Posts Tagged ‘Antarctica’

More on Antarctic sea ice

April 11, 2013

I wrote a post recently on Antarctic sea ice where I only included sea ice extent at minimum month, mostly because it was a nice figure.

Anyway, I just noticed this nicer figure from a new paper in the International Journal of Climatology (Tareghian and Rasmussen, 2013):

Quantile regressions (20th, 50th, and 80th percentiles) and standard linear regression of monthly sea ice extent (1979–2010) for (a) the Northern Hemisphere and (b) the Southern Hemisphere

Quantile regressions (20th, 50th, and 80th percentiles) and standard linear regression of monthly sea ice extent (1979–2010) for (a) the Northern Hemisphere and (b) the Southern Hemisphere from Tareghian and Rasmussen (2013)

This shows the difference in the annual trends as well as the seasonal ones.

Reference

Tareghian, R. and Rasmussen, P. (2013), Analysis of Arctic and Antarctic sea ice extent using quantile regression. Int. J. Climatol., 33: 1079–1086.

Antarctic sea-ice growth in Nature Geoscience

March 31, 2013

There’s a new paper on Antarctic sea-ice in Nature Geoscience so I thought it might be a good time to have a quick look at how this story has developed recently. (The short version: the small increase in Antarctic sea ice is most likely a result of a complicated coruination of: density changes in the surface layer of the Southern Ocean (a result of temperature and salinity changes), which stops warm deep water reaching the surface and melting the sea ice; changing wind patterns (partially a result of ozone depletion) that leads to ice drift increase and increased sea ice extent; and melting from the bottom of ice shelves adding cool water to the ocean surface layer that further reduces deeper warm water reaching the surface and reducing sea ice melt.)

Firstly, though, why does anyone care about Antarctic sea-ice? Well, it’s been increasing a little recently. This isn’t quite what you’d expect in a changing climate where the change is being driven by an increase in energy being retained in the system by an increase in greenhouse gases in the atmosphere.

In contrast, we often hear about sea-ice loss in the Arctic, which is often linked with climate change. However, if we compare the Arctic loss with the Antarctic gain you can see that one is clearly more of an issue than the other:

Sea ice extent in Arctic and Antarctic

Sea ice extent for the month when its at its minimum (i.e. the end of local summer) Source: James Hansen

Nonetheless, its still interesting to think about why Antarctic sea ice is increasing.

The best theory around five years ago (Zhang 2007) was that the surface layer of the Southern Ocean was changing density and that this stops the warmer water below getting to the surface and melting the sea ice. The change in density itself is driven by a couple of factors, such as increased evaporation from warmer Southern waters that increases rainfall in the seas around Antarctica and freshens the water and changes in salinity driven by the sea ice changes themselves (sea ice rejects salt as it forms).

Last year, the big advance was the use of satellite observations to show that changing wind patterns in the Southern Hemisphere (which I’ve written about here) were driving the sea ice extent increase via increased ice drift (Holland and Kwok 2012).

The new paper (Bintanja et al. 2013) shows that melting from the bottom of ice shelves – where the Antarctic glaciers flow out over the ocean – produces a layer of cold water that stops warmer water below reaching the surface and slowing sea-ice growth. Although,  Holland (of Holland and Kwok) isn’t convinced that the experiments are a good demonstration of the mechanism.

So this is quite a complicated situation to understand. However, the ocean and air temperatures around Antarctica aren’t decreasing so that isn’t the reason for the sea ice increase, even though it may seem like the most obvious. To find out the real reason you need to dig a little deeper.

How much does Antarctica contribute to sea level rise? (And how should that be communicated?)

October 26, 2012

There’s an interesting new paper in Nature (King et al.) this week that looks at how much the Antarctic continental ice contributes to sea level changes. It initially caught my eye as it uses data from the GRACE satellites, which are very cool! They are twin satellites that can detect tiny changes in the distance between one another. These distance changes are driven by changes in the gravity field so it is then possible to work out how that relates to changes in mass at Earth’s surface.

