by Judith Curry
Rapidly melting Arctic sea ice, growing Antarctic sea ice, and concerns about the melting Thwaites glacier – can all of this be explained by anthropogenic global warming?
Two recently published papers provide insights and some food for thought:
The ocean’s role in polar climate change: asymmetric Arctic and Antarctic responses to greenhouse gas and ozone forcing
John Marshall, Kyle C. Armour, Jeffery R. Scott, Yavor Kostov, Ute Hausmann, David Ferreira, Theodore G. Shepherd and Cecilia M. Bitz
Abstract. In recent decades, the Arctic has been warming and sea ice disappearing. By contrast, the Southern Ocean around Antarctica has been (mainly) cooling and sea-ice extent growing. We argue here that interhemispheric asymmetries in the mean ocean circulation, with sinking in the northern North Atlantic and upwelling around Antarctica, strongly influence the sea-surface temperature (SST) response to anthropogenic greenhouse gas (GHG) forcing, accelerating warming in the Arctic while delaying it in the Antarctic. Furthermore, while the amplitude of GHG forcing has been similar at the poles, significant ozone depletion only occurs over Antarctica. We suggest that the initial response of SST around Antarctica to ozone depletion is one of cooling and only later adds to the GHG-induced warming trend as upwelling of sub-surface warm water associated with stronger surface westerlies impacts surface properties. We organize our discussion around ‘climate response functions’ (CRFs), i.e. the response of the climate to ‘step’ changes in anthropogenic forcing in which GHG and/or ozone-hole forcing is abruptly turned on and the transient response of the climate revealed and studied. Convolutions of known or postulated GHG and ozone-hole forcing functions with their respective CRFs then yield the transient forced SST response (implied by linear response theory), providing a context for discussion of the differing warming/cooling trends in the Arctic and Antarctic. We speculate that the period through which we are now passing may be one in which the delayed warming of SST associated with GHG forcing around Antarctica is largely cancelled by the cooling effects associated with the ozone hole. By mid-century, however, ozone-hole effects may instead be adding to GHG warming around Antarctica but with diminished amplitude as the ozone hole heals. The Arctic, meanwhile, responding to GHG forcing but in a manner amplified by ocean heat transport, may continue to warm at an accelerating rate.
Published in Proc. Roy. Soc. full manuscript available [here].
There is some good background information in the Introduction:
Over the last few decades, the two polar regions of our planet have exhibited strikingly different behaviours, as is evident in observed decadal trends in surface air temperature. The Arctic has warmed, much more than in the global average, primarily in winter, while Arctic sea-ice extent has decreased dramatically. By contrast, the eastern Antarctic and Antarctic plateau have cooled, primarily in summer, with warming over the Antarctic Peninsula and Patagonia. Moreover, sea-ice extent around Antarctica has modestly increased.
Many mechanisms are at work in ‘Arctic amplification’. A positive snow and sea-ice albedo feedback plays a significant role in amplifying the warming signal. The albedo feedback operates in summer when solar radiation is maximal. Where sea ice is lost and water is exposed, warming due to absorbed shortwave radiation can be large and enhance sea-ice loss through lateral melt. In addition to these processes, the warmed ocean mixed layer delays sea-ice growth, and thus influences wintertime surface temperatures through a thinner ice pack. Because the Arctic atmosphere is stably stratified by thermal inversion at the surface, any warming that occurs there does not reach far up into the troposphere. Moreover, the surface energy balance is very sensitive to processes going on in the planetary boundary layer and cloud radiative processes. Additionally, as is emphasized in the work presented here, the climate of the polar caps is determined by more than regional and vertical energy balance, as lateral advection of heat by atmosphere and ocean circulation also plays a significant role.
The area poleward of the 70° N latitude circle receives more energy due to atmospheric transport than it does from the Sun. Moreover, this lateral heat-flux convergence is largely balanced by outgoing infrared radiation, with surface fluxes contributing a relatively small amount to the energy budget. The sensitivity of poleward atmospheric heat transports to climate change is currently under debate: polar amplification reduces meridional temperature gradients, which might be expected to reduce meridional atmospheric heat transport from lower latitudes, thus counteracting a portion of the amplification. Some studies argue that anomalous atmospheric heat transport, mainly due to increased moisture, have given rise to greater atmospheric warming above the surface of the Arctic. However, the validity of the analysed atmospheric trends on which such studies are based is disputed. Beyond atmospheric heat transports, the high-latitude response to greenhouse forcing may involve anomalous ocean heat transport into the Arctic; as we shall see, this occurs even if a weakened meridional overturning circulation (MOC) diminishes the heat transport at lower latitudes. In addition, the ocean can act as a reservoir for the heat gained in summer while the sea ice retreats, storing it through winter months.
