Atmosphere & Weather

2.1 Diurnal energy budgets

Structure of the atmosphere

The atmosphere consists of several layers, each with different properties. All our weather takes place in the troposphere.

Daytime and night-time energy budgets

An energy budget is the balance between incoming and outgoing energy. This can be on a local or global scale. Note that the figures you will see for the energy budgets will depend on the data source, so don't worry if you see values that are slightly different.

A diurnal energy budget refers to the daily amount of energy entering and leaving a system (see the figure below for an example).

The daytime energy budget contains six components:

• Incoming (short wave) solar radiation. This is also known as insolation

• Long wave radiation

• Reflected solar radiation

• Surface absorption

• Sensible heat transfer (heat that moves from warmer to colder objects by conduction or convection)

• Latent heat transfer (energy released or absorbed during a change of state e.g. heat taken in by the process of evaporation)

In contrast, the night-time energy budget only has four factors, because there is no sun present to allow for insolation or surface absorption:

• Long wave radiation

• Sub-surface supply (absorbed energy returning to Earth's surface)

• Sensible heat transfer

• Latent heat transfer (e.g. heat released by condensation as dew forms in the early morning)

Factors affecting the diurnal energy budget

Insolation

The sun emits short-wave (ultraviolet) radiation but not all of this energy emitted reaches Earth's surface. Some is scattered by particles of soot and dust straight back into space, some is reflected back to space, and some is absorbed by atmospheric gases such as ozone and oxygen at higher levels in the atmosphere, and carbon dioxide and water vapour nearer the surface of Earth. Cloud cover has a significant effect on the amount of insolation reaching Earth's surface. The less cloud cover, the more radiation reaches the surface of Earth. The angle of the sun also has an impact on the amount of insolation received. For example, when the sun is high in the sky, more insolation is received than when it is lower (around 450 watts/m²).

Reflected solar radiation

The reflective property of an object is known as its albedo. Light-coloured features like snow, ice, and clouds have a higher albedo than darker objects, which absorb, rather than reflect, energy. Albedo is measured from 0 to 1. Black objects have zero albedo whereas white objects have an albedo of 1. This can also be expressed as a percentage.

Surface and sub-surface absorption

Darker surfaces absorb much more radiation than lighter surfaces. Heat that is transferred into the soil and bedrock during the day, when the sun is out, is then released back to the surface at night. This is by the process of conduction (the process by which heat energy is transmitted through collisions between neighbouring atoms or molecules).

Long-wave radiation

Energy lost from Earth into the atmosphere, and even space, is long-wave (infra-red) radiation. Greenhouse gases in the atmosphere, such as water and carbon dioxide, then absorb this radiation and consequently the atmosphere is heated. During the day, there is a net loss of energy from the surface because the outgoing long-wave radiation transfer is greater than the incoming long-wave radiation transfer. This is known as the net long-wave radiation balance. Clouds absorb long-wave radiation and re-radiate it back to Earth. This is the reason why in a desert, there is such a large diurnal temperature range - the cloudless skies at night mean that there is a net loss of long-wave radiation and subsequent cold, even freezing temperatures (the world's deserts average 38°C in the day and -3.9°C at night).

Sensible heat transfer

Heat that is transferred by direct conduction or convection (the exchange of heat energy by the development of a liquid (fluid or gas) between regions of various temperatures) is referred to as sensible heat transfer. Air is a very poor conductor so only a small amount next to Earth's surface is warmed by conduction. Air that is warmed rises as the molecules have expanded and become lighter. As this air rises, it cools, becomes more dense and sinks. This cycle then repeats.

Latent heat transfer

Latent heat transfer refers to heat that is either given out, or taken in, when water changes state. The diagram below illustrates this.

Clouds

Clouds can have a significant impact on the amount of energy received. An overcast sky can block up to 80% of insolation. Clouds therefore have a net cooling effect. However, high, thin clouds, like cirrus clouds, allow some long-wave radiation to pass through them and warm Earth's surface. Cumulonimbus clouds neither heat or cool overall.

Dew

When surface temperatures drop enough to cause condensation, the air becomes saturated, and dew can form. When more moisture is introduced, condensation will occur.

Absorbed energy returned to Earth

Some of the energy that is received by Earth is absorbed by greenhouse gases such as water vapour. This raises the temperature. Ground temperatures can vary significantly between day and night. Air that is close to the ground is warmed through conduction. Due to friction with the surface of Earth, air movement is slower.

