Watt about the sea ice?

Bob Tisdale has a recent post on Watts Up With That (WUWT) called Model-Data Comparison: Hemispheric sea ice area. In this, Bob compares reductions in northern hemisphere sea ice and increases in southern hemisphere sea ice. He also compares the data with model results and concludes that the models correctly predict northern hemisphere losses, but states

Too bad for the modelers that our planet also has a Southern Hemsiphere.

Now, it may well be that models have done a bad job of predicting changes in a southern hemisphere sea ice. Is this relevant to global warming? I don’t really think it is. I thought I would, therefore, give a physicsy interpretation of the relevance of sea ice to global warming.

Now, I should state that I’m going to be discussing specifically global warming; the increase in energy going into the climate system. I’m not referring to climate change or only to global surface temperatures. I’m also not an expert at this, so – as always – am happy to be corrected by those who know more than I do.

Sea ice plays two roles in global warming. One is that it takes energy to melt ice (and energy is released when it refreezes) and as the ice coverage changes, it changes the albedo of that area. This is because ice reflects almost all incoming radiation, while the darker ocean absorbs almost all incoming radiation.

Let’s start by first considering the Arctic. There is no land at the north pole so all of the polar ice in the northern hemisphere is sea ice. If the Earth were in some kind of equilibrium (quasi-steady-state) then we would expect there to be some sea ice that was essentially permanent and that the rest of the sea ice would melt during the summer (absorbing energy) but then, in winter, we’d expect all this to refreeze, releasing as much energy as was absorbed in melting the ice. Of course, there will be variations but, essentially, we’d expect – on average – the melting and freezing of the ice to be energetically neutral. The same applies to the albedo. In summer, the reduction in sea ice will reduce the albedo (less incoming radiation will be reflected) and in winter it will increase again (although this isn’t that significant because there isn’t much sunlight reaching the Arctic in the northern winter). However, if the change in ice coverage is – on average – the same year-on-year then the albedo, on average, is the same.

What is actually happening in the Arctic? Below is a figure showing the change in Arctic ice volume from PIOMAS simulations together with Cryosat2 data. What this shows is that the ice volume in the Arctic has been decreasing both in summer and in winter. In other words, there is a net decrease in the amount of ice at the northern pole indicating that there has to be a net increase in the amount of energy associated with the melting and freezing of ice in the Arctic. Additionally, the reduction in ice coverage also means that the amount of exposed ocean in the Arctic has been increasing with time and hence there is an associated net reduction in albedo and more energy absorbed by the Arctic oceans. That’s my understanding of the significance of Arctic sea ice with respect to global warming. It can be a direct indicator of global warming.

Plot showing Arctic ice volume from PIOMAS simulations and from Cryosat2 data.

So, what about Antarctic sea ice? Well, firstly the Antarctic is a continent that is surrounded by sea ice. My understanding is that during the summer almost all the sea ice melts (not all, but most). This means that even if the winter sea ice is increasing (releasing more energy this year than last year), all this extra sea ice simply melts the following summer, absorbing all this excess energy again. On average, therefore, there is no real change in the energy associated with melting and freezing Antarctic sea ice. What about the associated albedo? Well any increase in winter sea ice doesn’t have much effect because not much sunlight reaches the Antarctic in the Southern winter. In the summer most of the sea ice melts irrespective of the amount present during winter, so there isn’t much net change in the area of exposed ocean. Hence, the change in winter sea ice in the Antarctic doesn’t particularly influence the albedo. Hence changes in Antarctic sea ice don’t play much of a role in global warming. Changes could be an indicator of climate change, but aren’t a particular good indicator of global warming specifically.

That’s really all I was going to say. I hope I’ve expressed it clearly and hope I’ve got it essentially correct. Essentially the point I was trying to get across is that changes in Arctic sea ice volume can be a direct indicator of global warming while changes in Antarctic sea ice are not, necessarily, a particularly relevant indicator of global warming.