Guest Post by Willis Eschenbach
As always, I get distracted by the daily news. The weather news today is a lovely rainy morning here in drought-plagued California, we got just under an inch (2cm) in last night’s storm, and the outer world is green and happy. Regarding the climate news, Anthony highlighted a claimed recent darkening of the Greenland ice cap. This is said to be reducing the ice cap’s albedo, which is the percentage of sunshine reflected back to space, and thus leading to more solar absorption and more melting.
Being an inherently suspectful type oif fellow, I thought I’d take a look at the albedo and other anatomical features of Greenland. First, the big view. Let me start with a map showing the global “all-sky” albedo from the CERES satellite data. It shows the average of all satellite observations, both when the sky is clear and when it is cloudy.
Figure 1. All-sky average albedo, CERES top-of-atmosphere data Mar 2000 – Feb 2015.
Overall, the combination of the clouds and the surface reflect just under a third of all the sunlight that hits the planet. In general the albedo is smallest in the tropics and increases towards both poles. In Figure 1, you can see the inter-tropical convergence zone just above the equator. You can also see Greenland, bright red up near the north pole, with an average albedo of about 65%
The CERES data provides us with another view of the albedo, which is just the measurements taken when the sky is clear. Figure 2 shows that clear-sky albedo, the solar reflection from the surface when there are no clouds..
Figure 2. Clear-sky average albedo, CERES top-of-atmosphere data Mar 2000 – Feb 2015.
As you can see, without the clouds there is much less sunlight reflected from the surface. For example, the ocean reflects less than 10% of the incident sunlight … but even without clouds, Greenland still has an albedo of about 65% because like Antarctica, it has a permanent ice cap. It is the darkening of this Greenland ice cap that I set out to investigate.
Now, there’s a problem with measuring albedo near the poles. Albedo is a ratio. It is a fraction with reflected solar energy on the top and the incoming sunshine on the bottom. Most of Greenland is above the Arctic Circle. So when the sun gets to very near zero in the winter, the albedo gets very uncertain and averages get distorted. As a result, I look instead at the total amount of sunshine that is reflected from Greenland. The incoming sun is constant on an annual basis, so any change in the albedo will be reflected as a change in the total amount of sunshine reflected.
Figure 3 below shows the month-by-month changes in the all-sky reflections from Greenland. I masked out the ocean, so Figure 3 represents solar reflections of just the area of the island itself.
Figure 3. All-sky average reflected solar energy, CERES top-of-atmosphere data. Units are watts per metre squared (W/m2). Mean value is 119.8 W/m2. Top panel shows raw data. Middle panel shows the average seasonal variation in the data. Bottom panel shows the residuals, which are the raw data minus the seasonal component. Standard deviation of the residuals is indicated by the horizontal gold dashed lines.
The average amount of energy reflected by the clouds plus the surface is about 120 W/m2. There is no trend visible over the period, and the standard deviation of the residuals (bottom panel) is only about ± 2.5 W/m2.
“Ah”, I hear you thinking, “but that includes the clouds”. Indeed it does, it is not the surface albedo from the ice cap. I like to look at what is happening overall before I look at the specifics. Having seen that there is no overall albedo trend in Greenland, Figure 4 shows the Greenaland surface reflections when the sky is clear.
Figure 4. Clear-sky average reflected solar energy, CERES top-of-atmosphere data. Units are watts per metre squared (W/m2). Mean value is 115.9 W/m2. Top panel shows raw data. Middle panel shows the average seasonal variation in the data. Bottom panel shows the residuals, which are the raw data minus the seasonal component. Standard deviation of the residuals is indicated by the horizontal gold dashed lines.
I note first that the surface average reflection is about 116 W/m2, only slightly smaller than the 120 W/m2 we saw in the all-sky data in Figure 3. This shows that the albedo of the surface and the albedo of the clouds are quite similar, with the clouds reflecting slightly more than the ice cap
And just like with the all-sky data, there is no trend in the surface data either. There is no indication at all of the claimed darkening of the surface.
Finally, I was interested in what to me was the most curious feature of Figure 4. This is the large dip in surface reflection in the summer of 2012 that reaches a minimum in July. I seemed to remember some oddity that year, and a bit of searching found this from the National Snow and Ice Data Center:
An intense Greenland melt season: 2012 in review
February 5, 2013
Greenland’s surface melting in 2012 was intense, far in excess of any earlier year in the satellite record since 1979. In July 2012, a very unusual weather event occurred. For a few days, 97% of the entire ice sheet indicated surface melting.
Now, we know from Figure 2 that water has a much lower albedo than ice. So we can see that meltwater on the icecap reduced the reflection of sunlight, and led to the 2012 summer drop in reflected solar energy shown in the CERES data.
