Guest post by Jos Hagelaars. Dutch version is here.
This was the title of a discussion that was held on the recently launched website ClimateDialogue regarding the possible causes of the decline in Arctic sea ice over the past decades. Three experts participated in this discussion: Walt Meier, Research Scientist at the NSIDC, Judith Curry, professor at Georgia Institute of Technology and Ron Lindsay, Senior Principal Physicist at the Polar Science Center of the University of Washington.
In this blog post I will start off with a description of the observations of the Arctic region, followed by a short overview of the potential causes of the decline in Arctic sea ice, incorporating the views of the three experts as they were expressed on ClimateDialogue. The final parts concern the uniqueness of this decline in a historical perspective and the possibility of having an ice-free Arctic in the summer in the not too distant future.
Observations of the Arctic region since 1979
Since 1979 the Arctic region has been extensively monitored by satellites. They detect e.g. the ice surface area, the extent of the area covered with ice and also the total amount or volume of ice. The results of these observations are startling. For example, sea ice area and the amount of perennial (multi-year) ice has decreased dramatically over the past 3 decades, as is visualized by the images in figure 1 and 2, generated by NASA (see here and here).
There are several institutes that collect data from the Arctic region. For example, NSIDC develops data sets about the ice area and the ice extent (the well-known English term for the outer edge of the area covered with ice) and data on the total volume of sea ice is incorporated in the PIOMAS model. In 2012 the September average ice extent dipped below 4 million km², which is about half of what it was in 1979, see figure 3. Ice volume shows a comparable rapid decrease.
Air temperatures in the Arctic region are measured via surface stations and also – albeit indirectly – with satellites. Figure 4 shows the annually averaged temperature data for the Arctic region from NASA GISTEMP and the satellite data from UAH. The former gives a warming trend of 0.53 °C/decade and the latter of 0.47 °C/decade (1979-2012; Arctic region). This is much higher than the increase in global temperatures, which is 0.16 °C/decade for GISTEMP and 0.14 °C/decade for UAH. Since 1979 the Arctic has warmed about 3.3 times faster than the earth in general. This so-called “Arctic Amplification” is partially caused by the disappearance of sea ice and the effect this has on regional albedo (the so-called ice-albedo feedback).
Processes affecting the amount and extent of Arctic sea ice
A multitude of processes affects the sea ice, such as wind, ocean currents, the temperature of the atmosphere and the temperature of sea water. The three experts on ClimateDialogue agree that greenhouse gases, through an increase in the surface temperature of the atmosphere and the oceans, play a major role in the sea ice decline in recent decades.
Walt Meier for instance puts it like this in his guest blog:
“.. the multi-decadal decline in all seasons, and in virtually all regions cannot be explained without the long-term warming trend that has been attributed to anthropogenic greenhouse gas (GHG) emissions.”.
Two thirds of the melting of the ice is caused by melting from below (at the edges) – through contact with ocean water – and one third is caused by surface melting (Steele et al. 2010[P23]). The warming of the oceans is therefore of major importance for the long-term trend in Arctic sea ice.
Besides the long-term (multi-decadal) warming trend, natural variation plays a big role in the shorter term (e.g. a decade or less). This view was in principle shared by all three experts, but it also gave rise to a lot of ambiguities and uncertainties on ClimateDialogue. For example, the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) affect the amount of sea-ice[P9]. The Atlantic Meridional Overturning Circulation (AMOC) affects the sea ice extent in the Greenland Sea, the Barents Sea and the Kara Sea[P11]. The same applies to the Pacific Decadal Oscillation (PDO) regarding the sea ice in the Bering Sea.
The evidence for the impact of greenhouse gases and other factors on the sea ice comes from modeling studies.
Kay et al. 2011[P13] show that for the period 1979-2005 approximately 50% of the decline in sea ice is caused by natural variation. Interestingly, this study also shows that the decline of the sea ice can temporarily be stopped:
“The computer simulations suggest that we could see a 10-year period of stable ice or even an increase in the extent of the ice.”
But also that the ice will eventually disappear:
“When you start looking at longer-term trends, 50 or 60 years, there’s no escaping the loss of ice in the summer”.
Day et al. 2012[P12] show that the variation in the AMO can explain 5-30% of the downward trend in the sea ice extent in the September month and that the influence of the AO thereon is small.
A conclusion from the correlation study of Notz en Marotke 2012[P8] is:
“We find that the available observations are sufficient to virtually exclude internal variability and self-acceleration as an explanation for the observed long-term trend, clustering, and magnitude of recent sea-ice minima. Instead, the recent retreat is well described by the superposition of an externally forced linear trend and internal variability. For the externally forced trend, we find a physically plausible strong correlation only with increasing atmospheric CO2 concentration. Our results hence show that the observed evolution of Arctic sea-ice extent is consistent with the claim that virtually certainly the impact of an anthropogenic climate change is observable in Arctic sea ice already today.”
