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Polar Amplification

Guest commentary by Cecilia Bitz, University of Washington

“Polar amplification” usually refers to greater climate change near the pole compared to the rest of the hemisphere or globe in response to a change in global climate forcing, such as the concentration of greenhouse gases (GHGs) or solar output (see e.g. Moritz et al 2002). Polar amplification is thought to result primarily from positive feedbacks from the retreat of ice and snow. There are a host of other lesser reasons that are associated with the atmospheric temperature profile at the poles, temperature dependence of global feedbacks, moisture transport, etc. Observations and models indicate that the equilibrium temperature change poleward of 70N or 70S can be a factor of two or more greater than the global average.

The Arctic Climate Impacts Assessment (ACIA) summarized the evidence for amplification in Arctic surface warming with the statement, “Over the past 100 years, it is possible [33-66% confidence] that there has been polar amplification, however, over the past 50 years it is probable [66-90% confidence]” (ACIA, 2005, p 22). This uncertainty in the evidence of polar amplification should not be confused with the evidence for significant warming in the Arctic. Arctic warming is both highly significant and substantial — it is just not possible to say with very high confidence yet that the Arctic has warmed more than the rest of the hemisphere or globe. The purpose of this posting is to explain why there is sometimes an absence of evidence for polar amplification.

Dramatic changes in ice at the poles are often cited as evidence of recent climate change — changes such as thinning and retreat of the sea ice, collapse of ice shelves along the Antarctic Peninsula and on the north coast of Greenland, melting permafrost, etc. Are these changes evidence of polar amplification? Not really. Because ice is mostly confined to the high latitudes, it is impossible to compare the magnitude of changes in ice across many latitudes. Therefore it seems a mistake to consider changes in ice as prima facie evidence of polar amplification. Instead, I would argue that changes in ice are a consequence of warming, though they may play a role in causing polar amplification. The term “polar amplification” should therefore be reserved to describe the amplification of surface temperature changes.

Zonal Mean Temperature change - Manabe and Stouffer (1980)
Fig 1. Latitude-height distribution of the zonal mean difference in annual mean temperature (K) in response to the quadrupling of CO2. [From Manabe and Stouffer (1980)] (click to enlarge)

Manabe and Stouffer (1980) first popularized the phrase “polar amplification” to describe the amplified rate of surface warming at the poles compared to the rest of the globe in their climate model’s response to increasing GHG levels. Their early climate model had a simple ocean component that only represented the mixed layer of the water. Their model had roughly symmetric poleward amplification in the two hemispheres, except over the Antarctic continent, where they argued the ice is too thick and cold to melt back (see Fig 1). Both poles warmed more at the surface than the midlatitudes or equatorial regions. Figure 1 also shows that polar amplification only occurs below about 300mb (ie, only in the troposphere). The vertical profile of warming in the lower atmosphere and cooling in the stratosphere from GHGs is well understood as a function of increased absorption and re-radiation.

Temperature trends deg C/century A1B scenarios
Fig 2. Linear surface warming trend in °C per century from NCAR CCSM3 averaged from 9 ensemble members using the SRES A1B scenario. (click to enlarge)

Observed polar climate change from the instrumental record is not symmetric. Except along the Antarctic Peninsula (Vaughan et al., 2003), most evidence of significant warming is from the Arctic. In addition, total sea ice extent in the Southern Ocean has had no significant trend since satellites began taking data in 1979 (Cavalieri et al 2003). Newer climate models generally also have very modest or no polar amplification over the Southern Ocean and Antarctica in hindcasts of the last century. The presence of a deep and circulating ocean component is key because ocean heat uptake increases most in the Southern Ocean as the climate warms (see Gregory 2000). The asymmetry at the poles does not however result from a difference in feedback strength associated with the ice or atmosphere. In fact, when these same climate models are run to equilibrium (in the same way that Manabe and Stouffer ran their model so that ocean heat uptake is not a factor) the hemispheres have nearly equal polar amplification.

