快猫短视频

Why do we have different weather in different parts of the world, and at different times of year? The tilt of the spinning Earth, and the distribution of land and sea, are chiefly responsible

CLIMATE change has become a major scientific issue of the 1990s, with growing concern about the prospect of global warming as we move into the 21st century. But what is changing and how? Before we can even begin to answer these questions we need to know how climate works.

The simplest definition of climate is the 鈥渁verage weather鈥� of a region.

Some regions experience much the same temperature throughout the whole year, others have hot summers and cold winters; some regions are likely to experience rainfall in any month, while others tend to have well-defined wet seasons; and so on.

The energy that drives all these weather patterns comes from the Sun, and it is absorbed on Earth mainly in the tropics, near the equator.

Some of the energy is then distributed to higher latitudes by causing the atmosphere and oceans to circulate. These circulation patterns are strongly affected by the geographical distribution of land and sea, the tilt of the Earth, and the way it spins on its axis.

Simple circulation

Earth and Sun

THE EARTH is a roughly spherical planet orbiting the Sun in a roughly elliptical orbit once every year at an average distance of 150 million kilometres. Although the orbit is slightly elliptical, and we are a little closer to the Sun in some months than in others, this is not the cause of seasonal variations in weather. If it were, the entire globe would experience winter at the same time. In fact, the closest approach to the Sun 鈥� 147 million kilometres 鈥� occurs in the northern hemisphere winter on 3 January.

The farthest distance from the Sun 鈥� 153 million kilometres 鈥� occurs in the northern hemisphere summer on 3 July.

If our planet were a uniform sphere, spinning with its axis perpendicular to a line joining the centre of the Earth to the centre of the Sun, it would experience a very simple pattern of atmospheric circulation. This idealised pattern is the basis for understanding the actual circulation of the atmosphere. It is driven by the difference in solar heating between the tropics and the poles.

Because the curvature of the Earth makes the planet鈥檚 surface slope away from the Sun on either side of the equator, the amount of incoming solar energy falling on a square metre of the Earth鈥檚 surface at the equator is spread over a successively larger area at successively greater distances from the equator. So high latitudes are cooler than low latitudes. In exactly the same way, the Sun鈥檚 warming effect is less in the early morning and late evening (when its rays strike the ground at a shallow angle) than at noon, when the Sun is high overhead.

Incoming solar energy is chiefly at visible wavelengths, which are not absorbed in the Earth鈥檚 atmosphere. When this radiation warms the surface of the Earth, the surface radiates energy in its turn, but at longer wavelengths, chiefly in the infrared. Much of this energy is absorbed in the lower atmosphere, which is warmed as a result 鈥� by the so-called 鈥済reenhouse effect鈥� (see Inside Science No.13). This initiates convection.

Hot air for cold

Hadley cell

WARM AIR rising near the equator cools and moves outward at high altitudes before descending again in the sub-tropics at around 30 degrees of latitude, on either side of the equator. Winds at low altitudes return air towards the equator to replace the rising air, completing a circulation pattern on either side known as the Hadley cell, named after the 18th century British pioneer of meteorology, George Hadley.

Polewards of the region dominated by Hadley-type circulation, the pattern of atmospheric convection is more complicated, but as a rough guide it can be thought of as two further cells in each hemisphere.

Overall, warm air is transferred polewards, and is replaced by cold air moving towards the equator. Rising air is associated with low pressure at the surface, and produces rain as the rising air cools; descending air is associated with high pressure at the surface, and is dry. Even such a simple picture of atmospheric circulation can explain broad features of climate, such as the warm, wet tropics and the desert regions just outside the tropics. But even on this idealised picture, because of the rotation of the Earth, winds at low altitudes do not blow due north-south.

Each point on the surface of the Earth completes one rotation in 24 hours.

Near the poles, this involves describing a tiny circle at a very slow linear speed. But at the equator it involves a speed of 470 metres per second 鈥� and at a latitude of 60 degrees a linear speed of 224 metres per second, always from west to east. Air that originates near the equator, and is carried polewards by convection, carries with it a momentum of the high west-east speed it started out with. And when it returns to the surface, it is moving eastward faster than it should be at that latitude. So the winds blowing polewards from the equatorial region are deflected to the east. This is why the prevailing pattern of weather at the latitude of Britain or New Zealand, for example, brings weather systems from the west.

