Melting of the Arctic sea ice**************************************EXCERPTS*********************************************
Melting of the Arctic sea ice
The Arctic sea ice extent has been decreasing steadily for the past three decades. Scientists discuss the potential causes of this decrease.
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Climate Dialogue editorial staff
Rob van Dorland, KNMI
Bart Strengers, PBL
Marcel Crok, freelance science writer
Map of the Arctic Ocean. Credit: National Snow and Ice Data Center
Introduction Arctic sea ice ∨
Melting of the Arctic
What are the causes of the decline in Arctic sea ice? Is it dominated by global warming or can it be explained by natural variability?
Introduction
Over the period 1979–2012, the Northern Hemisphere minimum sea ice extent for September—the end of the summer melt season— has declined by more than 11% per decade and the trend appears to be steeper for the last decade with record lows in 2007 and 2012. The decrease in winter sea ice extent is less strong, but the amount of thicker, older ice has decreased as well and therefore the decrease in total sea ice volume is even stronger than sea ice extent, both in summer and winter.
What is the cause?
Several studies have suggested that the decline in arctic sea ice is at least partly caused by global warming. An oft cited paper by Stroeve (2007)[1] and also more recent studies (i.e. Rampal 2011)[2] show that greenhouse forced climate models greatly underestimate the observed trend in arctic sea ice. A more recent study by Stroeve (2012)[3] shows that progress has been made in this area, but the climate models still cannot account for the full extent of the Arctic sea ice decline.
This could be explained as “it’s worse than we thought.” However it could also be interpreted as “models are yet unable to realistically simulate the sea ice behavior” and thus unable to conclude on the dominant role of global warming in the decline of Arctic sea ice. The low performance of climate models may be due to the difficulty in accounting for natural variations, or the physics associated with positive feedbacks. For example, it is not fully clear yet what role different oscillations have played like the Arctic Oscillation, the North Atlantic Oscillation and the Pacific Decadal Oscillation. Also, other anthropogenic forcings like black carbon could be important.
Discussion
A central question for the discussion is what is causing the recent decline in arctic sea ice. And can these processes be related to the emission of anthropogenic greenhouse gases?
Questions that are relevant for the discussion:
1) What are the main processes causing the decline in Arctic sea ice?
2) How unusual is the current decline in historical perspective?
3) What is the evidence for a substantial role of “global warming” in the current Arctic sea ice decline?
4) What is the evidence for a substantial role of natural variability (AO, AMO, NAO, PDO)?
5) What percentage of the recent decline would you attribute to anthropogenic greenhouse gases?
6) Do you think the Arctic could be ice free in the (near) future and when do you think this could happen?
[1] Stroeve, J.; Holland, M. M.; Meier, W.; Scambos, T.; Serreze, M. (2007). "Arctic sea ice decline: Faster than forecast". Geophysical Research Letters 34 (9): L09501
[2] IPCC climate models do not capture Arctic sea ice drift acceleration: Consequences in terms of projected sea ice thinning and decline; P. Rampal , J. Weiss , C. Dubois , J.-M. Campin; Journal: Journal of Geophysical Research , vol. 116, 2011, DOI: 10.1029/2011JC007110
[3] Stroeve, J., V. Kattsov, A. Barrett, M. Serreze, T. Pavlova, M. Holland, and W. N. Meier (2012a), Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations, Geophys. Res. Lett., doi:10.1029/2012GL052676
Guest blog Walt Meier ∨
Arctic sea ice decline: past, present, and futureWalt Meier, Research Scientist, National Snow and Ice Data Center
Over the last 30+ years, Arctic sea ice has declined precipitously, particularly during summer. Summer extent has decreased by ~50%, including most of the older, thicker ice [1, 2]. Globally warming temperatures have been the primary cause of the long-term decline in Arctic sea ice [3]. Many processes effect the sea ice on several temporal spatial scales – e.g., winds, ocean currents, clouds – but the multi-decadal decline in all seasons, and in virtually all regions (Bering Sea in winter being an exception) cannot be explained without the long-term warming trend that has been attributed to anthropogenic greenhouse gas (GHG) emissions.
