Is it the albedo effect? Is it increased ice dynamics? Dutch researchers add a third player to better understand why Arctic melting is happening as fast as it is.
The clue is in the winter polar night skies, they say.
Climate models indicate Arctic melting ‘should’ be more or less linear with global temperature rise. Satellite observations however show something else is going on – the melting is happening much faster.
Accelerating sea ice drift
The 2007 IPCC model projection would underestimate the rate of 21st century melting by as much as a factor 4, another research team last week stated in their Journal of Geophysical Research publication. They point to the current models’ poor performance replicating the fact that sea ice is actually afloat on an ocean, and that with ice thinning and increased wind fetch over open water (so higher waves, more calving) more and more chunks drift away via the Greenland current.
So there we would have a positive feedback on ice melting. The less ice you have, the more dynamic it gets, the more ice the system would lose (to the warm Atlantic).
Albedo effect downplayed
The ice dynamics feedback is important as it supports observed rates of annual melting – at a time when model studies have just recently suggested the albedo feedback [which is for instance illustrated in this recent NASA video] would lack the power. [The coupled Nature publication was even as optimistic as to suggest that if we manage to keep atmospheric GHG concentrations to below 450 ppm CO2 equivalents -seen the IEA requirements for that?- we could see Arctic sea ice recovery from 2035 onwards.]
If however we explain rapid annual melting as caused by a combination of mainly albedo and dynamics feedbacks – we’re left with a riddle.
In the High North albedo, Earth’s reflectivity, is only relevant in summer. That’s when the Sun shines. In winter it’s not only cold, the skies are also pitch-black, for months on end. Really the climate couldn’t care one bit what colour you’d paint the snow and ice.
The same goes for the dynamics feedback. As the ice progressively thins, its movement accelerates. But each winter the cold returns and ice at least to some extent thickens and expands, closing off much of the water movement around the Arctic Ocean’s surface.
Summer-winter discrepancy increasing, but not enough
So if dynamics and albedo are dominant feedbacks to the current Arctic melting, one would not only expect the Arctic to lose more ice as years go by, but also for the difference between the summer and winter situations to increase. Well, to make this easier, first of all that is actually what seems to be going on. Witness for instance the ice curve of 2007 – that delivered that world-shaking record. [Here you get a similar picture of other recent years.]
But, climatologists say, if the Arctic climate system were that simple and straightforward, there should be more recovery in winter, after the summer losses. Just last March the smallest winter ice maximum was reached since orbital observations began – a clear indication something is going on outside the summer months. Would something perhaps handicap the Arctic in its ability to shed excess heat?
A thick blanket of cold keeps the Arctic tucked under
To that three researchers, Richard Bintanja, Rune Grand Graversen and Wilco Hazeleger, of the Royal Netherlands Meteorological Institute (KNMI*) have found a convincing answer: thermal inversion in the lower Arctic atmosphere. As the sun-deprived Arctic surface cools rapidly in winter, so does the layer of air right above it.
Apparently largely independent of altering positive and negative phases in the Arctic Oscillation (which do affect temperature differences throughout the Arctic troposphere) this situation persists over the winter months.
Having a cold layer close to the surface has two effects on the energy balance of the Arctic climate system in winter. The cold air and the (presumed) sea ice beneath it prevent summer heat stored in the Arctic Ocean from escaping to the cosmos. But because of the temperature inverse warmer air higher up in the atmosphere also radiates less energy away to space, as it is busy warming the cold air layer underneath it – ‘pointing’ infrared radiation downwards, instead of up to the stars.
In other words: in winter the summer heat cannot escape to space – but is instead recycled within the Arctic climate system.
4 steps forward, 3 steps back (repeat each year)
With the onset of spring and the new melting season the system still carries part of the energy of the previous melting season (and extra energy carried on from lower latitudes) allowing summer feedbacks to assert a progressive influence.
The KNMI study was published in Nature Geoscience on Sunday. There the researchers write the following:
“We find that the surface inversion […] intensifies Arctic amplification, because the ability of the Arctic wintertime clear-sky atmosphere to cool to space decreases with inversion strength. Specifically, we find that the cold layers close to the surface in Arctic winter, where most of the warming takes place, hardly contribute to the infrared radiation that goes out to space. Instead, the additional radiation that is generated by the warming of these layers is directed downwards, and thus amplifies the warming. We conclude that the predominant Arctic wintertime temperature inversion damps infrared cooling of the system, and thus constitutes a positive warming feedback.”
From how we understand this with that last bit the researchers mean the Arctic inversion is not (as had been previously suggested) a negative climate feedback. With the remaining positive summer feedbacks the annual result would still be accelerating ice loss, and thus constitute a net positive feedback.
We however wonder, would there be a direct winter feedback too. Could climate change affect the intensity of the Arctic winter thermal inversion – and if so, would it make it stronger, or weaker?**
[*) You may remember them as that research institute that beat NCAR by 6 weeks in their (largely affirmative) explanations for missing ocean heat. These people have an understanding for temperature layers.]
[**) Well actually there could be an answer to that question, KNMI climate researcher and lead author Richard Bintanja says. The strength of the winter thermal inversion could decline, models show, under continuation of the present climate change - as (helped by the very mechanism) the Arctic surface (and the surface air layer) warms faster than the air (higher) above it. However marginal that effect may be within the wider complexity of the system, to us it seems that would at least count as some good news.]
© Rolf Schuttenhelm | www.bitsofscience.org