To a person on the surface of the Earth, it is as if there were a force, known as the Coriolis force, pushing winds from west to east, and increasing the further the winds move from the equator. Like centrifugal force, the effect is perfectly real to anyone in the rotating 鈥渇rame of reference鈥�; but because it is entirely caused by rotation, the Coriolis force is sometimes called, confusingly, a 鈥渇ictitious鈥� force.

Similarly, air returning towards the equator at low level is imprinted with the linear speed associated with the rotation at higher latitudes. In effect, these prevailing winds 鈥� the trade winds 鈥� are being overtaken by the surface of the rotating Earth as they move to lower latitudes, so that instead of blowing at right angles to the equator they blow diagonally to the west.

The equator they blow towards, however, is the meteorological equator, which follows the apparent movement of the Sun north and south of the equator with the seasons. This region, also known as the Inter-tropical convergence zone (ITCZ) is where the northerly and southerly winds meet. The seasonal variations arise because the Earth is tilted at 23.5掳 out of the perpendicular in its orbit around the Sun. The hemisphere that is tilted towards the Sun is warmed more intensely both because the Sun rises higher in the sky and because the Sun is above the horizon for more than 12 hours each day. In the opposite hemisphere, the noonday Sun is lower on the horizon at corresponding latitudes, and there are more than 12 hours of night. Because the Earth maintains its orientation in space as it orbits the Sun, northern and southern hemispheres each experience summer and winter in turn.

Between latitude 23.5掳N (the Tropic of Cancer) and latitude 23.5掳S

(the Tropic of Capricorn) the Sun will be directly overhead at noon on at

least one day in the year. Outside the tropics, this never happens. The Sun is

overhead at the Tropic of Cancer at noon on 22-23 June each year, and overhead

at the Tropic of Capricorn on 22-23 December. Halfway between these dates, the

noonday Sun is overhead at the equator, and the whole planet experiences 12

hours of day and 12 hours of night. The atmospheric circulation pattern has to

adjust constantly to the resulting changes in the energy distribution over the

planet. (The major wind belts shift with the migrating ITCZ.)

Further complications to the simple circulation pattern are caused by the

presence of mountains deflecting low level winds. Different ways in which

oceans, land and icecaps respond to the warming influence of the Sun are also

to be accounted for. But the other major influence on climate is the

circulation of the oceans.

All at sea

Gyrating currents

THE GREAT surface ocean currents are generated by the influence of the

prevailing winds, blowing steadily across the sea. Like the atmospheric

circulation, they are influenced by the rotation of the Earth; but unlike the

winds they are completely unable to cross land masses. So the dominant pattern

of surface ocean currents is a roughly circular flow, or gyre, within each of

the great ocean basins. The sweep of these currents is clockwise in the

northern hemisphere and anticlockwise in the south. The main exception to this

pattern is the circumpolar current that flows around Antarctica from west to

east; there is no equivalent current in the northern hemisphere because of the

intervening land masses.

Within the circulation of the gyres, water piles up into a dome, so that in

the Sargasso Sea, for example, sea level is about a metre higher than on the

nearest coast. The effect of the rotation of the Earth is as if there were a

force pushing everything in the ocean basin westward, and this piles water up

on the western edge of ocean basins, so that sea level in the Caribbean, for

example, is slightly higher than on the Pacific side of the Panama Canal. Like

water slopping in a bucket, as the Earth rotates from west to east the water

piles up at the 鈥渂ack鈥� of the ocean basins. This effect squeezes the gyres up

against the western sides of the ocean basins, producing narrow, fast-flowing

currents such as the Gulf Stream. By contrast, the return flow on the eastern

sides of the basins is slower and more diffuse.

The Gulf Stream in particular transports heat northward and then eastward

across the North Atlantic ocean very efficiently. This is why the British

Isles have milder winters than most regions at comparable latitudes, such as

southern Argentina. Other ocean currents are less reliable than the Gulf

Stream. The atmospheric-oceanic circulation over the Pacific basin seems to

be able to exist in either of two states, one with warm surface water in the

west and cold surface water in the east, and one with the pattern reversed. In

either case, the warm region of ocean surface produces a convection pattern

with winds blowing in to the warm region from the other side of the ocean.

These winds push warm surface water in to the warm region, exposing colder

deep water behind them and maintaining the pattern. But, for reasons that are

still not fully understood, from time to time the pattern breaks down and

reverses. This brings pronounced changes to the weather of regions as far

apart as Australia and South America.