2.2 The global energy budget

The global energy budget

The sun is the driving force behind all atmospheric processes. It provides energy in the form of insolation. However, little heat is generated from this and most atmospheric heat is gained from Earth.

The latitudinal pattern of radiation: excesses and deficits

The atmosphere is an example of an open system i.e. a system where energy can cross its boundary to the surrounding environment and vice versa. Insolation enters the system and re-radiation leaves it. Before anthropogenic (human) influences, the atmosphere had been in balance and was not getting any warmer or colder. This is achieved by radiation (the emission of electromagnetic waves), convection and conduction.

The figure below shows the latitudinal pattern of radiation. There is a surplus of heat between 40° and the Equator, and a deficit between 40° and the poles. Without the transfer of this energy by winds and ocean currents, the area with a surplus of heat would keep getting warmer, and the area with a deficit of heat would become increasingly colder. The aatmosphere is largely heated from below as some outgoing radiation is trapped by greenhouse gases. This is known as the greenhouse effect and without it, Earth would be uninhabitable. The problem we have caused is the enhanced greenhouse effect (significant warming of the planet).

Overall, the pattern of annual temperature is that there is a decline in temperature with latitude to the poles. However, there are important anomalies such as the Andes and those areas influenced by cold ocean currents, such as Namibia. In January, the highest temperatures are found in Austalia (around 30°C) and the coldest in areas like Siberia (around -40°C). In July, the Sahara experiences the highest temperatures (around 40°C) and the southern hemisphere experiences colder temperatures.

Atmospheric transfers: wind belts and ocean currents

Pressure and wind belts

Pressure, measured in millibar (mb) and indicated on a surface pressure chart by isobars, is an indicator of changes in weather. High pressure is around 1000 mb wheras low presssure is around 900 mb. Lower pressure indicates poorer whether and higher pressure indicates more stable atmopsheric conditions. In the northern hemisphere, there are greater seasonal contrasts of pressure than in the southern hemisphere.

The poles are regarded as areas of high pressure, although this is reduced by altitude in Antarctica. At aroud 30°N and 30°S, there is an areas of subtropical high pressure (in summer this is usually sporadic around South Africa and Australia, and is even more discontiuous in the northern hemisphere due to the land). Over the oceans, the Azores and Pacific highs are discrete areas of high pressure. The Sahara, south-western USA and southern Asia are continental areas that experience large fluctuations of pressure i.e. low presssure in summer and high pressure in winter due to overheating from the influence of land's specific heat capacity.

At the Equator, pressure is low, coinciding with high levels of insolation. This zone of low pressure reaches India in July, but is just south of the Equator in January due to the lack of land in the southern hemisphere. The doldrums are areas of low pressure. Pressure is usually lower in temperate areas than subtropical areas. During winter in the northern hemisphere, there is high pressure over Canada and Siberia due to the coldness of the land, and over Iceland and other ocenic areas there is low pressure.

The intertropical convergence zone (ITCZ) is the point where winds between the Tropis converge. It is several hundred kilometres wide and consists of low pressure. The rising air stimulates convection.

Prevailing winds indicate the most common wind direction. In the UK, this is from the south-west. Low latitude winds between 10° and 30° are mostly easterlies and the weather is predictable - warm, dry mornings and showers in the afternoons. Showers are heavier in summer driven by convection. Easterlies are strongest when pressure is around 700 mb, not at ground level. Ahead of the easterly wave, air is subsiding. Convergence occurs at the easterly wave, where air ascends, due to the low pressure. Easterlies are important in the formation of hurricanes.

An examples of a monsoon (a seasonal reversal of winds corresponding to changes in precipitation) is when wind is blowing from high pressure from continental Asia in winter, but pulls the southern trade winds into low pressure in summer. Monsoons always blow from cold to warm regions. The summer monsoon and the winter monsoon determine the climate for most of India and Southeast Asia. A monsoon is therefore influneced by the reversal of land and sea temperatures.

Between 35° and 60° westelies dominate around 25% of Earth's surface. Here there are rapidly developing and dissipating depressions.

Winds belts transfer around 80% of surplus heat energy, and are composed of air masses that move from high to low pressure. Air masses are separated by frontal zones where there is a significant difference in the characteristics of the air. Air masses develop characteristics from what they flow over e.g. moisture from the sea in their source region (where they began). Tropical air masses are warm, whereas polar or Arctic are cold. Continental air masses are dry, whereas maritime air masses contain moisture. Examples of air masses are shown in the table below. Note the nomenclature for abbreviating them.