They find a strong long-term correlation between the CO2 forcing and the ice extent in the September month, alongside low correlations with the PDO and the AO.
All three experts agree that greenhouse gases played a major role in the long-term decline of the Arctic sea ice in recent decades, although they find it difficult to put a precise number on it.
In the discussion greenhouse gases are alluded to as the human influence, but there are more anthropogenic influences (e.g. cooling by aerosols), so a breakdown in greenhouse gases on the one hand and natural variability on the other hand wouldn’t make a lot of sense, as it doesn’t add up to 100%. It is thus likely that with greenhouse gases the net human influence is meant, since it is implicitly assumed that the total should be 100%. [updated Oct 2014]
A range of 50-70% with an uncertainty of 20% is probably a reasonable average of what the three experts think the human contribution to Arctic sea ice melt is, see the quotes below.
“My assessment is that it is likely (>66% likelihood) that there is 50-50 split between natural variability and anthropogenic forcing, with +/-20% range.”
“The disagreement seems to arise if we are each forced to pick a single attribution value: mine would be 50%”
“CCSM4 simulations show about a 50% decline in ice volume since the 1960’s with a typical ensemble spread on the order of 15%, so the CCSM4 runs indicate the decline in ice volume is about 3 times the natural variability, or about 70% of the decline is due to greenhouse gases. The decline in volume seen in the PIOMAS simulations is also very consistent, particularly if one focuses just on the Arctic Ocean since the late 1980’s. So I would go on the high side of the percentage loss due to greenhouse gases for ice volume and less for ice extent, maybe near 50%.”
“The 50-70% range for GHGs that Judith mentions is probably a reasonable spread in capturing the potential range.”
However, there is indeed a difference in judgment between Judith Curry and the other two experts on the human influence. In my opinion Meier explains his position most clearly. He refers to Day et al., who arrive at a 70-95% anthropogenic contribution, and gives some strong arguments in his guest blog:
– The decline in sea ice correlates with the increase in global temperature.
– The decline is outside the range of normal variability over the past several decades and probably over the past several centuries.
– The decline is pan-Arctic, with all regions experiencing declines throughout all or most of the year.
– Climate model simulations cannot explain the decline without taking greenhouse gases into account.
– There does not appear to be another mechanism to sufficiently explain the long-term decline.
Curry indicates that the greenhouse gases undoubtedly contribute to the reduction of sea ice, but she highlights the uncertainties therein, especially for the short term. According to her, there is a complex interplay between natural variation and CO2 forcing together with complex interactions between ocean dynamics, heat transport and sea ice dynamics driven by winds and ocean currents. To support her estimated 50-50 division between anthropogenic and natural causes she highlights the high degree of uncertainty, although uncertainty in itself only causes a wide error margin and is no reason for a given estimate.
Lindsay also refers to Day et al. and says that the evidence 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. the sun or volcanoes.
But above all Lindsay says:
“I believe fundamentally the main process causing the decline in Arctic sea ice is increasing greenhouse gases.”.
How unique is the current decline in sea ice in historical perspective?
The most complete data sets are of course the satellite measurements. There is a fairly complete coverage from ice charts dating back to the 1950s and there are Russian and Danish ice charts dating back to the 1930s[T1]. The 1930s are often cited as a warm period, but this was mainly confined to the Atlantic[T2]. For the years preceding the 1930s we must rely on proxy data to get an idea of the ups and downs of the Arctic sea ice.
In a recently published paper by Meier et al. [P24] the sea ice extent from 1953 onwards is mapped and then linked to satellite data. Figure 8, taken from this paper, clearly shows that the decline of Arctic sea ice has started in the 1970s, not coincidentally at the same time that global surface temperatures started to increase fast.
A study by Kinnard from 2011[P21] shows that the decline of the Arctic sea ice extent is unique since at least the last 1450 years and that it corresponds to the surface temperatures in the Arctic.
There are indications that the last time sea ice extent in the Arctic was this low, was during the Holocene Thermal Maximum, about 8000 year BP. See the review paper by Polyak et al. from 2010[P5].
A good summary of the history of Arctic sea ice can, as so often, be found at SkepticalScience.
An ice-free Arctic.
The common definition of ice-free comes from Wang & Overland[P18,P19], who indicate that ‘nearly sea ice free’ implies a maximum surface area of less than 1 million square kilometers. It is uncertain when the Arctic will become ice-free for the first time. In their 2012 paper Wang & Overland come up with the following indication, based on CMIP4 models:
“Applying the same technique of model selection and extrapolation approach to CMIP5 as we used in our previous paper, the interval range for a nearly sea ice free Arctic is 14 to 36 years, with a median value of 28 years. Relative to a 2007 baseline, this suggests a nearly sea ice free Arctic in the 2030s.”
