Does Antarctica initiate glaciations?

Attempts to explain the last million years of glacial cycles have concentrated mainly on the growth of Northern Ice sheets. Traditionally the summer insolation at 65N is thought to control the growth and extent of ice sheets, with interglacials coinciding with summer melt-back. A new paper ‘Hemispheric sea ice distribution sets the glacial tempo‘ argues instead that Antarctic sea-ice sets  the timing of glaciations.

The Southern Hemisphere has a higher capacity to grow sea ice than the Northern Hemisphere, where continents block growth. New research shows that the expansion of Southern Hemisphere sea ice during certain periods in Earth’s orbital cycles can control the pace of the planet’s ice ages. Credit: Jung-Eun Lee / Brown University

Variations in the earth’s orbit (Milankovitz cycles) change the intensity of polar summer insolation and consequent summer melt back. Obliquity affects both poles equally, whereas precession of the equinoxes  is modulated by the earth’s eccentricity. The extent of sea ice in the Arctic is limited by the closeness of northern continents, whereas Antarctica is not. Lee et al. argue that at minimum summer insolation over Antarctica, amplified by high eccentricity,  leads to a rapid growth in annual sea ice coverage in the southern hemisphere. This reduces the earth’s net albedo enough to initiate a new ice age leading consequently also to the growth in northern ice sheets.  This asymmetry in the response of sea ice to precession between North and South poles is enough, they argue, to kick start a new ice age.

Currently perihelion coincides with the Antarctic summer insolation at a maximum. Despite this an asymmetry in sea ice cover averaged between 1980-2015 is evident.

Sea ice extent (million km2) NH SH
Max 13.27 16.39
Min 5.40 2.57
Difference 7.87 13.82

The authors use a climate model to calculate that June perihelion leads to sufficient increase in summer antarctic sea ice to reduce the net annual insolation for the earth, triggering a new ice age thus reducing albedo.

drop in net shortwave energy flux at the top of the atmosphere for the clear (dotted) and all-sky (solid) conditions from the June perihelion and to the December perihelion calculated by climate model GFDL CM2.1.  They compare different CO2 levels and ‘All sky’ is the realistic situation in the Antarctic.
Credit: Jung-Eun Lee / Brown University

The paper does not discuss how ice ages end, but proposes that glaciations can only start when the precession term is amplified by maximum eccentricity  leading to the 100,000y cycles.  Until 1 million years ago obliquity alone drove the glacial cycle until the earth cooled below a threshold. This is a nice idea to explain the 100k problem but does it work? Here are the calculated summer maxima of insolation calculated from the work of Laskar et al.

Maximum and total solar insolation calculated at the poles during last 600,000 years. The Arctic and Antarctic summer insolation are modulated by eccentricity and out of phase by half a precession cycle. Both the total annual insolation and the N-S asymmetry clearly show the underlying effect of the 41,000 obliquity signal.

One problem is that eccentricity itself follows a 400,000 years super cycle so that some summer minima in the Antarctic (dashed line) nearer minimum eccentricity  are actually larger than others at maxima eccentricity. Obliquity affects both poles equally which is why it alone was sufficient to drive glacial cycles fro the previous 3 million years.

Now let’s see if  minima in  Antarctic insolation match up to the onset of glaciations.

There is indeed some correlation but it is not fully convincing. Basically the problem is that there is only a 12000y gap between maximum northern and maximum southern polar summer insolation. So why should only 1 in 6 southern minima initiate a new glacial cycle and likewise 1 in 6 northern minima herald a new interglacial? Perhaps only those minima coincident with minimum obliquity count. This leads to a slightly better correlation with cooling. If we now just look at the last glacial cycle and compare the Greenland (NP) ice data with that from Antarctica, there is further insight.

Comparison of Antarctic temperature and dust measurements(top) with Greenland data (bottom). The dashed curve is the Summer insolation at the South Pole while the Blue curve is that at the North Pole. Interglacials are triggered by a rise in NP insolation.

It is remarkable how similar the overall Arctic/Antarctic  temperature dependence is. Whatever initiates a glaciation and terminates it is a global phenomena without significant time delay. This favours a global change in albedo and probably also connected heat flow from Antarctica to the Arctic and vice versa through the AMOC. Is the fall in temperature 12000 years ago in the Antarctic caused by by the fall in summer insolation? If so then the next glacial cycle will surely begin within 5000y as Antarctic summer insolation is currently in decline.

The new proposal in this paper is that sea ice growth is unhindered in the Antarctic Ocean but limited in extent in the Arctic. This produces an imbalance in global albedo whenever Antarctic summer insolation reaches an (eccentricity enhanced)  minimum. However despite this nice idea, I am dubious that this paper is a breakthrough in understanding glacial cycles. It still seems to need the Arctic to end ice ages even if the Antarctic starts them. Yet something else is needed as well to short circuit those intermediate insolation minima which apparently have little effect until the every 100 ky eccentricity peaks. This is the same problem as relying on 65N insolation to pace glaciations.

Posted in Climate Change, climate science, Ice Ages | Tagged , | 4 Comments

Central England Temperatures 1659-2016

There has been no change in UK average temperatures in summer(JJA) or in Spring(MAM) for the last 367 years. The two hottest summers were 1826 (17.6C) and 1976(17.8C). I remember 1976 as the perfect summer with two months of continuous sunshine, causing a severe drought.  The two coldest winters were 1740 (-0.73C) and 1963 (-0.07C). 1963 was the perfect time to be a small child aged 10, sledging every weekend. These extremes have not been exceeded for the last 40 years.