Unique
The current decline appears to be unique in at least the last 5000 years. While the consistent satellite record began only in 1979, earlier partial records indicate decreased extent in the Russian Arctic and the Greenland and Barents seas during the 1930s [4]. However, these reductions were regionally and temporally variable, unlike the pan-Arctic decline seen in recent decades. There are also indications that the North American side of the Arctic did not experience warm temperatures and thus low sea ice conditions during those years [5]. Thus, the 1930s period appears to be more of a regional event, as opposed to the pan-Arctic warming and sea ice decline we’re seeing now.
Earlier than the 1930s, proxy records from paleographic data (e.g., sediment cores) are essential to understand Arctic-wide sea ice conditions. The indicate reduced sea ice extent at levels near or possibly below current conditions during the Holocene Maximum, a period between 5000 and 10,000 years ago [6], though these are far from comprehensive. The next earliest potential period when ice conditions might have been low as now was during the Eemian period, the previous interglacial, about 130,000 years ago when temperatures were quite warm.
Global warming
The evidence for a substantial role of “global warming” in the current sea ice decline comes from the fact that the decline (1) correlates with the global warming temperatures over the past several decades, (2) is outside the range of normal variability over the past several decades and likely over the past several centuries, (3) the decline is pan-Arctic, with all regions experiencing declines throughout all or most of the year. Also, model simulations of sea ice cover consistently show a response of declining sea ice to increasing GHGs (albeit slower than the observed decline); conversely, model runs over the last 30 years without GHG forcing do not show a decline [7, 8]. Finally, there does not appear to be a mechanism to sufficiently explain the long-term decline without including the effect of GHGs [9].
Arctic Oscillation
Along with the long-term GHG forcing, there is substantial natural variability in the sea ice system. The winter mode of the Arctic Oscillation (AO) or North Atlantic Oscillation (NAO) has been linked to summer sea ice conditions and the amount of multiyear ice in the Arctic [10]. A strong winter positive mode results in greater outflow of multiyear ice, resulting in a summer ice cover more prone to melt. Conversely, a negative mode tends to retain more multiyear ice, resulting in higher summer extents. However, the AO typically has a 3-7 year cycle, which does not correspond to the long-term trend. In addition, in recent years, the influence of the AO appears to have been broken, or at least weakened. For example, the 2009-2010 winter had the lowest AO on record (since 1950), and yet the summer 2010 minimum was among the lowest in the satellite record [11].
The Atlantic Meridional Overturning Circulation (AMOC) has some influence on sea ice extents in the Greenland, Barents, and Kara Seas, and thereby some effect on the total ice extent, particularly in winter [12]; it may explain some of the recent declining trend in sea ice [13]. Likewise, the Pacific Decadal Oscillation (PDO) affects winter ice conditions in the Bering Sea, but not elsewhere. These naturally varying climate oscillations cannot sufficiently explain the long-term decline in sea ice.
Attribution
It is difficult to put a precise number on how much of the decline is due to GHGs. There is strong natural variability, which is seen in observations and in model simulations. It is likely that at least some of the acceleration of the loss of sea ice in the past ~10 years is due to natural variability. A modeling study [14] suggested that about half of the observed September sea ice trend from 1979-2005 could be explained by natural variability, with the rest attributable to GHGs. There may also be some influence of black carbon, though how much is unclear.
Ice-free
The Arctic has been seasonally ice-free in the past under temperatures not much higher than in recent years. With continued GHG forcing and resulting increased temperatures, the Arctic will again become seasonally ice-free*. When this will occur is highly uncertain due to a number of factors. First, the ice cover has high interannual variability. Model simulations indicate periods of rapid ice loss, such as we have seen in the last decade [15]. However, periods of stasis or even increasing extent over several years are possible [14].
A couple of recent model studies have indicated a long-term linear response to GHG forcing [16, 17], suggesting that the recent acceleration in the decline is temporary and due to natural variability. Such a response would first result in ice-free conditions near the end of the century. However, there are reasons to believe that a linear response is unlikely due to feedbacks and the response of the ocean to the loss of sea ice [18]. Over the satellite record, the observations are declining much faster than GCMs have indicated [7, 8]. IPCC models that match the historical record most closely indicate ice-free conditions by sometime in the 2030-2050 timeframe [19, 20]. This range seems reasonable, though it may not encompass the full range of possibilities.
