Posts Tagged ‘ice ages’

A model to explain the end of an ice age (but not yet to predict when one may start)

February 23, 2017

That the onset of glacial (cold) and interglacial (warm) periods on earth are a consequence of the Milankovitch cycles is almost certain. Researchers have now developed a model which seems to be able to explain why and when glacial periods end to give interglacial conditions. Exactly what cause glacial conditions to be triggered remains to be discovered.

P. C. Tzedakis, M. Crucifix, T. Mitsui, E. W. Wolff. A simple rule to determine which insolation cycles lead to interglacials. Nature, 2017; 542 (7642): 427 DOI: 10.1038/nature21364

AbstractThe pacing of glacial–interglacial cycles during the Quaternary period (the past 2.6 million years) is attributed to astronomically driven changes in high-latitude insolation. However, it has not been clear how astronomical forcing translates into the observed sequence of interglacials. Here we show that before one million years ago interglacials occurred when the energy related to summer insolation exceeded a simple threshold, about every 41,000 years. Over the past one million years, fewer of these insolation peaks resulted in deglaciation (that is, more insolation peaks were ‘skipped’), implying that the energy threshold for deglaciation had risen, which led to longer glacials. However, as a glacial lengthens, the energy needed for deglaciation decreases. A statistical model that combines these observations correctly predicts every complete deglaciation of the past million years and shows that the sequence of interglacials that has occurred is one of a small set of possibilities. The model accounts for the dominance of obliquity-paced glacial–interglacial cycles early in the Quaternary and for the change in their frequency about one million years ago. We propose that the appearance of larger ice sheets over the past million years was a consequence of an increase in the deglaciation threshold and in the number of skipped insolation peaks.

Onset of Interglacials Tzedakis et al

Onset of Interglacials Tzedakis et al

Science Daily reports:

…. In a new study published today in Nature, researchers from UCL (University College London), University of Cambridge and University of Louvain have combined existing ideas to solve the problem of which solar energy peaks in the last 2.6 million years led to the melting of the ice sheets and the start of a warm period.

During this interval, Earth’s climate has alternated between cold (glacial) and warm (interglacial) periods. In the cold times, ice sheets advanced over large parts of North America and northern Europe. In the warm periods like today, the ice sheets retreated completely.

It has long been realised that these cycles were paced by astronomical changes in the Earth’s orbit around the Sun and in the tilt of its axis, which change the amount of solar energy available to melt ice at high northern latitudes in summer.

However, of the 110 incoming solar energy peaks (about every 21,000 years) only 50 led to complete melting of the ice sheets. Finding a way to translate the astronomical changes into the sequence of interglacials has previously proved elusive. 

Professor Chronis Tzedakis (UCL Geography) said: “The basic idea is that there is a threshold for the amount of energy reaching high northern latitudes in summer. Above that threshold, the ice retreats completely and we enter an interglacial.”

From 2.6 to 1 million years ago, the threshold was reached roughly every 41,000 years, and this predicts almost perfectly when interglacials started and the ice sheets disappeared. Professor Eric Wolff (University of Cambridge) said: “Simply put, every second solar energy peak occurs when the Earth’s axis is more inclined, boosting the total energy at high latitudes above the threshold.”

Somewhere around a million years ago, the threshold rose, so that the ice sheets kept growing for longer than 41,000 years. However, as a glacial period lengthens, ice sheets become larger, but also more unstable.

The researchers combined these observations into a simple model, using only solar energy and waiting time since the previous interglacial, that was able to predict all the interglacial onsets of the last million years, occurring roughly every 100,000 years.

Dr Takahito Mitsui (University of Louvain) said: “The next step is to understand why the energy threshold rose around a million years ago — one idea is that this was due to a decline in the concentration of CO2, and this needs to be tested.”

The results explain why we have been in a warm period for the last 11,000 years: despite the weak increase in solar energy, ice sheets retreated completely during our current interglacial because of the very long waiting time since the previous interglacial and the accumulated instability of ice sheets. …..

Milankovitch Cycles (Wikipedia)

What would cause the current interglacial to end remains to be discovered. It’s only my speculation of course but I suspect that a trigger event is probably needed. Possibly 2 or 3 major (VEI >6) volcanic eruptions over a short period, with large amounts of dust, which in turn led to a a few “years without summers”, could provide such a trigger for an unstoppable process. However the onset of full glacial conditions would still take a few thousand years. The availability of high energy densities would probably make it (relatively) easy for humans to continue to thrive and prosper (as they have done through other glacial periods with much lower energy availability).