Zonal Mean Temperature change - IPCC AR4
Fig 3. Zonal mean surface temperature change at 2080-2099 relative to 1980-1999 for models archived at PCMDI for the IPCC AR4. The temperatures are normalized by the global mean to emphasize the relative polar amplification across models. (click to enlarge)

In addition to the asymmetry from ocean heat uptake, models have been used to attribute a considerable fraction of the warming on the Antarctic Peninsula and the lack of warming elsewhere on Antarctica to a decreasing trend in stratospheric ozone levels in the past few decades (Shindell and Schmidt, 2004). Due to the successful treaty to reduce ozone-depleting chlorofluorocarbons (CFCs) emissions, ozone levels in the stratosphere are expected to recover over Antarctica by about 2040, so eventually Antarctica begins to warm somewhat in climate model predictions of the 21st century (see Fig 2) (WMO, 2002).

It is well established that climate models exhibit Arctic amplification of future climate change (see Fig 3). In addition, averaging over numerous hindcasts from a single model started with differing initial conditions (known as an ensemble average) yields significant Arctic amplification in simulations of the last century. However, single realizations with a climate model do not universally show Arctic amplification in the past [Bitz and Goosse, in prep]. Thus models and observations alike show uncertainty in Arctic amplification in the last century. Nonetheless Arctic amplification in models (and most likely in nature too) is a robust result of forced climate change, provided the forcing is sufficiently large to overcome internal climate variability. Antarctic amplification only occurs if a model is run long enough so ocean heat uptake in the Southern Ocean does not damp the positive feedbacks and if trends in stratospheric ozone do not cause compensatory cooling. Predictions with climate models indicate that Arctic amplification will be significant (above the 95% confidence level) in one to two decades, while significant Antarctic polar amplification will take much longer. A lack of polar amplification over relatively short periods of time is therefore understandable and does not undermine the credibility of climate models or climate theory in any way.

References:
ACIA, 2005. Arctic Climate Impacts Assessment, Cambridge University Press, New York, U.S., 1042pp.
Cavalieri, D. J., C. L. Parkinson, and K.Y. Vinnikov, 30-Year satellite record reveals contrasting Arctic and Antarctic decadal sea ice variability. Geophys. Res. Lett., doi:10.1029/2003GL018031, 2003.
Gregory, J.M., Vertical heat transport in the ocean and their effect on time-dependent climate change, Clim. Dyn., 16, 501-515, 2000.
Holland, M.M., and C.M. Bitz, Polar amplification of climate change in the Coupled Model Intercomparison Project, Clim. Dyn., 21, 221–232,2003.
Manabe, S. and R.J. Stouffer, Sensitivity of a global climate model to an increase of CO2 concentration in the atmosphere, J. Geophys. Res., 85, 5529–5554, 1980.
Moritz R.E., C.M. Bitz, and E.J. Steig, Dynamics of recent climate change in the Arctic, Science, 297, 2002.
Shindell, D.T. and G.A. Schmidt, Southern Hemisphere climate response to ozone changes and greenhouse gas increases, Geophys. Res. Lett. doi:10.1029/2004GL020724, 2004.
Vaughan, D. G. et al. Recent rapid regional climate warming on the Antarctic Peninsula. Climatic Change, 60, 243-274 2003.

117 Responses to “Polar Amplification”

  1. 101
    Pat Neuman says:

    Re 100. THE MILD TEMPERATURES OVER THE PAST 4 WEEKS HAVE BEEN DUE TO TWO MAIN FACTORS. FIRST…WIDESPREAD CLOUD COVER DURING THE NIGHT HAVE
    PREVENTED LOWS FROM FALLING TOO FAR FROM THE DAYTIME HIGHS. THE
    CLOUDS KEEP MUCH OF THE HEAT FROM ESCAPING INTO SPACE OVERNIGHT.
    ADDITIONALLY…THE JET STREAM HAS REMAINED IN CANADA DURING MUCH OF
    THIS PERIOD. THIS HAS KEPT THE COLD ARCTIC AIRMASSES CONFINED TO THE
    POLAR REGIONS.
    http://www.crh.noaa.gov/crnews/display_story.php?wfo=fgf&storyid=1109&source=0

    Stephen,

    It’s what they (NWS) don’t say that the public doesn’t hear.

