Posts Tagged ‘arctic sea ice

15
Nov
13

Global warming “hiatus” – it might be the ice

There has been a big guessing why the heck the plot of global atmospheric temperatures did not go straight up the last 18 or so years. hiatus 1964-2012

I plotted the curve from 1964 – 2012 here, with a linear fit for the data from 1964 to 1998, the beginning of the so called “hiatus”. (Data are frome here, with the NASA GISS temperature set as basis.)

As you can see,  there is not much of a hiatus. But if you look sharply, you could see after 1999, the red data points, a slow-down of atmospheric (not global!) warming, resulting in a lag of about 0.1 K.

The big hiatus rumoring through the press can be seen only, when we fit our straight line to a shorter period between 1985 and 1998, which is statistically questionable.

hiatus 1964-2012 fit1985-1998

Doing that means assuming, that atmospheric (not global!) warming did accelerate in the middle period. Now, we get a temperature lag of about 0.2 K against the expectation.

A lot of interesting work has been done to find an explanation for this fact, the most notable I know of by Foster and Rahmstorf, who calculated the deviations from the middle line caused by volcanoe eruptions and El-Nino/La-Nina events, the big southern pacific air and sea current oscillation, with linear contributions. They found out, that the “hiatus” can be explained quite well by these. Even more convincing was the simulation of Kosaka and Xie, who allowed for nonlinear feedback of El-Nino/La-Nina events to atmospheric temperature and got a theory – measurement accordance almost too good to be true. The British Met Office argued in a meticulous discussion, that a lot of the heat unaccounted for must be gone into the deep sea, which is no contradiction to the former.

arctic sea ice volumeBut there is one striking point, where a lot of energy has been dumped during the last 18 or so years: the arctic ocean. Let’s look at another plot: the volume of the ice floating on it (from here):

Between 1995 and today more or less 8000 km³ of perennial ice melted away. Could not the heat necessary to do this be reflected in the warming lag of the atmosphere? And it could.

Here is the copy of an excel sheet, the electronic equivalent of a back-of-envelope-calculation, in which I tried to find out, whether or not the ice loss could be correlated with the warming lag.

melting enthalpy of water 333 1,00E+03 J/kg
lost perennial ice volume since ca. 1995 8000 1,00E+09
density of ice 0,9 1,00E+03 kg/m³
lost perennial ice mass since ca. 1995 7200 1,00E+12 kg
heat used to melt lost ice mass 2397600 1,00E+15 J
2,3976 1,00E+21 J
mass of earth atmosphere 5,15 1,00E+18 kg
specific heat of air under standard conditions 1,05 1,00E+03 J/kgK
total heat capacity of the atmosphere 5,4075 1E+21 J/K
temperature decrease of the earth atmosphere (even distribution of heat contribution assumed) -0,44338419 1,00E+00 K
Difference between expected and real atmospheric temperature, ca. 0,1 1 K
antarctic volume gain rate 35,5 1,00E+09 m³/year
years since 1995 18
approximate gain since 1995 639 1,00E+09

The result is, that when the heat energy for the melting of the perennial arctic sea ice had been taken evenly from the hole of the atmosphere and from nothin else, and its specific heat capacity can be considered everywhere the same, a temperature lag of more than 0,4 K would have happened.

As is not so well known, the antarctic sea ice increased over the last decades, thus puzzeling once more the scientific community. Massonet and colleagues calculed this mass increase here. In the lower part of the spreadsheet, I looked at the size of this effect. It is not bigger than 10 % of the arctic loss.

This is all terribly incomplete of course. The atmosphere does not contribute evenly to the melting. A lot of the heat during summer melting comes not via the atmosphere, but directly from the solar radiation because of increasing surface absorption. It is quite possible, that a small shift in the regional radiation balance is responsible for the lions share of  the melting, as the sea ice simulation team of the University of Washington warns here. The atmosphere is pretty strongly coupled to the top layers of the continents and the seas, so those should have contributed to the melting, too.

On the other hand, the observed temperature lag is only 0.25 to 0.5 of what would be expected in an atmosphere-only model. So this seems to be somehow already accounted for.

It might not be the whole truth, but it’s worth looking at it.

09
Jan
13

Feedbacks, Feedbacks, awful, awful

There is this well known ice albedo feedback. Albedo is another name for the percentage of sunlight, which is reflected back into space. The albedo of ice is considerably bigger than that of the sea surface. When (not if) the summer sea ice vanishes, which is in full swing, the polar sea absorbs more sunlight. Result: climate change will be sped up generally and disproportionally in the arctic .

Lately, another important feedback mechanism has come to public attention: the spring and summer snow cover on land. In the course of the polar warming, it has shrunk, naturally, as can be seen in this graph (source):

june snow cover

It is interesting to compare the areas affected:

1980 ca.

2012 ca.

Minimum Sea Ice Area (Mio km²) ca.

5.5

2.5

June Snow Cover Area (Mio. km²) ca.

10

5

As You can see, the area drop of summer snow is a little bit less than the double of summer sea ice.