However, even after ice-free conditions are reached for the first time, whether it be 2050, 2030, or even earlier, high interannual variability will continue. It is likely that there will be some subsequent years with more than 1 million square kilometers of sea ice remaining at the end of summer. Thus, prediction of sea ice conditions, particularly on decadal scales will be a challenge. Regardless of when ice-free conditions first occur, impacts of the sea ice loss are already being felt within the Arctic (and likely outside of the Arctic). These impacts will continue to increase well before summer ice-free conditions occur.
*There will likely be at least some ice throughout the summer, thick ice that piles up along the Greenland coast, ice in protected bays and inlets. Thus, it is important to define what is meant by “ice-free”. Here I will accept the 1 million square kilometers used in Wang and Overland (2009, 2012) as a reasonable threshold.
Biosketch
Dr. Walt Meier is a research scientist at the National Snow and Ice Data Center (NSIDC), part of the University of Colorado Boulder’s Cooperative Institute for Research in Environmental Sciences. His research focuses on studying the changing sea ice cover using satellite sensors and investigating impacts of the declining Arctic sea ice on climate. Dr. Meier also serves as lead scientist for NSIDC’s sea ice datasets. He has participated in several national and international activities, including a lead author on the AMAP Snow, Water, Ice and Permafrost in the Arctic (SWIPA) assessment report published earlier this year. He received a B.S. degree in from the University of Michigan, Ann Arbor in 1991 and an M.S. and Ph.D. degree from the University of Colorado in 1992 and 1998 respectively. From 1999 to 2001 Dr. Meier served as a visiting scientist at the U.S. National Ice Center in Washington, DC where he researched improved products for operational support of vessels in and near ice-infested waters. From 2001 through 2003 he was an adjunct assistant professor at the U.S. Naval Academy in Annapolis, MD teaching undergraduate courses in remote sensing and polar science.
References
[1] NSIDC Arctic Sea Ice News and Analysis, http://nsidc.org/arcticseaicenews/.
[2] Maslanik, J., J. Stroeve, C. Fowler, and W. Emery (2011), Distribution and trends in Arctic sea ice age through spring 2011, Geophys. Res. Lett., 38, L13502, doi:10.1029/2011GL047735.
[3] Overland, J.E., M. Wang, and S. Salo (2008), The recent Arctic warm period, Tellus, doi: 10.1111/j.1600-0870.2008.00327.x.
[4] Mahoney, A. R., R. G. Barry, V. Smolyanitsky, and F. Fetterer (2008), Observed sea ice extent in the Russian Arctic, 1933–2006, J. Geophys. Res., 113, C11005, doi:10.1029/2008JC004830.
[5] Overland, J.E., M.C. Spillane, D.B. Percival, M. Wang, H.O. Mofjeld (2004), Seasonal and regional variation of pan-Arctic surface air temperature over the instrumental record, J. Climate, 17, 3263-3282.
[6] Polyak, L., and several others (2010), History of sea ice in the Arctic, Quaternary Sci. Rev., 29, 1757-1778, doi: 10.1016/j.quascirev.2010.02.010.
[7] Stroeve, J., M.M. Holland, W. Meier, T. Scambos, and M. Serreze (2007), Arctic sea ice decline: Faster than forecast, Geophys. Res. Lett., 34, L09501, doi:10.1029/2007GL029703.
[8] Stroeve, J.C., V. Kattsov, A. Barrett, M. Serreze, T. Pavlova, M. Holland, and W.N. Meier (2012), Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations, Geophys. Res. Lett., 39, L16502, doi:10.1029/2012GL052676.
[9] Notz, D. and J. Marotzke (2012), Observations reveal external driver for Arctic sea-ice retreat, Geophys. Res. Lett., 39, L08502, doi:10.1029/2012GL051094.
[10] Rigor, I.G., J.M. Wallace, and R.L. Colony (2002), Response of sea ice to the Arctic Oscillation, J. Climate, 15, 2648-2663.
[11] Stroeve, J. C., J. Maslanik, M. C. Serreze, I. Rigor, W. Meier, and C. Fowler (2011), Sea ice response to an extreme negative phase of the Arctic Oscillation during winter 2009/2010, Geophys. Res. Lett., 38, L02502, doi:10.1029/2010GL045662.
[12] Mahajan, Salil, Rong Zhang, Thomas L. Delworth (2011), Impact of the Atlantic Meridional Overturning Circulation (AMOC) on Arctic surface air temperature and sea ice variability. J. Climate, 24, 6573–6581, doi: 10.1175/2011JCLI4002.1.