The appearance of this July 2012 event in the CERES data supports the validity of the data, and also shows that the data should be more than precise enough to show any trend in the solar reflection over the fifteen-year period of the record … and despite that, there is no such trend visible.
Go figure … I don’t know why the original researchers are claiming a darkening of Greenland, but I’m unable to find it in the CERES data.
w.
My Usual Request: If you disagree with me or anyone, please quote the exact words you disagree with. I can defend my own words. I cannot defend someone else’s interpretation of some unidentified words of mine.
My Other Request: If you think that e.g. I’m using the wrong method on the wrong dataset, please educate me and others by demonstrating the proper use of the right method on the right dataset. Simply claiming I’m wrong doesn’t advance the discussion.
The residuals demonstrate sizeable peaks. The solar component is pretty constant. The differencies in the peaks represent more or less meltwater. The process of melting ice can be described as intial melting by changes in the atmospheric condition and accelerated by the subsequent change in albedo.
The number of peaks are limited so it seems that the local conditions are controlling the entire process
Tks Willis,
The take that I find important is the surface melt attributed to July 2012.
In decades to come, that will appear in ice cores as a paraconformity or disconformity or unconformity, a happening very well known to stratigraphers in geology. (Wiki defines terms. The period in which normal deposition was not happening was named a “hiatus” in anticipation of future fun: not).
Of interest is not so much a single season with melting, but a succession of seasons with melting, when non-melting is showing above and below in the core. The hiatus would be expected in almost any case where drilling was done where ice cover is transient. Less expected would be a hiatus in a well frozen area like inland Antarctica.
When we do not know the length of the hiatus in ice core, we have a timing problem because in ice core, as opposed to rock core, one relies much on layer counting to determine ages and time intervals. Hiatus can mean missing layers, missing years, a mistimed sequence.
It would be so easy to miss an unconformity in ice core. How can a core logger tell easily between an unconformity and a plastic flow layer of reduced thickness?
There is even the possibility of a hiatus lasting thousands of years. Therefore emphasis is placed on regional or global markers, where for example dust deposits from volcanos as matched in ice cores in time and space.
Personally, I think that there is too little mention of the possibility, even likely occurrence, of a hiatus or two in the ice core literature. Is this because it is overlooked, because it is seldom found in the chosen locations, or why? A recognised hiatus in Antarctic ice has important implications.
From readers, I’d be grateful for references in ice core literature to unconformities etc and hiatus periods.
Geoff.
Actually a major melt unconformity would be very conspicuous since it would collect all insolubles (dust, volcanic ash, micrometeorites etc) from the missing period in a single layer. Unconformities do occur and are a major problem in the deeper part of the cores, but they are caused by deformation of the ice as it moves over uneven bedrock.
Thanks, tty
The dust etc. ‘should’ accumulate and aid detection, but one can envisage scenarios where accumulation ‘might not’ happen. Any references in mind?
Glacier Girl (a Lockheed P-38F) was lost in 1942, 10 miles from the South-Eastern shore of Greenland on the ice sheet. Search began in 1981, it was found in 1988 using ground penetrating radar, two miles away under 264 feet of ice. It was dug out and flown again eventually.
Therefore average annual ice accumulation at that site was almost 6 feet for half a century, meaning much more snow, because it gets compacted into ice once buried.
I can hardly believe a single season’s surface melt may have lasting effect on albedo under such circumstances.
Nope, under 264 feet of compacted snow. The pressure is only high enough to turn the firn into solid ice at about 300 feet. Details here:
http://www.iceandclimate.nbi.ku.dk/research/flowofice/densification/
Planes were found at a depth of 80 m.
http://images.slideplayer.com/15/4554181/slides/slide_13.jpg
Average density of firn down to that depth is much higher than that of fresh snow, because as pressure builds up, it gets compressed ever more.
To reach a depth of 80 m in half a century at least ten feet of annual snowfall is needed. It is never going to melt back sufficiently during the next summer to expose darker layers beneath, no way.
The Dark Snow project also found a reduction in albedo in recent years.
http://darksnow.org
I was poking around and discovered back in the 1960s for Cold War military exercises they were flying B52s constantly over Greenland. B52s belch out incredible amounts of soot, Ive seen it firsthand with the B52s that flew out of KI Sawyer, one smokey one also shown in the attached pic. I gotta wonder if a lot of that soot can be traced back to the B52s.
http://gizmodo.com/5083001/us-air-force-abandoned-nuclear-bomb-in-greenland
Official name was Operation Chrome Dome, ran from 1960 to 1968.
https://en.m.wikipedia.org/wiki/Operation_Chrome_Dome