Econstudentlog

The Ice Age (II)

I really liked the book, recommended if you’re at all interested in this kind of stuff. Below some observations from the book’s second half, and some related links:

“Charles MacLaren, writing in 1842, […] argued that the formation of large ice sheets would result in a fall in sea level as water was taken from the oceans and stored frozen on the land. This insight triggered a new branch of ice age research – sea level change. This topic can get rather complicated because as ice sheets grow, global sea level falls. This is known as eustatic sea level change. As ice sheets increase in size, their weight depresses the crust and relative sea level will rise. This is known as isostatic sea level change. […] It is often quite tricky to differentiate between regional-scale isostatic factors and the global-scale eustatic sea level control.”

“By the late 1870s […] glacial geology had become a serious scholarly pursuit with a rapidly growing literature. […] [In the late 1880s] Carvill Lewis […] put forward the radical suggestion that the [sea] shells at Moel Tryfan and other elevated localities (which provided the most important evidence for the great marine submergence of Britain) were not in situ. Building on the earlier suggestions of Thomas Belt (1832–78) and James Croll, he argued that these materials had been dredged from the sea bed by glacial ice and pushed upslope so that ‘they afford no testimony to the former subsidence of the land’. Together, his recognition of terminal moraines and the reworking of marine shells undermined the key pillars of Lyell’s great marine submergence. This was a crucial step in establishing the primacy of glacial ice over icebergs in the deposition of the drift in Britain. […] By the end of the 1880s, it was the glacial dissenters who formed the eccentric minority. […] In the period leading up to World War One, there was [instead] much debate about whether the ice age involved a single phase of ice sheet growth and freezing climate (the monoglacial theory) or several phases of ice sheet build up and decay separated by warm interglacials (the polyglacial theory).”

“As the Earth rotates about its axis travelling through space in its orbit around the Sun, there are three components that change over time in elegant cycles that are entirely predictable. These are known as eccentricity, precession, and obliquity or ‘stretch, wobble, and roll’ […]. These orbital perturbations are caused by the gravitational pull of the other planets in our Solar System, especially Jupiter. Milankovitch calculated how each of these orbital cycles influenced the amount of solar radiation received at different latitudes over time. These are known as Milankovitch Cycles or Croll–Milankovitch Cycles to reflect the important contribution made by both men. […] The shape of the Earth’s orbit around the Sun is not constant. It changes from an almost circular orbit to one that is mildly elliptical (a slightly stretched circle) […]. This orbital eccentricity operates over a 400,000- and 100,000-year cycle. […] Changes in eccentricity have a relatively minor influence on the total amount of solar radiation reaching the Earth, but they are important for the climate system because they modulate the influence of the precession cycle […]. When eccentricity is high, for example, axial precession has a greater impact on seasonality. […] The Earth is currently tilted at an angle of 23.4° to the plane of its orbit around the Sun. Astronomers refer to this axial tilt as obliquity. This angle is not fixed. It rolls back and forth over a 41,000-year cycle from a tilt of 22.1° to 24.5° and back again […]. Even small changes in tilt can modify the strength of the seasons. With a greater angle of tilt, for example, we can have hotter summers and colder winters. […] Cooler, reduced insolation summers are thought to be a key factor in the initiation of ice sheet growth in the middle and high latitudes because they allow more snow to survive the summer melt season. Slightly warmer winters may also favour ice sheet build-up as greater evaporation from a warmer ocean will increase snowfall over the centres of ice sheet growth. […] The Earth’s axis of rotation is not fixed. It wobbles like a spinning top slowing down. This wobble traces a circle on the celestial sphere […]. At present the Earth’s rotational axis points toward Polaris (the current northern pole star) but in 11,000 years it will point towards another star, Vega. This slow circling motion is known as axial precession and it has important impacts on the Earth’s climate by causing the solstices and equinoxes to move around the Earth’s orbit. In other words, the seasons shift over time. Precession operates over a 19,000- and 23,000-year cycle. This cycle is often referred to as the Precession of the Equinoxes.”

