the 2 °C target.

Specifically, the new Hansen et al. study — titled “Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming could be dangerous” — warns that even stabilizing at 2 °C warming might well lead to devastating glacial melt, multimeter sea level rise and other related catastrophic impacts. The study is significant not just because it is peer-reviewed, but because the collective knowledge about climate science in general and glaciology in particular among the co-authors is quite impressive.

Besides sea level rise, rapid glacial ice melt has many potentially disastrous consequences, including a slowdown and eventual shutdown of the key North Atlantic Ocean circulation and, relatedly, an increase in super-extreme weather. Indeed, that slowdown appears to have begun, and, equally worrisome, it appears to be supercharging both precipitation, storm surge, and superstorms along the U.S. East Coast (like Sandy and Jonas), as explained here.

It must be noted, however, that the title of the peer-reviewed paper is decidedly weaker than the discussion paper’s “Ice melt, sea level rise and superstorms: evidence from paleoclimate data, climate modeling, and modern observations that 2 °C global warming is highly dangerous.” The switch to “could be dangerous” is reminiscent of the switch (in the opposite direction) from the inaugural 1965 warning required for cigarette packages, “Caution: Cigarette Smoking May Be Hazardous to Your Health” to the 1969 required label “Warning: The Surgeon General Has Determined that Cigarette Smoking Is Dangerous to Your Health.”

And yes I’m using the analogy to suggest readers should not be sanguine about the risks we face at 2 °C warning. Based on both observations and analysis, the science is clearly moving in the direction that 2 °C warming is not “safe” for humanity. But as Hansen himself acknowledged Monday on the press call, the record we now have of accelerating ice loss in both Greenland and West Antarctica is “too short to infer accurately” whether the current exponential trend will continue through the rest of the century.

Hansen himself explains the paper’s key conclusions and the science underlying them in a new video:

The fact that 2 °C total warming is extremely likely to lock us in to sea level rise of 10 feet or more has been obvious for a while now. The National Science Foundation (NSF) itself issued a news release back in 2012 with the large-type headline, “Global Sea Level Likely to Rise as Much as 70 Feet in Future Generations.”  The lead author explained, “The natural state of the Earth with present carbon dioxide levels is one with sea levels about 70 feet higher than now.” Heck, a 2009 paper in Science found the same thing.

What has changed is our understanding of just how fast sea levels could rise. In 2014 and 2015, a number of major studies revealed that large parts of the Antarctic and Greenland ice sheets are unstable and headed toward irreversible collapse — and some parts may have already passed the point of no return. Another 2015 study found that global sea level rise since 1990 has been speeding up even faster than we knew.

The key question is how fast sea levels can rise this century and beyond. In my piece last year on Hansen’s discussion draft, I examined the reasons the Intergovernmental Panel on Climate Change (IPCC) and scientific community have historically low-balled the plausible worst-case for possible sea level rise by 2100. I won’t repeat that all here.

The crux of the Hansen et al. forecast can be found in this chart on ice loss from the world’s biggest ice sheet:

Antarctic ice mass change from GRACE satallite data (red) and surface mass balance method (MBM, blue). From Hansen et al.

Hansen et al. ask the question: if the ice loss continues growing exponentially how much ice loss (and hence how much sea level rise) will there be by century’s end? If, for instance, the ice loss rate doubles every 10 years for the rest of the century (light green), then we would see multi-meter sea level rise before 2100? On the other hand, it is pretty clear just from looking at the chart that there isn’t enough data to make a certain projection for the next eight decades.

The authors write, “our conclusions suggest that a target of limiting global warming to 2 °C … does not provide safety.” On the one hand, they note, “we cannot be certain that multi-meter sea level rise will occur if we allow global warming of 2 °C.” But, on the other hand, they point out:

There is a possibility, a real danger, that we will hand young people and future generations a climate system that is practically out of their control.
We conclude that the message our climate science delivers to society, policymakers, and the public alike is this: we have a global emergency. Fossil fuel CO2 emissions should be reduced as rapidly as practical.

I have talked to many climate scientists who quibble with specific elements of this paper, in particular whether the kind of continued acceleration of ice sheet loss is physically plausible. But I don’t find any who disagree with the bold-faced conclusions.

Since there are a growing number of experts who consider that 10 feet of sea level rise this century is a possibility, it would be unwise to ignore the warning. That said, on our current emissions path we already appear to be headed toward the ballpark of 4-6 feet of sea level rise in 2100 — with seas rising up to one foot per decade after that. That should be more than enough of a “beyond adaptation” catastrophe to warrant strong action ASAP.