 

Ancient humans coped with massive climate change (without the IPCC)

October 1, 2013

Many people today seem to like to live in the fear of an impending catastrophe. The fears are all artificial and always include fanciful predictions of doom. Fears of uncontrollable population explosions, food shortages and starvation, of energy crises and depletion of all resources and of course of catastrophic global warming. And they give rise to such utterly useless bodies as the IPCC.

The period from before the last interglacial, the Eemian and through to the current interglacial in the Holocene has seen the rise of Anatomically Modern Humans and, starting from Africa, the peopling of the world. Anatomically modern humans make their appearance in Africa during an even earlier interglacial at around 250,000 years ago. They saw a descent into glacial conditions with global temperatures dropping about 6 °C and sea levels  dropping by some 150m. Then around 130,000 to 135,000 years ago a very rapid (relatively) climate change ocurred as the conditions of the Eemian were established.  Global temperatures increased by some 7 °C and sea levels rose by upto 170m. Temperatures were warmer than today and sea levels were higher.They didn’t just survive this change – they thrived. They made their way through the Sahara (perhaps through ancient green river corridors) and established themselves in the North and North-East of Africa. At this time sea-levels were high and crossing over into Europe or to Arabia would not have been possible. Both these crossings would have been made at earlier periods by the precursors of AMH and such groups would have given rise to the Neanderthals in Europe and the Denisovans in Asia. When sea levels allowed and perhaps driven by desertfication they crossed into Arabia. From Africarabia they moved across the globe – again perhaps driven by desertification of Arabia.

All these predecessors of ours – some ancestors and some distant cousins – not only survived but actually thrived. They had no IPCC to warn them of looming catastrophe if sea levels rose by 20 cm or temperatures rose by 1.5 °C. Not realising their dangers they still coped with changes of 7 °C and sea-levels of 170 m. Of course they were not without their resources. They had fire. They could probably speak but they had not been contaminated by the written word and were not corrupted by IPCC reports. They may have had some primitive form of rafts but they had no boats and the wheel was unknown. They had stone tools and their version of WMD consisted of many spears. They just coped with the weather and whatever it threw at them. They didn’t waste time predicting the climate and living in the fear of their own predictions. They had other more real fears to worry about.

Former interglacials

The period after the Eemian and upto the present day is particularly interesting.  For most of the time the world was in the grip of glacial conditions. Even as the climate changed and the world started warming up, there were sudden spikes of climate in the reverse direction as with the Younger Dryas. It was in this glacial period that AMH left Africa and then peopled the entire globe. It was not a period of stable climate and their expansion and growth took place in an environment of frequent and violent change. Real population increase started some time before the neolithic when we were still hunter-gatherers or semi-nomadic herders.

Age of Human Expansion

Age of Human Expansion

Of course in North Africa and the Middle East and Asia where much of the action took place for AMH there was little danger of advancing ice sheets. But there was the constant risk of sudden desertification, the drying up of fresh water resources and the sudden loss or appearance of new coastal land as sea levels increased or decreased. Rainfall patterns would have changed. Landscapes would have been transformed from forests to savannahs to deserts and back again. The only recourse available to humans of that time was to move to a more viable location whenever their survival was threatened.

And as they did that they populated the world and they prospered.

But they could have been stopped in their tracks if they had had the benefit of an IPCC.

“Carbon dioxide involved but not determinative in 100,000 year glacial cycles”

August 10, 2013

We are still struggling to explain what initiates an ice age (glaciation) and what causes them to end and the ice sheets to withdraw giving the interglacials.

interglacials

That the Milankovitch cycles and variations of insolation are involved in the onset and retreat of glacial periods is clear but the “how” is still elusive. Now a new paper describes a model where the ice sheets and the mutual feedbacks with climate are considered.

“Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles”.

Abe-Ouchi A, Saito F, Kawamura K, Raymo ME, Okuno J, Takahashi K, Blatter H: Insolation-driven 100,000-year glacial cycles and hysteresis of ice-sheet volume. Nature, 2013, 500: 190-193, doi: 10.1038/nature12374

ETH Press Release: 

Ice ages and warm periods have alternated fairly regularly in the Earth’s history: the Earth’s climate cools roughly every 100,000 years, with vast areas of North America, Europe and Asia being buried under thick ice sheets. Eventually, the pendulum swings back: it gets warmer and the ice masses melt. While geologists and climate physicists found solid evidence of this 100,000-year cycle in glacial moraines, marine sediments and arctic ice, until now they were unable to find a plausible explanation for it.