    More humidity and clouds in winter and a polar jet spending more time in Canada than in the U.S., in winter, are global warming signatures.

  2. 102
    joel Hammer says:

    are global warming signatures

    Oh good. Perhaps you could spell out:
    1. Which climate events are NOT evidence of global warming. So far, every climate event is taken as evidence for global warming, whether it hot or cold, dry or wet, etc. For example, is the really cold winter in Russia this year a signature of global warming, or, has it got something to do with the mild winter in the USA? You know, simple minds think alike. Is the jetstream dumping all the cold air in Russia this winter instead of giving some of it to the USA?

    2. Could you name some weather events which have NOT yet happened but will occur and be signatures of global warming. If those events never happen, will those non-events be evidence of no global warming.

    3. If the jetstream moves South next winter, will you say that this is a sign of NO global warming?

    I always got the impression that global warming was going to be a gradual affair, but listening to you people talk, global warming is here already and is making huge climate and temperature differences as we speak. Were you computer models that far off?

    [Response: I wonder how many times we need to repeat this – individual weather events are not proof of anything. Neither Russia being cold last week, or New York being warm the week before are, in and of themselves attributable to climate change. Claims that ‘scientists’ have claimed that they are, are just strawmen arguments. Whether GW is rapid or gradual, the effects will be seen in the statistics of weather, not the individual events themsleves. -gavin]

  3. 103
    Stephen Berg says:

    Re: #101,

    Indeed. I was speaking with my honours advisor about that very topic, regarding the same overly mild winter we’ve been experiencing in Southern Manitoba.

    The media is sure a failure in bringing this to the forefront. Here’s a perfect example of the type of event which solidifies the IPCC’s argument, yet the media is not doing anything to help ordinary people realise that climate change is here and it needs immediate action. Money talks while common sense and proper scientific discussion have left the building.

  4. 104
    Pat Neuman says:

    re 102

    Joel wrote: … “For example, is the really cold winter in Russia” …

    Public weather and news broadcasters in the Twin Cities have been making similar statements … that the cold air in Russia this January just balances out the warm January in Minnesota/Wisconsin. Is there specific data to support those judgment calls being broadcast to the public? Why aren’t they making reference to NASA and NOAA annual and monthly global temperature summaries… instead of cherry pic-kin the Russian reports?

    The averages that I’ve been making references to are 100-120 year temperature plots at official climate stations. What do they have in Russia to make accurate comparisons to in historical records? Density and quality of data are important.

    My update temp. average plots (1888-2005, monthly, annual) are at: http://pg.photos.yahoo.com/ph/patneuman2000/my_photos

    Also at the link above are photos which I took of fossils found at Fossil Butte National Monument which lived in a subtropical environment at the same location (Wyoming) as today, 52 million years old. Recent climate in Wyoming is dry and frozen in winter months (but warming).

  5. 105

    #102, I want to pick up on an important point raised by Gavin. Statistics may convince most scientists about proving GW, but analyzing stattistics does not resonate well with most lay people. There should be key signature events, well forecasted in advance which may help focus world wide attention on the suject and bring it out a ctritical mass of people to steer away from a passive GW interest, to a full fledged understanding and response enabling to counter the effects of GH gases before it is too late. Or
    if it is too late, at least help kick start programs removing GH gases from our atmosphere. It turns out, that short of Broadway and 42nd having boat taxis like Venice, there are signature GW pre-catastrophic events to look for. Unfortunately, they are found in mostly in the Polar regions, those in populated areas are not as clear cut. Polar Amplification is one of them, and its seems a done deal, although Dr Bitz thinks otherwise. Shrinking Multi-year Arctic Ocean ice is another, also a “fait accomplie”. But the problem with Polar events is the population impact they foster, Northerners and Antarcticans have no world wide clout. What is left is signature events which have a huge impact amongst a greater population, short of much higher sea level increases, it will be indeed too late, unless everyone understands statistics.

    Are there more pre-catastrophic signature events which would have a huge cognitive impact? I can only think of mountain glaciers vanishing completely amongst densily populated areas.

  6. 106
    Pat Neuman says:

    Wayne, you made some good points (105.).