The mix of ongoing processes in the Antarctic is rather different from those in the Arctic. The dramatic depletion of the Antarctic ozone since the late 1970s has introduced a major perturbation to the radiative balance of the stratosphere with a wide range of consequences for climate. There is strong evidence that ozone loss has significantly altered the climate of the Southern Hemisphere troposphere, including the surface, with implications for ocean circulation, the cryosphere and coupled carbon cycle. Observations indicate a poleward shift of the Southern Hemisphere atmospheric circulation over the past few decades, predominantly in late spring and summer. This shift has been attributed to polar ozone depletion in the Antarctic lower stratosphere. The observed changes have the structural form of the Southern Annular Mode (SAM) in its positive phase: the surface wind maximum, the storm tracks, and the edge of the Hadley cell all shift poleward. While similar changes, with the same sign, have been reproduced in models under GHG warming scenarios they are also found in response to imposed ozone depletion. In fact, on the basis of GCM studies in which both forcings were included, separately and together, it is believed that ozone depletion has been the primary cause of the observed wind changes. In the future, assuming ozone depletion weakens as expected, the effects of GHG and ozone forcings may no longer act in the same sense on surface winds.
Changes in the Southern Hemisphere westerlies (and SAM) have been linked to changes in sea-surface temperatures (SSTs) and sea-ice extent around Antarctica on interannual time scales. A positive SAM induces an overall transient cooling through the enhanced Ekman transport of cold surface waters northward from Antarctica promoting sea-ice growth. There is, however, debate about the cause of the observed decadal trends in sea-ice extent, which show a small net expansion around Antarctica but large regional trends of opposing sign. Coupled models suggest that initial (interannual) cooling around Antarctica induced by a positive SAM reverses to one of warming as time proceeds [36–39]. The warming tendency and sea-ice retreat is a consequence of enhanced upwelling of warm water from depth around Antarctica associated with strengthening westerly winds. Natural variability may also be playing a role in the observed signals, even if trends in the SAM itself were to be absent.
The links between the upwelling of deep water in the Southern Ocean (SO) and the Southern Hemisphere westerly winds and consequences for climate have long been an area of active research. Although changes in the slope of density surfaces in the Antarctic Circumpolar Current (ACC) cannot yet be detected, ocean observations indicate a freshening of Antarctic Intermediate Water and a substantial warming of the SO equatorward of the ACC at all depths which may be linked to atmospheric forcing. Modelling studies and theory, however, suggest that eddy transport in the ACC can partially compensate for changes in Ekman transport ameliorating changes in the strength of the MOC.
Enhanced communication of the interior ocean with the surface could have marked effects on the Earth’s climate through changes in rates of heat and carbon sequestration as well as consequences for ice shelves around Antarctica which may be vulnerable to enhanced upwelling of warm water from depth. The stratification of the SO is also delicately poised and sensitive to changes in the freshwater balance.
Geothermal sources of melt
Phys.org reports: Researchers find major West Antarctic glacier melting from geothermal sources. Excerpts:
Thwaites Glacier, the large, rapidly changing outlet of the West Antarctic Ice Sheet, is not only being eroded by the ocean, it’s being melted from below by geothermal heat, researchers at the Institute for Geophysics at The University of Texas at Austin (UTIG) report in the current edition of the Proceedings of the National Academy of Sciences.
The findings significantly change the understanding of conditions beneath the West Antarctic Ice Sheet where accurate information has previously been unobtainable.
The Thwaites Glacier has been the focus of considerable attention in recent weeks as other groups of researchers found the glacier is on the way to collapse, but more data and computer modeling are needed to determine when the collapse will begin in earnest and at what rate the sea level will increase as it proceeds. The new observations by UTIG will greatly inform these ice sheet modeling efforts.
Using radar techniques to map how water flows under ice sheets, UTIG researchers were able to estimate ice melting rates and thus identify significant sources of geothermal heat under Thwaites Glacier. They found these sources are distributed over a wider area and are much hotter than previously assumed. The geothermal heat contributed significantly to melting of the underside of the glacier, and it might be a key factor in allowing the ice sheet to slide, affecting the ice sheet’s stability and its contribution to future sea level rise.
The cause of the variable distribution of heat beneath the glacier is thought to be the movement of magma and associated volcanic activity arising from the rifting of the Earth’s crust beneath the West Antarctic Ice Sheet.
The glacier is retreating in the face of the warming ocean and is thought to be unstable because its interior lies more than two kilometers below sea level while, at the coast, the bottom of the glacier is quite shallow.
Because its interior connects to the vast portion of the West Antarctic Ice Sheet that lies deeply below sea level, the glacier is considered a gateway to the majority of West Antarctica’s potential sea level contribution.
The collapse of the Thwaites Glacier would cause an increase of global sea level of between 1 and 2 meters, with the potential for more than twice that from the entire West Antarctic Ice Sheet.
“The combination of variable subglacial geothermal heat flow and the interacting subglacial water system could threaten the stability of Thwaites Glacier in ways that we never before imagined,” Schroeder said.
“Evidence for elevated and spatially variable geothermal flux beneath the West Antarctic Ice Sheet,” by Dustin M. Schroeder, Donald D. Blankenship, Duncan A. Young, and Enrica Quartini. PNAS, 2014: http://www.pnas.org/cgi/doi/10.1073/pnas.1405184111
Journal reference: Proceedings of the National Academy of Sciences
The Marshall et al. paper provides a mechanism for cooling in the Antarctic, and the Shroeder paper provides a mechanism for geothermal heating of the WAIS. Clearly, there is a lot going on that cannot be explained directly or even indirectly by warming from greenhouse gases. Climate models don’t simulate correctly the ocean heat transport and its variations, and they certainly don’t simulate geothermal heat sources. Integrating these factors with radiative forcing is needed to start making sense of climate change in the polar regions.