The world needs to understand the plausible worst-case scenario for climate change by 2100 and beyond — something that the media and the IPCC have failed to deliver. And the world needs to understand the “business as usual” set of multiple catastrophic dangers of 4 °C if we don’t reverse course now. And the world needs to understand the dangers of even 2 °C warming.

So kudos to all of these scientists for ringing the alarm bell: James Hansen, Makiko Sato, Paul Hearty, Reto Ruedy, Maxwell Kelley, Valerie Masson-Delmotte, Gary Russell, George Tselioudis, Junji Cao, Eric Rignot, Isabella Velicogna, Blair Tormey, Bailey Donovan, Evgeniya Kandiano, Karina von Schuckmann, Pushker Kharecha, Allegra N. Legrande, Michael Bauer, and Kwok-Wai Lo.

http://thinkprogress.org/climate/2016/03/22/3762111/climate-scientists-global-emergency/

MUST READ: The Antarctic Half of the Global Thermohaline Circulation Is Faltering

by FishOutofWater, Daily Kos, April 10, 2013

The sudden cooling of Europe, triggered by collapse of the global thermohaline circulation in the north Atlantic and the slowing of the Gulf Stream has been popularized by the movies and the media. The southern half of the global thermohaline circulation is as important to global climate but has not been popularized. The global oceans' coldest water, Antarctic bottom water forms in several key spots around Antarctica. The water is so cold and dense that it spreads out along the bottom all of the major ocean basins except the north Atlantic and Arctic. Multiple recent reports provide strong evidence that the formation of Antarctic bottom water has slowed dramatically in response to massive subsurface melting of ice shelves and glaciers. The meltwater is freshening a layer of water found between depths of 50 and 150 meters. This lightened layer is impeding the formation of Antarctic bottom water, causing the Antarctic half of the global thermohaline circulation to falter.

Update from the comments

I have been asked what's going to happen in response to the faltering of the thermohaline circulation around Antarctica. This post is based on a synthesis of very recent research reports. The key report, that found the layer of fresh water between 50 and 150 meters deep, was just published. Deward Hastings explained, in a comment, how disruptive this lens of freshened water could be to the earth's climate system and our models of it:

it IS complicated, and confusing

That lens of (relatively) fresh water that is forming around Antarctica is challenging, and changing, almost everything in global circulation patterns.  It freezes sooner (and at a higher temperature).  That shields the water from the wind, and reduces wind-driven mixing.  It reduces, perhaps to the point of stopping altogether, the present global ocean circulation patterns.  That in turn will change global atmospheric weather.

Nobody knows exactly what comes next.  We've never seen it happen, and our models, not terribly accurate in describing the world we know, are completely untested in the coming world that we don't know.

Without a constant flow of cold water from the poles the Abyss will warm . . . and without cold slowly rising from the Abyss the mid-ocean and ocean surface will warm (already happening).  That will lead to more evaporation (driving a different haline circulation in the tropics) and stronger tropical winds driving different surface currents and greater mixing.

Pretty much everything changes as a result . . . pretty much everywhere.  After it's all over some places will have it better and some worse.  While it's changing everywhere will be worse, because there is no way to know what to expect (except that it won't be what you've prepared for).

The best guesses we can make now about the effects of this melt layer are based on paleoclimatology research. Possible effects, based on paleoclimatology studies, are presented in the last few paragraphs. The results of these new studies will be challenging climate modelers for many years.

Sea ice extent has been increasing around Antarctica. In September 2012, while Arctic sea ice was at record low levels, Antarctic sea ice extent hit a record high. Climate skeptics jumped on the Antarctic record as evidence of cooling, while sea ice researchers blamed it on the wind.

Since the start of the satellite record, total Antarctic sea ice has increased by about 1% per decade. Whether the small overall increase in sea ice extent is a sign of meaningful change in the Antarctic is uncertain because ice extents in the Southern Hemisphere vary considerably from year to year and from place to place around the continent. Considered individually, only the Ross Sea sector had a significant positive trend, while sea ice extent has actually decreased in the Bellingshausen and Amundsen Seas. In short, Antarctic sea ice shows a small positive trend, but large scale variations make the trend very noisy.

NSIDC scientist Ted Scambos said, "Antarctica's changes—in winter, in the sea ice—are due more to wind than to warmth, because the warming does not take much of the sea ice area above the freezing point during winter. Instead, the winds that blow around the continent, the "westerlies," have gotten stronger in response to a stubbornly cold continent, and the warming ocean and land to the north."
 

Several recent reports, however, paint a more complex and disturbing picture where the intensifying winds are speeding up below surface currents bringing more above freezing water in contact with deep ice around Antarctica. Twenty of the ice shelves and many of the glaciers that feed them are melting from below.

