[Introduction]
The Earth's climate varies considerably from place to place, and it's very important to us. One of the most important things about climate is the temperature, and in this package you will be given the power to change it. Not of course the real temperature, but the temperature calculated by a climate model - a model that is at your command.

[A climate model]
The global mean surface temperature depends on the radiation the Earth's surface receives from the Sun. It also depends on the infrared radiation received from the atmosphere. These are the main energy gains. The temperature also depends on the losses, notably the infrared radiation leaving the surface, and heat transferred upwards through convection.

There are many factors on which these gains and losses depend. All the important ones are included in the climate model in this package, though only some of them are available for you to change.

[The factors]
Here is a list of the important factors in the model that you can change. To get information on each of these factors you can click on its icon. Try it now.

[The model]
The model consists of a set of mathematical equations. These represent the input of energy from the Sun to the Earth's atmosphere and surface, the various ways in which energy is transferred between the atmosphere and surface, the loss of energy from the atmosphere and surface to space.

When the model is running, it takes the value of the solar constant and of all the other factors, and does a lot of calculations. The outcome is the new steady state global mean surface temperature.

[Coupling between factors]
So far, when you've changed a factor, all the other factors stayed fixed. This enabled you to explore the effect of each factor one at a time. Nature is a bit more complicated than this. For example, suppose that there was an increase in the solar constant. The immediate effect would be an increase in the Earth's surface temperature. But this would then cause a decrease in the fraction of the Earth's surface covered in ice and snow, and an increase in the water content. This is called coupling between factors.

[Variation with latitude and season]
Up to now we've only been concerned with the global mean surface temperature. In fact, as we all know, temperatures vary from place to place, and from month to month. More elaborate climate models can handle these variations. You can now explore the data from one such model.

[Animation 1]
This is the Earth at a particular position in its orbit. The column shows a certain amount of solar radiation , and it falls on a region of the Earth's surface at the equator. A region with the same area but well away from the equator gets less solar radiation, as shown by the narrower column, and the region centred on the south pole gets none at all. A region with the same area centred on the north pole gets a small amount of radiation. The Earth moves around the Sun, with its rotation axis in a fixed direction. Six months later the Earth has moved to here. A region at the equator still receives more solar radiation than a region with the same area nearer the poles. Averaging over the whole year, the further the region from the equator, the less solar radiation it receives. It is for this reason that the mean surface temperature generally decreases as latitude increases.

[Animation 2]
Consider a region at a northern latitude, and a region with the same area and just as far from the equator, but at a southern latitude. Here, the southern region receives less solar radiation than the northern region. It is winter in the southern hemisphere and summer in the northern hemisphere. The Earth moves around the Sun, with its rotation axis in a fixed direction. Six months later the Earth has moved to here. The southern region now receives more solar radiation than the northern region. It is now summer in the southern hemisphere and winter in the northern hemisphere. The temperature changes through the year are thus the result of the fixed tilt of the Earth's rotation axis, combined with the motion of the Earth around the Sun.

[Conclusion]
You have now seen the effects on the Earth's surface temperature of changing several factors that determine climate. The models used here are simplified to fit into your computer, and so the calculated temperature changes are not the most accurate we could have calculated, but they are illustrative - they are broadly right.

There are however two aspects of climate that haven't been included in this package. First, we've only considered the temperature aspect, not rainfall, winds etc. Second, we haven't considered variations in climate with longitude. These variations can be large. For example, though north west Europe and central Canada have the same latitude they have different longitudes, and central Canada has a much more extreme climate than north west Europe.

Longitude variations in climate arise from longitude variations in the Earth's surface - oceans, continents, mountain ranges and the like. But though we haven't considered longitude variations, nor aspects such as rainfall and so on, you have nevertheless made an important start in exploring the richly varied climate of our planet.