The local phenomenon of a warming of the ocean surface near South America

is called El Ni帽o; it is related to an overall pattern of changes, which

occurs roughly every two years, and is called the Southern Oscillation. There

is some evidence that the Southern Oscillation influences weather on a global

scale, including the intensity of the African monsoon.

Into the depths

The oceanic conveyor belt

IN ADDITION to the influence of its surface currents, the ocean influences

climate through its own pattern of convection. Like the atmosphere, the ocean

is warmed at the surface. But whereas the surface of the Earth is at the

bottom of the atmosphere, it is at the top of the ocean. So the water that is

warmed by the Sun in the tropics is already at the top of the ocean, and

cannot rise by convection. Instead, oceanic convection is driven from the

polar regions, where cold, salty water sinks down into the depths and makes

its way towards the equator.

The densest water forms in the region near Antarctica. There, seawater

freezes to form ice at a temperature of around -1.9 degrees Celsius; the ice

is freshwater, so the seawater left behind is more salty and therefore more

dense. It flows down the continental shelf to form a great current known as

the Antarctic Bottom Water. This cold, dense water sweeps around Antarctica at

a depth of about 4 kilometres, and spreads branches out into the deep basins

of the Atlantic, Pacific and Indian Oceans. These deep, cold currents then

link into a network of currents carrying water around the world.

Wallace Broecker, of the Lamont-Doherty Geological Observatory, in New

York, likens the system to an 鈥渙ceanic conveyor belt鈥�, which at present works

to the benefit of the North Atlantic. The surface currents that bring warm

water to the North Atlantic can be traced all the way back to the Indian and

Pacific Oceans. The water gives up its heat to cold winds blowing across the

North Atlantic from Canada, and sinks, starting the upside down convection of

the deep ocean current systems. The amount of heat it gives up is nearly one

third as much, each year, as the region receives from the Sun; this mainly

benefits Europe, which is downwind of the ocean. Cold, deep currents then

carry the circulation back into the Indian and Pacific Oceans, where the water

rises once more (pushed by the flow behind it) and warms as it begins the long

journey back to the North Atlantic.

The pattern is maintained by salt. Because the conveyor operates in this

way, the North Atlantic is warmer than the North Pacific, so there is

proportionately more evaporation there. The water left behind by the

evaporation has a higher concentration of salt, so it is denser, which

encourages it to sink. The resulting cold, deep flow starts out as the North

Atlantic Deep Water Current, an oceanic 鈥渞iver鈥� that carries a flow of water

twenty times that of all the rivers of the world put together. Eventually, it

carries salt into the Pacific where it is diluted, reducing the density of the

water in the flow.

The whole pattern is self-sustaining. Furthermore, the pattern would be

equally self-stabilising if the conveyor were to run in reverse, warming the

North Pacific instead of the North Atlantic. But if the conveyor belt were to

shut down, the land around the North Atlantic would be cooler by about

6掳颁.

Climatologists have found evidence of sudden 鈥渇lips鈥� in climate during the

late stages of the most recent Ice Age, which ended about 15 000 years ago.

There were several fluctuations in temperature around the North Atlantic, and

the last cold spell of the Ice Age, known as Younger Dryas, ended 10 720 years

ago. Northern Greenland then recorded a rise in temperature of 7掳C, but

the same region also warmed by 4掳C during the 1920s.

There is a suggestion 鈥� no more than a suggestion, at present 鈥� that such

climate flips from one stable state to another may be associated with

reversals of the oceanic conveyor belt, like the Southern Oscillation but on a

much grander scale. And Broecker has expressed concern that such a flip could

happen again, perhaps as a result of human interference with natural climate

systems by adding to the burden of greenhouse gases in the atmosphere.

Keeping cool: Ice Ages

Effects of continental drift

THE THIRD physical feature determining climate now, after the circulation

of the air and sea, is the geography of the globe. Stretching a point

slightly, it is even possible to talk about the 鈥渃irculation鈥� of the land, as

well as the air and sea, because the geography is not fixed but changes as

continents drift about the surface of the Earth through the effects of plate

tectonics (see Inside Science No.6). The key influence of the land on global

climate today is its role in obstructing the flow of water to the poles.