Researchers used 4.5 million measurements made by a laser instrument mounted on NASA’s ICESat satellite to map the changing thickness of almost all the floating ice shelves around Antarctica, revealing the pattern of ice-shelf melt across the continent. Of the 54 ice shelves mapped, 20 are being melted by warm ocean currents, most of which are in West Antarctica.

Figure 2. Antarctic ice-shelf ice-thickness change rate DT/Dt, 2003–2008.

Seaward of the ice shelves, estimated average sea-floor potential temperatures (in uC) from the World Ocean Circulation Experiment Southern Ocean Atlas (pink to blue) are overlaid on continental-shelf bathymetry (in metres)30 (greyscale, landward of the continental-shelf break, CSB) Grey circles show relative ice losses for ice-sheet drainage basins (outlined in grey) that lost mass between 1992 and 2006 (after ref. 2)

 The melting from below is creating a layer of relatively fresh water 50-150 meters below the surface around Antarctica. This layer of light fresh water is floating above a  salty layer below. When ice forms at the surface in the Antarctic winter it creates cold dense salty water that tends to sink to the bottom, forming bottom water. However, this layer of light melt water is tending to block the water in the top 50 meters from sinking. The area of Antarctic sea ice has expanded because the layer of cold water has stayed on top and expanded outwards instead of sinking. Melting from below has created 2 stratified cold layers in the top 150 meters.

Note the bright pink area in the top 25 meters between 65° and 70° S. This top layer is becoming more saline. Brine is rejected from ice when sea ice forms. It isn't sinking because it is ponding above a freshening layer located at depths between 50 and 150 meters.

The freshened water column around Antarctica has become more stable between depths of 100 and 150 meters. This increasing stability is impeding the formation of Antarctic bottom water. Water that does sink is freshened through incorporation of glacial melt water.

Figure 3.  Austral winter half-year (April–September) zonal mean trends (1985–2010) of observed salinity, vertical density gradient and potential temperature, in the Southern Ocean. a, salinity; b, vertical density gradient; c, potential temperature. Contours indicate the 1985–2010 mean state (psu; kg m^-4, °C). Colouring (bright or faint) indicates whether the trend is significant (yes or no) at p<0:1 65="" 70="" a="" according="" analysis="" and="" based="" between="" brine="" due="" en3="" font="" forms.="" from="" ice="" in="" increase="" is="" likely="" met="" most="" near-surface="" observations.="" observations="" ocean="" office="" on="" rejection="" s="" salinity="" sea="" situ="" sub-surface="" t-test.="" taken="" the="" to="" two-sided="" were="" when="" which="">

Analysis of potential temperatures, which are temperatures adjusted for the effects of increasing pressure with depth, shows the surface water in the top hundred meters is cooling over a vast area from 40° S to 80° S while the water in that vast area below 150 meters is warming.

These results show a trend towards reversal of vertical motions around Antarctica. Intermediate water is welling up around Antarctic melting ice form below creating a freshened layer. Strengthening winds are blowing the cold surface water away from Antarctica. Bottom water formation, caused by the sinking of cold salty water formed by brine rejection, is declining.

The results of this study are confirmed by a detailed study of anthropogenic tracers in the Weddell sea. Chlorofluorocarbon (CFC) observations showed increasing average ages of the deep water in the sea from 1984–2010. The average age increased because because bottom water formation, and outflow from the Weddell sea, declined.

...we find that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27 years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15–21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%. About 88–94% of the age increase in WSBW near its source regions (1.8–2.4 years per year) is explained by the age increase of WDW (4.5 years per year). As a consequence of the aging, the anthropogenic Carbon increase in the deep and bottom water formed in the Weddell Sea slowed down by 14–21% over the period of observations.

The decline in Antarctic bottom water formation, combined with the southward expansion of warm subtropical water in the south Pacific and south Indian oceans has led to the rapid heating of intermediate and deep ocean water in the southern hemisphere.

Figure: Ocean Heat Content from 0 to 300 meters (grey), 700 m (blue), and total depth (violet) from ORAS4, as represented by its 5 ensemble members. The time series show monthly anomalies smoothed with a 12-month running mean, with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical colored bars indicate a two year interval following the volcanic eruptions with a 6 month lead (owing to the 12-month running mean), and the 1997–98 El Niño event again with 6 months on either side. On lower right, the linear slope for a set of global heating rates (W/m2) is given.

A new study of ocean warming has just been published in Geophysical Research Letters by Balmaseda, Trenberth and Källén (2013).  There are several important conclusions which can be drawn from this paper.

