The energy balance of the earth determines the global climate. The main source of energy for the earth is the sun. As the sun's rays do not hit the poles at the same intensity as they do at the equator, a temperature difference arises, and the process of balancing this out regulates the global atmospheric circulation. Greenhouse gas emissions caused by human activity are changing this delicate balance of energy, and creating global warming.
Factors that drive the climate system
The sun heats the earth up
The main supplier of energy to the earth is our sun. It has a surface temperature of around 5,700 °C, and radiates at an average rate of 1,367 watts per square meter towards our planet, a measure which is known as the solar constant. The amount of solar radiation which reaches a particular location on earth is primarily dependent on the angle of the earth's axis in relation to the sun. Moreover, half of the globe is always in shadow. For these reasons, the solar energy at the outer edge of the earth's atmosphere has an average rate of 340 watts per square meter. The angle of the earth's axis in relation to the sun is also responsible for our seasons.
The sun's influence changes over the course of time. In earth's early history, ice ages and warm ages alternated with each other roughly every 100,000 years. The reason for this is the earth's orbit around the sun. We talk about the Milankovitch cycles, named after their discoverer. Short-term influencing factors are at work against the backdrop of these slow cycles, so, in cycles of 11 years, for instance, we have alternating phases of more intense and then weaker solar activity. This cycle is caused by the repeated appearance of sun spots. The solar constant is therefore not actually constant, but fluctuates around the average value mentioned above of 1,367 watts per square metre. At present, the sun's activity is decreasing somewhat.
Natural drivers of climate
Besides the sun, there are a whole range of other, natural factors that affect the energy balance of our earth, and with it, our climate. Around a third (100 watts per square metre) of incoming solar radiation is reflected back into space by the earth's surface, and by clouds and aerosols. Aerosols are tiny particles in the air that consist of liquid or solid matter (soot, dust, sulphur compounds, etc.). In nature, it is primarily volcanoes which inject these into the atmosphere, but they are also emitted by oceanic organisms, as well as from other sources. Aerosols and clouds thus have a cooling effect on the earth's climate as they reflect the solar radiation. The reflectivity at a particular location is also heavily dependent on the condition of the earth's surface: snow and ice-covered areas reflect a great deal of radiation, as the surface is very light in colour, whereas there is less reflection off dark areas such as oceans. A simple example will serve to explain this: if you sit on a black chair in the sun, it will become uncomfortably warm in a much shorter time than if you were sitting on a white chair.
The surface of the earth and the atmosphere, which have an average temperature of around 15°C, radiate energy in the form of longwave radiation into space (239 watts per square metre). Greenhouse gases in the atmosphere absorb the reflected longwave radiation, and thus heat up our atmosphere. Without greenhouse gases, the temperature would be about -18°C at the earth's surface.
As the incoming solar radiation at the poles is weaker than at the equator, a temperature difference is created. This drives the oceanic currents and atmospheric circulation. The planet attempts to balance out this difference by means of heat transfer. For instance, the gulf stream transports heat from the Gulf of Mexico to northern Europe. Warm fronts bring warm air to the north, and cold fronts bring cold air from the north in a southerly direction.
Global circulation and climate zones
At the equator, where the incoming radiation from the sun is at its strongest all year round, are the tropics with their rainforests. They are fed by rainfall in the so-called Intertropical Convergence Zone (ITCZ). This zone is characterised by tropical low-pressure regions. The zone shifts slightly over the course of the year, back and forth across the equator, from north (Northern Hemisphere summer) to south (Southern Hemisphere summer). This is why it is mostly hot and humid in northern Australia in December, while in West Africa the hottest and most humid time of the year is primarily in July.
The warming of the earth's surface in the ITCZ leads to an updraft, to the generation of high storm clouds, and thus to localised heavy precipitation. At the tropopause (the upper boundary of the troposphere, or the weather-layer of the atmosphere, at about 10 km about the earth's surface) the rising air is transported in the direction of the poles. Above the subtropics, the air sinks and becomes much drier in the process. The subtropics are consequently dominated by high pressure regions. These regions are where our planet's largest deserts are found today (the Sahara, the Arabian Desert, the Atacama Desert in South America, the Gobi Desert in Central Asia, and the central Australian outback).
In the mid-latitudes, the "westerlies" region, areas of low-pressure are generated that are responsible for an exchange of air masses and, consequently, heat transport. This zone is characterised by a moderate, mild climate, and is in fact responsible for Switzerland's climate.
The cool climate in the sub-polar land regions (e.g. Scandinavia, Siberia and Alaska) means that the predominant biome in those areas is tundra - low-growing vegetation that is adapted to long periods of cold weather. Sea ice and large ice sheets are found in the coldest regions of the planet - at the poles. These areas are heavily affected by anthropogenic climate change.
The vast differences between the various climate zones are highlighted by global record measurements. The World Meteorological Organisation (WMO) holds an archive of all global weather and climate records. Here is a small sample of them:
The highest temperature ever recorded was 56.7°C, in 1913 in California (USA). The lowest temperature, an unbelievable -89.2°C was recorded in 1983 at the Vostok weather station in Antarctica.
The highest level of precipitation within one day fell in January 1966 on La Réunion (1825 mm). That is about the average annual precipitation in the Swiss Alpine foothills. The largest amount of annual precipitation ever was experienced in India: in Cherrapunji. Between August 1860 and July 1861, 26,470 mm (26.5 m) of precipitation fell in the 12-month period. Most places in Switzerland do not see that much precipitation in 10 years.
There was nowhere drier for longer than in Arica, Chile, where not even one drop of rain was recorded between 1903 and 1918 - for 173 months.
The heaviest hailstone ever recorded weighed 1.02 kg (found in Bangladesh).
Many more records can be found here:
Humans are a major climate driver
The planet is becoming increasingly warm. The main reason for the global warming of the past 50 to 60 years is the change in the energy balance of the earth due to human activity. Humans cause emissions of aerosols and greenhouse gases - primarily carbon dioxide. The natural sink processes that take place, for example, in plants, and the absorption of carbon dioxide by the oceans are not able to compensate fully for anthropogenic emissions. This leads to a build-up of carbon dioxide in the atmosphere. As a result, longwave radiation towards space is reducing, thus intensifying the greenhouse effect. Oceans and the earth's surface therefore have to absorb more energy, and consequently heat up. This also increases evaporation and the sensible heat flux. The latter directly affects the temperature at the earth's surface which means that we are able to feel this long-term warming trend. It is worthy of note that solar activity is currently at a lower level, and so cannot explain the warming of the past decade.
Humans also influence climate in other ways. For example, changes in land use by humans lead to changes in the condition of the ground, which can have a direct impact on the outgoing radiation from the earth's surface. Dark areas such as forests that have burned down as a result of bush fires absorb more heat and also reduce the amount of heat that is radiated back into space, thus creating an additional warming effect. The melting of polar ice also leads to intensified warming in those regions when the very dark surface of the ocean is revealed which once lay underneath the highly reflective snow and ice. These are examples of what is known as positive feedbacks on the climate - i.e. the warming that has primarily been caused by humans is now intensifying of its own accord.