The albedo of a surface is a measure of its ability to reflect solar energy. Darker surfaces tend to absorb most of the incoming solar energy and have low albedos. The albedo of the ocean surface in high latitudes is commonly about 10 per cent — in other words, it absorbs 90 per cent of the incoming solar radiation. In contrast, snow, glacial ice, and sea ice have much higher albedos and can reflect between 50 and 90 per cent of incoming solar energy back into the atmosphere. The elevated albedos of bright frozen surfaces are a key feature of the polar radiation budget. Albedo feedback loops are important over a range of spatial and temporal scales. A cooling climate will increase snow cover on land and the extent of sea ice in the oceans. These high albedo surfaces will then reflect more solar radiation to intensify and sustain the cooling trend, resulting in even more snow and sea ice. This positive feedback can play a major role in the expansion of snow and ice cover and in the initiation of a glacial phase. Such positive feedbacks can also work in reverse when a warming phase melts ice and snow to reveal dark and low albedo surfaces such as peaty soil or bedrock.”

“At the end of the Cretaceous, around 65 million years ago (Ma), lush forests thrived in the Polar Regions and ocean temperatures were much warmer than today. This warm phase continued for the next 10 million years, peaking during the Eocene thermal maximum […]. From that time onwards, however, Earth’s climate began a steady cooling that saw the initiation of widespread glacial conditions, first in Antarctica between 40 and 30 Ma, in Greenland between 20 and 15 Ma, and then in the middle latitudes of the northern hemisphere around 2.5 Ma. […] Over the past 55 million years, a succession of processes driven by tectonics combined to cool our planet. It is difficult to isolate their individual contributions or to be sure about the details of cause and effect over this long period, especially when there are uncertainties in dating and when one considers the complexity of the climate system with its web of internal feedbacks.” [Potential causes which have been highlighted include: The uplift of the Himalayas (leading to increased weathering, leading over geological time to an increased amount of CO2 being sequestered in calcium carbonate deposited on the ocean floor, lowering atmospheric CO2 levels), the isolation of Antarctica which created the Antarctic Circumpolar Current (leading to a cooling of Antarctica), the dry-out of the Mediterranean Sea ~5mya (which significantly lowered salt concentrations in the World Ocean, meaning that sea water froze at a higher temperature), and the formation of the Isthmus of Panama. – US].

“[F]or most of the last 1 million years, large ice sheets were present in the middle latitudes of the northern hemisphere and sea levels were lower than today. Indeed, ‘average conditions’ for the Quaternary Period involve much more ice than present. The interglacial peaks — such as the present Holocene interglacial, with its ice volume minima and high sea level — are the exception rather than the norm. The sea level maximum of the Last Interglacial (MIS 5) is higher than today. It also shows that cold glacial stages (c.80,000 years duration) are much longer than interglacials (c.15,000 years). […] Arctic willow […], the northernmost woody plant on Earth, is found in central European pollen records from the last glacial stage. […] For most of the Quaternary deciduous forests have been absent from most of Europe. […] the interglacial forests of temperate Europe that are so familiar to us today are, in fact, rather atypical when we consider the long view of Quaternary time. Furthermore, if the last glacial period is representative of earlier ones, for much of the Quaternary terrestrial ecosystems were continuously adjusting to a shifting climate.”