Ocean currents

Ocean currents transfer around 20% of surplus heat energy. They are driven mainly by prevailing surface winds (the most common wind direction in a particular area). The effect off this is to create gyres (circular ocean currents), which flow clockwise in the northern hemisphere and anti-clockwise in the southern hemisphere. The effect of this rotation is for water in the oceans to push westward.

The general pattern of ocean currents is that warm, less dense water, heated by the Sun at the Equator moves towards the poles where it cools, becomes more dense and returns to the Equator for the process to begin again. Any winds that blow over warm ocean currents pick up moisture as they are warmed. The Gulf Stream is one of the reasons why the UK has mild winters and cool summers. New York, which although further south than London, is colder in winter than London because of the influence of the Labrador Current flowing paralell with the coastline, bringing with it cold water from the north.

The global ocean conveyor belt is the slow circulation of water at depth by ocean currents. This is driven by convection and salinity, as opposed to winds. The ice at the poles is fresh water, so the sea water left behind is much denser as it is saltier. At the poles, this cold, salty water sinks and flows towards the Equator where it is heated, becomes less dense, and lighter. Warm surface waters from the Equator flow towards the poles and this cycle continues. The North Atlantic is warmer than the North Pacific, due to differences in salinity, so there is more evaporation here. The salty, denser water left behind by evaporation is transported to the Pacific where it becomes diluted.

Seasonal variations in temperature, pressure and wind belts: the influence of latitude, land/sea distribution, and ocean currents

Latitude

At the Equator, the Sun's rays are directly overhead all year round (a high angle of incidence) and this leads to more concentrated heating. At the poles, the Sun's rays are at lower angles, and insolation is spread out over a larger area. As a result the Equator is hotter than the poles.

The thickness of the atmosphere also has an impact on temperature with latitude. The higher the latitude, the greater the thickness of the atmosphere. As a result, reflection, absorption and scattering of solar radiation increase with increased latitude. The exception is in the polar regions where the air is very clean.

Earth's orbit

Earth's orbit around the Sun affects the amount of insolation different latitutes receive. This is referred to as seasonal variation. Extremes of daylength occur around 21 December and 21 June. These are called solstices and are where at 66.5° latitude, there is either 24 hours of light or darkness, depending on whether this is in the northern or southern hemisphere. At the poles this is most extreme, when, in winter, there are six months of continual darkness.

The equinoxes occur on 21 March and 23 September, named the spring and autumn equinox, respectively. This is when all latitudes are bisected equally by the circle of solar illumination.

Land/sea distribution

Land and sea have different specific heat capacities (the amount of heat needed to raise the temperature of 1g of a substance by 1°C). Land has a low specific heat capacity and sea water has a high specific heat capacity. It takes five times more heat to raise the temperature of water by 2°C than land. Water, therefore, takes longer to warm up and cool down than land. Furthermore, continentality means that, due to land's low specific heat capacity, places near the coast have small seasonal temperature ranges e.g. cool summers and winters, whereas places inland experience large seasonal temperature ranges e.g. cold winters and warm summers. See the two climate graphs below for Edinburgh, UK, and Moscow, Russia, which are roughly on the same latitude but due to continentality, have very different climates.

It is important to note here that the mean annual thermal equator (the point of highest mean surface air temperature) is actually at 5°N, not the Equator, due to the heating of the continents in the northern hemisphere. The northern hemisphere is commonly referred to as the land hemisphere and the southern hemisphere as the water hemisphere, as the northern hemisphere contains 60% water whereas the southern hemisphere contains 80% water.

The movement of the thermal equator shifts the belts of planetary winds and pressure systems to the north and to the south annually. It extends further towards the poles on land due to the low specific heat capacity of land, which means it heats up faster.

The global pattern of pressure and winds

Global atmospheric circulation

Air that is heated expands and becomes less dense whereas colder air is more dense as the molecules are more compressed. This causes low and high pressure respectively. It is this differential heating of the planet that produces wind. Wind is the movement of air between high and low pressure across a pressure gradient (two areas that have different pressures). Isobars connect areas of each equal pressure on a synoptic chart. If the lines are close together, this indicates stronger winds than if the lines are far apart.

If air simply moved from high to low pressure, then we should experience winds blowing from the poles to the Equator. This is true to some extent, but it is not that simple.

There are three distinct cells (sometimes referred to as the tri-cellular model) in global atmospheric circulation: the polar cell and the Hadley cell (named after English lawyer and amateur meteorologist, George Hadley), both driven by thermal processes, as well as the Ferrel cell (named after William Ferrel, an American meteorologist), which is not, and is influenced by the cells to the north and south.

