As the sea ice retreats, exposed water absorbs more sunshine, heating the water and causing further melt of the sea ice in a vicious cycle. This appears to be the dominant positive feedback loop in the Arctic, although snow retreat may contribute nearly as much to the warming of the Arctic generally.
This cycle is shown in the purple square in the diagram below, marked "Vicious cycle ".
|The basic strategy for preventing an exponentially worsening situation is to cool the Arctic sufficiently to halt Arctic warming and prevent a record sea ice minimum in 2013 [click to enlarge image].|
However there are longer term impacts and threats, in particular:
(i) Greenland Ice Sheet destabilisation,
(ii) accelerated methane discharge,
(iii) loss of biodiversity and habitat, and
(iv) making it more difficult to keep to global warming targets.
As the snow and sea ice retreat from their levels in the 70s, more solar energy is absorbed. Taking the 70s as the baseline (zero forcing), this year's retreat produced as much as 0.4 petawatts of climate forcing averaged over the year. Much of this heat energy is retained in the Arctic, causing ice to melt and sea and land temperatures to rise. As temperatures rise, there will be slightly more thermal radiation into space, dissipating some of this energy. However most of this heat energy will slowly dissipate across the planet - and 0.4 petawatts is equivalent to half the forcing producing by anthropogenic CO2 emissions (1.6 watts per square metre). Peter Wadhams has estimated that the sea ice retreat by itself is equivalent to the forcing from 20 years of CO2 emissions, thus making it much more difficult for the global temperature to be kept below the so-called safe limit of 2 degrees warming.
However these long term effects are somewhat academic, if the immediate impact is to raise food prices far above a safe level.
It is much easier to think about and quantify the longer term impacts of Arctic warming than the more immediate impacts. This is a trap for the unwary. Therefore AMEG is trying to bring the world's attention to the immediate impacts, as they turn out to be colossal even this year, and are likely to be worse in 2013 and even worse than that in 2014.
It is clear that abrupt climate change has started, but not in the way we had been told to expect. Yes, there would be more climate extremes as the planet heated, but we were expecting a linear or near linear behaviour of the climate system, with gradual temperature change over the century. Instead we have striking non-linearity, with exponential growth in frequency and severity of climate extremes. This non-linearity is almost certain to have arisen from the exponential decline in sea ice, as shown in the PIOMAS sea ice volume trend. The trend is for September ice to fall to zero by 2015. Thus we can expect one month without sea ice in 2015, with the possibility for this event in 2014 or even in 2013.
Apart from volcanic eruptions and earthquakes with their step changes of state, the behaviour of the sea ice is possibly the most non-linear part of the Earth System because the melting is a threshold process. Until recently it was not well understood how the retreat of sea ice could cause a commensurate increase in weather extremes. But now it has become clear.
The retreat of sea ice is causing a non-linear rise in Arctic temperature, so that it is now rising at about 1 degree per decade, which is about 6x faster than global warming, reckoned to be rising at between 0.16 and 0.17 degree per decade. The temperature gradient between the tropics and the Arctic has reduced significantly over the past decade.
It now appears that the polar jet stream behaviour is critically dependent on this gradient. As the gradient diminishes, the jet stream meanders more, with a bigger amplitude of the Rossby waves, with peaks further north and troughs further south. This effect alone produces weather extremes - hot weather further north than normal and cold weather further south than normal. But as well as meandering more, the jet stream is also tending to get stuck in so-called 'blocking patterns', where, instead of moving gradually eastwards, the jet stream wave peak or wave trough stays in much the same place for months.
As a climate scientist, one might have expected a reduced gradient between tropics and pole to have some effect on weather systems, because there is less energy to drive them. The normal pattern comprises 3 bands of weather systems around the planet for each hemisphere, with each band having 'cells' of circulating air. The air rises at the tropics, falls at the next boundary, rises at the next, and falls at the pole. There has to be an odd number of bands, so that there is air rising at the equator and falling at the poles. The jet streams are at the boundary between the bands.
Such a pattern is visible on Jupiter but with many more bands. The number of bands is dependent on the temperature difference between equator and poles. When the temperature gradient reduces, one would expect the number of bands to reduce. For the Earth System it would have to reduce from 3 bands to 1 band! Thus as the gradient reduces the weather systems spread in a chaotic manner, meandering more wildly. This is exactly what has been observed.
The sticking of the jet stream must be associated with non-uniformities of surface topology and heat distribution. Thus highs and/or lows are getting stuck over some feature or other, while the jet stream meanders around them.
Thus there is a reasonable explanation for how we are getting weather extremes, simply as a result of a reduced temperature gradient between Arctic and tropics. Another argument that has been given, most notably by Professor Hansen, is that the extreme weather events are simply a result of global warming - i.e. a general rise in temperature over the whole surface of the planet. Global warming can indeed explain a gradual increase in the average intensity of storms (whose energy is derived from sea surface warming) and in the peaks of temperature for heat waves. But global warming does not explain the observed meandering of the jet stream and associated weather extremes, both hot/dry and cold/wet. On the other hand the warming of the Arctic can explain the increased meandering of the jet stream and the non-linear increase of extreme events.
Since this hypothesis seems reasonable, it is fitting that the precautionary principle should be applied when it comes to trends. The forecasting of extreme events must take into account the trend towards more extreme events as the Arctic warms. And the Arctic is going to be warming about twice as fast in 2015 as it is this year, because of sea ice retreat.
Furthermore the change from a basic 3 bands to 1 band is likely to be a threshold process, i.e. highly non-linear.
This all adds up to a picture of abrupt climate change, which is happening now.
We must be prepared to deal with the direct impact on food prices, in order to try and avoid so much social rest that it leads to international conflict.
But we must also attack the underlying cause of the climate extremes, and for this we have no option but to cool the Arctic with the necessary cooling power firstly to halt Arctic warming and the sea ice retreat "as soon as possible", and secondly to gradually bring the temperature and sea ice back to the state they were in decades ago - the state required to return the planet's climate and temperature to the old norm, under which civilisation flourished for thousands of years.
In view of the rapid deterioration in the food situation, "as soon as possible" has to be for next year - with the aim of preventing a worse retreat of sea ice in the summer. The financial cost of a year's deterioration runs into hundreds of $billions, but we also have a cost in human misery measured in perhaps a billion people moving into starvation or near starvation. On top of that, we have the possibility that if we don't act quickly, it may be too late - i.e. we will have passed the point of no return.
Thus our geoengineering to cool the Arctic is worthy of a war effort to obtain large scale deployment in a matter of months.