Sea Ice & Sea Shanties

The National Snow and Ice Data Centre (NSIDIC) publishes the extent of daily Sea Ice coverage for both the Arctic and Antarctic. These are derived from  from meteorological satellite images. Often you see more alarming plots of  rapidly disappearing Sea Ice based instead on  ‘area’ , for example that produced by Cryosphere Today. The difference is that ‘area’ subtracts all surface pixels which identify from space as being water rather than ice inside a 25 km^2 grid cell. During  summer, ‘melt pools’ appear naturally on top of solid ice and as a result the area method treats these as open water, thereby exaggerating apparent ice loss. As NSIDC itself writes:

Scientists at NSIDC report extent because they are cautious about summertime values of ice concentration and area taken from satellite sensors. To the sensor, surface melt appears to be open water rather than water on top of sea ice. So, while reliable for measuring area most of the year, the microwave sensor is prone to underestimating the actual ice concentration and area when the surface is melting. To account for that potential inaccuracy, NSIDC scientists rely primarily on extent when analyzing melt-season conditions and reporting them to the public.

Therefore the temptation of those who want Arctic warming  to appear dramatic will tend to use area.  This realisation of this tendency only resulted after a twitter exchange with @GreatWhiteCon and his supporters. As a result I will use extent data from NSIDC for the rest of this post.   First lets see all the daily values of Ice extent since 1978

Daily Sea Ice extent from 1978. The Arctic is shown in dark blue and the Antarctic shown in light blue. The top graph shows the global balance (NH+SH)

Daily Sea Ice extent from 1978. The Arctic is shown in dark blue and the Antarctic shown in light blue. The top graph shows the global balance (NH+SH)

Antarctica sea ice almost completely melts every summer, but gains a massive  ~ 15 million km^2 each winter. This is twice the seasonal range of the Arctic, where a core amount (5-7 million km^2) survives each summer as perennial ice. This perennial ice has been slowly reducing, but even today remains at about 5 million km^2. If you just look by eye at the trend then it should be another 30 years before the Arctic perhaps becomes Ice free in summer. The following graph now shows how the minimum annual global coverage which occurs around February each year has changed since 1979.

Annual-extremes

There has been a reduction of about 1.5 Million square kilometers globally. Next we look at the Sea Ice ‘anomalies’, which is the difference between the measured daily values and the average for each day as calculated from 1978 to 2010. The concept is similar to temperature anomalies and is intended to show changes in trends over time.

Anomalies

So the Arctic has lost around 1.6 million km^2 whereas Antarctica has gained about 1 million km^2. While a loss in sea ice is expected in climate models I am not aware of any which can realistically explain the sudden increase in Antarctica. Globally though the net result is a loss of around 0.6 million km^2. This must affect  the  radiative balance globally slightly on earth since ice has a much larger albedo than water.

The message is that the next time you see some scary graph of vanishing Arctic ice   always first check whether it refers to ‘Extent’ or ‘Area’. There  is a very large difference between the two!

Here below the Storify recording made by Jim Hunt.

 

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Next Ice delayed?

A new paper led by Potsdam authors has received much publicity because it claims that human emissions will delay the next ice age for 100,000y. This is based on model simulations which predict another glacial period starting naturally only in 50,000y time.

Using an ensemble of simulations generated by an Earth system model of intermediate complexity constrained by palaeoclimatic data, we suggest that glacial inception was narrowly missed before the beginning of the Industrial Revolution. The missed inception can be accounted for by the combined effect of relatively high late-Holocene CO2 concentrations and the low orbital eccentricity of the Earth7. Additionally, our analysis suggests that even in the absence of human perturbations no substantial build-up of ice sheets would occur within the next several thousand years and that the current interglacial would probably last for another 50,000 years. However, moderate anthropogenic cumulative CO2 emissions of 1,000 to 1,500 gigatonnes of carbon will postpone the next glacial inception by at least 100,000 years. Our simulations demonstrate that under natural conditions alone the Earth system would be expected to remain in the present delicately balanced interglacial climate state, steering clear of both large-scale glaciation of the Northern Hemisphere and its complete deglaciation, for an unusually long time.