The Coriolis force (the force exerted by Earth's rotation) deflects these prevailing winds to the right in the northern hemisphere, and the left in the southern hemisphere (Ferrel's Law).

A geostrophic wind flows parallel to the isobars on surface pressure chart. It develops when there is no surface friction. As the wind flows from high to low pressure, this exerts a pressure gradient force which is balanced by the Coriolis force. Winds can experience friction from the drag exerted by Earth's surface. Another force that acts on winds is centrifugal force, which is the outward force exerted on a body moving in a circle. This acts at right angles to a wind wind, pulling objects outwards, meaning that air flow is faster around areas of high pressure.

The upper westerlies and Rossby waves

The upper westerlies are fast-moving high-level winds between 30° and 50°. They occur due to a strong north-south pressure gradient created by different temperatures as well as the fact that the Coriolis force increases towards the poles, causing the air to deflect to the east as a geostrophic wind. It is this deviation that causes Rossby waves to form. The upper westerlies are very important in balancing Earth's energy budget through the horizontal movement of air.

Carl-Gustaf Rossby, a Swedish-born American meteorologist, put forward a theory in the 1940s that was later developed by Erik Palmén in 1951, of meandering rivers of air formed by westerly winds around 10km into the troposphere. Rossby waves are much slower than the air moving through them (a jet stream). If the meanders are large enough, this can lead to the separation of pools of either warm or cold air. Heat is then transferred to the poles and cooler air to the tropics. There are three to six waves in each hemisphere and they are formed by thermal differences between land and sea, as well as major relief barriers e.g. the Andes, the Tibetan Plateau and the Rockies. Here the wave is compressed to form a wave trough. When there is a high pressure ridge in the upper airflow, there is a low pressure trough at the surface and vice versa. Air speeds up and diverges at the poles and slows and converges at the Equator. An example of the impact of this is the 2014 Winter Olympics, held in Sochi, where the Russian hosts had to provide artificial snow because Rossby waves had created a ridge over Europe causing unusually high temperatures for the time of year.

Jet streams

Jet streams are strong, regular winds about 10km high in the troposhere that blow west to east. They flow much faster than Rossby waves at around 250 km/h. They are discontinuous but can be thousands of kilometres long and are often seasonal. There are two jet streams in each hemisphere: one between 30° and 50° (the polar jet stream, the fastest) and the other between 20° and 30° (the sub-tropical jet stream). However, they can extend between 30° and 70° in each hemisphere and are at their lowest in latitudes in winter.

El Niño

One phenomenon, known in Spanish as 'The Boy', or 'Christ Child', sees a dip in the jet stream's position in the Eastern Pacific. This causes a reversal in the trade winds blowing from South America to Australia. Consequntly, there is less upwelling of cold water in the Eastern Pacific and less warm surface water in the Western Pacific. This causes droughts in Australia and Indonesia, and heavy rainfall in North and South America.

2.3 Weather processes and phenomena

Atmospheric moisture processes: evaporation, condensation, freezing, melting, deposition and sublimation

Atmospheric moisture exists in three states or phases: solid, liquid and gas (vapour). Energy is used in these phase changes which occur either through a change in temperature or the amount of water vapour in the air. During evaporation, heat is absorbed therefore cooling the surrounding air, and when water freezes, heat is released. Sublimation (think dry ice, although this involves carbon dioxide) is the conversion between a solid and the gaseous phases of matter, with no intermediate liquid stage. The processes of condensation and melting both release latent heat, causing a rise in temperature. Deposition occurs when a gas turns into a solid e.g. hoar frost. When this happens, heat is absorbed.

Evaporation

When water is heated, it changes to a gas when the air is unsaturated. Increases in temperature, dry air and calm conditions all increase evaporation rates.

It is useful to note that humidity is a measure of the amount of moisture in the air. Absolute humidity is the actual amount of water vapour in a given volume of air whereas relative humidity is a measure (in %) of how much water vapour the air is holding (absolute humidity) compared with the maximum amount it could hold at that temperature and pressure.

Condensation

Condensation occurs when either the dew point (the temperature to which air must be cooled to become saturated with water vapor) has been reached or enough water vapour has been evaporated in an air mass. There needs to be condensation nuclei (small particles such as dust, soot or ash around 0.0001mm) in the air around which condensation can form. These particles are hygroscopic (they have an affinity for water).Saturation happens through the addition of water vapour into the air e.g. wind blowing over a warm sea, or by cooling. This can happen in a number of ways:

• Contact cooling (conduction cooling) - this happens when warm, moist air comes into contact with a cold object below the dew point e.g. a cold sea or a surface that has lost heat through terrestrial radiation.