King et al. aren’t the first to use GRACE to look at Antarctic mass change but they have used a new model of the way ice sheets affect the Earth’s surface. When this new model is used, you get quite a low number for the contribution of Antarctic mass loss to global sea level: 0.19mm ± 0.05mm (this is less than half of previous GRACE estimates of Antarctic mass loss to global sea level).

The first result I found for global average sea level rise for 1993-2009 was: 3.3 ± 0.4 mm per year (thanks wikipedia!) so you can see that it is a small contribution.

Anyway, I tweeted a link to this paper from my @Antarctic_news twitter account and then noticed a story about the paper in the Sydney Morning Herald and tweeted a link to that as well.

Someone quickly pointed out that the headline in the SMH was wrong – it said Antarctica was contributing 1mm to global sea level when it should be less than that (0.19mm ± 0.05mm). It turned out that Ben Cubby, who wrote the SMH article, had already noticed the mistake (and our tweets) and the headline was corrected by the next day. This is why the article has a rather clumsy headline now!

But the chat on twitter didn’t end there. Quite a few tweets were exchanged between myself, Ben and Barry Woods, who felt that Ben should have said ~0.2mm per year in his article rather than “less than a millimetre per year”, which is what he did say (and was probably why the sub-editor made a mistake with the headline).

Personally, I feel that either (~0.2mm or less than 1mm) would have been ok so tried to defend Ben’s choice of words. Both options sound quite small and, without the context of average global sea level change (which I doubt many people hold in their head), the more accurate figure doesn’t really add much. Moreover, the full passage that includes the “less than a millimetre per year” bit gives some important qualitative information that does contextualise the result:

While the continent contains enough frozen water to raise global sea levels by 59 metres should it ever all melt, the findings show it is currently contributing less than a millimetre per year. Professor King said the findings showed that sea levels had already been rising faster than they had for centuries without much extra water from the Antarctic ice sheet.

That last bit, which I’ve emboldened, seems to convey that the Antarctic contribution is small in comparison to global changes without using either of the numbers (i.e. 0.19mm ± 0.05mm and 3.3 ± 0.4 mm per year).

Someone else suggested that is was in the interest of “environmental activists” to maximise the contribution of Antarctica to sea level rise but I’m not sure that even makes sense: the view from King et al. seems even more worrying i.e. sea levels are rising without a large contribution from Antarctica.

So what was the point of this blog post? Maybe it was so that I could articulate my thoughts without twitter’s 140 character limit but I was also wondering what other people thought about how to communicate findings like this. Should journalists always use the figures straight from papers or are phrases like “less than a millimetre” ok if they make the article more accessible?

King MA, Bingham RJ, Moore P, Whitehouse PL, Bentley MJ, & Milne GA (2012). Lower satellite-gravimetry estimates of Antarctic sea-level contribution. Nature PMID: 23086145

WUWT alarmism?

December 6, 2010

Watts Up With That? recently published a post about an improvement to a method developed by Steig et al. (2009). This paper aimed to identify temperature trends over the data sparse Antarctic. The improved method has been accepted for publication in Journal of Climate, which is a decent achievement.

Firstly, I think its great that this exchange of ideas is happening in the peer-reviewed literature and not only on blogs.

I say this because, as Watts demonstrates, blogs can be used to insinuate things that are not the case.

For example, there is a quote from one of the paper authors in the post:

“I would hope that our paper is not seen as a repudiation of Steig’s results, but rather as an improvement.”

Yet Watts decided to title his post “Skeptic paper on Antarctica accepted – rebuts Steig et al”. Whilst I realise the difference between “rebut” and “repudiate”, it strikes me as poor form.

There also seems to be a tone of indignation in Watts’ part of the post about how long it took to get the paper through peer review and that one of the “difficult” reviewers had probably been involved with the initial paper:

“Anyone want to bet that reviewer was a “[hockey] team” member?”

I don’t understand why Watts is surprised about this: if you contribute something novel to the literature then the peer review process assesses that work against itself; if, on the other hand, you criticise and amend other people’s work then it would be irresponsible of the journal editor not to send the paper to one of the people being questioned.