    In Minnesota, based on observed temperatures and temperature forecasts for continued well above the historical average for the month, January of 2006 will likely be the warmest since official record keeping began in the 1800s. But as you said above, “There should be key signature events, well forecasted in advance which may help focus world wide attention on the suject and” … “before it is too late”.

    The record breaking January warm temperatures were not forecasted well in advance. Even if they were, I doubt there would have been world wide focus. Media weather people (local channels for NBC, ABC) have been suggesting in public broadcasts that cold air in Russia and Alaska can explain the record warmth this January in Minnesota, making no reference in their broadcasts to global warming or regional climate change trends.

    A few years ago, I made an effort, mostly on personal time, to report on increasing humidity, and increased snowmelt rates from latent heat released in condensation. I asked others to look into increasing heat indexes in summer with higher humidity. I think what may be needed is to predict a great heat wave with high humidity (France in Aug 2003 not predicted). But, it’s unlikely that a great heat wave will be predicted from federal agencies in the U.S., unless there is a drastic change in the way federal, state, university and private sector meteorologists in the U.S. view and speak out about the effects of greenhouse warming on climate now and in the future.

  7. 107
    Stephen Berg says:

    Re: #106, “The record breaking January warm temperatures were not forecasted well in advance.”

    You’re right. NOAA predicted a very normal January, even a half-month in advance.

  8. 108
    Coby says:

    Re predictions: GISS predicted a record breaking global temperature 10 months in advance. That did not get any notice.

    Most of the other things you want predicted are weather, and weather is not climate. In terms of dramatic, attention focusing events, this year’s hurricane season and in particular Katrina and Rita probably can’t be topped (til next year?). But again it is weather and you can not reasonably pin the blame on global warming even if it is reasonable to suggest GW made it more likely and/or a bit more intense. Doing so, as we saw lots of journalism try, can result in a lot of backlash ammunition for denialists.

    I agree that melting glaciers are probably one of the best “reality checks” for your average American. But how long before Glacier National Park has no glaciers? When the evidence is that clear we will be very far down a very dangerous path.

  9. 109

    There is some extremely interesting 3d Polar weather/chemistry of late. Namely low stratospheric Ozone concentrations over NW Russia North of the Scandinavia, just below there is very warm Arctic air (http://www.thepoles.com/news.php?id=1445), while surface weather further South being coolish. All while at the same time extremely high concentrations of Ozone above the Canadian archipelago, with SAT’s just below just below about “normal” (-35 C) in very calm surface winds, but as many of us know, unbelievably warm weather South of 50 degrees North.. . It all seems tied a bit in a way with Dr Bitz Polar Amplification explanation, especially about ozone concentrations, I have been studying this present puzzle, and have many missing pieces avoiding me to see a complete picture of this moment. Wonder if any one can explain any connections between the pieces, if any??.

  10. 110
  11. 111

    I have heard enough about this very warm winter on the North American side, having a chuckle or two with such statements that winter “tipped over” towards the other side of the Pole, giving a more “normal” like winter on the Russian side. Must add on some Polar context with this apparent strange climate. Usually on a normal NH winter, there is a trans-continental continous zone of cold air, which now appears cut off, but first for a pretty standard Meteorologist point of view:

    “So what happened to winter? The answer is pretty simple: the cold air (which morphs around the North Pole during the winter) has stayed on the opposite side of the world.” http://wwwa.accuweather.com/adcbin/public/community_blog.asp?webcaster=COMMUNITY&date=2006-01-25_1444

    For more than a month Alaska and Siberia shared different air zones, unusual, but understandable by a warmer Arctic Ocean which is acting as a barrier between the continents, unless the two continents Air masses are bridged, usually by a massive Arctic Ocean High North of Alaska (now disappeared), this warmer than usual winter will continue. There is a fair amount of media depicting how bad it is on the Russian/East European side, it should be considering the temperature differences between North America and Russia, but then again it may not be such a terrible winter throughout the NH because Cold air is having such a hard time establishing itself. Many shrug off this year, saying “will get it colder next year” , perhaps premature speculation because of the apparent Arctic Ocean multi-year ice is on a shrinking trend. Therefore if this is the future, I see a problem with the Polar Ampllification model projections, as depicted above, not quite reflecting this present complex but real scenario.