• Radiation cooling - this happens when air loses heat to space through long-wave radiation e.g. from gases and clouds in the atmosphere.

• Expansion cooling - this happens when air expands after it is forced to rise into thinner air. As the gasses expand, they cool resulting in the surrounding air cooling, too.

Causes of precipitation: convection, frontal and orographic uplift of air, and radiation cooling

Precipitation

Precipitation refers to any form of moisture deposition from the atmosphere i.e. rain, snow, sleet, dew and hail. Other than dew, all forms of precipitation require clouds to form first. For precipitation from clouds to fall to Earth's surface, the droplets need to grow big enough and therefore heavy enough. There are two theories that explain how this happens:

Collision theory

Within a cumulonimbus cloud, the water droplets are all different sizes and are carried by rising and falling air currents. When droplets collide with each other, this causes them to become larger and therefore heavier. Three processes can occur:

• coalescence - two water droplets collide and become rain;

• aggregation - tow ice crystals collide and join together to form snow;

• accretion - an ice crystal and a water droplet join forming hail.

Bergeron-Findeisen theory

This theory describes the growth of cloud droplets that occurs between temperatures of -12°C and -40°C, when air is between ice and water saturation. At these temperatures water droplets evaporate and are deposited on ice crystals. These grow into snowflakes until they are large enough to fall as snow as long as the temperature of the atmosphere remains below freezing. If these snowflakes fall through warm air (~2°C) they will melt into rainfall. Most summer rainfall in mid-latitudes forms in this way.

Convectional rainfall

As the land is heated, it heats the air above it which then rises, cools and condenses and, if it continues to rise, will then causes rainfall. This type of rainfall is very common around the ITCZ and in the summer months in temperate areas.

Frontal rainfall

When warm and cold air masses meet the warm air is forced to rise above the cold air, which is more dense, and therefore sinks. This is common along the polar fronts in mid-latitude. Here, low pressure systems called depressions form. As the warm front passes, this causes a long period of precipitation and as the cold front moves through a short period of heavy rain occurs. The warm front and cold front of this depression cause rainfall as the cold air is moving faster so catches the warm front, forcing the warm air upwards. This forms an occluded front with a long period of rainfall.

Orographic rainfall

Air that is forced to rise over a relief barrier such as a mountain cools, condenses and forms rain. On the leeward side (downwind) of the mountain, there is a rain shadow (an area of significantly reduced rainfall) as the air warms and consequently expands and can hold more moisture. Increases of precipitation occur with an increase in altitde up until around 2 km, after which the air temperature is so low, rainfall decreases.

Radiation cooling

Radiation cooling occurs on cloudless nights with little wind so the land surface loses heat rapidly and the air remains in contact with the valley sides long enough so it cool by conduction respectively. This chilled air sinks as it is more dense and the coldest air collect on the valley floor. This creates a temperture inversion with the warmer air above the colder air. This can result in radiation fog, dew and ice forming on valley floors. Ice forms when the ground surface is cooler than freezing point.

Types of precipitation: clouds, rain, hail, snow, dew and fog

Clouds

Clouds are composed of millions of tiny water droplets. There are three main types of cloud:

• stratus - these are layered, and form when there is little vertical uplift;

• cumulus - these are fluffy clouds that form where there is localised uplift;

• cirrus - these are high, wispy clouds formed when condensation occurs at high levels.

White clouds let light pass through them and will not produce precipitation, although if at a low level, these could produce fog. Low-level clouds indicate poor weather.

The cloud types that produce precipitation are nimbostratus (thick, dark, layered clouds up to 5000m high) and cumulonimbus (dark grey, dense clouds that tower up to the top of the troposphere). The cumulonimbus cloud spreads out at the tropopause producing an anvil head as the cold air cannot rise through the warm air of the stratosphere above it. These clouds are driven by convection. The anvil head is composed of ice crystals at the top and water droplets lower down.

Banner clouds are caused by the relief of the land. Moist, uplifted air streams only reach condensation level at the very summit of mountains. As the air sinks and warms, the cloud dissipates. The clouds are influenced by the shape of the object they are lifted over.

