Past Climates

Information on remotely past climates is obtained by means of considerable ingenuity (Table 15.3). As an example, layers in 7.6 metres of sediment in a lake near sea-level at 42°S in Chile can be individually dated as far back as 43,000 BP, and the type of pollen in a layer indicates either rainforest (with over 4,000 mm/a of rain) or dry conditions, with less than 1,000 mm/a, compared with the current 2,600 mm/a. (It is notable that the Chilean climates seem to have varied in parallel with those in Tasmania and New Zealand, judging by pollen data there also.)

All such evidence needs careful evaluation. Most of the techniques listed in Table 15.3 are only indirect and inaccurate, and even instrument measurements before about 1900 warrant caution because of differences of equipment and site. Also, the evidence may apply only locally; a high rainfall at one latitude does not necessarily imply wetness everywhere, but perhaps a change of ocean circulation

(Section 11.5), a shift in the global circulation pattern (Figure 12.17) or a change in the track of frontal disturbances (Section 13.3). Another example concerns indications of a fall in sea-level. This might mean less ocean water due to the locking up of water in polar ice in cold periods, or could have been due to rising of the land locally. Likewise, evidence of a dwindling glacier (implying a rate of accretion by precipitation less than the rate of loss by sublimation and melting) might mean either reduced precipitation or more warmth or both, or may be associated with an interlude between periodic surging of the ice. Proof of global changes requires the concurrence of several lines of evidence from several locations.

Earliest Climates

Section 1.2 refers to the origin of the atmosphere. Since then, fossil algae in the oldest sedimentary rocks (Table 15.3) and evidence

Table 15.3 Some methods of assessing past climates (BP means 'before present', i.e. before 1950)

Data

Variables measured

Region

Time (years bp)*

Climatic inferencest

Sedimentary rockst

Appearance and fossil content

-

At least 100 million

Rainfall and sea-level

Geomorphic features

Shape and elevation of terrain

-

10 million

Temperature, rainfall, and sea-level

Ocean sediments

Types and isotopes of

Oceans

10 million

Sea-surface temperature

plankton fossils§

Ash and sand

Shallow oceans

200,000

Wind direction

Ice cores||

Depth and isotopes of layers

Antarctica and

200,000

Temperature,

Greenland

precipitation and solar activity

Lake sediments

Varves^I

Midlatitudes

About 100,000

Temperature and rainfall

Pollen type**

Species amount

50°S-70°N

100,000

Temperature, rain

Ancient soil type

Composition

Low and midlatitudes

100,000

Temperature, rain

Glaciers

Length

Global

20,000

Temperature, precipitation

Archaeology

Various

Global

Over 10,000

Various

Boreholesft

Temperature

Various

About 10,000

Temperature

Tree ringstt

Ring width

Mid- to high latitudes

8,000

Temperature, rain

Proxy records§§

Phenology, sailing logs etc.

Europe and Asia

Over 1,000

Various

Instrument measurements

Various

Global

300

Various

* This is the earliest time that can be assessed by means of the given technique. See Note 1 5.L about radiometric dating of evidence, t Sedimentary rocks are those deposited initially as grains of sand, dust, etc., turning over the course of time into shale, etc. t The sea-level is an indirect indication of global temperature, since a low level means low temperatures when water is locked up in polar ice. § Columns of sediment drilled from the sea bed contain shells, including those of microscopic snail-like creatures called foraminifera. Various species flourish near the surface of the sea according to the temperature there. Also, they contain oxygen, which occurs in two isotopes—,60 is most common, ,sO is rare. The ratio of ,80/,60 is greater when the water vapour (which eventually deposits as ice) forms over a warmer sea surface.

| Ice cores display the annual cycle, through variations in acidity and concentrations of dust (there is more acid and dust in winter), especially in Greenland. Extremely cold years whose dates are known from other evidence act as markers in the ice-core record. Cores have been drilled through the Antarctic ice cap as deep as 3 km, indicating conditions over 200,000 years. Shorter cores have also been retrieved from the ice on Mt Kilimanjaro in Africa and some high peaks in the Andes. H Varves are layers seen in lakes fed by meltwater, examined by a technique developed in Sweden. A layer of relatively coarse texture is deposited in spring and summer, from material brought in streams from the melting snow and ice. A thin layer of clay settles each autumn/winter when the melting stops, there is no streamflow and the lake freezes over. The various sediments deposited in one year constitute a varve. The layer's thickness and coarseness are greater if extra warmth increases the rate of melting and hence the streamflow. In one example, a sediment 1 6 m thick contained 250 varves, showing conditions over that number of years. Sediments from various lakes can be correlated by finding similar sequences of thin and thick layers, and overlappinq records provide an extended history of the climate. » The typical habitat of the dominant species of plant suggests the type of climate at the time, tt See Section 3.5