• Completely contrary to the popular contrarian myth, global warming has accelerated, with more overall global warming in the past 15 years than the prior 15 years.  This is because about 90% of overall global warming goes into heating the oceans, and the oceans have been warming dramatically.

• As suspected, much of the 'missing heat' Kevin Trenberth previously talked about has been found in the deep oceans.  Consistent with the results of Nuccitelli et al. (2012), this study finds that 30% of the ocean warming over the past decade has occurred in the deeper oceans below 700 meters, which they note is unprecedented over at least the past half century.

As the earth has warmed in response to the effects of increasing levels of greenhouse gases the southern subtropical belt in the oceans and atmosphere has expanded, tightening the rings of winds and ocean currents around Antarctica. Enormous volumes of warm subtropical water have been added to the southern ocean at depths greater than 300 meters (greater than approximately 1,000 feet).

Observed temperature trends in the Indian Ocean present complex patterns that cannot be explained by surface heating alone. The heat storage has apparently increased more in the southern part than in the northern part of the Indian Ocean (Levitus et al., 2005), although this result may be biased by the sparse data coverage, particularly in the south (Harrison & Carson, 2007). The strongest warming is found near the subtropical front and extends as deep as 800 m; it is not directly linked to surface heating but rather due to a southward shift of the oceanic gyre circulation and associated thermal structure (Alory et al., 2007).

Another recent detailed study of the water properties of the southern ocean has independently determined that the southern branch of the global thermohaline circulation has slowed dramatically, contributing to a large uptake of heat by the deep southern ocean.

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the Meridional Overturning Circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW.

Rates of change in AABW-related circulation are estimated in most of the world’s deep
ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The
Southern Ocean is losing water below θ = 0 °C at a rate of -8.2 (±2.6) × 106 m3/s.

The budget calculations and global contraction pattern are consistent with a global scale slowdown of the bottom, southern limb of the MOC.

The slowdown of the southern branch of the thermohaline circulation and the cooling of the surface waters close to Antarctica are enhancing the thermal gradient from the tropics to the pole, speeding up the winds in the southern hemisphere. These increases in wind speeds are likely increasing the flow of water from the Pacific to the Atlantic ocean, enhancing the northward flow of water, salt and heat from the south to the north Atlantic. Moreover, the southward movement of the subtropical front allows more flow of the Agulhas current around the south African capes from the Indian ocean to the south Atlantic.

Thus, increased melting of Arctic sea ice may be related to declines in Antarctic bottom water formation. Likewise, the cool Pacific, warm Atlantic pattern causing increased U.S. droughts and storminess in the north Atlantic may be tied to these changes in ocean circulation patterns. Paleoclimate studies have consistently shown oscillations between Antarctic and north Atlantic bottom water formation and between relative coolness around Antarctica and north Atlantic warmth.

The Arctic melt down that is far exceeding model predictions is connected to the slow down in Antarctic bottom water formation. Climate modelers will be challenged to model the connections and the details. The cooling waters around Antarctica, while apparently good news, are not. The rapid melting of the Arctic will be enhanced.

http://www.dailykos.com/story/2013/04/10/1200602/-The-Antarctic-Half-of-the-Global-Thermohaline-Circulation-Is-Faltering

Global Contraction of Antarctic Bottom Water between the 1980s and 2000s, by S. G. Purkey & G. C. Johnson, J. Climate, 25 (2012)

Purkey, Sarah G., Gregory C. Johnson, 2012: Global Contraction of Antarctic Bottom Water between the 1980s and 2000s*. J. Climate, 25, 5830–5844.

doi: http://dx.doi.org/10.1175/JCLI-D-11-00612.1

Global Contraction of Antarctic Bottom Water between the 1980s and 2000s*

Sarah G. Purkey and Gregory C. Johnson

School of Oceanography, University of Washington, and NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

Abstract

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the meridional overturning circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW. Rates of change in AABW-related circulation are estimated in most of the world’s deep-ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The Southern Ocean is losing water below θ = 0°C at a rate of −8.2 (±2.6) × 10⁶ m³ s⁻¹. This bottom water contraction causes a descent of potential isotherms throughout much of the water column until a near-surface recovery, apparently through a southward surge of Circumpolar Deep Water from the north. To the north, smaller losses of bottom waters are seen along three of the four main northward outflow routes of AABW. Volume and heat budgets below deep, cold θ surfaces within the Brazil and Pacific basins are not in steady state. The observed changes in volume and heat of the coldest waters within these basins could be accounted for by small decreases to the volume transport or small increases to θ of their inflows, or fractional increases in deep mixing. The budget calculations and global contraction pattern are consistent with a global-scale slowdown of the bottom, southern limb of the MOC.