[13] Day, J.J., J.C Hargreaves, J.D. Annan, and A. Abe-Ouchi (2012), Sources of multi-decadal variability in Arctic sea ice extent, Env. Res. Lett., 7, 034011, doi: 10.1088/1748-9326/7/3/034011.
[14] Kay, J. E., M. M. Holland, and A. Jahn (2011), Inter-annual to multi-decadal Arctic sea ice extent trends in a warming world, Geophys. Res. Lett., 38, L15708, doi:10.1029/2011GL048008.
[15] Holland, M.M., Bitz, C.M. and Tremblay, B. (2006), Future abrupt reductions in the summer Arctic Sea ice. Geophys. Res. Lett. 33, L23503, doi:10.1029/2006GL028024.
[16] Tietsche, S., D. Notz, J. H. Jungclaus, and J. Marotzke (2011), Recovery mechanisms of Arctic summer sea ice, Geophys. Res. Lett., 38, L02707, doi:10.1029/2010GL045698.
[17] Amstrup, S.C., E.T. DeWeaver, D.C. Douglas, B.G. Marcot, G.M. Durner, C.M. Bitz, and D.A. Bailey (2010), Greenhouse gas mitigation can reduce sea-ice loss and increase polar bear persistence, Nature, 468, 955-958, doi: 10.1038/nature09653.
[18] Maslowski, W., J.C. Kinney, M. Higgins, and A. Roberts (2012), The future of Arctic sea ice, Ann. Rev. Earth and Planetary Sciences, 40, 625-654, doi: 10.1146/annurev-earth-042711-105345.
[19] Wang, M., and J. E. Overland (2009), A sea ice free summer Arctic within 30 years?, Geophys. Res. Lett., 36, L07502, doi:10.1029/2009GL037820.
[20] Wang, M. and J. E. Overland (2012), A sea ice free summer Arctic within 30 years: An update from CMIP5 models, Geophys. Res. Lett., 39, L18501, doi:10.1029/2012GL052868.
Guest blog Judith Curry ∨
On the decline of Arctic sea ice
Judith Curry
I applaud the Dutch Ministry for establishing Climate Dialogue, and I am very pleased to participate in this inaugural dialogue on the decline of Arctic sea ice.
At my blog Climate Etc. http://judithcurry.com , I’ve written four lengthy articles on Arctic sea ice over the past 18 months:
Pondering the Arctic Ocean. Part I: Climate Dynamics
Likely causes of recent changes in Arctic sea ice
Reflections on the Arctic sea ice minimum: Part I
Reflections on the Arctic sea ice minimum: Part II
This essay presents an overview of my perspective on this topic; see the original articles for more details and references to scientific publications.
Observations
The conventional understanding of Arctic sea ice extent shows a general retreat of seasonal ice since about 1900, and accelerated retreat of both seasonal and annual ice during the latter half of the 20th century. Hints that this understanding may be overly simplistic in view of the uncertainties and ambiguities in the period prior to satellites are described in this presentation by John Walsh about plans for a gridded sea ice product back to 1870. Further, I’ve recently had some discussions about this with a historian that is investigating historical reports of sea ice extend during the period 1920-1950. He has found reports of reduced wintertime extent during this period, and a general lack of data from the Russian sector. While this material is not yet published, it reminds us that prior to 1979, we do not have a reliable data set of global sea ice extent. The lack of such a data set hampers our ability to test our ideas about the impact of natural variability versus anthropogenic forcing on sea ice variability and change.
Analysis of climate dynamics and sea ice physical processes
The following factors impact the sea ice fate during the melt season:
▪ Thickness and compactness of sea ice at the beginning of the melt season: ice that starts out thinner is more easily melted away. Further, first year ice has different optical and thermodynamic characteristics than multi-year ice.
▪ Transport of ice through the Fram Strait (between Greenland and Europe), which depends on a combination of atmospheric and ocean circulation patterns
▪ Weather patterns that act to either break up or consolidate the ice
▪ Radiative forcing (which is dominated by the cloud patterns)
▪ Melting from below by warm ocean currents.
▪ Melting from above by warm atmospheric temperatures.