  12. 112

    Small follow up on 111, a good Astronomer friend of mine recognized unbelievably clear air in my recent horizon pictures, this is a signature of isolated cold polar air zones, meaning that Arctic air masses are not huge and more numerous, rather than consolidated. For the case of Resolute Bay, the air is more Greenlandic, mainly kept in a closed loop counterclockwise circulation around Greenland. Usually it is more circumpolar, with haze from Russia/North Alaska, mainly seen as a milkish white horizon caused by ice crystals at this time of the year. I attempt to characterize more what’s happening at http://www.eh2r.com. The horizon picture shown has very brisk colours devoid of hazy distortions. If the air mass becaomes more transcontinental, these colours fade substantially. Finally the Artic Ocean atmospheric surface flows seems more like tidal events, closely matching daily tidal waves from the North Atlantic……

  13. 113
    Pat Neuman says:

    re 112 Wayne, in looking at your photographs, it would give me eerie feelings to be there.

    Cecilia Bitz starts her article with … “Polar amplification” usually refers to greater climate change near the pole compared to the rest of the hemisphere or globe in response to a change in global climate forcing, such as the concentration of greenhouse gases (GHGs) or solar output (see e.g. Moritz et al 2002).

    I think warming amplification occurs in the higher latitudes in response to increasing concentrations of greenhouse gases, but I think that in the initial years of an episode of higher solar radiation, the amplification in temperatures may be highest in the middle latitudes.

    I base that on my evaluation of temperature and dewpoint data in the early-mid 1930s, when warm and very dry summer conditions occurred in the Great Plains and Midwest (especially 1931 and 1936), and in the Northwest (mainly 1934).

    NASA data shows high global anomalies for July in 1931 and 1936. July global anomalies were also high in 1940 and 1941. Humidity at Minneapolis was also high in 1940, 1941. Thus I think it’s like that the warm 1940, 1941 period was influenced by warm El Ninos, which may have been a result of the surges in solar radiation during hot, dry and absence of clouds period dust bowl year in the early-mid 1930s.

    103 Years of Twin Cities Dew Point Temp. Records: 1902-2005
    http://climate.umn.edu/doc/twin_cities/mspdewpoint.htm

    1888-2005 climate station plots for the U.S.
    http://pg.photos.yahoo.com/ph/patneuman2000/my_photos

    Global temperatures from met stations (land areas)
    http://data.giss.nasa.gov/gistemp/tabledata/GLB.Ts.txt

  14. 114

    113 Pat, must be quite a scene being so warm in Mn, I can’t study far back, too much stuff to work on right now. Ya, smaller isolated climatic zones may trigger all kinds of fascinating phenomena, sort of breaking the usual weather patterns to bring out a new world climatic order. There is no sun here yet , but there is still much warmer air, some guys commenting on RC are too attached to solar effects, rather forgetting feedback processes which act like sunshine. Eastern Europe is warming up just nicely, I suspect more surprises as winter never rages in some parts….

  15. 115
    Hank Roberts says:

    New Scientist:
    “Antarctic ice sheet is an ‘awakened giant'”
    * 13:38 02 February 2005
    * Jenny Hogan, Exeter
    The massive west Antarctic ice sheet, previously assumed to be stable, is starting to collapse, scientists warned on Tuesday…..

    http://www.newscientist.com/article.ns?id=dn6962&print=true

  16. 116
    Pat Neuman says:

    re 114

    Wayne,

    It’s more like a shame than a scene here in Minneapolis-Chanhassen. There’s only a few patches of snow left, and now it’s raining again.

    The government meteorologists and TV broadcasters continue to say absolutely nothing about climate change / global warming.

    Note: Updated 1820-2006 January mean temperatures:
    Minneapolis is there now, other stations will be there tomorrow,
    at:
    http://pg.photos.yahoo.com/ph/patneuman2000/my_photos

  17. 117

    […] regions. The Arctic is very likely to warm at a higher rate than the global mean (Polar amplification), and the precipitation is expected to increase while the sea-ice will be reduced. The Antarctic is […]