The Anglian Glaciation

I seem to have returned to an old obsession. Tying to understand the underlying causes and timings of ice ages is as frustrating as it is fascinating.

By far the largest glaciation that occurred over Britain was 480,000 years ago in what geologists call the ‘Anglian’ ice age. Glac-limits-UK Two thirds of Britain and Ireland were then covered by a thick ice sheet extending from the west coast of Ireland across the North Seas all the way to Norway. The British Ice sheet reached as far south as  Oxfordshire and Essex diverting the original course of the River Thames, which flowed north to the Wash, to its current position through what is now London. Geological evidence for this can be seen in old destroyed river beds and layers of boulder clay laid down by glaciers.

The second largest glaciation was the most recent one which reached its maximum extent at LGM 25,000 years ago. It is called by geologists the ‘Devensian’ . It turns out that these two glacial cycles were very similar, despite being spaced by 400,000 years, because of a regular cycle in the earth’s eccentricity. When eccentricity is low glaciations are always longer and deeper. To see how this happens, we can start by comparing the last 800,000 years of ice-core data as shown below.

Note the similarity between the two deepest saw-tooth glacial cycles namely the last one and the Anglian. Both terminated at weak eccentricity maxima implying low precession forcing terms.

Note the similarity between the two deepest saw-tooth glacial cycles namely the last one and the Anglian. Both terminated at weak eccentricity maxima implying low precession forcing terms. The top graphs show summer insolation maxima at both poles and in red the total insolation at either pole. This total insolation follows exactly the 41,000y obliquity cycle

The ‘Anglian’ ice volume, as measurement by Benthic Fora dO18, exceeded that of the LGM, and proves that this was also a global phenomena. The Anglian interglacial also lasted far longer than normal, at least 25,000 years, and this is most likely because of the weak value of eccentricity. A very similar situation is re-occurring now, luckily for us, because an even lower eccentricity peak will occur in 30,000 years time. Low eccentricity mainly acts to suppress the effects of precession, leaving just the 41000 year obliquity cycle to cause seasonal changes at the poles. For reasons that are not yet fully clear,  this alone has not been sufficient to end ice ages for a million years. From 5 million  to 1 million years ago glaciations followed the 41,000 year obliquity cycle, but as the earth cooled and glaciations became more extensive this then switched to a 100k year cycle. No-one has yet convincingly explained quite why this occurred.

The reference calculations of Laskar et al. which solve the solar system multi-body gravitational orbital equations also show a remarkable 2.8 million year cycle in eccentricity. I have never seen this mentioned before. We are entering a periodicity very similar to that which occurred just under 3 million years ago.

LA2010 calclations of the earth's eccentricity over a 4 million year period spanning the present day.

LA2010 calculations of the earth’s eccentricity over a 4 million year period spanning the present day.

So what happened 2.8 million years ago? The climate was much warmer then and the glacial cycle was driven by the 41ky obliquity cycle. Yet at that exact time two successive glaciations missed a beat doubling the glacial cycle to 82,000 y.

Combined fits compared to data 900-1500 ya (see http://clivebest.com/blog/?p=2775)

Combined fits compared to data 900-1500 ya (see Phenomenology of Ice Ages)  The red arrow marks a low eccentricity  cycle exactly similar to the current one. Two successive glaciations  lasted about twice the obliquity period ~80,000 y although the earth was much warmer and levels of  CO2  higher.

Further evidence that eccentricity over the last million years subtly modulates all glaciations can be seen by comparing the net annual average insolation the earth receives from the sun. Despite Keppler’s law there is still a small variation in the net solar insolation that the earth receives, dependent on eccentricity and slightly modulated by obliquity. The values shown below are all calculated using  the Laskar’s 2010 orbital solution model.

Global Average Insolation

The change in net solar forcing change is very small ( <0.5 W/m2) and unlikely to be as important as changes in how the radiation is distribution, but it is rarely mentioned. When eccentricity is low glaciations become more severe, more sawtoothed and yet somehow still end with a bang. How do they end? Is this perhaps when Gaia intervenes to restore a climate that is more conducive to life on earth?

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‘Hyper-Anthropocene’

Just when you thought it was safe to go out again and enjoy life a bit – forget it!  We will soon be hit by a climate catastrophe on a scale previously un-dreamed of. You and I, or our grandchildren, are going to have to pay the price  for our extravagant lifestyles. At least that is if you believe James Hanson et al.’s new paper.

A fundamentally different climate phase, a “Hyper-Anthropocene”, began in the latter half of the 18th century as improvements of the steam engine ushered in the industrial revolution (Hills, 1993) and exponential growth of fossil fuel use. Human-made climate forcings now overwhelm natural forcings. CO2, at 400 ppm in 2015, is off the scale. CO2 climate forcing is a reasonable approximation of the net human forcing, because forcing by other GHGs tends to offset negative human forcings, mainly aerosols (Myhre et al., 2013). Most of the CO2 growth occurred in the past several decades, and three-quarters of the ~ 1 deg.C global warming since 1850.
Our analysis paints a very different picture than IPCC (2013) for continuation of this Hyper-Anthropocene phase, if GHG emissions continue to grow. In that case, we conclude that multi-meter sea level rise would become practically unavoidable, probably within 50–150 years. Full shut- down of the North Atlantic Overturning Circulation would be likely within the next several decades in such a climate forcing scenario. Social disruption and economic consequences of such large sea level rise, and the attendant increases in storms and climate extremes, could be devastating. It is not difficult to imagine that conflicts arising from forced migrations and economic collapse might make the planet un- governable, threatening the fabric of civilization.

One of his arguments is the proposal that a sudden increase in sea level occurred at the end of the last inter-glacial – the Eemian. As a result they predict for us an exponential sea-level rise of several meters and Armageddon type  super-storms to hit us within the next few decades. You have been warned!

Supposedly a late surge in spring Antarctic insolation at the end of the Eemian interglacial led to sudden collapse of the ice sheets and a surge in sea-levels. Interglacials always start with an increase in summer insolation over northern ice sheets (65N) and end when summer insolation drops. Hansen has decided that it is really insolation in early spring that matters, probably because it fits his storyline better.

Hansen

Note the vertical straight lines for CO2 at 0 !

The timing with summer insolation is not as good as it appears here since maxima occur about 6000 years later than those shown. Hansen’s argument is that the second southern peak in insolation shown in blue at 121.1 ky ago was enough to accelerate strong melting in Antarctica shutting down AMOC. A similar fate awaits us because we have increased CO2 forcing.

Warming in polar regions might be expected to reduce North Atlantic storms which are driven by strong meridional temperature gradients. Hansen’s argument though seems to be that excessive Greenland melt water will instead cool the North Atlantic, dampening the AMOC. Europe will get colder and storms will therefore get worse. That will serve us right for hoping that the future might bring barmy Mediterranean temperatures to Britain! Instead London will soon have to be abandoned as sea level rise accelerates, while our western coasts will be pounded by  super-storms with waves over 100ft high all within the next 50 years.

Oh and there’s nothing we can do about it either, because CO2 levels won’t fall enough over the next 1000 years to make any difference anyway. It’s all the fault of James Watt for inventing the steam engine.

Posted in AGW, Climate Change, climate science, Ice Ages, Science | Tagged , | 8 Comments

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.

 

Posted in AGW, Climate Change, climate science, Ice Ages, Oceans | Tagged , | 84 Comments