Monthly averaged temperatures for winter(DJF), Spring (MAM), Summer (JJA) and Autumn (SON). Data curtesy of the UK Met Office

If you are looking for  evidence of climate change in the UK then you will notice a possible small upturn of < 1C since 1950 during Autumn and Winter. This supports the argument made in the last post that any small increase in CO2 forcing mainly changes meridional heat transport during winter. It could also be due to the urban heating effect from buildings during winter months.

Here is the annual data. The one you see plotted say by Ed Hawkins is instead as temperature ‘anomalies’ relative to a 1961-1990 average. This is baseline is represented by the bottom red line.

CET Annual average temperatures. The two red lines show the 1961-1990 average used by Hadley and CRU to define anomalies.

On that basis you could argue that the  UK has ‘warmed’ by 0.6 ± 0.4C since 1960. That increase is due to changes in  autumn and winter temperatures.

Update: On request of Joe Public here is the comparison to CO2 levels in the atmosphere as measured at  Mauna-Loa extrapolated back to  1750 (280 ppm)

Posted in AGW, Climate Change, UK Met Office | 21 Comments

Do Global Temperatures make sense?

How would you measure the temperature of the earth from mars? Well you could measure the infra red spectrum of the earth and then fit it to a black body spectrum to derive T. If you did that you would get an answer of about 252K, unchanging with time. The greenhouse effect keeps the effective average surface warmer at 288K, but what does this actually mean and how can we measure if it changes?  This is not a simple question.

The earth’s  temperature at any location is never in equilibrium. It changes daily, seasonally and annually. Incoming solar radiation varies enormously especially near the poles which receive more energy per day in summer than the equator.

Solar radiation flux dependency on latitude and seasons. This falls to zero inside arctic circles in winter

The earth cools primarily by moving heat from hot tropical regions towards high latitudes where net IR radiation loss cools the planet, thus maintaining a certain temperature profile.

ERBE measurements of radiative imbalance.

Rising CO2 levels modify that radiation imbalance profile slightly. Surface temperatures in the tropics are not really warming at all. Any excess heat induces more clouds and more convection while surface temperatures remain constant. What really happens is that the meridional radiation profile changes. Slightly more heat is transported polewards so that hot places are shifting more heat to cold places which are doing the warming. If CO2 levels stop rising then a new temperature and radiation profile would rather quickly be reached. This is then called ‘climate change’ but any such changes are concentrated in colder regions of the world. The global ‘temperature’ itself is not changing, but instead the global distribution of temperature is changing.

Temperatures at the poles during  6 months of darkness would fall well below -150C if there was no atmosphere, similar to the moon. Instead heat is constantly being transported from lower latitudes by the atmosphere and ocean and so that temperatures never fall much below -43C. If more heat is transported northwards than previously, then minimum  temperatures must rise, and this is exactly what we observe in individual measurements.

The main problem with all the existing observational datasets is that they don’t actually measure the global temperature at all. Instead they measure the global average temperature ‘anomaly’.  This is because measuring any average global temperature is “biased” by the distribution of stations, whereas measuring an average anomaly (\Delta T )  is supposedly not. Each monthly station ‘anomaly’ is actually the difference between the measured monthly temperature and so-called “normal” monthly values.  In the case of Hadcrut4 these normal values are the 12 monthly averages calculated from 1961 to 1990 at each station.

The basic assumption being made when using anomalies is that global warming is  a universal, location independent phenomenon which can be measured by averaging all station anomalies together, wherever they might be distributed.  So in principal this then implies that global warming could be measured by just one station alone, which is clearly nonsense.

The use of anomalies introduces a new bias because they are now dominated by the larger ‘anomalies’ occurring at cold places in high latitudes. The reason for this is obvious, because all extreme seasonal variations in temperature occur in northern continents, with the exception of Antarctica. Increases in anomalies are mainly due to an increase in the minimum winter temperatures, especially near the arctic circle. To take an extreme example here is the monthly temperature data and calculated anomalies for Verkoyhansk in Siberia.

Annual temperatures vary from -50C in winter to +20C in summer. That is a seasonal range of 70C each year, and a year to year anomaly variation of ~8C is normal. The only global warming effect evident is a slight increase in the minimum winter temperatures since 1900. That is not due to any localised enhanced greenhouse effect but rather to an enhanced meridional heat transport.  Temperatures in equatorial regions meanwhile have only ~4C seasonal variations, and show essentially no warming trend.

Long term changes in temperature anomalies occur mainly in northern continents in winter months. This is not because the earth as a whole is warming up but rather that meridional heat transport from the equator to the poles has increased and the largest effect on ‘anomalies occurs in winter. The average absolute temperature of the earth’s surface is unknown. Basing the evidence for climate change on the 150 year trend in global averaged temperature anomalies still biases the result towards higher latitudes where most of the stations are located. Is it any wonder then that climate scientists are tempted to further amplify the impression of global warming by artificially infilling the Arctic, rather than say Africa or South America. For some reason they are not so keen to infill the much larger Antarctica, because surprisingly it isn’t warming

This is an animation of recent monthly temperature anomalies which demonstrates how most variability in anomalies occur over northern continents.

Note the El Nino’s in 1998 and 2015.

 

Posted in AGW, Climate Change, climate science | Tagged | 26 Comments