Currently the earth’s orbit is in a low eccentricity cycle which is similar to the one 400,000 years ago. We can therefore make an analogy with that interglacial to determine when this one will end. The growth of northern ice sheets begins when summer insolation is insufficient to completely melt back winter expansion, starting within the arctic circle. An expanding ice surface reduces albedo thereby leading to less absorption of solar energy. This ice albedo feedback effect accelerates the growth of ice sheets, reducing CO2 levels until another ice age begins. Something else is needed to end deep glaciations. A likely mechanism is that described in the previous post. Low CO2 levels lead to the death of boreal forests, soil erosion and dust storms which deposited on the ice sheets break the ice-albedo feedback loop.

We are now 15,000 years into the current interglacial (12,000 if you exclude Younger Dryas). What will cause it to end ? The answer is a large reduction in summer insolation inside the arctic circle. Currently the precession of the equinox is unfavorable for northern summers since it coincides with aphelion (largest distance from the sun). We are lucky that the earth’s orbit is close to circular and obliquity low so that the effect is rather small, although the earth has indeed cooled a little since 8000 years ago.  We narrowly avoided another glaciation a couple of hundred years ago. Now polar summer insolation is beginning to increase again.

NEXT-ICE-AGE

Figure 1

The northern hemisphere will naturally warm slightly for about another 4000 years before another dip in summer insolation will slide us back into another glaciation. We can see when this will happen by comparing the current interglacial to that 400,000 years ago. The earth eccentricity follows a 420,000 year ‘amplification’ cycle superimposed on the 100,000 year cycle. Ice ages tend to follow the 100,000 year cycle but their strength depends on the super-cycle. About 400,000 years ago we had a similar pattern of eccentricity to the current one leading to weaker precession peaks. Changes in obliquity are constant and simply increase/decrease net radiation at both poles and extend the arctic/antarctic circles.

The arrow in figure 1 shows the interglacial 400Ky ago compared to average June/July insolation at the north pole (blue curve). Note how similar it is to the Holocene.  It ended when summer insolation fell below 520 W/m2. Currently  summer insolation is ~540 W/m2. Another precession dip will occur in about 15,000 years time when summer insolation will again fall below 520 W/m2 reaching 515 W/m2. This is when the next ice age is most likely to begin. The Potsdam authors of the new paper seem to want to ignore this dip preferring instead to wait another 50,000 years for a slightly bigger drop of ~511 W/m2.  The aim of their paper though is to  claim this too will be delayed (in their model) because we have pumped too much CO2 into the atmosphere !

 

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Does Gaia end Ice Ages?

Orbital changes to solar radiation inside the Arctic circle alone are insufficient to end all ice ages.  A new paper by Ralf Ellis proposes an elegant solution to the mystery of how ice ages terminate. Life itself could provide the ultimate thermostat control.

Variations in insolation at high latitudes occur in tune with the 23Ky precession cycle, but their strength is strongly modulated by the 41Ky obliquity cycle and the 100Ky eccentricity cycle. Glaciations used to follow the 41Ky obliquity cycle, but for the last 800Ky the maximum summer insolation in the arctic has not been enough to fully terminate an ice age. Ice sheets in the Northern hemisphere have  grown so large that the ice-albedo feedback suppresses solar melting alone. Something else is now  needed to break the ice albedo feedback effect. The basic proposal is as follows.

As ice sheets expand and ocean temperatures fall  CO2 levels will eventually fall dangerously below 200ppm,threatening the survival of plants based on C3 photosynthesis. Boreal forests and  temperate grasslands will begin to die back exposing soil to weathering. Desertification ensues and strong winds will transport dust storms over the ice sheets. With very little annual snowfall during a glacial maximum, this dust layer will build up and thereby will reduce the ice albedo. The next  ‘Grand Summer’  precession cycle at maximum eccentricity will now be able to  rapidly melt back the ice sheets, because finally lower albedo ensures more heat is absorbed by the ice sheets.  Does  the data actually support this hypothesis ?

Figure 1: The blue curve is a calculation of the maximum insolation in summer at the north pole. The ice volume data is the benthic d18O stack. In red is the Epica temperature anomaly data from Antarctica. In yellow is the Epica CO2 data and in purple their dust data. The black curve is the change in the Earth's orbital eccentricity. The temperature, dust, and ice volume data have all been scaled for comparison purposes.