▪ Geographic distribution of the sea ice, which depends on a combination of all of the above
And all this is complicated by the fact that the minimum sea ice extent in an individual season doesn’t simply reflect that season’s weather processes, but also reflects the decadal history of sea ice characteristics, sea ice export and atmospheric and oceanic circulation patterns. And the sea ice extent itself influences the atmospheric and oceanic circulation patterns. Hence, the sea ice characteristics tend to be out of equilibrium with the thermal forcing in a particular year.
Older ice
Here’s the basic story as I see it. During the late 1980s and early 1990s, the circulation patterns favored the motion of older, thicker sea ice out of the Arctic. This set the stage for the general decline in Arctic sea ice extent starting in the 1990′s. In 2001/2002, a hemispheric shift in the teleconnection indices occurred, which accelerated the downward trend. A local regime shift occurred in the Arctic during 2007, triggered by summertime weather patterns conspired to warm and melt the sea ice. The loss of multi-year ice during 2007 has resulted in all the minima since then being well below normal, with a high amplitude seasonal cycle. After 2007, there was another step loss in ice volume in 2010. In 2012, the basic pattern of this new regime was given a ‘kick’ by a large cyclonic storm in early August.
Anthropogenic
So, what is the contribution of anthropogenic global warming to all this? It’s difficult to separate it out. The polar regions are extra sensitive to CO2 forcing and water vapor feedback, owing the low amounts of water vapor. However, any radiative forcing from greenhouse gases is swamped by inter-annual variability in cloud radiative forcing. In the bigger picture sense, greenhouse forcing is involved in complex nonlinear ways with the climate regime shifts. So there is undoubtedly a contribution from CO2 forcing, but it is difficult to find any particular signal in this year’s record minimum, other than the contribution of greenhouse warming to a longer term trend. In the overall scheme of what is going on with the sea ice, I think 2007 was the most significant event, followed by 2010. The big event in 2012 was the cyclonic storm, and the impact on ocean mixing may turn out to be more significant than the sea ice minimum.
There is a complex interplay between natural internal variability and CO2 forcing, with complex interactions among ocean dynamics and heat transport, sea ice dynamics forced both by atmospheric winds and ocean currents, and atmospheric thermodynamic forcing acting to determine recent variations in multi-year sea ice extent. Hence sorting dynamical versus thermodynamic factors and attribution to increased greenhouse gases is not at all straightforward.
So . . . what is the bottom line on the attribution of the recent sea ice melt? My assessment is that it is likely (>66% likelihood) that there is 50-50 split between natural variability and anthropogenic forcing, with +/-20% range. Why such a ‘wishy washy’ statement with large error bars? Well, observations are ambiguous, models are inadequate, and our understanding of the complex interactions of the climate system is incomplete.
Whence an ‘ice free’ Arctic?
‘Ice free’ is put in quotes, because ‘ice free’ as commonly used doesn’t mean free of ice, as in zero ice. The usual definition of ‘ice free’ Arctic is ice extent below 1 M sq. km (current minimum extent is around 3.5 M sq. km). This definition is used because it is very difficult to melt the thick ice around the Canadian Archipelago. And the issue of ‘ice free’ in the 21st century is pretty much a non-issue if your require this thick ice to disappear.
What do the climate models have to say? Several recent papers have analyzed the CMIP5 simulations, and find near ice free conditions by mid-century, and even as early as the 2030’s. Whereas sea ice models are becoming quite sophisticated, most recently in terms of the radiative transfer, melt ponds, and aerosols, prediction of sea ice is hostage to predictions of the chaotic atmospheric and oceanic dynamics.
For the next two decades, natural variability will almost certainly trump any direct effects from anthropogenic warming by a long shot. The current sea ice situation does not seem stable, but it is not at all clear whether we can expect a reversion to the (more recently) normal state or yet a larger ice loss.
The issue is whether the ice is now sufficiently thin that it would be difficult to reverse the decline. Growing and diminishing the sea ice pack are not symmetric processes: ice export that contributes to diminishing the sea ice pack does not have a reverse counterpart; at best you stop the export and stop the decrease.
So the question then becomes what processes could contribute to a recovery of the Arctic sea ice on the time scale of two decades?
Recovery (?)