Keywords: Southern Ocean, Meridional overturning circulation, Abyssal circulation, Bottom currents/bottom water, Thermohaline circulation, Climate variability

Received: October 20, 2011; Accepted: January 19, 2012

* Pacific Marine Environmental Laboratory Contribution Number 3771.

Corresponding author address: Sarah G. Purkey, School of Oceanography, Box 357940, University of Washington, Seattle, WA 98195-7940. E-mail: sarah.purkey@noaa.gov

Readers, a must-read: The Antarctic Half of the Global Thermohaline Circulation Is Faltering

byFishOutofWaterFollowforOcean Advocates,  Daily Kos, April 10, 2013

The sudden cooling of Europe, triggered by collapse of the global thermohaline circulation in the north Atlantic and the slowing of the Gulf Stream has been popularized by the movies and the media. The southern half of the global thermohaline circulation is as important to global climate but has not been popularized. The global oceans' coldest water, Antarctic bottom water forms in several key spots around Antarctica. The water is so cold and dense that it spreads out along the bottom all of the major ocean basins except the north Atlantic and Arctic. Multiple recent reports provide strong evidence that the formation of Antarctic bottom water has slowed dramatically in response to massive subsurface melting of ice shelves and glaciers. The meltwater is freshening a layer of water found between depths of 50 and 150 meters. This lightened layer is impeding the formation of Antarctic bottom water, causing the Antarctic half of the global thermohaline circulation to falter.

Update from the comments

I have been asked what's going to happen in response to the faltering of the thermohaline circulation around Antarctica. This post is based on a synthesis of very recent research reports. The key report, that found the layer of fresh water between 50 and 150 meters deep, was just published. Deward Hastings explained, in a comment, how disruptive this lens of freshened water could be to the earth's climate system and our models of it:

it IS complicated, and confusing

That lens of (relatively) fresh water that is forming around Antarctica is challenging, and changing, almost everything in global circulation patterns.  It freezes sooner (and at a higher temperature).  That shields the water from the wind, and reduces wind-driven mixing.  It reduces, perhaps to the point of stopping altogether, the present global ocean circulation patterns.  That in turn will change global atmospheric weather.

Nobody knows exactly what comes next.  We've never seen it happen, and our models, not terribly accurate in describing the world we know, are completely untested in the coming world that we don't know.

Without a constant flow of cold water from the poles the Abyss will warm . . . and without cold slowly rising from the Abyss the mid-ocean and ocean surface will warm (already happening).  That will lead to more evaporation (driving a different haline circulation in the tropics) and stronger tropical winds driving different surface currents and greater mixing.

Pretty much everything changes as a result . . . pretty much everywhere.  After it's all over some places will have it better and some worse.  While it's changing everywhere will be worse, because there is no way to know what to expect (except that it won't be what you've prepared for).

The best guesses we can make now about the effects of this melt layer are based on paleoclimatology research. Possible effects, based on paleoclimatology studies, are presented in the last few paragraphs. The results of these new studies will be challenging climate modelers for many years.

Sea ice extent has been increasing around Antarctica. In September 2012, while Arctic sea ice was at record low levels, Antarctic sea ice extent hit a record high. Climate skeptics jumped on the Antarctic record as evidence of cooling, while sea ice researchers blamed it on the wind.

Since the start of the satellite record, total Antarctic sea ice has increased by about 1% per decade. Whether the small overall increase in sea ice extent is a sign of meaningful change in the Antarctic is uncertain because ice extents in the Southern Hemisphere vary considerably from year to year and from place to place around the continent. Considered individually, only the Ross Sea sector had a significant positive trend, while sea ice extent has actually decreased in the Bellingshausen and Amundsen Seas. In short, Antarctic sea ice shows a small positive trend, but large scale variations make the trend very noisy.

NSIDC scientist Ted Scambos said, "Antarctica's changes—in winter, in the sea ice—are due more to wind than to warmth, because the warming does not take much of the sea ice area above the freezing point during winter. Instead, the winds that blow around the continent, the "westerlies," have gotten stronger in response to a stubbornly cold continent, and the warming ocean and land to the north."

Several recent reports, however, paint a more complex and disturbing picture where the intensifying winds are speeding up below surface currents bringing more above freezing water in contact with deep ice around Antarctica. Twenty of the ice shelves and many of the glaciers that feed them are melting from below.

Researchers used 4.5 million measurements made by a laser instrument mounted on NASA’s ICESat satellite to map the changing thickness of almost all the floating ice shelves around Antarctica, revealing the pattern of ice-shelf melt across the continent. Of the 54 ice shelves mapped, 20 are being melted by warm ocean currents, most of which are in West Antarctica.