Using computer simulations, a Japanese, Swiss and American team including Heinz Blatter, an emeritus professor of physical climatology at ETH Zurich, has now managed to demonstrate that the ice-age/warm-period interchange depends heavily on the alternating influence of continental ice sheets and climate.

“If an entire continent is covered in a layer of ice that is 2,000 to 3,000 metres thick, the topography is completely different,” says Blatter, explaining this feedback effect. “This and the different albedo of glacial ice compared to ice-free earth lead to considerable changes in the surface temperature and the air circulation in the atmosphere.” Moreover, large-scale glaciation also alters the sea level and therefore the ocean currents, which also affects the climate. 

As the scientists from Tokyo University, ETH Zurich and Columbia University demonstrated in their paper published in the journal Nature, these feedback effects between the Earth and the climate occur on top of other known mechanisms. It has long been clear that the climate is greatly influenced by insolation on long-term time scales. Because the Earth’s rotation and its orbit around the sun periodically change slightly, the insolation also varies. If you examine this variation in detail, different overlapping cycles of around 20,000, 40,000 and 100,000 years are recognisable (see box).

Given the fact that the 100,000-year insolation cycle is comparatively weak, scientists could not easily explain the prominent 100,000-year-cycle of the ice ages with this information alone. With the aid of the feedback effects, however, this is now possible.

The researchers obtained their results from a comprehensive computer model, where they combined an ice-sheet simulation with an existing climate model, which enabled them to calculate the glaciation of the northern hemisphere for the last 400,000 years. The model not only takes the astronomical parameter values, ground topography and the physical flow properties of glacial ice into account but also especially the climate and feedback effects. “It’s the first time that the glaciation of the entire northern hemisphere has been simulated with a climate model that includes all the major aspects,” says Blatter.

Using the model, the researchers were also able to explain why ice ages always begin slowly and end relatively quickly. The ice-age ice masses accumulate over tens of thousands of years and recede within the space of a few thousand years. Now we know why: it is not only the surface temperature and precipitation that determine whether an ice sheet grows or shrinks. Due to the aforementioned feedback effects, its fate also depends on its size. “The larger the ice sheet, the colder the climate has to be to preserve it,” says Blatter. In the case of smaller continental ice sheets that are still forming, periods with a warmer climate are less likely to melt them. It is a different story with a large ice sheet that stretches into lower geographic latitudes: a comparatively brief warm spell of a few thousand years can be enough to cause an ice sheet to melt and herald the end of an ice age.

The Milankovitch cycles

The explanation for the cyclical alternation of ice and warm periods stems from Serbian mathematician Milutin Milankovitch (1879-1958), who calculated the changes in the Earth’s orbit and the resulting insolation on Earth, thus becoming the first to describe that the cyclical changes in insolation are the result of an overlapping of a whole series of cycles: the tilt of the Earth’s axis fluctuates by around two degrees in a 41,000-year cycle. Moreover, the Earth’s axis gyrates in a cycle of 26,000 years, much like a spinning top. Finally, the Earth’s elliptical orbit around the sun changes in a cycle of around 100,000 years in two respects: on the one hand, it changes from a weaker elliptical (circular) form into a stronger one. On the other hand, the axis of this ellipsis turns in the plane of the Earth’s orbit. The spinning of the Earth’s axis and the elliptical rotation of the axes cause the day on which the Earth is closest to the sun (perihelion) to migrate through the calendar year in a cycle of around 20,000 years: currently, it is at the beginning of January; in around 10,000 years, however, it will be at the beginning of July.

Based on his calculations, in 1941 Milankovitch postulated that insolation in the summer characterises the ice and warm periods at sixty-five degrees north, a theory that was rejected by the science community during his lifetime. From the 1970s, however, it gradually became clearer that it essentially coincides with the climate archives in marine sediments and ice cores. Nowadays, Milankovitch’s theory is widely accepted. “Milankovitch’s idea that insolation determines the ice ages was right in principle,” says Blatter. “However, science soon recognised that additional feedback effects in the climate system were necessary to explain ice ages. We are now able to name and identify these effects accurately.”

