What's the weather like today? It's a question we often ask, because the weather can vary so much—and because the nature of the weather is so fundamental to our lives. But despite the variability in the weather, we assume a constancy in the climate. Weather is not the same as climate. Climate is the long-term average of weather in a particular place—taking into account all the small fluctuations and the seasonal changes. See Weather and climate—what's the difference?.

Because climate is only meaningful over a long timeframe and across all the seasons, changes to it are much harder to detect. This is clear if we pick one particular day of the year at random—for example, Christmas Day, and examine the weather on that day over 20 years. Obviously there will be considerable variation between the years. We cannot predict the weather on that day a year ahead, because the factors determining daily weather change rapidly. But we can appreciate what it means to talk about the climate on that day. In Australia we know that it will be summer then, and therefore the weather will operate within certain 'boundaries' expected for summer. A mid-winter's day brings different expectations. The same holds true in monsoonal tropical areas with wet and dry seasons.

Climate change means that the boundaries in the weather that we expect in a particular location and season are changing. But such changes are slow and subtle, and because climate can only be measured over decades it is often not possible to give an instant answer to the question of whether and when a climate has changed. It is rather like asking for the precise moment when day becomes night as the sun sets.

There are many reasons why climate can change, and this section deals with the main ones. Most of the reasons are natural, and we know that in the long geological past the earth's climate has altered many times. Causes range from subtle shifts in the earth's orbit and the angle of its axis of rotation, to changes in the output of heat from the sun. Regional climate can also change slowly for quite natural reasons, such as alterations to ocean currents or the incredibly slow drift of continents. More rapid, temporary climate change can be caused by strong volcanic eruptions leaving fine dust high in the atmosphere that weakens the sunlight for months at a time.

The most important determinant of any planet's temperature is its distance from the sun. This cannot be changed, other than by slow changing of an orbit over millennia or by catastrophic impact. The second most important factor is the composition and volume of the planet's atmosphere. Some gases act rather like a blanket, keeping heat from leaving the surface. Others are quite transparent to departing heat. We know from studying the atmospheres of Mars and Venus, and the respective conditions and temperatures there, how important atmospheric composition can be in affecting temperature.

Subtle alteration to the composition of our own atmosphere has recently occurred, and the main cause is the burning of carbon-containing fuels (coal, oil and methane gas). The carbon dioxide gas released by the combustion of these substances is a heat-trapping gas (known as a greenhouse gas). An increase in its concentration in our atmosphere will change conditions on the planet—in broad terms the earth will retain more heat. This is considered to be the main reason why climate is changing around the world.