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p. 11. What is climate?free

  • Mark Maslin

Abstract

The climate we live in controls where and when we get our food, what we wear, the houses we live in, and what diseases we might catch. ‘What is climate?’ shows how crucial climate is to human life and explains what influences it. A key cause is that the Equator receives more of the sun's energy than the poles do. About a third of the solar energy we receive is reflected back into space. Climate is mainly controlled by two basic relationships between the Earth and the sun: the tilt of the Earth's axis of rotation, which causes the seasons; and the daily rotation of the Earth.

Introduction

Climate affects everything we do in life, from the clothes we wear to the diseases we catch. This is because as humans we only feel comfortable within a very narrow range of temperature and humidity. This comfort zone ranges from about 20˚C to 26˚C and from 20 to 75 per cent relative humidity (see Figure 1). However, we live almost everywhere in the world, meaning that conditions are frequently outside this comfort zone, and we have learnt to adapt our clothing and dwellings to maintain our comfort. So while you may think the clothes you have hanging in your wardrobe simply reflect your fashion taste or lack of, in reality they reflect the climate in which you live and how it changes throughout the year. So you have a thick padded coat for a Canadian winter and a short-sleeved shirt for a business meeting in Rio. Our wardrobes also give hints about where we like to take our holidays. If you are a budding Polar explorer then there will very warm Arctic clothes hanging up—if you love sunning yourself on the beach, then there will be shorts or a bikini instead.

1. Human comfort and climate

Our houses are also built with a clear understanding of local climate. In England almost all houses have central heating as the outside temperature is usually below 20˚C, but few have air conditioning as temperatures rarely exceed 26˚C. On the other p. 2hand, in Australia most houses have air conditioning but rarely central heating. Climate also affects the structure of our cities and how transport systems around the world operate. In Houston, Texas, there is a network of 7 miles of underground tunnels connecting all the major downtown buildings; this is fully climate controlled and links 95 heavily populated city blocks. People use the tunnel when it is raining or hot outside, because for at least 5 months of the year the average temperature in Houston is above p. 330˚C. Similarly there are underground malls in Canada to avoid the problems of heavy snow and extreme cold.

Climate controls where and when we get our food, because agriculture is controlled by rainfall, frost, and snow, and by how long the growing season is, which includes both the amount of sunlight and the length of the warm season. So in a simplified way, rice is grown where it is warm and very wet, while wheat can grow in much more temperate climes. The climate can also affect the quality of our food, for example it is well known that the very best vintages of French wine are produced when there are a few short sharp frosts during the winter, which harden the vines, producing excellent grapes. Farmers can also ‘help’ the local climate, for example by growing tomatoes in a greenhouse or by irrigating the land to provide a more constant supply of water.

Climate also influences where there will be extreme weather events such as heatwaves, droughts, floods, and storms. However in many cases our perception of extreme events is determined by local conditions, so for example in 2003 northern Europe was hit with a ‘heatwave’ and 100˚F (37.8˚C) was recorded for the first time ever in England. However in countries of the tropics a heatwave would not be recorded until temperatures were above 45˚C. Climate also has a large effect on our health, as many diseases are temperature and humidity controlled. For example incidences of influenza, commonly called the flu, reach a peak in winter. Since the Northern and Southern Hemispheres have winter at different times of the year, there are actually two different flu seasons globally each year. The influenza virus migrates between the two hemispheres after each winter, giving us time to produce new vaccinations based on the new strain of flu that has appeared in the previous six months in the other hemisphere. There have been many arguments about why flu is climate controlled and the theory is that during cold dry conditions the virus can survive on surfaces longer and so be more easily transmitted between people. Another suggestion is that vitamin D might provide some resistance or p. 4immunity to the virus. Hence in winter and during the tropical rainy season, when people stay indoors, away from the sun, their vitamin D levels fall and incidences of influenza increase.