Throughout most of Earth history, water from the tropics has been able to

penetrate to polar latitudes, keeping even the polar regions free from ice.

But the polar ice cover of the present-day climate is a good reflector of

incoming solar energy. Albedo, a measure of the reflecting power of a surface,

is as much as 80 per cent at the polar icecap as compared with 40 per cent for

the Earth as a whole. Partly because of this, and partly because the ice cover

acts as an insulating lid on the warmer water below, removing the Arctic ice

cap would have a dramatic impact on climate.

Hermann Flohn, working for the International Institute for Applied Systems

Analysis, in Austria, suggests that to melt the Arctic ice it would be

necessary to warm the world by 4掳C. This increase in temperature at the

edge of the floating ice cap would make it melt back, exposing more dark water

which could absorb more solar energy and speed the meltback, until all the ice

had gone. The average surface temperature of the air over the Arctic today is

about -34掳C. But, says Flohn, removing the ice cover 鈥� initially due to a

warming of just 4掳C 鈥� would increase the temperature of wintertime surface

air over the Arctic to +4掳C, a rise of 38掳C compared with the

present day.

Ice ages have been rare on Earth during most of its 4.5 billion year

history. Land has only rarely drifted over one or the other of the poles,

cutting off the flow of warm water from the tropics and providing a base on

which falling snow can build up to make ice sheets. Antarctica today provides

the archetypal example of such a rare geographical event. The cooling and

freezing of Antarctica, associated with changes in ocean currents about that

time, began some 40 million years ago, and a large Antarctic icecap had formed

by 15 million years ago.

But it is much more difficult to create the conditions required to freeze

the ocean at the pole, and floating icecaps like that of the Arctic Ocean

today are far more rare than glaciated continents. Even 3 million years ago,

the Arctic Ocean was still free from ice. The 鈥減ermanent鈥� northern icecap that

seems so natural to us may have appeared as recently as between 3 and 5

million years ago, as the northern continents jockeyed into position around

the polar sea, cutting off the northward flow of warm water.

The pattern of climate that has existed for the past few million years,

with ice at both poles, may never have occurred before. As a result, the Earth

has been in a frozen state, an Ice Epoch, for about three million years.

During that Ice Epoch, the glaciers have repeatedly advanced as full Ice Ages

have developed and retreated slightly during warmer intervals, known as

interglacials. These cyclic variations in climate are linked with regular

changes in the orbital geometry of the Earth, changing the balance of solar

heating between the seasons. In round terms, each Ice Age lasts for about 100

000 years and each interglacial lasts for some 15 000 years. We live in an

interglacial that began about 15 000 years ago.

Climate zones temperate and intemperate

Equatorial weather is always the same 鈥� reliably hot and wet. Outside

this region, at latitudes around 20掳 where the Hadley circulation produces

descending air, the climate is equally predictable 鈥� hot and dry. Descending

air gets hot because the pressure increases as it piles onto the surface

below, in much the same way that the air inside a bicycle pump becomes hot

when it is compressed.

Between the dry desert regions and the wet tropical regions, the weather

is less consistent throughout the year, because the latitudes of desert and

tropical rain shift with the seasons. So some regions, such as northwest

India, are dry for much of the year but experience tropical rainfall during

the monsoon season.

Polewards from the hot deserts, the weather pattern is dominated by

westerly winds, which bring rainfall (especially in winter) to regions on the

western sides of continents. This is the 鈥渕editerranean鈥� type of climate,

found in the Mediterranean itself, California and parts of Australia. Further

north and south lie the temperate climate zones and the continental climate

regions. Cold, polar weather completes the broad picture at the highest

latitudes.

The temperate zones, between the polar cold and the mediterranean

warmth, experience variations in climate analogous to the variations in the

monsoon regions, with marked variations in temperature.

Further reading

A good basic text is Everyday Meteorology, by A. A. Miller and M. Parry

(Hutchinson, revised edition 1975). For more about ice ages and the influence

of the changing geography of the Earth on its climate, see Children of the

Ice, by John and Mary Gribbin (Basil Blackwell, 1990). Climate is put in its

geophysical context in Chapter 13 of the Hutchinson Encyclopedia of the Earth,

edited by Peter Smith (1985), and climate zones are described in more detail

in The Weather Book, by Ralph Hardy, Peter Wright, John Gribbin and John

Kington (Mermaid Books, 1985).