Thunderstorms

When absolute or conditional instability occurs at great heights, causing strong updraughts in cumulonimbus clouds, thunderstorms develop. They are especially common in warm, tropical areas as the air can hold large amounts of water. Lightening may well occur as a result of the charges between clouds and/or the ground (upper parts of a cloud are positive and the lower parts are negative). Thunder is caused by the rapid expansion and vibration of the column of air created. The three main stages of a thunderstorm can be seen below.

Rain

If liquid water falls and reaches the ground with a diameter between 0.5mm and 5mm, it is termed rain. Drizzle refers to rainfall less than 0.5mm in diameter. The intensity, duration, amount, seasonality and effectiveness can all vary.

Hail

Hail is formed by raindrops being carried up and down in vertical air currents in large cumulonimbus clouds and is defined as alternate concentric rings of clear/opaque ice. Ascending air is frozen to form ice pellets. Drops of super-cooled water collide with these ice pellets to freeze round them forming concentric rings. If they are carried higher then water vapour will freeze directly onto them. Freezing and partial melting may occur many times before the pellet is large and heavy enough to fall from the cloud. The largest ever recorded hailstone (by diameter) was 20cm in Vivian, South Dakota, USA on the 23 July 2010. Hail is common in summer in mid-continental interiors.

Snow

Snow is frozen precipitation. When water vapour condenses at a temperature below freezing-point, snow is formed. The resulting crystals are either flat hexagonal plates or prisms. If condensation continues then snowflakes are formed from the combination of these crystals. If the lower atmosphere is cold enough then these snowflakes will reach the ground without melting. Within a cloud, there needs to be lots of moisture, a sufficiently low temperature and fairly calm conditions for snow to form. Very cold air, however, contains little moisture so the heaviest snowfall usually occurs when warm, moist air is force over very high mountains or at a front when warm, moist air comes into contact with very cold air.

Dew

Dew is the direct deposition of water droplets through condensation onto a surface. This usually happens when night-time radiation makes the ground cool, which cools the air above it. Conditions necessary for dew formation include air with a high humidity, clear skies, and calm conditions. High pressure is ideal.

Fog

Fog refers to a cloud at ground level. It is created in calm conditions. When humid air is cooled to dew point, condensation occurs and a dense cloud of water droplets forms. There are four types of fog:

• Radiation fog - this type of fog forms when the ground has cooled due to radiation.

• Advection fog - this type of fog forms when warm air passes over a cold surface e.g. the Great Banks off Newfoundland, Canada, experience up to 100 fog days a year due to warm air from the Gulf Stream meeting cold air from the Labrador Current.

• Frontal fog - this type of fog forms when a warm and cold air mass meet (when localised cold air blows over warm water, this creates steam fog).

• Hill fog - this type of fog forms when air is forced to rise up a steep slope and cools.

In urban areas smog (an intensified haze caused by pollutants) can form especially where coal is a primary sources of energy generation such as in cities in China.

2.4 The human impact

The enhanced greenhouse effect and global warming: the evidence, possible cause and atmospheric impacts

The greenhouse effect

The greenhouse effect is not a bad thing, in fact, it is needed for life on Earth. The average temperature on the Moon (without an atmosphere) is -18°C. On Earth (with an atmosphere) this is 15°C. Greenhouse gases (a gas that contributes to the greenhouse effect by absorbing infrared radiation) include carbon dioxide, water vapour (the most abundant greenhouse gas), methane, ozone, chloroflurocarbons (CFCs) and nitrous oxide. These gases trap outgoing long-wave radiation but allow incoming short-wave radiation to pass through them (like a greenhouse) therefore resulting in a warming of the temperature of the lower atmosphere. The problem is the enhanced greenhouse effect.

The enhanced greenhouse effect

Humans have released greenhouse gas emissions into the atmosphere (anthropogenic causes). This has warmed the atmosphere at an alarming rate (the enhanced greenhouse effect) and has resulted in global warming.

The level of carbon dioxide in the atmosphere has risen from 313 ppm (parts per million) in 1958 to 400 ppm in May 2013. During ice ages, CO₂ levels were around 200 ppm, and during the warmer interglacial periods, they hovered around 280 ppm.

Radiative forcing is, in simple terms, the warming effect of a gas. Since pre-industrial (1880s) levels, current global average temperatures are 0.7°C higher. Whilst there have been periods However, the rate of average global surface warming from 1998 to 2012 was slower than it had been for two to three decades leading up to it due to natural variability, volcanic eruptions, and relatively low solar activity (Lindsey, 2018). Mount Pinatubo caused global temperatures to drop by 0.4°C in 1992, having previously emitted 20 million tonnes of sulphur dioxide into the atmosphere in 1991.