Hot and cold Earth

The climate of our planet is caused by the Equator of the Earth receiving more of the sun's energy than the poles. If you imagine the Earth is a giant ball, the closest point to the sun is the middle or the Equator. The Equator is where the sun is most often directly overhead and it is here that the Earth receives the most energy. As you move further north or south away from the Equator, the surface of the Earth curves away from the sun, increasing the angle of the surface of the Earth relative to the sun. This means the sun's energy is spread over a larger area, and thus causes less warming. If we lived on a flat disc we would get much more energy from the p. 5sun—about 1,370 Watts per square metre (W/m2)—instead the planet surface averages about 343 W/m2 (Figure 2) due to its curved nature. The Earth also receives a very small fraction of the energy pumped out of the sun. If you consider how small the Earth is compared with the sun, for every Watt we receive from the sun, it emits 2 billion Watts. This is why in many science fiction novels the authors imagine a strip or even a sphere around a star to collect all that energy that is simply being lost into space.

2. Solar energy distributed over a sphere

About one-third of the solar energy we receive is reflected straight back into space. This is because of ‘albedo’, which means how reflective is a surface. So, for example, white clouds and snow have a very high albedo and reflect almost all of the sunlight that falls on them, while darker surfaces such as the oceans, grassland, and rainforest absorb a lot more energy. Not only do the poles receive less energy than the Equator, but they also lose more energy back into space (Figure 3): the white snow and ice in the Arctic and Antarctic p. 6have a high albedo and bounce a lot of the sun energy back into space. On the other hand, the darker much less reflective vegetation at lower latitudes absorb a lot more energy. These two processes working together mean that the tropics are hot and the poles are very cold. Nature hates this sort of energy imbalance, so energy, in the form of heat, is transported by both the atmosphere and oceans from the Equator to both poles, and this affects the climate.

3. Energy transfer away from the Equator driven by angle of solar radiation

Earth in space

Our climate is controlled by two fundamental facts about the relationship between the Earth and the sun. The first is the tilt of the Earth's axis of rotation, which causes the seasons. The second factor is the daily rotation of the Earth that provides us with night and day and drives the circulation of both the atmosphere and the oceans.

The Earth's axis of rotation is tilted at an angle of 23.5˚ and results in a seasonal difference in the amount of energy received by each hemisphere throughout the year. The seasonal changes are by far the largest effect on climate. It is amazing to think that if the Earth were not tilted and stood straight up on its axis then we would not have spring, summer, autumn, and winter. We would not have the massive change in vegetation in the temperate latitudes and we would not have the monsoon and hurricane seasons in the tropics. The reason for the seasons is the change in the angle of the sunlight hitting the Earth through the year. If we take 21 December as an example, the Earth's axis is leaning away from the sun, so the sunlight hitting the Northern Hemisphere is at a greater angle and spreading its energy over a wider area. Moreover the lean is so great that in the Arctic the sunlight cannot even reach the surface and this produces 24 hours of darkness and winter in the Northern Hemisphere. However, everything is opposite in the Southern Hemisphere, since it is then leaning towards the sun and hence the sunlight is more directly overhead. This means that Antarctica is bathed in 24 hours of p. 7sunlight and people in Australia have Christmas dinner on the beach, while topping up their tan. As the Earth moves round the sun, taking about 365.25 days (hence the leap year every fourth year), the angle of the axis stays in the same place. Hence when it comes to June the Earth's axis is leaning towards the sun, so the Northern Hemisphere has lots of direct sunlight and thus summer, while the Southern Hemisphere is shielded from the sunlight and is plunged into winter.

p. 8If we follow the sun through a year we can see how this tilt affects the Earth through the seasons. If we start at 21 June the sun is overhead at midday at the Tropic of Cancer (23.4˚N), the northern summer solstice. The angle of the sun moves southward until 21 September when it is overhead at midday over the Equator, the equinox or autumn equinox in the Northern Hemisphere. The sun appears to continue southward and on 21 December it is overhead at midday at the Tropic of Capricorn (23.4˚S) the southern summer solstice. The sun then appears to move northward until it is directly overhead at midday at the Equator on the 21 March the equinox or spring equinox in the Northern Hemisphere and so the cycle continues (see Figure 4).

4. Solstice and equinox caused by the tilt of the Earth

The seasons signal by far the most dramatic change in our climate; if we take for example New York, winter temperatures can be as low as -20˚C while summer temperatures can be over 35˚C—a 55˚C temperature difference. Moreover as we will find out the seasons are one of the major reasons for storms.