Figure 2 | Antarctic ice-shelf ice-thickness change rate DT/Dt, 2003–2008.

Seaward of the ice shelves, estimated average sea-floor potential temperatures (in uC) from the World Ocean Circulation Experiment Southern Ocean Atlas (pink to blue) are overlaid on continental-shelf bathymetry (in metres)30 (greyscale, landward of the continental-shelf break, CSB) Grey circles show relative ice losses for ice-sheet drainage basins (outlined in grey) that lost mass between 1992 and 2006 (after ref. 2).

The melting from below is creating a layer of relatively fresh water 50-150 meters below the surface around Antarctica. This layer of light fresh water is floating above a  salty layer below. When ice forms at the surface in the Antarctic winter, it creates cold dense salty water that tends to sink to the bottom, forming bottom water. However, this layer of light melt water is tending to block the water in the top 50 meters from sinking. The area of Antarctic sea ice has expanded because the layer of cold water has stayed on top and expanded outwards instead of sinking. Melting from below has created 2 stratified cold layers in the top 150 meters.

Note the bright pink area in the top 25 meters between 65° and 70° S. This top layer is becoming more saline. Brine is rejected from ice when sea ice forms. It isn't sinking because it is ponding above a freshening layer located at depths between 50 and 150 meters.

The freshened water column around Antarctica has become more stable between depths of 100 and 150 meters. This increasing stability is impeding the formation of Antarctic bottom water. Water that does sink is freshened through incorporation of glacial melt water.

Figure 3.  Austral winter half-year (April–September) zonal mean trends (1985–2010) of observed salinity, vertical density gradient and potential temperature, in the Southern Ocean. a, Salinity. b, Vertical density gradient. c, Potential temperature. Contours indicate the 1985–2010 mean state (psu; kg m^-4, °C). Colouring (bright or faint) indicates whether the trend is significant (yes or no) at p<0:1 65="" 70="" a="" according="" analysis="" and="" based="" between="" brine="" due="" en3="" font="" forms.="" from="" ice="" in="" increase="" is="" likely="" met="" most="" near-surface="" observations.="" observations="" ocean="" office="" on="" rejection="" salinity="" sea="" situ="" sub-surface="" t-test.="" taken="" the="" to="" two-sided="" were="" when="" which="">

Analysis of potential temperatures, which are temperatures adjusted for the effects of increasing pressure with depth, shows the surface water in the top hundred meters is cooling over a vast area from 40°-80° S, while the water in that vast area below 150 meters is warming.

These results show a trend towards reversal of vertical motions around Antarctica. Intermediate water is welling up around Antarctic melting ice from below, creating a freshened layer. Strengthening winds are blowing the cold surface water away from Antarctica. Bottom water formation, caused by the sinking of cold salty water formed by brine rejection, is declining.

The results of this study are confirmed by a detailed study of anthropogenic tracers in the Weddell sea.   Chlorofluorocarbon (CFC) observations showed increasing average ages of the deep water in the sea from 1984–2010. The average age increased because because bottom water formation, and outflow from the Weddell sea, declined.

...we find that all deep water masses in the Weddell Sea have been continually growing older and getting less ventilated during the last 27 years. The decline of the ventilation rate of Weddell Sea Bottom Water (WSBW) and Weddell Sea Deep Water (WSDW) along the Prime Meridian is in the order of 15–21%; the Warm Deep Water (WDW) ventilation rate declined much faster by 33%. About 88–94% of the age increase in WSBW near its source regions (1.8–2.4 years per year) is explained by the age increase of WDW (4.5 years per year). As a consequence of the aging, the anthropogenic Carbon increase in the deep and bottom water formed in the Weddell Sea slowed down by 14–21% over the period of observations.

The decline in Antarctic bottom water formation, combined with the southward expansion of warm subtropical water in the south Pacific and south Indian oceans has led to the rapid heating of intermediate and deep ocean water in the southern hemisphere.

Figure: Ocean Heat Content from 0 to 300 meters (grey), 700 m (blue), and total depth (violet) from ORAS4, as represented by its 5 ensemble members. The time series show monthly anomalies smoothed with a 12-month running mean, with respect to the 1958–1965 base period. Hatching extends over the range of the ensemble members and hence the spread gives a measure of the uncertainty as represented by ORAS4 (which does not cover all sources of uncertainty). The vertical colored bars indicate a two year interval following the volcanic eruptions with a 6 month lead (owing to the 12-month running mean), and the 1997–98 El Niño event again with 6 months on either side. On lower right, the linear slope for a set of global heating rates (W/m2) is given.