Download video: 

Simulated ice sheet change during the last glacial cycle (mov file, video: Abe-Ouchi et al. 2013)

Abstract: The growth and reduction of Northern Hemisphere ice sheets over the past million years is dominated by an approximately 100,000-year periodicity and a sawtooth pattern (gradual growth and fast termination). Milankovitch theory proposes that summer insolation at high northern latitudes drives the glacial cycles, and statistical tests have demonstrated that the glacial cycles are indeed linked to eccentricity, obliquity and precession cycles. Yet insolation alone cannot explain the strong 100,000-year cycle, suggesting that internal climatic feedbacks may also be at work. Earlier conceptual models, for example, showed that glacial terminations are associated with the build-up of Northern Hemisphere ‘excess ice’, but the physical mechanisms underpinning the 100,000-year cycle remain unclear. Here we show, using comprehensive climate and ice-sheet models, that insolation and internal feedbacks between the climate, the ice sheets and the lithosphere–asthenosphere system explain the 100,000-year periodicity. The responses of equilibrium states of ice sheets to summer insolation show hysteresis, with the shape and position of the hysteresis loop playing a key part in determining the periodicities of glacial cycles. The hysteresis loop of the North American ice sheet is such that after inception of the ice sheet, its mass balance remains mostly positive through several precession cycles, whose amplitudes decrease towards an eccentricity minimum. The larger the ice sheet grows and extends towards lower latitudes, the smaller is the insolation required to make the mass balance negative. Therefore, once a large ice sheet is established, a moderate increase in insolation is sufficient to trigger a negative mass balance, leading to an almost complete retreat of the ice sheet within several thousand years. This fast retreat is governed mainly by rapid ablation due to the lowered surface elevation resulting from delayed isostatic rebound, which is the lithosphere–asthenosphere response. Carbon dioxide is involved, but is not determinative, in the evolution of the 100,000-year glacial cycles.

Since last ice age, warming and cooling have been caused by ocean currents

January 16, 2011

A new paper in Science giving ocean currents in the Atlantic their due (and without finding it necessary to appeal to tales of carbon dioxide). Perhaps the science is not so settled after all!

File:Oceanic gyres.png

The five major ocean-wide gyres — the North Atlantic, South Atlantic, North Pacific, South Pacific, and Indian Ocean gyres. Each is flanked by a strong and narrow “western boundary current,” and a weak and broad “eastern boundary current”: Wikimedia

The Deglacial Evolution of North Atlantic Deep Convection. by D. J. R. Thornalley, S. Barker, W. S. Broecker, H. Elderfield, I. N. McCave.  Science, 2011; 331 (6014): 202 DOI: 10.1126/science.1196812

Science Daily reports:

… Scientists have long suspected that far more severe and longer-lasting cold intervals have been caused by changes to the circulation of the warm Atlantic ocean currents themselves.

Now new research led by Cardiff University, with scientists in the UK and US, reveals that these ocean circulation changes may have been more dramatic than previously thought. The findings, published January 14, 2011 in the journal Science, show that as the last Ice Age came to an end (10,000 — 20,000 years ago) the formation of deep water in the North-East Atlantic repeatedly switched on and off. This caused the climate to warm and cool for centuries at a time.

The circulation of the world’s ocean helps to regulate the global climate. One way it does this is through the transport of heat carried by vast ocean currents, which together form the ‘Great ocean conveyor’. Key to this conveyor is the sinking of water in the North-East Atlantic, a process that causes warm tropical waters to flow northwards in order to replace the sinking water. Europe is kept warmer by this circulation, so that a strong reduction in the rate at which deep water forms can cause widespread cooling of up to 10 degrees Celsius. ….. The new results suggest that the Atlantic ocean is capable of radical changes in how it circulates on time scales as short as a few decades.

Dr Thornalley said: “These insights highlight just how dynamic and sensitive ocean circulation can be. Whilst the circulation of the modern ocean is probably much more stable than it was at the end of the last Ice Age, and therefore much less likely to undergo such dramatic changes, it is important that we keep developing our understanding of the climate system and how it responds when given a push.”

Paper Abstract:

Deepwater formation in the North Atlantic by open-ocean convection is an essential component of the overturning circulation of the Atlantic Ocean, which helps regulate global climate. We use water-column radiocarbon reconstructions to examine changes in northeast Atlantic convection since the Last Glacial Maximum. During cold intervals, we infer a reduction in open-ocean convection and an associated incursion of an extremely radiocarbon (14C)–depleted water mass, interpreted to be Antarctic Intermediate Water. Comparing the timing of deep convection changes in the northeast and northwest Atlantic, we suggest that, despite a strong control on Greenland temperature by northeast Atlantic convection, reduced open-ocean convection in both the northwest and northeast Atlantic is necessary to account for contemporaneous perturbations in atmospheric circulation.

 


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