tt The tree ring is wider in a wet year for a tree growing in an arid climate, but for a warm year for one in a cold climate. The temperature may also be inferred from the ratio of ,3C (i.e. the carbon isotope with seven neutrons, Note 15.L) to normal ,2C in the wood. §§ Proxy records are a substitute for climatic data, i.e. historic records that reveal climatic fluctuations. For instance: the wheat price in Europe (since AD 1 200), the height of the Nile in Cairo (since AD 622), the blooming date of cherry trees in Kyoto, Japan (since AD 81 2), the occurrence or sea-ice off Iceland (since AD 860).

Plate 15.1 Evidence on past climates can be derived from the analysis of ice, and the bubbles of air within it, from various depths in Antarctica. The photograph shows a rod of ice being handled in an ice cave, after having been extracted by means of a vertical hollow drill mounted at the surface. The drill may be many hundreds of metres long, allowing the examination of ice of great age.

Plate 15.1 Evidence on past climates can be derived from the analysis of ice, and the bubbles of air within it, from various depths in Antarctica. The photograph shows a rod of ice being handled in an ice cave, after having been extracted by means of a vertical hollow drill mounted at the surface. The drill may be many hundreds of metres long, allowing the examination of ice of great age.

from later times all suggest that the climate of the globe as a whole has never differed by more than a few degrees from what it is nowadays. Despite this, there have been important changes of climate, such as the Ice Ages when ice sheets covered north-western Europe and much of Canada, and the locking up of water as polar ice lowered sea-levels by as much as 150 metres. The Ice Ages occurred around 700 million years before the present (i.e. 700 mBP), 300 mBP, and during the last 2 mBP (Figure 15.7).

The climates of places changed greatly when the Earth's crust shifted and cracked apart, millions of years ago. Australia was in the 'tropics' 340 million years ago (Note 15.J) and grew the lush vegetation which became coal. Africa was over the South Pole in 300 mBP. What is now the middle of Australia was at 50°S by 280 mBP, and cold enough for considerable glaciers. Australia separated from the Indian subcontinent around 130 mBP, and the present Simpson Desert was under water between 110-65 mBP. New Zealand separated off around 50 mBP and Antarctica some 20 million years later, deflecting the ocean currents which affect climates. Australia is currently drifting northwards at a rate of about two centimetres annually.

The global mean temperature around 3.5 mBP was about 3 K higher than now. This period (the 'Pliocene Climatic Optimum') was followed

Figure 15.7 Variations of global climates since 800 mBP.

by a gradual cooling, culminating in the latest Ice Age. Temperatures in the southern hemisphere during the coldest part of this Ice Age were 3-10 K lower than now, depending on location.

Pleistocene

The 1.8 million years since the last Ice Age constitute the Quaternary period, a time too brief for continental drift to be an explanation of considerable variations of climate. The period is divided into the Pleistocene until 10,000 years ago (which more or less coincided with the evolution of human beings since Homo erectus in Africa in 1.9 mBP), and then the more recent Holocene.

There have been several major cold periods called glaciations (Figure 15.7) in the last 900 millenia, i.e. since 900 kBP. Each lasted about a hundred millenia and culminated in a notably cold time, along with discernible minor coolings each forty and twenty-two millenia, attributed to the Milankovic variations of the Sun's orbit (Section 2.2). The glaciations have alternated with warmer interglacialperiods. Some of the switches between the alternative regimes appear to have been remarkably rapid, e.g. 5 K in a hundred years. As far as we can tell, our present era, the Holocene, is just another interglacial period.

The interglacial previous to the current one is called the Eemian interglacial and lasted some 20,000 years, until about 120 kBP. Temperatures in Europe were around 2 K warmer than now. There was more rain, and hippopotamus wallowed in the Thames. The increase of Antarctic temperatures was closely parallelled by an increase of carbon dioxide in the atmosphere from 190 ppm to 280 ppm. Also, Antarctic ice was greatly reduced in area, so that sea-levels were about 6 m higher than nowadays. The abrupt end of that warm period was possibly triggered by a change in the direction of the ocean conveyor belt in the North Atlantic (Figure 11.19).

The subsequent most-recent glaciation involved cooling from 120 kBP until about 20 kBP (Figure 15.8), with a parallel reduction of carbon dioxide, suggesting a greenhouse effect (Sections 1.3 and 2.7). The overall lowering of temperature by about 8 K did not progress steadily, but consisted of six minor coolings, each followed by a smaller warming. The eventual cooling at 20 kBP froze so much water on glaciers and at the poles that the sea-level had fallen to about 120 m below what it is now.