A new study of ocean warming has just been published in Geophysical Research Letters by Balmaseda, Trenberth, and Källén (2013).  There are several important conclusions which can be drawn from this paper.

• Completely contrary to the popular contrarian myth, global warming has accelerated, with more overall global warming in the past 15 years than the prior 15 years.  This is because about 90% of overall global warming goes into heating the oceans, and the oceans have been warming dramatically.

• As suspected, much of the 'missing heat' Kevin Trenberth previously talked about has been found in the deep oceans.  Consistent with the results of Nuccitelli et al. (2012), this study finds that 30% of the ocean warming over the past decade has occurred in the deeper oceans below 700 meters, which they note is unprecedented over at least the past half century.

As the earth has warmed in response to the effects of increasing levels of greenhouse gases the southern subtropical belt in the oceans and atmosphere has expanded, tightening the rings of winds and ocean currents around Antarctica. Enormous volumes of warm subtropical water have been added to the southern ocean at depths greater than 300 meters (greater than approximately 1000 feet).

Observed temperature trends in the Indian Ocean present complex patterns that cannot be explained by surface heating alone. The heat storage has apparently increased more in the southern part than in the northern part of the Indian Ocean (Levitus et al. 2005), although this result may be biased by the sparse data coverage, particularly in the south (Harrison & Carson 2007). The strongest warming is found near the subtropical front and extends as deep as 800 m; it is not directly linked to surface heating but rather due to a southward shift of the oceanic gyre circulation and associated thermal structure (Alory et al. 2007).

Another recent detailed study of the water properties of the southern ocean has independently determined that the southern branch of the global thermohaline circulation has slowed dramatically, contributing to a large uptake of heat by the deep southern ocean.

A statistically significant reduction in Antarctic Bottom Water (AABW) volume is quantified between the 1980s and 2000s within the Southern Ocean and along the bottom-most, southern branches of the Meridional Overturning Circulation (MOC). AABW has warmed globally during that time, contributing roughly 10% of the recent total ocean heat uptake. This warming implies a global-scale contraction of AABW.

Rates of change in AABW-related circulation are estimated in most of the world’s deep
ocean basins by finding average rates of volume loss or gain below cold, deep potential temperature (θ) surfaces using all available repeated hydrographic sections. The
Southern Ocean is losing water below θ = 0 °C at a rate of -8.2 (±2.6) × 106 m3 s-1.

The budget calculations and global contraction pattern are consistent with a global scale slowdown of the bottom, southern limb of the MOC.

The slowdown of the southern branch of the thermohaline circulation and the cooling of the surface waters close to Antarctica are enhancing the thermal gradient from the tropics to the pole, speeding up the winds in the Southern Hemisphere. These increases in wind speeds are likely increasing the flow of water from the Pacific to the Atlantic ocean, enhancing the northward flow of water, salt and heat from the south to the north Atlantic. Moreover, the southward movement of the subtropical front allows more flow of the Agulhas current around the south African capes from the Indian ocean to the south Atlantic.

Thus, increased melting of Arctic sea ice may be related to declines in Antarctic bottom water formation. Likewise, the cool Pacific, warm Atlantic pattern causing increased U.S. droughts and storminess in the north Atlantic may be tied to these changes in ocean circulation patterns. Paleoclimate studies have consistently shown oscillations between Antarctic and north Atlantic bottom water formation and between relative coolness around Antarctica and north Atlantic warmth.

The Arctic melt down that is far exceeding model predictions is connected to the slow down in Antarctic bottom water formation. Climate modelers will be challenged to model the connections and the details. The cooling waters around Antarctica, while apparently good news, are not. The rapid melting of the Arctic will be enhanced.

http://www.dailykos.com/story/2013/04/10/1200602/-The-Antarctic-Half-of-the-Global-Thermohaline-Circulation-Is-Faltering

"Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya," by K. I. Ohshima et al., Nature Geosci., (2013); doi:10.1038/ngeo1738

Nature Geoscience, (2013); doi:10.1038/ngeo1738

Antarctic Bottom Water production by intense sea-ice formation in the Cape Darnley polynya

• Kay I. Ohshima,
• Yasushi Fukamachi,
• Guy D. Williams,
• Sohey Nihashi,
• Fabien Roquet,
• Yujiro Kitade,
• Takeshi Tamura,
• Daisuke Hirano,
• Laura Herraiz-Borreguero,
• Iain Field,
• Mark Hindell,
• Shigeru Aoki and
•  Masaaki Wakatsuchi