The possible causes

Carbon dioxide (CO₂) has increased from around 315ppm in 1950 to over 400ppm, and is expected to reach 600ppm by 2050. It is linked to human activities e.g. burning of fossil fuels and deforestation. Destroying trees also stops oxygen being converted into carbon dioxide.

Methane (CH₄) has increased by 0.5 to 2% per year and is the second largest contributor to global warming after carbon dioxide. Contributors are cattle grazing (cattle give off around 80 million tonnes of methane per year), rice farming (paddy fields contribute 150 million tonnes of methane a year) and melting permafrost

Chloroflurocarbons (CFCs) are synthetic chemicals that destroy ozone (O₃) and absorb long-wave radiation. They are 10 000 times more efficient at trapping heat than CO₂. They are increasing at a rate of 6% per year, despite having been banned since the mid-1990s in many countries. They are used in aerosols such as hairsprays, fridges and in making foam plastics.

Nitrous oxide (N₂O) is a minor component of the atmosphere that is used in nitrate fertiliser and released when burning fossil fuels and burning vegetation (deforestation).

The global warming potential (GWP), that is, the heat absorbed by any greenhouse gas in the atmosphere, as a multiple of the heat that would be absorbed by the same mass of carbon dioxide, varies for each greenhouse gas. Methane has a value of 11, nitrous oxide 270 and some CFCs up to 7000.

The evidence and impacts

Berkeley Earth (2020) concluded that 2019 was the second warmest year on Earth since 1850. The global mean temperature in 2019 was colder than 2016, but warmer than every other year that has been directly measured. 2016 remains the warmest year in the period of historical observations. Year-to-year rankings are likely to reflect short-term natural variability, but the overall pattern remains consistent with a long-term trend towards global warming. Nine of the ten warmest years have occurred since 2000. Between 1947 and 1975 there was a period of cooling, however, this is largely attributable to increased pollution which would have caused more condensation nuclei and thus denser clouds reflecting more sunlight back into space. Since the 1970s, each decade has been warmer than the previous one. Early warming (1910 to 1940) is thought to be largely natural but latterly attributable to increased greenhouse gases in the atmosphere.

In the UK, a new record daily maximum temperature was reached on 19 July 2022, with 40.3°C recorded at Coningsby, Lincolnshire, exceeding the previous record by 1.6°C.

Ocean salinity and temperature - ocean temperature have increased over the past 20 years as oceans store a large percentage (60%) of heat from the enhanced greenhouse effect. The deep oceans have also been affected. Drier areas with high evaporation rates e.g. western Indian Ocean have become saltier. Wetter areas e.g. North Atlantic have become more diluted with fresh water. Melting glaciers and ice sheets enhance this effect. The Great Ocean Conveyor Belt could, as a result, stop. This would make Europe much colder.

Melting sea ice and glaciers - the Arctic is suffering the most from increased temperatures. Since 1980, the March extent has decreased by around 2.4% per decade to a record low in 2012. Antarctica, however, saw its third highest sea ice record in 2015 since records satellite began and sea ice extent has been increasing since the 1970s (Parkinson and Cavalieri, 2012).

Most mountain glaciers have been shrinking since the mid-1800s. In 2013, due to increased snowfall, all Nepalese glaciers were growing as well as three Norwegian glaciers. West Antarctica and Greenland are seeing rapid melting of their glaciers, resulting in increased sea levels. The East Antarctic ice sheet is increasing.

Rising sea levels - between 1992 and 2010, global sea levels rose by an average of 3.1mm per year. In low-lying areas such as the Netherlands, Bangladesh and Egypt, around 200 million people could be displaced. In its 2019 report, the IPCC projected a 0.6m to 1.1m rise of global sea level by 2100 (or around 15 mm per year) if greenhouse gas emissions remain at high rates. By 2300, seas could stand as much as 5m higher under the worst-case scenario. If countries do cut their emissions significantly, the IPCC expects a 0.3m to 0.6m rise in global sea level by 2100.

Ocean acidity - oceans have absorbed around 50% of all the carbon dioxide released by human activity, forming carbonic acid. The pH of the ocean surface has fallen from 8.2 to 8.1 since 1750.

Biological evidence/impacts - there are numerous examples of biological indicators of warming such as the increase in mosquitos, and therefore malaria, in southern Europe due to a warming climate.