Figure 1: The blue curve is a calculation of the maximum insolation in summer at the north pole. The ice volume data is the benthic d18O stack. In red is the Epica temperature anomaly data from Antarctica. In yellow is the Epica CO2 data and in purple their dust data. The black curve is the change in the Earth’s orbital eccentricity. The temperature, dust, and ice volume data have all been scaled for comparison purposes. Click to expand

The last glacial cycle is particularly interesting because the Great Summer maxima are suppressed by low values of orbital eccentricity. This results in the classic sawtooth shape showing how two insolation maxima had very little effect on ice volume growth. However, when CO2 levels fall below ~220 ppm the deposition of dust increases dramatically. The onset of the next summer maximum triggered the current interglacial. We are very lucky that human civilization developed during an era of low orbital eccentricity since the current summer minimum insolation in the arctic is very weak. Otherwise we would already be heading for the next ice age. The most similar interglacial occurred 400,000 years ago and lasted ~30,000 years (see figure 2). That too shows a large dust peak prior to rapid warming.

I think Ellis’s proposal is attractive and it is likely to be the correct explanation for the last glacial termination, as no other explanation so far makes sense to me. However the evidence is less strong for the previous glacial cycle. What is clear is that whenever CO2 levels fall below 200ppm there is a steep increase in dust deposition. The lack of moisture and low CO2 levels kills off boreal forests and soil turns to dust. The previous glacial cycle shows a long period of dust but without a final peak. However, the final Great Summer insolation is very strong, reaching an average daily insolation at the pole of 600W/m2. Note however that  a similarly large excursion during the last ice age had no effect because CO2 levels were still high without any dust deposition on the ice sheets.

Figure 2. The last 800,000 years of glacial cycles. Curves are the same as in figure 1. The current interglacial is most similar to that 400,000 years ago. Click to expand

Figure 2. The last 800,000 years of glacial cycles. Curves are the same as in figure 1. The current interglacial is most similar to that 400,000 years ago. Click to expand

Figure 2 above shows the last 800k years and demonstrates why it is eccentricity that drives the long-term glacial cycle. What is also evident is that there are always peaks of dust deposition whenever CO2 levels fall below 200ppm. However, the data implies that it is only when eccentricity is small that the dust-albedo effect becomes critical. Otherwise the boost to a Great Summer Insolation caused by large eccentricity is usually sufficient to terminate a glaciation. Note that the glacial ‘cycle’ from -630Ky to -480Ky essentially consisted of three mini-cycles, each broken by an enhanced Great Summer Insolation, because eccentricity remained continuously above current values.

The IPCC AR5 report is closer to a  ‘CO2 controls everything’ scenario, because they see the feedback of CO2 forcing feedback as the ‘extra’ mechanism for ending ice ages. However cooler oceans absorb more CO2 and rises in CO2 follow those first in temperature. There clearly must be some enhanced CO2 greenhouse feedback, but rises in humidity(H2O) with warming are probably far more important. Taken together rising CO2 and H2O levels  boost the regrowth of boreal forests thereby ending dust storms. Life rapidly recovers following a deep shock. Gaia lives on.

In conclusion Ralf Ellis has made a very interesting proposal which I believe could well be the correct explanation for the rapid recovery from the last glacial minimum (LGM), and also that of 400,000 years ago. During the LGM CO2 levels reached dangerously low levels of ~180 ppm causing arid desertification  as temperate trees and savannah died off. The resulting dust storms then deposited huge amounts of dust on the ice sheets increasing its albedo. The consequent Great Summer Insolation, coinciding with maximum eccentricity, finally melted back the ice sheets through reduced albedo, perhaps aided by increasing CO2 and H2O. However, the evidence is far less strong for previous glacial cycles. A fall in dust induced ice-albedo is less likely to be the primary cause of interglacial warming. This is because the orbital eccentricity was much larger and  was sufficient to melt back ice sheets for the  Great summer maxima.

Posted in Climate Change, climate science, Gaia, Ice Ages, Science | Tagged | 22 Comments