Abstract

The formation of Antarctic Bottom Water—the cold, dense water that occupies the abyssal layer of the global ocean—is a key process in global ocean circulation. This water mass is formed as dense shelf water sinks to depth. Three regions around Antarctica where this process takes place have been previously documented. The presence of another source has been identified in hydrographic and tracer data, although the site of formation is not well constrained. Here we document the formation of dense shelf water in the Cape Darnley polynya (65°–69° E) and its subsequent transformation into bottom water using data from moorings and instrumented elephant seals (Mirounga leonina). Unlike the previously identified sources of Antarctic Bottom Water, which require the presence of an ice shelf or a large storage volume, bottom water production at the Cape Darnley polynya is driven primarily by the flux of salt released by sea-ice formation. We estimate that about 0.3–0.7×10⁶ m³ s⁻¹ of dense shelf water produced by the Cape Darnley polynya is transformed into Antarctic Bottom Water. The transformation of this water mass, which we term Cape Darnley Bottom Water, accounts for 6–13% of the circumpolar total.

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http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo1738.html

4th source of Antarctic Bottom Water pinpointed with help of tagged southern elephant seals

Tagged seals help find missing piece in global climate puzzle

Researchers pinpoint fourth known source of bottom water, a crucial oceanic heat-sink.

• by Richard A. Lovett, Nature News,

24 February 2013

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Southern elephant seals fitted with satellite-linked instruments similar to the one above helped oceanographers map deep currents off Antarctica.

MARTIN BIUW

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By tracking the voyages of elephant seals off Antarctica, and with the help of satellite imaging and undersea sensors, researchers have discovered a long-elusive source for the deep-ocean streams of cold water that help to regulate the Earth's climate.

Antarctic bottom water (AABW) is cold, highly saline water that forms near the shores of Antarctica. Being denser than typical seawater, it sinks to the depths and then moves north insluggish currents that spread across the globe.

Three sources of AABW were known until now. The first, in the Weddell Sea, was found in 1940; two others were found in the Ross Sea and along the Adélie Coast of East Antarctica in the 1960s and ‘70s. But for years, researchers have suggested that these were not the only ones. In particular, water samples from an area called the Weddell Gyre contain atmospheric pollutants known as chlorofluorocarbons (CFCs), indicating that the deep water came into contact with the air far too recently to have been carried there from one of the known AABW sinks.

Now, Kay Ohshima, a physical oceanographer at Hokkaido University in Sapporo, Japan, and his colleagues have traced that water to a fourth AABW source, in the Cape Darnley polynya. Their results are published today in Nature Geoscience¹.

Polynyas are regions of open water near sea ice that are kept from freezing by wind and currents that sweep newly formed ice away. Polynyas have relatively high salinity, because most of the salt in sea water is expelled as it freezes.

Armed with the hypothesis that the missing source might be such a polynya, the researchers used satellite sensors to hunt for polynya regions where ice formed particularly rapidly. When satellite data suggested that Cape Darnley might be a candidate, the researchers moored instruments on the seabed, hoping to spot the descending current. In addition, they relied on data from elephant seals (Mirounga leonina) tagged with instruments that monitor ocean conditions.

“The seals went to an area of the coastline that no ship was ever going to get to, particularly in the middle of winter,” says Guy Williams, a physical oceanographer at the Antarctic Climate and Ecosystems Cooperative Research Centre in Hobart, Australia, and a co-author of the study.

The elephant seals confirmed the researchers' hunch. “Several of the seals foraged on the continental slope as far down as 1,800 metres,” he says, “punching through into a layer of this dense water cascading down to the abyss. They gave us very rare and valuable wintertime measurements of this process.”

The new finding fills a gap in researchers’ understanding of the Southern Ocean’s role in global climate, “including carbon dioxide, temperature, the stability of the Antarctic ice sheet and changes in sea level", says Richard Alley, a geophysicist at Pennsylvania State University in University Park, who was not part of the study.

Still, Williams and Ohshima say that the Cape Darnley polynya represents, at most, about one-eighth of the world’s AABW, and that other, similar sources might remain to be discovered.

Michael Meredith, a polar oceanographer at the British Antarctic Survey in Cambridge, UK, who wrote an accompanying commentary on the study, says that if the total rate of AABW formation declines, the resulting changes in cold-water circulation could have important effects on global climate, letting the ocean depths warm and thereby changing the rate of heat exchange between Antarctica and the tropics. Moreover, he says, sea levels could rise — owing to the fact that water expands as it warms — and temperature changes could affect deep-sea ecosystems.

Nature

 

doi:10.1038/nature.2013.12488

References

1. Ohshima, K. I. et al. Nature Geosci. http://dx.doi.org/10.1038/ngeo1738 (2013).
   
   
   http://www.nature.com/news/tagged-seals-help-find-missing-piece-in-global-climate-puzzle-1.12488