This facilitated human migration across the globe as early as 40 kBP, in particular into South America via a land bridge between Asia and America (now the Bering Strait) and into Australia across what is now the Torres Strait. New Zealand was a single island.

The cooling had several other consequences. Glaciers were probably active in the Snowy Mountains in south-east Australia in 30 kBP, and South Africa seems to have become wetter after 32 kBP. Global-average temperatures were about 5 K lower than at present by 20 kBP, but 6-8 K lower in South America, 5-8 K in the southern half of Australia and about 10 K lower in Antarctica, causing polar ice to extend as far north as 45°S at some longitudes, i.e. to much lower latitudes than nowadays (Figure 11.7). Ice covered much of Tasmania in 18 kBP. Even in the tropics the average cooling was about 4K, or yet more in the tropical highlands, so that the treeline in Papua New Guinea was about 1,000 m lower than now.

At the same time, there was extensive aridity in South Africa and Australia; much of the Simpson Desert of sand dunes in central Australia was formed around 18 kBP and lake levels were low. Most of South America also had less rain. The general aridity during the cold period is attributed to a reduced area of ocean and reduced evaporation from a cooler water surface (Note 4.C).

The Earth's orbit around the Sun gradually changed between 18-10 kBP, causing a 6 per cent decrease of summertime radiation and a similar increase in winter, notably at high latitudes. So polar mean temperatures had increased by about 11 K around 15 kBP. South Africa was initially wetter than now (during 17-15 kBP), but the climate became arid during 12-10 kBP, when there was a global cooling by about 1.5 K for 500 years or so (a period called the 'Younger Dryas', Figure 15.9). Australia was also drier than now (except inland in the south-east), probably on account of a shift southwards in the latitudes of the

Figure 15.8 Changes in the carbon-dioxide concentration in air bubbles within ice from various depths (i.e. times since 160,000 BP) beneath Vostok, Antarctica, along with the temperature during bubble formation. The temperature is estimated from the ratio of the amount of 'heavy' oxygen (with a molecular weight of 18) to that of normal oxygen, with a molecular weight of 16 (Note 15.L). The ratio depends on the global sea-surface temperature at the time the bubbles were trapped.

Figure 15.8 Changes in the carbon-dioxide concentration in air bubbles within ice from various depths (i.e. times since 160,000 BP) beneath Vostok, Antarctica, along with the temperature during bubble formation. The temperature is estimated from the ratio of the amount of 'heavy' oxygen (with a molecular weight of 18) to that of normal oxygen, with a molecular weight of 16 (Note 15.L). The ratio depends on the global sea-surface temperature at the time the bubbles were trapped.

subtropical anticyclones (Section 13.6) and the midlatitude westerlies, in addition to weaker year-end monsoons in northern Australia. The sea was around 90 metres below present levels in 15

Holocene

The relatively warm times since 10 kBP are known as the Holocene, the era of humanity's domination. The warming led to a rise of sea-level, so that only the northern tip of Queensland was joined to Papua New Guinea by 8 kBP. At that time, New Zealand was wetter on the west side than now, while the east side was drier. Lake levels worldwide were high during 8-6 kBP, South Africa was relatively wet, Australia became wetter than now and global temperatures continued to rise. The time of highest global temperatures is known as the Altithermal (or Climatic Optimum). It occurred during 5-6 kBP in the northern hemisphere, but may have been a thousand years or so earlier in the southern. Temperatures during the Altithermal were about 2 K higher than now in New Zealand and Papua New Guinea, for instance, but tropical sea-surface temperatures were no higher than now. (Climate

Altithermal

Last

Little

/ \

Glacial

/

10000 5000 0

years before present

10000 5000 0

years before present

Figure 15.9 Greatly generalised variation of global climates during the Holocene, showing the Younger Dryas cold spell between 11—10 kBP, and a general warming after 10 kBP, except for four cool spells of which the last was the Little Ice Age.

changes have generally been greatest at high latitudes.) The sea had risen to 3-10 m below the level nowadays.

Western Australia was wetter than at present in the later part of the Altithermal, most lakes in Australia were relatively full, and rainforest replaced drier vegetation over much of northern Australia. Similarly in New Zealand, the southeast coast of South America and the South African coast opposite Madagascar. Simultaneously, there was dry warmth in Argentina and parts of south-east Australia.

There was cooling after the Altithermal until about 3,000 BP, and then temperatures were about 2 K lower than now for the next two millenia, at least in south-east Australia.