So, can we infer that the Arctic sea ice is caught in an irreversible ‘spiral of death’? Here are some processes that would contribute to a recovery of the sea ice:
▪ Reduction of the sea ice export through the Fram Strait
▪ Reduction of warm water inflow from the Atlantic and Pacific
▪ Fewer clouds in winter and/or more clouds in summer
▪ Less snow fall on ice in autumn and more in spring
▪ Less soot transported to the Arctic
▪ No rainfall on snow covered ice before mid-June
▪ Fewer storms in summer causing ice breakup, and more storms in autumn/early winter causing ice ridging/rafting
These processes depend on both random weather patterns and the teleconnection climate regimes. Can I predict how this might go over the next two decades? Heck no, other than that I suspect that the cool phase of the PDO will persist and at some point probably within two decades we will switch to the cool phase of the AMO.
And then there are the known unknowns: what solar radiation will do (looks like cooling), volcanoes are always a wild card, and then there are the less known unknowns such as cosmic ray effects, magnetic field effects, etc. And in terms of climate shifts, there may be something happening on much longer time scales (e.g. the Atlantic Meridional Overturning Circulation) that could influence the next climate regime shift. Focusing on CO2 as the dominant influence on the time scale of two decades seems very misguided to me.
Does ‘ice free’ matter?
The first issue to debunk is that an ‘ice free’ Arctic is some sort of ‘tipping point.’ A number of recent studies find that in models, the loss of summer sea ice cover is highly reversible.
The impact of September sea ice loss on the ice albedo feedback mechanism is interesting. The minimum sea ice occurs during a period when the sun is at low elevation, so the direct ice albedo effect isn’t all that large. Less sea ice in autumn means more snowfall on the continents, which can have a larger impact on albedo. The impacts of the freeze-thaw over the annual cycle influences ocean circulations. But sea ice would continue to freeze and thaw on an annual cycle.
Clouds would change, atmospheric circulation patterns would change. The net effect on climate outside the Arctic Ocean would be what? More snow during winter on the continents is the most obvious expected change. But we really don’t know.
There would likely be regional triggers that could feedback onto larger scale regime shifts. Would any of these patterns or extreme events fall outside the envelope of what we have seen over the past century? Hard to know.
Would melting sea ice trigger some sort of clathrate methane release into the atmosphere? Well in terms of thawing permafrost, it seems like more snow fall on the continents would inhibit permafrost thawing. Same for the stability of the Greenland ice cap.
These are all qualitative speculations, but I am not seeing a big rationale for climate catastrophe if the see ice melts.
Biosketch
Judith Curry obtained her Ph.D. in Geophysical Sciences from the University of Chicago. She is currently Professor and Chair of the School of Earth and Atmospheric Sciences at the Georgia Institute of Technology, and is also President of Climate Forecast Applications Network LLC. Her research addresses a range of topics in atmospheric and climate science, including sea ice and the climate dynamics of the Arctic http://curry.eas.gatech.edu/climate/arctic.htm. She is a Fellow of the AAAS, the American Geophysical Union, and the American Meteorological Society. She has recently served on the U.S. National Research Council Space Studies Board and Climate Research Committee. She currently serves on the U.S. Department of Energy Biological and Environmental Research Committee and the Earth Science Subcommittee of the NASA Advisory Council. Curry is an active spokesperson on issues related to integrity in scientific research. She is the proprietor of a blog Climate Etc. judithcurry.com, which is a forum to discuss topics related to climate science and the science-policy interface.
Guest blog Ron Lindsay ∨
Melting of the Arctic
Ron Lindsay
I usually choose to focus on the mean ice thickness within the Arctic Ocean as opposed to all ice-covered seas because this limited system is better defined. The atmospheric forcing is more uniform over the region, the volume-thickness relationship is well established since it is a defined region and any residual summer ice will mostly be found there. In addition, ice thickness is a much more consistent climate indicator than ice area or extent. Trends in thickness explain much more of the inter-annual variability than sea ice area and even more than extent.
If we want to understand the fate of summer ice, it is the Arctic Ocean we need to look at. The peripheral seas in many respects don’t matter. I use the ice thickness estimates from the retrospective studies using the PIOMAS ice-ocean model. I am the first to say models are far from perfect, but the general pattern of ice thickness simulated by this model has been shown to be pretty good when compared to observations (Schweiger et al 2011[1]). If anything, the model may be a little conservative in estimating the decline in ice thickness.
