I recently posted about the difference in sea ice changes between the Arctic and Antarctic. A comment appeared from Dave Burton which included some nonsense trying to claim that the Antarctic sea ice gain is more important for climate than the much greater Arctic loss. He happened to be the same Dave Burton associated with “NC-20,” a group which tried to get the North Carolina state legislature to outlaw any official use of sea level rise forecasts other than the extrapolation of a linear trend based on past observations.
I informed him that I intended to do a post about his claims on sea level rise. I even promised, at his request, that I would not censor his replies to that post. He starting commenting prolifically on the sea-ice post, with no fewer than 15 comments in the space of about 48 hours. He seemed exceptionally eager to engage — I got the impression he could hardly wait to argue with us about our silly ideas that the next century would bring much greater sea level rise than the last, an ominous threat to the future.
When the sea-level post appeared four days ago, he suddenly went silent. We’ve heard not a peep from Dave Burton since. I guess the cat got his tongue.
Among all the talk about sea level, we haven’t forgotten about sea ice. Let’s talk about Burton’s original comment:
daveburton | September 24, 2012 at 3:45 pm | Reply
It is true that total sea ice extent is down. Compared to the 1979-2008 average for this date, global sea ice is currently down about 7%:
Of course, total sea ice extent varies by about 30% over the course of a year, so a 7% drop isn’t really very much. Moreover, Antarctic sea ice extent (which is high this year) is more important than the Arctic sea ice extent (which is low this year) because:
1. Antarctic sea ice is a better climate indicator than Arctic sea ice, because Arctic sea ice is greatly affected by factors other than climate, Antarctic sea ice less so. Antarctic sea ice is anchored to a continent, rather than blown around by the vagaries of the wind. Plus, this year, a big, unusual August storm that broke up the Arctic ice. And,
2. As an albedo-based feedback mechanism, Antarctic ice is more important than Arctic ice, because Antarctic ice is at less extreme latitudes.
I’ll leave it to readers to dispute most of his claims. In this post, I’d like to compare the albedo feedback between Arctic and Antarctic sea ice changes.
First of all, for most of the year higher latitudes receive less solar energy than lower latitudes. But deep in the heart of summer this is no longer true. In fact at the point of midsummer, the poles receive more solar energy than any other place on earth. Here’s the solar insolation as a function of latitude, for various times of year from northern midwinter (southern midsummer) to northern midsummer (southern midwinter) (note: this does not include the correction for varying earth-sun distance, it’s merely to illustrate the latitude dependence at a given time of year):
During the summer months, when ice albedo feedback really counts, higher latitudes really do get more solar energy than lower latitudes. When the insolation depends strongly on latitude, with higher latitudes getting much less solar energy, the total insolation is so small that the impact of albedo change is small.
Another factor is that earth is farthest from the sun during northern hemisphere summer (when albedo change counts most) but closest to the sun in southern hemisphere summer. That has lead some to believe that the impact of albedo change will be enhanced in the southern hemisphere. This is an understandable mistake; in southern hemisphere summer earth really does receive more solar energy per unit of time because we really are closer to the sun. But the rate of travel of the earth along its orbit is inversely proportional to the square of our distance from the sun — exactly the same proportion by which solar energy changes — so the northern hemisphere summer may be weaker than the southern, but it also lasts longer. For total insolation, the two factors cancel each other so at any given latitude, the annual-total solar energy input is unaffected (a fact which was emphasized by Peter Huybers in relation to ice age cycles). So no, proximity to the sun during southern hemisphere summer doesn’t enhance the ice albedo effect.
If we really want to know the relative impact of ice albedo change between hemispheres, the thing to do is: the math.
I took daily values of sea ice area from NSIDC. I modelled the Arctic ice pack as a circle around the pole of the same area, and the Antarctic ice pack as a ring around the Antarctic continent, with the edge of the Antarctic continent as being at latitude 70S. This is only a crude representation of the geometery of the sea ice packs, but it’s at least correct to first order.
Then I computed, for each day, the sun’s declination (its altitude above or below the celestial equator). I also computed the earth-sun distance so I could apply that correction to the solar insolation. This enabled me to compute the total amount of solar radiation hitting parts of the earth which are covered by sea ice, which I call “sea ice insolation“. I did so separately for the northern and southern hemispheres.
For both hemispheres, sea ice insolation shows a strong annual cycle. So I computed the average total sea ice insolation for each year, for each hemisphere, corrected for earth-sun distance and angle of elevation of the sun. This gives the annual average sea ice insolation in TW (teraWatts). We can then look at the trends in sea ice insolation, to see which pole has lost or gained more in terms of solar energy impacting sea-ice-covered regions.
It’s quite a complicated calculation, so it’s possible I’ve made an error. But the amounts are certainly in the right ballpark, and I’ve done orbital calculations for decades, so I suspect I got it right. The data cover the time span from 1979 through 2011 (2012 isn’t over yet so is not included).
And here’s the result: annual average sea ice insolation, together with linear trend lines, for both hemispheres:
Since 2012 is incomplete, this doesn’t include this year’s ever-so-slightly record high Antarctic sea ice or way-astounding record low Arctic sea ice. But you can see the trends. Clearly. The trend has shown an increase in Antarctic sea ice insolation of about 53 TW, and a decrease in Arctic sea ice insolation of about 329 TW. That’s over 6 times as great.
If we spread 53 TW over the entire earth we get a global average of 0.10 W/m^2. So even if the difference between ice and ocean albedo were equal to 1 in the southern hemisphere (i.e., Antarctic sea ice were perfectly reflective while ocean was completely absorbing) the net global climate forcing would amount to -0.10 W/m^2. But sea ice isn’t perfectly reflecting, not even in the southern hemisphere where the sea ice is often snow-covered, and ocean is not perfectly absorbing. If the top-of-atmosphere (TOA) albedo difference between sea-ice-covered and open ocean areas is 0.2, then the global climate forcing from Antarctic sea ice changes would be about -0.02 W/m^2.
If we spread 329 TW over the entire earth we get a global average of 0.65 W/m^2. If the TOA albedo difference between sea ice and open ocean is 0.2, then the global climate forcing from Arctic sea ice changes would be about +0.13 W/m^2.
And it turns out that 0.2 is not a bad figure for the TOA albedo difference, according to Hudson 2011 (JGR, 116, D16102, DOI:10.1029/2011JD015804):
In fact Hudson states that “Results show that the globally and annually averaged radiative forcing caused by the observed loss of sea ice in the Arctic between 1979 and 2007 is approximately 0.1 W m-2,” so my crude calculation is certainly in the right ballpark. As I said, this is a pretty complex calculation so I may have made an error, but I certainly ended up in the right neighborhood according to Hudson.
I’m gonna go out on a limb and suggest that Dave Burton didn’t do this calculation. In fact let me take a wild guess — he doesn’t know how. But he still felt qualified to lecture us about the albedo impact of Antarctic vs Arctic sea ice changes. And he certainly feels qualified to lecture the North Carolina state legislature about planning for future risk from sea level rise.
My opinion: Dave Burton is typical of the vast majority of fake skeptics about climate science. He really, really isn’t qualified to discuss the subject. But not only does he do so, he tries hard to influence public policy about it. This is why governments worldwide have been so effectively paralyzed — because there are so many voices, speaking so loud, whose influence (on both politicians and the general public) is way out of proportion to their qualifications even to understand, let alone advise, on the subject. Dave Burton is hardly unique in this respect, he’s actually typical. The loudest voices seem to be those of the most ignorant.
Just my opinion.
I suggest to the people of North Carolina that taking Dave Burton’s advice on scientific topics is a very bad idea.