The atmospheric impacts

There are numerous atmospheric impacts attributed to global warming: a positive feedback loop exists with melting Arctic sea ice and a reduction in albedo; rainfall patterns will vary; droughts will occur in the USA, Australia, South Africa and the Mediterranean; heat waves will affect urban areas in particular (in Europe, 30 000 people died in the hot summer of 2013); permafrost will continue to melt in Siberia and release methane and more frequent and violent storms are going to occur.

Arguments against global warming and the complexity of the problem

There are many natural causes of global warming and climate change, and some argue that all of these may still be doing so, despite anthropogenic forces. However, what is unequivocal, is that Earth is currently warming at an alarming rate. Natural variations include variations in Earth's orbit around the Sun, precession and angle of tilt (Milankovitch cycles), as well as sunspot activity and atmospheric dust e.g. volcanic activity. Climate change is a complex problem not least because of the timescales involved and the complexity of the interactions taking place and the feedback mechanisms involved.

Combatting climate change

The Stern Report (2006) found that:

• climate change is fundamentally altering the planet;

• the risks of inaction are high;

• time is running out.

Global warming could dent GDP by as much as 20% but dealing with the problem would cost only 1% i.e. £184 billion.

To help combat this, the Paris Agreement was set up in 2015 at the UN Climate Change Conference in Paris, France. 174 countries signed the agreement and pledged to limit global warming to 2°C. The Kyoto Protocol of 1997 was limited as it did not cover all counties. In 1979, the first world conference on climate change was help in Geneva, Switzerland. The Intergovernmental Panel on Climate Change (IPCC) was established by the World Meteorological Organization and the United Nations Environment Programme in 1988.

Case study: London

Urban climates

Heat from industry, buildings and vehicles react very differently with concrete and glass than vegetation. Due to the low albedo of these surfaces heat is absorbed in the day and released at night. Pollutants help trap radiation in urban areas and air pollution is high.

In urban areas there is a relative lack of moisture due to a lack of vegetation and a high drainage density due to sewers and drains combined with high temperatures in the inner city. There are more intense storms in cities, particularly in hot summer evenings and night due to greater convection and instability. There are higher incidents of thunder but less snowfall due to the higher temperatures. As there is little energy used for evapotranspiration, there is more energy available to heat the atmosphere. Cities often have reduced visibility due to industrial haze. In summer, cloud cover is greater due to convection and there is a greater chance of radiation fog and smog in winter due to pollution.

Causes

The urban heat island effect is when an urban area is significantly warmer than the surrounding rural area due to various heat sources such as buildings, reduced turbulence (although building do create a funnelling effect), pollution etc.

Pollution domes (soot, dust, and chemical emissions that become trapped in the air above urban spaces) may also develop in urban areas. At night, the pollution dome can prevent some long-wave radiation from escaping and keep urban areas warmer than surrounding rural areas.

Impacts

London's temperature in the city centre are significantly above those at the rural-urban fringe. May minimum temperatures in these areas can reach 11°C compared with 5°C respectively. The temperature gradient in the west of London is more gentle due to the high density of infrastructure. Low-density infrastructure areas result in a steep temperature gradient and vice versa. Pollution domes in London can filter incoming solar radiation, thereby reducing the impact of heat during the day. At night, the dome may trap heat therefore raising the temperature. There is an absence of strong winds in the city centre, and heat islands often develop under blocking anticyclones in summer. Humidity is lower in London than surrounding areas.

Bibliography

The following books, links and journal articles are useful for further reading around the topic:

Berkely Earth (2020) Global Temperature Report for 2019 http://berkeleyearth.org/2019-temperatures/

Christpherson, R. W. (2009) Geosystems: An introdution to physical geography. 7th ed. New Jersey: Pearson.

Fretwell, M., Kelly, D. and Nanson, J. (2015) Geography. Oxford: OUP.

Grotzinger, J. and Jordan, T. (2010) Understanding Earth. New York: W. H. Freeman.

Intergovernmental Panel on Climate Change

Lindsey, R. (2018) Did global warming stop in 1998? https://www.climate.gov/news-features/climate-qa/did-global-warming-stop-1998

Parkinson, C. L. and Cavalieri, D. J. (2012) Antarctic sea ice variability and trends, 1979–2010. The Cryosphere, 6, 871-880.

Stern Review

Mel Strong's YouTube channel - some great footage and lectures on clouds and the atmosphere.

Nagle, G. and Guinness, P. (2016) Geography. 2nd ed. London: Hodder.

Whittow, J. B. (2000) Dictionary of Physical Geography. 2nd ed. London: Penguin.