Greenhouse gases
I believe fundamentally the main process causing the decline in Arctic sea ice is increasing greenhouse gases. Evidence for the role of greenhouse gases must come primarily from modeling studies. Only those can help us separate natural climate variations from variations caused by changes in greenhouse gases or other external forcing mechanisms (e.g. sun or volcanoes). Examining past climate records and asking questions about the forcing mechanisms responsible for changes can also help. For example there is some evidence that there was less sea ice about 9000 years ago when solar insolation was stronger.
But teasing out the actual mechanisms for the decline is very tricky (e.g. whether it is changes in the ocean or in the atmosphere or both or what processes are responsible). Observational evidence is difficult to interpret, since the decline itself modifies the lower atmosphere and the surface fluxes. What is cause and what is effect? One piece of evidence though is the high correlation (R = -0.72, 57 years through 2011) between the rate of melt (including export) in the Arctic Ocean and melt season (May to September) surface temperatures in the rest of the hemisphere, from 20N to 60N (NCEP-R1). The surface temperatures south of the Arctic are likely less influenced by ice loss and the trends are likely more influenced by global forcings. Sea ice basically responds to hemispheric conditions and is not on its own trajectory.
Unknowable
But the actual detailed mechanisms for the decline are currently unknowable. The trend in the latent heat content of the ice in September is less than 0.5 W/m2/year in September. It seems this means annual change in the net heat balance of the ice and the changes in the mean annual melt and growth rates are much too small to be accurately measured by any observational system that looks at the entire region. That the PIOMAS model gives reasonably good estimates of the mean ice thickness trends is because it has been tuned to the atmospheric forcing fields that we use. So while the total melt and growth rates must be about right (to get the correct thickness) an analysis of the individual components of the surface or oceanic heat fluxes and their trends likely won’t provide a useful answer. Because of the small shifts in energy needed to melt the ice it is perhaps no surprise that there is a wide scatter in sea ice trends between climate models.
Unusual
The current decline in ice extent and volume is highly unusual. Maybe the best way to show this is in the consistency in the trends. The linear trend in the September ice thickness from 1987 to 2012 explains 90% of the variability, much more than any other comparable interval since 1948. The observational record for sea ice is more spotty before this time, though researchers are piecing together a more comprehensive picture from various ship observations. So far that picture doesn’t suggest that large variations in sea ice extent were anything but regional over the last 120 years or so.
A reconstruction based on proxy records suggest that sea ice extent is now the lowest in 1450 years (Kinnard, 2011[2]). A recent review of the current state of knowledge (Polyak et al, 2010[3]) concludes: “This ice loss appears to be unmatched over at least the last few thousand years and unexplainable by any of the known natural variabilities.”
The evidence for a substantial role of “global warming” in the current Arctic sea ice decline is very strong, both from observations and from modeling studies. Of course neither can “prove” the role of greenhouse gases but there is overwhelming evidence it is true. To refute the evidence from models, one would have to show that they wildly underestimate natural variability (w.r.t to sea ice). Even in the NCAR CCSM4 which is one of the CMIP5 models with the highest “natural” variability, the sea ice extent trend over the last 30 years is still 50% due to greenhouse gases (Kay et al. 2011[4]).
Natural variability
For those that think this is not the case, they need to show some evidence that there are alternative explanations. Comparing ice volume instead of sea ice extent greatly reduces the natural variability compared to the trend and shows an earlier and more definitive separation than ice area between models run with or without increased greenhouse gas forcings (Schweiger et al 2011[5]).
While natural variability is very important for determining the ice extent, primarily through the action of the winds, I see a very consistent trend in the mean ice thickness with relatively little year-to-year variations. So while natural variability can strongly influence the ice area and extent, I doubt there is a strong component in the variability of the mean ice thickness within the Arctic Ocean. In the peripheral seas the winds are very important for determining heavy or light ice years and hence in these areas the variability associated with circulation changes can be very large, both for ice extent and thickness. There is evidence that an extended positive phase of the AO during the early 1990’s helped flush out older thicker ice and helped set up the subsequent decline in sea ice (Rigor et al. 2002[6], Lindsay et al. 2005[7]). Since then the ice decline cannot be explained by variations in the AO. Evidence from models (Day et al. 2012[8]) indicates that the AO may not play much of a role in sea ice variability. That same study suggest that the AMO indeed may indeed play a significant role in sea ice decline and that as much as 3%/decade of the 10%/decade trend in September sea ice extent between 1979-2010 may be due to AMO variability. There is also some observational evidence that sea ice extent may be influenced by the AMO but none of this evidence suggest that an arctic-wide change in ice extent as seen over the last decade is possible due to these type of modes of natural variability alone. It also appears ice thickness within the Arctic Ocean is less closely tied to the AMO.