Annual layers within 160 m of ice at 5,670 m on top of Quelccaya in South America (Section 3.2) indicate little of the normal November-April deposition of snow around AD 700, and during AD 1050-1500, but wet conditions from AD 1500-1700 followed by dryness till 1850. Conditions were colder than now during the period 1540-1880, but then there was an abrupt warming within a couple of years.

A worldwide cooling occurred between about 1450 and 1850, called the Little Ice Age (LIA). It appears to have been the fourth of similar cool periods, spaced almost equally apart since the last glacial period (Figure 15.9). The LIA happened to include the time of the Maunder Minimum, when there were hardly any sunspots, and the end of the LIA coincided with the termination of the second period of few sunspots, the Dalton Minimum (Figure 2.8). There was an increased difference between summer and winter on top of Quelccaya, i.e. less moisture in the colder air of winter along with more melting in summer. Tree rings in Chile confirm that the LIA occurred there at about the same time as in the northern hemisphere, though the evidence of cooling is less striking. New Zealand stalagmites from that period indicate temperatures only about 0.7 K cooler than now, and glaciers there were longer than usual. Some parts of the world were drier and some wetter during this time of cooling. Nowhere was the LIA continuously cold: it was simply a period which included many cold episodes.

The Latest Two Millenia

Relative warmth prevailed in Europe between AD 900-1200, with temperatures a fraction of a degree above those now. Stalagmites in New Zealand, caves dating from AD 1200 indicate an average temperature of 10.2°C, compared with 9.4°C now, and tree rings in Tasmania also indicate this Medieval Warm Period (or 'Little Climatic Optimum').

Indications

This brief survey of the patchy and confusing evidence on past climates leads to the following provisional summary:

1 The temperature of the globe as a whole has varied within at most 10 K, for millions of years. Climates during recent centuries are amongst the warmest during the Quaternary period.

2 However, conditions at any spot have varied considerably. The climate of a place is not fixed, if we look beyond the experience of a single generation.

3 Temperatures (and by implication rainfall) can change rapidly from those of an Ice Age to those of an interglacial, or vice versa. Ice-core data from Greenland imply a switch in less than a hundred years, or perhaps only thirty. There is much to be explained about this process.

4 Changes of climate have not been uniform around the globe. They appear to have been greater at high latitudes than at low, and in the northern than the southern hemisphere.

5 Cool times tend to mean dry times in most places, presumably because of less evaporation from the oceans (Section 4.2). Thus, buried desert sand-dunes in tropical Africa and Australia extend beneath the present sea-level, so they were formed when the sea was low during a glaciation but when the land was dry. Nevertheless, the relationshp between aridity and temperature is not simple; inland dryness could also arise in warm periods because of increased evaporation from the ground.

Possible Causes of Change

There appear to be several factors causing alterations of climate, all acting together:

1 There is the random element inherent in the atmosphere and in the occurrence of volcanic eruptions.

2 There are the regular rhythms due to the Earth's spin and its orbit around the Sun, which account for daily and seasonal changes, modified by persistence. Milankovic variations of solar radiation (Section 2.2) are unlikely by themselves to cause much climate alteration but they may have triggered positive feedbacks sufficient to cause change (e.g. Note 2.J and Note 7.A). There are also other processes not quite so regular, like sunspot fluctuations (Section 2.2), the rotation of oceanic gyres (Section 11.5), the Southern Oscillation (Section 12.7) and the continuous processes of mountain building and erosion. In addition, there are occasional surges of enormous amounts of ice from the Antarctic glaciers into the sea, which might account for sudden coolings in the southern hemisphere, lasting for years.

3 There may be alternative patterns of energy flows between the atmosphere, hydrosphere, cryosphere and biosphere, each stable within limits but triggered into another regime by a sufficient perturbation. The Walker circulation (Figure 12.17) is an example of a system with two alternative, almost self-maintain-ing, sets of wind and ocean temperature conditions. If this analogy is applicable to the atmosphere as a whole, it would be useful to know how great an excursion from the normal can be accommodated without setting off a catastrophic change, such as a run-away greenhouse effect due to more of the Earth's oceans being converted to water vapour, which is a greenhouse gas.

4 In addition to chance, rhythmic processes and the possible onset of an abrupt acceleration of existing trends as a result of positive feedbacks, there is now another factor affecting climatic change. This is the influence of human activities, such as urban heating (Section 3.7), alteration of the surface albedo and roughness by deforestation and agriculture, and modification of the air's chemical composition by urban or industrial air pollution (Section 14.7).

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