Hot Topic has devoted a lot of posts to events in the Arctic over the last northern hemisphere summer. The loss of sea ice was dramatic – there was 25% less ice in September than the previous record, set in 2005. The little graph to the left shows just far off the trend line last year’s September area really was. And as I posted yesterday, recent studies suggest that the Arctic is primed for more significant losses in the near future. If the reduction in summer sea ice continues, there are some pretty major implications for the climate of the northern hemisphere and for our modelling of the global climate, and it’s those things that I want to consider in this post. Please note: I am not a climate scientist, and there are a lot of ifs and handwaves in this argument, but bear with me…
What really got me going was the news from the American Geophysical Union conference in December that half of the volume of sea ice had melted between 2004 and 2007. That’s a lot of ice, and it set me thinking about the “heat budget” for the Arctic. Assume for a moment that the Arctic climate is stable. Every summer, the ice melts back to roughly the same area, and every winter it refreezes to the same, larger, area. The freeze/thaw cycle is determined by the seasonal changes in incoming solar radiation and winter radiation of heat out to space (and quite a few other things, like albedo, but let’s keep it simple). Heat gained in summer is matched by heat lost to space and to the atmosphere over winter. The key thing to keep in mind is that the winter heat losses are essentially fixed.
Now lets add some heat. We’ll ignore changes in the relationship of the earth to the sun, the Milankovitch cycles, because they operate over much longer time scales than we’re interested in here. There are several ways to get heat into the Arctic. We can increase the amount of greenhouse gases in the atmosphere: this will reduce the heat lost to space, especially in winter. We can increase the amount of warm air and water vapour moved into the Arctic by the atmosphere through weather patterns, and we can increase the amount of heat carried into the Arctic by the oceans, especially the North Atlantic. There are feedbacks, too, that speed up the process, such as the “albedo flip” when white snow and ice is replaced by dark, heat absorbing water. If more heat is going in than can be lost each winter, then the Arctic will accumulate heat. Where does this heat go? It melts ice. All of these factors can be seen at work in the Arctic today.
To change water to ice, it has to lose a lot of energy – heat. Wikipedia explains enthalpy of fusion, also known as latent heat of fusion:
The heat of fusion can be observed if you measure the temperature of water as it freezes. If you plunge a closed container of room temperature water into a very cold environment (say âˆ’20 Â°C), you will see the temperature fall steadily until it drops just below the freezing point (0 Â°C). The temperature then rebounds and holds steady while the water crystallizes. Once completely frozen, the temperature will fall steadily again.
The reverse is also true. To melt the ice, you have to put heat in. You don’t see much temperature change until the ice is all gone. I’m not going to attempt any back-of-the-envelope calculations based on estimates of ice volume losses and the heat of fusion of water (333.55kJ/kg), but for the Arctic to lose as much ice as it has in the last ten years, the overall heat budget must be strongly positive.
The most aggressive predictions for the loss of the remaining summer sea ice are those made by Wieslaw Maslowski of the US Navy – a researcher with access to sea ice data from nuclear submarines patrolling in the Arctic ocean. Maslowski’s ice model suggested that the ice could all be gone by 2013, based on data up to 2004. In other words, prior to the huge losses of 2005 and 2007. “You can argue that maybe our projection of 2013 is already too conservative,” he told the BBC. Even if he is wrong in detail, other workers in the field have been rapidly revising their estimates for the loss of summer sea ice, bringing them nearer and nearer to Maslowski. And the consequences for what we think we know about the likely future state of the global climate are profound.
Once the last multi-year ice is gone, the Arctic will be much more like the Antarctic, where the sea ice melts away every summer, and then reforms every winter. The difference is the heat accumulation – which will continue. All the positive feedbacks will remain positive, and the Atlantic won’t suddenly stop shipping heat north of Spitzbergen because the ice has gone. The heat that’s been going into the latent heat of fusion will go into raising the temperature of the Arctic Ocean. How fast that might happen, I don’t know enough to speculate, but the end state could look like the Paleocene-Eocene Thermal Maximum (PETM) when the summer ocean temperatures could have been as high as 23C. Not much chance of winter ice forming in a sub-tropical Arctic…
When the summer sea ice is reduced to a few little chunks clinging to northern Greenland and the Canadian archipelago (within ten years?) the autumn freeze-up will start progressively later as heat continues to accumulate, and the spring melt will begin earlier. As the Arctic Ocean warms, the climate of the entire northern hemisphere will change – and in ways that no-one can project, because none of the models get the current ice loss – and therefore Arctic heat budget – right. The current crop of climate models aren’t very good at modelling the PETM Arctic – and they’re not doing a good job of keeping up with the current rate of change. This means that IPCC projections of likely climate change are all old news. Until the models can produce the changes we’re seeing, and then progress them into a future where the Arctic is seasonally – and eventually annually – ice free, they can’t give us good information about the sort of global and regional climate states we might see over the next 100 years.
There is a real danger that events are overtaking the science. A rapidly warming Arctic will bring significant – and rapid – changes to the northern hemisphere climate, and the potential for some very nasty feedbacks (melting permafrost releasing methane, methane hydrates boiling up from the sea bed) to kick in. We will not be immune down south. The impacts of an increase in rate of sea level rise will affect us all, and the economic dislocation of rapid change could be huge.
I am aware that this will be interpreted as “alarmist” by sceptics, and those who prefer to take what they feel to be a “moderate” view of the impacts of climate change. If there are holes in my argument, tell me. Perhaps the amount of heat going north is really not that great, and the ice will bounce back towards the long term trend line, and will still be around in 50 years. I hope it is – I hope I lose my bet with Stoat. But I don’t think I will. At the very least, the scenario I’ve described here needs to be considered as a “worst case” and evaluated scientifically and strategically. And there needs to be a lot of urgent work on the modelling of oceanic and atmospheric heat transport into the Arctic Ocean. Really urgent work, because I can’t see how any amount of emissions reductions are going to stop this happening.