As shown by Stroeve et al. (2012[9]), the CMIP5 models as a group underestimate the sea ice trend. But there is no requirement that reality should follow the group mean nor that any model that reproduces the observed trend any better than another is indeed the preferable model. In fact, there are ensemble members that match reality fairly well (e.g. Schweiger et al., 2011) but that shouldn’t fool us in to believing them more. Given that the CCSM4, a model with rather large (and possibly excessive) natural variability pins 50% of sea ice loss on greenhouse gases, probably more than this is due to the greenhouse emissions.
Ice free
The Arctic will likely be largely ice free at the end of some summers within a decade or two. Small bits of ice might remain some years, but they may not matter for much. Current research does not support the notion of any “tipping” points for summer sea ice so if we somehow magically could turn off the forcing that comes from greenhouse gases, sea ice would likely grow back relatively quickly. Unfortunately that is not likely to happen. Winter ice will remain for a long time, a century or more. How long probably depends mostly on the future rate of greenhouse gas emissions.
Biosketch
Ron Lindsay is an Arctic climatologist at the Polar Science Center Applied Physics Laboratory of the University of Washington in Seattle. Lindsay is interested in how the sea ice in the Arctic moves, grows, and decays in response to changing environmental conditions and how the changes in the ice pack are impacting the atmosphere above. To pursue these research themes he uses a wide variety of in situ and remote sensing data and numerical models. In support of these interests he has joined the IceBridge science team to help direct a NASA program to monitor ice thickness from aircraft. He is also developing a capability for modeling the response of the atmosphere to changing pack ice conditions in order to understand the extent to which the heat absorbed in the open water areas in the summer slows the growth of ice in the winter. Lindsay has been conducting Arctic research for over 35 years and has been with the Polar Science Center since 1988.
[1] Schweiger, A., R. Lindsay, J. Zhang, M. Steele, H. Stern, and R. Kwok. 2011. Uncertainty in Modeled Arctic Sea Ice Volume. J. Geophys. Res., doi:10.1029/2011JC007084
[2] Kinnard, C., C. Zdanowicz , D Fisher, and E. Isaksson, 2011: Reconstructed changes in Arctic sea ice over the past 1,450 years, Nature, 509-512, doi 10.1038/nature10581.
[3] Polyak, L., and several others (2010), History of sea ice in the Arctic, Quaternary Sci. Rev., 29, 1757-1778, doi: 10.1016/j.quascirev.2010.02.010.
[4] Kay, J. E., M. M. Holland, and A. Jahn (2011), Inter-annual to multi-decadal Arctic sea ice extent trends in a warming world, Geophys. Res. Lett., 38, L15708, doi:10.1029/2011GL048008.
[5] Schweiger, A., R. Lindsay, J. Zhang, M. Steele, H. Stern, and R. Kwok. 2011. Uncertainty in Modeled Arctic Sea Ice Volume. J. Geophys. Res., doi:10.1029/2011JC007084
[6] Rigor, I.G., J.M. Wallace, and R.L. Colony, Response of Sea Ice to the Arctic Oscillation, J. Climate, v. 15, no. 18, pp. 2648 – 2668, 2002.
[7] Lindsay, R. W. and J. Zhang, 2005: The thinning of arctic sea ice, 1988-2003: have we passed a tipping point?. J. Climate, 18, 4879–4894.
[8] Day, J.J., J.C Hargreaves, J.D. Annan, and A. Abe-Ouchi (2012), Sources of multi-decadal variability in Arctic sea ice extent, Env. Res. Lett., 7, 034011, doi: 10.1088/1748-9326/7/3/034011.
[9] Stroeve, J.C., V. Kattsov, A. Barrett, M. Serreze, T. Pavlova, M. Holland, and W.N. Meier (2012), Trends in Arctic sea ice extent from CMIP5, CMIP3 and observations, Geophys. Res. Lett., 39, L16502,doi:10.1029/2012GL052676.