The CMIP5 models do a better job in simulating the decline of the sea ice than the older CMIP3 models (as used for IPCC AR4), see Stroeve et al. 2012[P7]. Figure 10, based on a presentation of Stroeve, indicates the difference between the different models and the observation of Arctic sea ice extent in the September month. The green dot in this figure stands for sea ice extent in September 2012. It is clear that models still partially underestimate the decline of the sea ice, although the observations may not fall significantly outside the range of model calculations.
Of course the three experts on ClimateDialogue do not come up with a certain date upon which the Arctic would become ice-free and they point at the uncertainty which is mainly caused by natural variation. They do indicate that, logically, natural variation will continue to play a role after the Arctic has become ice-free. Therefore, an ice-free year will be followed with years where more or less sea ice will be present. That the Arctic will become ice-free in the summer months is now almost certain, again nicely put into words by Meier:
“The timing is still uncertain but it changes things from a “if the Arctic loses summer sea ice” to “when the Arctic loses summer sea ice”.”
The area covered with sea ice has declined sharply the last 30 years and will continue to decline in the long run. This has consequences for the rest of the world, as can be read in this excellent summary by Neven: “Why Arctic sea ice shouldn’t leave anyone cold”.
This continuous decline of Arctic sea ice certainly has an impact on the Arctic climate and also on the weather in our part of the world, beautifully expressed by Professor Jennifer Francis[P25]:
The question is not whether sea-ice loss is affecting large-scale atmospheric circulation…
…it’s how can it not?
Many thanks to Neven and Bart.
Text references Climatedialogue.org:
[T1] Walt Meier – http://www.climatedialogue.org/melting-of-the-arctic-sea-ice/#comment-38
While our most complete dataset, the one we have the highest confidence in, is the passive microwave record, there is fairly complete coverage from operational ice charts back to at least the mid-1950s. And there are Russian ice charts for the Eurasian Arctic back to the early 1930s. Though not complete, these do extend the record and I think provide some sense of the interannual and decadal natural variability of the ice. There are indications of lower ice in the 1930s in the Russian Arctic [Reference 4 in my post], suggesting the influence (AMO?) of a multi-decadal cycle, at least in the Russian Arctic. But the data show a different character in terms of the seasonality and regionality of the lower ice conditions compared to the recent decline.
[T2] Walt Meier – http://www.climatedialogue.org/melting-of-the-arctic-sea-ice/#comment-237
Before the 1950s, the 1930s are often mentioned as a warm period. However, this is primarily in the Atlantic region, where observations were more common. Ice charts from the Denmark (http://nsidc.org/data/docs/noaa/g02203-dmi/) and Russia, indicate some periods of low summer ice, but on a more regional scale than we see now.
Peer-review paper references provided by the experts on ClimateDialogue.org :
[P1] 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.
[P2] Overland, J.E., M. Wang, and S. Salo (2008), The recent Arctic warm period, Tellus, doi: 10.1111/j.1600-0870.2008.00327.x.
[P3] 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.
[P4] 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.
[P5] 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.
[P6] 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.
[P7] 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.
[P8] Notz, D. and J. Marotzke (2012), Observations reveal external driver for Arctic sea-ice retreat, Geophys. Res. Lett., 39, L08502, doi:10.1029/2012GL051094.
[P9] Rigor, I.G., J.M. Wallace, and R.L. Colony (2002), Response of sea ice to the Arctic Oscillation, J. Climate, 15, 2648-2663.
[P10] 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.
[P11] 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.
[P12] 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.
[P13] 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.
[P14] 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.
[P15] 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.
[P16] 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.
[P17] 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.
[P18] 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.
[P19] 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.
[P20] 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
[P21] 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.
Figuur 3.2 Kinnard et al
[P22] 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
[P23] 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.
[P23] M.Steel, J.Zhan, W.Ermold, Mechanisms of summertime upper Arctic Ocean warming and the effect on sea ice melt, Journal of Geophysical Research, VOL. 115, C11004, doi:10.1029/2009JC005849, 2010
[P24] W. N. Meier, J. Stroeve, A. Barrett, and F. Fetterer, A simple approach to providing a more consistent Arctic sea ice extent time series from the 1950s to present, The Cryosphere, 6, 1359-1368, 2012, doi:10.5194/tc-6-1359-2012
[P25] J. Francis, S. Vavrus, Evidence linking Arctic amplification to extreme weather in mid-latitudes, Geophysical Research Letters, Vol. 39, L06801, doi:10.1029/2012GL051000, 2012
Video Weather and Climate Summit – Jennifer Francis.
[P26] D. Kaufmann et al, Recent warming reverses long-term Arctic cooling, Science 4 September 2009, Vol. 325, no. 5945, doi:10.1126/science.1173983
Website 2000 Years of Climate Variablity from Arctic Lakes