Moving heat around the Earth

The second big factor affecting the climate of the Earth is its daily rotation. First this plunges the Earth in and out of darkness causing massive changes in diurnal temperature. For example the Sahara desert during summer can have daytime temperatures of over 38˚C (100˚F) and then nighttime lows of 5˚C (40˚F); while Hong Kong has a diurnal temperature range of little more than 4˚C (7˚F). Depending on the season, different areas also get varying amounts of daylight. The days can vary from 24 hours’ daylight to 24 hours’ darkness at the poles to around 12 hours’ sunlight every day at the Equator. This change in the daylight compounds the seasonal contrasts, because not only during summer do you get more direct ‘overhead’ sunlight but also you get it for much longer.

But the spinning of the Earth also makes the transport of heat away from the Equator more complicated. This is because the p. 9spinning of the Earth makes everything else including the atmosphere and oceans turn. The simple rule is that rotation of the Earth causes the air and ocean currents to be pushed to the right of the direction they are travelling in the Northern Hemisphere and to the left of the direction they are travelling in the Southern Hemisphere. This deflection is called the Coriolis effect and its strength increases the further you go towards the poles.

An everyday example of this, which is always quoted, is the way water flows down a plughole or a toilet. In the Northern Hemisphere water is said to flow clockwise down the plughole while in the Southern Hemisphere it is anti-clockwise. However, I hate to tell you that the direction the water drains out of your bath or toilet is not related to the Coriolis effect or to the rotation of the Earth. Moreover no consistent difference in rotation direction between toilets in the Northern and Southern Hemispheres has been observed. This is because the Coriolis effect has such a small influence compared with any residual movement of the water and the effect of the shape of the container. This also mean the wonderful cottage industry of communities living on the Equator showing tourists the Coriolis effect is simply done by a sleight of hand. For example in Kenya there are big signs up telling you when you are crossing the Equator; if you care to stop at the road side locals will happily pour water from a bucket into a large funnel and seeming to demonstrate clearly that it goes a different way round when you are standing on one side of the sign than when you are standing on the other. However, this change is all in the wrist and how the water is poured in, affecting which way it goes round. Still, even though it is completely fake, I love these demonstrations as it means loads of locals and tourists get to hear about the Coriolis effect!

Back to climate, so why do the ocean currents and winds have this deflection? Imagine firing a missile from the Equator directly north. Because the missile was fired from the Earth which is spinning eastward, the missile is also moving east. As the Earth p. 10spins the Equator has to move fast through space to keep up with the rest, as it is the widest part of the Earth. As you go further north or south away from the Equator the surface of the Earth curves in, so it does not have to move as fast to keep up with the Equator. So in one day the Equator must move round 40,074 km (the diameter of the Earth) a speed of 1,670 km/hour, while the Tropic of Cancer (23.4˚N) has to move 36,750 km, with a speed of 1,530 km/hour, and the Arctic circle (66.6˚N) has to move 17,662 km so has a speed of 736 km/hour. At the North Pole there is no relative movement at all so the speed there is 0 km/hour. A practical demonstration of this is if you hold hands with a friend and stand in the same place while spining them around, they will travel much faster than you do. Therefore the missile, fired from the Equator, has the faster eastward speed of the Equator; as it moves northward towards the Tropic of Cancer, the surface of the Earth is not moving as fast eastward as the missile. This gives the appearance that the missile is moving northeast as it is moving faster eastward than the area it is moving into. Of course the closer you get to the poles the greater this difference in speeds so the greater the deflection to the east (see Figure 5).

5. Coriolis effect due to relative movement of the surface of the Earth

Summary

The climate system is very straightforward. It is controlled by the different amount of solar energy received at the Equator and the poles. Climate is simply the redistribution of energy to undo this imbalance. It is the atmosphere and the oceans which undertake this redistribution, as we will see in Chapter 2. Complications are added because the Earth's axis of rotation is at an angle with respect to the sun, which leads to there being a strong season cycle. On top of this the Earth rotates every 24 hours, plunging the Earth in and out of darkness. It also means the redistribution of energy away from the Equator takes place on a spinning ball. This creates the Coriolis effect and helps to explain why nearly all weather systems seem to spin.