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What's causing Arctic amplification?

What the science says...

Decline in sea ice is the major driver of Arctic amplification. This is evidence by the pattern of atmospheric warming over the Arctic. Maximum warming occurs over the surface during winter while less surface warming is found in summer when heat is being used to melt sea ice. This pattern is consistent with sea ice amplification.

Climate Myth...

Melting ice isn't warming the Arctic

'The University of Melbourne study claims are not supported by the available data.  The authors seem to have jumped right into statistical analysis without proposing a physical mechanism that works.  Heat flows across differences in temperatures, yet the winter water temperature under the ice is fixed at -2C.  Thus elevated winter air temperatures should actually cause a reduction in heat flow out of the ocean.  Whatever is driving increases in winter Arctic temperatures is not heat coming out of the Arctic Ocean, which is covered with insulating ice.' (Steve Goddard)

The warming trend in the Arctic is almost twice as large as the global average in recent decades. This is known as Arctic amplification. What's the cause? Changes in cloud cover, increases in atmospheric water vapour, more atmospheric heat transport from lower latitudes and declining sea ice have all been suggested as contributing factors. A new paper The central role of diminishing sea ice in recent Arctic temperature amplification (Screen & Simmonds 2010) (here's the full paper) examines this question. The title is a bit of a give-away  - the decline in sea ice is the major driver of Arctic amplification.

The vertical profile of Arctic warming (i.e. - how much warming occurs at different altitudes) gives us insight into the underlying cause. If atmospheric heat transported from lower latitudes was the major driver, more warming would be expected at greater heights. On the other hand, if retreating snow and sea ice cover was the major cause, maximum warming would be expected at the surface. Figure 1 shows the simulated warming expected in each season if declining sea ice was the major cause of warming.

Figure 1: Temperature trends linked to changes in sea ice. Temperature trends over the 1989–2008 period averaged around circles of latitude for winter (a), spring (b), summer (c) and autumn (d). The trends are derived from projections of the temperature field on the sea ice time series.

Using higher resolution temperature data supplemented with updated satellite measurements, Screen 2010 analyse the observe warming trend in each season. What they find is maximum Arctic warming at the surface and that warming lessens with height in all seasons except summer. This vertical structure suggests that changes at the surface, such as decreases in sea ice and snow cover, are the primary causes of recent Arctic amplification.

Figure 2: Observation of temperature trends, 1989–2008. Temperature trends averaged around circles of latitude for winter (December–February; a), spring (March–May; b), summer (June–August; c) and autumn (September–November; d). Red shading indicates that the lower atmosphere has warmed faster than the atmospheric column as whole. Blue shading indicates that the lower atmosphere has warmed slower than the atmospheric column as a whole.

The surface warming is modest in summer because energy is used to melt remaining sea ice and warm the upper ocean. The majority of the winter warming is associated with changes in sea ice cover even though the sea ice declines at this time of the year are relatively small. During summer, the atmosphere loses heat to the ocean whereas during winter, the flux of heat is reversed. Reduced summer sea ice cover allows for greater warming of the upper ocean but atmospheric warming is modest. The excess heat stored in the upper ocean is subsequently released to the atmosphere during winter.

Another potential contributor to amplified warming that's investigated is changes in cloud cover. Spring is the only season that exhibits significant trends in Arctic average cloudiness and the trend is negative. However, decreased cloud cover is expected to cause surface cooling because clouds have a warming influence in spring. Thus no evidence is found of cloud cover changes contributing to recent near-surface Arctic warming.

Changes in atmospheric water vapour content may amplify Arctic warming. However, specific humidity trends are found only during summer and early autumn. The pronounced warming in winter and spring are not accompanied by increases in humidity. In fact, the evidence suggests part of the humidity increase is driven by enhanced surface moisture fluxes associated with sea ice reductions.

The empirical evidence from the past two decades reveals that declining sea ice cover and thickness have been great enough to enhance Arctic warming during most of the year. The emergence of strong ice–temperature positive feedbacks increases the likelihood of future rapid Arctic warming and sea ice decline.

Last updated on 26 June 2010 by James Wight. View Archives

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Comments 1 to 16:

  1. I'm confused, why is polar amplification considered a fingerprit of greenhouse warming, if the amplified warming is the result of sea ice loss. Almost every radiative forcing that causes global warming would be acompanied by polar amplification, because sea ice would melt regardless of whether the warming was caused by greenhouse gases or another positive radiative forcing. So why is amplified warming in high latitudes considered a fingerprint of greenhouse warming? Could please explain this?
  2. Where did you see that polar amplification was a "fingerprint of greenhouse warming"? The article doesn't seem to make that case.
  3. Arctic amplification is an expected (and now observed) result of an enhanced greenhouse effect, but not a unique 'fingerprint' of such. Actual 'fingerprints' specific to the enhanced greenhouse effect can be found here, here, and here.
  4. On a meta-note, Karamanski's I'm confused...please explain this? artifice is beginning to wear thin, now that we've read it on multiple topic threads. Here's Karamanski claiming to be "confused" and in need of help and he's already got a grip on Arctic amplification. I'd suggest not biting; this does not seem a person in need of assistance.
  5. @doug: yeah, I noticed that as well. The fact that Karamanski doesn't acknowledge the answers he's given makes me think he's not really interested in learning, but rather in wasting people's time (like TTTM, cruzn246 and KL). I could be wrong, but the pattern is there.
  6. doug_bostrom, archiesteel, Learning about the meticulous details of climate change fascinates me. I thought the question I asked above was a very good question. I frequently hear the idea that polar amplification is a signature of greenhouse warming and cannot be idicative of solar forcing(I know the sun cannot be causing the warming now). I heard it in the "Solar schmolar" video on the "its the sun" post. I would like to apologize for the misunderstanding between you two.
  7. I've been wrong before, Karamanski. Perhaps I'm just too grouchy today.
  8. "Arctic amplification is an expected (and now observed) result of an enhanced greenhouse effect" Isn't this amplification an outcome of the higher concentration and larger seasonal amplitudes of CO2 in the arctic? As the figures demonstrate, large parts of the Arctic see >390ppm for more months of the year than the lower latitudes. Peak CO2 occurs in April-May, as the melt season is getting underway.
  9. @Karamanski: I'm sorry as well if I mistook your eagerness to learn for disingenuous posturing. Now, in order to satisfy your curiosity to learn about climatology, you might consider looking at some other sites about this science, as this particular one here rather specializes in responding to popular skeptical arguments, and not provide a general introduction to climate science. Personally, I like Wikipedia as a general learning tool, but I'm sure there are other sites that focus on providing quality information on this subject. Perhaps others here can give you suggestions. Beyond that, you might have to look at online classes and/or trips to the library to learn the science in more detail...
  10. muoncounter #8, interesting images. There is a hemispheric seesaw in CO2 as spring and summer in each hemisphere allow plants to grow and absorb CO2 while fall and winter see plants dying and releasing CO2. I assume that is what is responsible for the high northern hemisphere CO2 levels in the images. Note however that the Arctic region isn't showing higher CO2 levels than much of the rest of the northern hemisphere... thus if this seasonal CO2 were responsible for Arctic amplification wouldn't we expect it to also be impacting most of North America? Also, we're talking about just a few PPM difference from the global average... the additional forcing from such a small change would be tiny. Basically something like; ln(394/390)*5.35 = 0.05 w/m^2. In any case, as the article above notes, the observed changes in snow and ice cover in the arctic would be expected to produce additional warming consistent with the measured increases. The arctic ocean used to be nearly completely ice covered all year round. Now about a third of it melts out each year and the other two thirds often have widely spaced bits of ice with plenty of ocean surface receiving direct sunlight. All that sunlight heating the Arctic ocean rather than reflecting off the ice generates a great deal of atmospheric warming.
  11. Karamanski #6: "I heard it in the "Solar schmolar" video on the "its the sun" post." If so then you misheard. What that video says is that an increase in solar activity would be characterized by (among other things) more pronounced warming at the equator than at the poles. This is obvious since more sunlight hits the equator for a longer period of the year. It would be accurate to say that the video 'leaves out' that Arctic amplification would occur in either case, but the point it actually makes, that global warming caused by an increase in total solar irradiance would be most pronounced at the equator while observed warming is most pronounced at the poles, is valid. There has not been equatorial warming in excess of warming at the poles or even the mid-latitudes. Ergo, it's not the Sun.
  12. Re: CBDunkerson (10) I noted your reference to the hemispheric seesaw in atmospheric CO2 and posted some available animations depicting it over on this thread. The Yooper
  13. I also read in a paper, Shakhova(2010) that there is more methane in the Arctic atmosphere than in the global atmosphere. Does this extra methane Arctic temperatures somewhat higher than they would be otherwise. And could methane release from the East Siberian Arctic Shelf and thawing tundra be partially contributing to Arctic amplification? (Something Screen (2010) did not cover)
  14. #10: "spring and summer in each hemisphere allow plants to grow and absorb CO2 while fall and winter see plants dying and releasing CO2." Of course. However, is biomass in Arctic latitudes sufficient to account for the largest amplitude seasonal swings on the planet? Or is CO2 essentially migrating in and out of the Arctic each season? Myneni et al. 1997 showed that it is sub-arctic latitudes (45-70N) that had the "greatest increase" in a consistent pattern of longer growing season, earlier snowmelt and higher amplitude CO2 cycles. (Don't you just love finding 13 year old papers that observed exactly what the skeptics/deniers are still insisting isn't happening?) "about a third of it melts out each year and the other two thirds often have widely spaced bits of ice with plenty of ocean surface receiving direct sunlight." Again, no question about that statement. But it suggests to me that summer melting is a consequence of warming; melting results in a feedback mechanism as open water has higher absorption than ice. It's this feedback that is the amplifier. But this article seems to imply the other way around: "Maximum warming occurs over the surface during winter while less surface warming is found in summer when heat is being used to melt sea ice." Shouldn't that be stated more clearly?
  15. If more energy has been 'used' to melt ice than predicted, does it follow that less energy will have gone into heating the ocean/atmosphere than predicted? If so, how much less 'heating' would we have seen since the ice decline parted company with the IPCC predictions?
  16. Arctic amplification is the phenomenon where changes in the net radiation balance due to greenhouse gas levels, for instance, tend to produce a larger increase in temperature near the poles than the planetary average.
    Sea ice helps to keep the Arctic atmosphere cold. Its whiteness reflects much of the Sun's energy back to space, and it physically insulates the Arctic atmosphere from the underlying Arctic Ocean. With less sea ice, the refrigerator door is left open-darker open water is exposed, which readily absorbs the Sun's energy in summer, heating the ocean and leading to even more melt. With less sea ice there is also less insulation, so that heat from the ocean escapes to warm the atmosphere in the autumn and winter.
    What are the main reasons that can cause the Arctic to heat faster? How it impacts human life?

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