Anyway, so what is the difference between the two analyses? Here are the plots that are provided before the paper is published properly:

The striking differences in the update are the increased positive trend on the peninsula and a “new” negative trend from the South Pole to the eastern Weddell Sea. The positive trend over most of Western Antarctica has also largely gone.

I expect Real Climate will post a response once the full paper is published so I don’t want to try to pick the methods apart here.

However, it struck me as a little odd that Watts was almost celebrating the re-affirmation of a massive warming on the Antarctic Peninsula!

Sure, the atmospheric dynamics of this region are very complicated and it’s not clear exactly what the distribution of temperature changes mean. But this “victory” seemed to focus more on getting one over the “hockey team” (ugh) rather than achieving something potentially useful.

ResearchBlogging.orgRyan O’Donnell, Nicholas Lewis, Steve McIntyre, Jeff Condon (2011). Improved methods for PCA-based reconstructions: case study using the Steig et al. (2009) Antarctic temperature reconstruction Journal of Climate, in press

Antarctic climate change – the exception that proves the rule?

March 1, 2010

Antarctica has been in the news recently because two large icebergs (one about 60 miles long and the other about 50) have broken off the continent. These “calving” events often occur naturally and these ones are probably not linked to climate change, although they might affect the global ocean circulation.

But I thought that this would be a good opportunity to have a look at the general climate situation in the South Pole region…

The clearest signal is rapid warming that has been seen on the Antarctic Peninsula (the bit that points up to South America) over the last 50 years.

The picture for the rest of the continent is not so clear, mainly because of the lack of data. For comparison, the USA has over 1000 climatological observing stations, some of which go back to the late 1800s; Antarctica currently has around 55 stations, very few of which go back to before 1957, (plus a similar number of automatic weather stations, which tend to not be maintained for long periods) and these data are used to represent a much bigger land area.

Antarctica compared to the USA[Image from NASA]

Nonetheless, there have been some high profile studies looking at Antarctic temperature trends, some finding cooling, some finding warming and nearly all being controversial.

So why is the warming on the Peninsula so clear?

The reason is that the warming is mostly driven by atmospheric circulation changes and not the increase in the greenhouse gas concentrations (although global climate change patterns forced by GHGs can include atmospheric circulation changes).

Ozone "hole"The key factor is that the ozone hole above the South Pole has changed the wind patterns – when ozone is removed from the stratosphere, less solar UV radiation is absorbed so the polar stratosphere cools. This increases the temperature change as you move away from the pole and, in turn, has changed the westerly (clockwise) winds that circle the pole – they are now further south and faster.

This wind pattern spreads down through the atmosphere towards the planet’s surface and has, therefore, brought more warm air from over the Southern Ocean to the Peninsula. This circulation change has less effect on the Antarctic interior and possibly even isolates it from the rest of the Earth system.

This climate change pattern is really interesting to study and we can even use ice core data from the Antarctic to look at how these winds have changed in the past – I’ve recently reviewed the literature on this subject (Russell and McGregor 2010).

Korhonen et al. (2010) have even found another mechanism of how these wind changes have affected the climate. As the wind speed over the ocean increases, it throws up more spray and this means that more clouds can form over the Southern Ocean and Antarctica (I’ll write a post later about how clouds form). If there are more, bright clouds around then these reflect away more incoming sunlight, which will cool the region beneath these clouds.

So, to bring all this together, if the Antarctic continent has been cooling (which isn’t clear) then this could be because the normal rules don’t apply to Antarctica. Does this mean that we can say that Antarctic climate change is the exception that proves the rule of GHG forced climate change?

Probably not, but it does highlight just how complicated the climate system is and how much more there is find out about it!

References:

ResearchBlogging.orgKorhonen, H., Carslaw, K., Forster, P., Mikkonen, S., Gordon, N., & Kokkola, H. (2010). Aerosol climate feedback due to decadal increases in Southern Hemisphere wind speeds Geophysical Research Letters, 37 (2) DOI: 10.1029/2009GL041320

ResearchBlogging.orgRussell, A., & McGregor, G. (2009). Southern hemisphere atmospheric circulation: impacts on Antarctic climate and reconstructions from Antarctic ice core data Climatic Change DOI: 10.1007/s10584-009-9673-4


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