As often, things are more complex than they appear at first sight:

  • The snow data are “extent” – data which means, that they contain 10 – 15 % snow free area.
  • The albedo of land is bigger than that of water (ca. 0.2 vs. ca. 0.1, source).
  • Contrary to intuition, the mean solar irradiation in mid summer in those areas, where the main snow melting difference occurs (around the 60° N) is lower than on the polar sea around the north pole!
  • The data for other months than June are not given in my source, probably in the paywalled original paper, so they might look less crassy.

On the other hand, the capacity of heat absorbtion of land is smaller than that of sea surface, which will direct the absorbed energy earlier into atmosphere instead of the water body. Also, the speed of recent summer snow loss seems to be higher than that of sea ice (source).

What is the bottom line of all this discussion?

It is safe to say, that the climate feedback effects of sea ice – and land snow cover decline are of comparable size and that the latter should definitely be reckoned with.

Is that something new?

For science –  not, besides that the rate of snow area loss is (AFAIK) as unforeseen as that of ice loss. For the public – yes, this aspect has scarcely been covered by media.

Are there effects other than general climate warming?

Yes, as with sea ice loss, the weather system is beeing changed, i.e. strength, position and movement of lows and highs. But knowledge about that aspect is still not quite settled. (How could it?) And don’t forget the perils of arctic methane release, which is still low compared to tropical release rates, but could be considerably accelerated by permafrost thaw.

Does this have any significance for our daily life?

No, besides the feeling of urgency becomes stronger to tap on the brake pedal with emissions.

What would be a nice thing concerning this?

Put a price on GHGs emission, be it by a comprehensive, working cap and trade system or tax.

27
Nov
10

News from polar ice

cryosat_klein

Sometimes, by a small mistake, the development of a field of science is blocked for years. This happened, when in 2005 the russian Rockot launch vehicle plunged into the polar sea because of a software error, together with ESAs Cryosat-1. The satellites only measurement device was a high precision radar surface elevation detector that should have made possible the rather exact determination of land ice surface elevation as well as sea ice thickness. Its measurements would have closed – or at least narrowed considerably – an information gap concerning polar sea ice, namely its volume.

For polar sea ice extension exists a consistent data series based on many years of satellite surveillance – for the more important volume, for which the thickness distribution is needed, there are only relatively scarce measurements by probes, submarine sonar measurements and aircraft overflights, interpolated by complex numerical simulations yielding a somewhat hypothetical field of thickness. The currently most important of these models is PIOMAS of the Polar Science Center at the University of Washington, whose results I posted here. The graph is looking apalling and basically says, that – with a little “luck” – we can celebrate the first ice free summer in the arctic in not more than four years. (Actually, there is at least one more ice thickness model I know of, PIPS of the Naval Research Laboratory, but I didn’t find any useful results of it in the internet until now.)

Those simulations have been compared to satellite measurements, to be more specific to those of IceSat in the years 2003 to 2008, and the consistency was rather damned good. Since then, there have been no further checks. IceSat has been decommissioned in August. NASA plans to launch another one in 2015, so all we have now is PIOMAS running on a less than rich data base, on thin ice so to say.

With the consequence, that the credibility of the results isn’t as high as it should be – I mean, the plot is alarming, but none of the big media takes even notice of it.

CryoSat would have solved this credibility problem.

It is remarkable, that ESA began only four months after the desaster to build a new satellite, that is, a copy of the old one with a clutch of enhancements, and shot it into orbit April 2010. The apparatus went through a period of tests and fine-tunings called “commissioning” , the end of which I want to report here.

This means, that we have to expect the first papers on ice volume determination in some weeks, the first tests of PIOMAS calculations with CryoSat-2 data.

Let’s see, whether this will have any reverberation in the media.

07
Aug
10

arctic sea without ice in7 years

This is of course a lurid title, a striking statement, which no responsible scientist would make, because it lacks some explanation.

I am going to give it herewith.

Strictly, the sentence should go like this:

If one extrapolates into the future the trend visible in the arctic ice volume data compiled since 1979, one comes to the conclusion, that the arctic sea will very probably be completely ice-free for the first time in september, the month of smallest ice volume, between 2015 and 2020.

After that, it it will possibly show some year ice cover in september but only for a short period of time.

The point of total ice-freeness lies  – extrapolating the present trend – somewhere around 2035, a datum, which has as things are a pretty high uncertainty on it.

To come to that I interpreted the data of the Polar Science Center of Washington University by calculating the somewhat enigmatic plot here back into absolute ice volume values:

plot_arct_sea_ice_volume_537_384

The seasonal changes are beautifully depicted and also the decrease of the mean value. If one continues the lower touching line by finger, one gets to region around 2017. You won’t get a more reliable value with mathematical methods. Crucial is whether you take the slight but clearly perceptible bending downward into account or not. In the latter case, you would land on the zero line 10 years later.

If you continue the upper touching line, you get to the above mentioned 2035, but of course in this longer time span the process may as well accelerate as slow down.

What does this mean?

Ice has a higher coefficient of reflection than water. Hence in polar summer, more sunlight will be absorbed. On the other hand ice emits during night and winter less thermal radiation into space than water. Both statements are valid only without the sky being cloud covered. So it is not a priori clear, whether the energy balance over the year of the ice-freeness is positive or negative. Anyway it will depend very much on the cloud cover pattern.

What is clear, is that the weather system of the northern hemisphere will change, because the air over the arctic region will no longer be cooled as much as hitherto.




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