“Greenland ice cores typically have very clear banding […] that corresponds to individual years of snow accumulation. This is because the snow that falls in summer under the permanent Arctic sun differs in texture to the snow that falls in winter. The distinctive paired layers can be counted like tree rings to produce a finely resolved chronology with annual and even seasonal resolution. […] Ice accumulation is generally much slower in Antarctica, so the ice core record takes us much further back in time. […] As layers of snow become compacted into ice, air bubbles recording the composition of the atmosphere are sealed in discrete layers. This fossil air can be recovered to establish the changing concentration of greenhouse gases such as carbon dioxide (CO2) and methane (CH4). The ice core record therefore allows climate scientists to explore the processes involved in climate variability over very long timescales. […] By sampling each layer of ice and measuring its oxygen isotope composition, Dansgaard produced an annual record of air temperature for the last 100,000 years. […] Perhaps the most startling outcome of this work was the demonstration that global climate could change extremely rapidly. Dansgaard showed that dramatic shifts in mean air temperature (>10°C) had taken place in less than a decade. These findings were greeted with scepticism and there was much debate about the integrity of the Greenland record, but subsequent work from other drilling sites vindicated all of Dansgaard’s findings. […] The ice core records from Greenland reveal a remarkable sequence of abrupt warming and cooling cycles within the last glacial stage. These are known as Dansgaard–Oeschger (D–O) cycles. […] [A] series of D–O cycles between 65,000 and 10,000 years ago [caused] mean annual air temperatures on the Greenland ice sheet [to be] shifted by as much as 10°C. Twenty-five of these rapid warming events have been identified during the last glacial period. This discovery dispelled the long held notion that glacials were lengthy periods of stable and unremitting cold climate. The ice core record shows very clearly that even the glacial climate flipped back and forth. […] D–O cycles commence with a very rapid warming (between 5 and 10°C) over Greenland followed by a steady cooling […] Deglaciations are rapid because positive feedbacks speed up both the warming trend and ice sheet decay. […] The ice core records heralded a new era in climate science: the study of abrupt climate change. Most sedimentary records of ice age climate change yield relatively low resolution information — a thousand years may be packed into a few centimetres of marine or lake sediment. In contrast, ice cores cover every year. They also retain a greater variety of information about the ice age past than any other archive. We can even detect layers of volcanic ash in the ice and pinpoint the date of ancient eruptions.”

“There are strong thermal gradients in both hemispheres because the low latitudes receive the most solar energy and the poles the least. To redress these imbalances the atmosphere and oceans move heat polewards — this is the basis of the climate system. In the North Atlantic a powerful surface current takes warmth from the tropics to higher latitudes: this is the famous Gulf Stream and its northeastern extension the North Atlantic Drift. Two main forces drive this current: the strong southwesterly winds and the return flow of colder, saltier water known as North Atlantic Deep Water (NADW). The surface current loses much of its heat to air masses that give maritime Europe a moist, temperate climate. Evaporative cooling also increases its salinity so that it begins to sink. As the dense and cold water sinks to the deep ocean to form NADW, it exerts a strong pull on the surface currents to maintain the cycle. It returns south at depths >2,000 m. […] The thermohaline circulation in the North Atlantic was periodically interrupted during Heinrich Events when vast discharges of melting icebergs cooled the ocean surface and reduced its salinity. This shut down the formation of NADW and suppressed the Gulf Stream.”

Links:

Archibald Geikie.
Andrew Ramsay (geologist).
Albrecht Penck. Eduard BrücknerGunz glaciation. Mindel glaciation. Riss glaciation. Würm.
Insolation.
Perihelion and aphelion.
Deep Sea Drilling Project.
Foraminifera.
δ18O. Isotope fractionation.
Marine isotope stage.
Cesare Emiliani.
Nicholas Shackleton.
Brunhes–Matuyama reversal. Geomagnetic reversal. Magnetostratigraphy.
Climate: Long range Investigation, Mapping, and Prediction (CLIMAP).
Uranium–thorium dating. Luminescence dating. Optically stimulated luminescence. Cosmogenic isotope dating.
The role of orbital forcing in the Early-Middle Pleistocene Transition (paper).
European Project for Ice Coring in Antarctica (EPICA).
Younger Dryas.
Lake Agassiz.
Greenland ice core project (GRIP).
J Harlen Bretz. Missoula Floods.
Pleistocene megafauna.

February 25, 2018 - Posted by | Astronomy, Engineering, Geology, History, Paleontology, Physics

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