Introduction to Climate Change

Climate is the long-term statistical expression of short-term weather. Climate can be defined as "expected weather". When changes in the expected weather occur, we call these climate changes. They can be defined by the differences between average weather conditions at two separate times. Climate may change in different ways, over different time scales and at different geographical scales. In recent times, scientists have become interested in global warming, due to mankind's impact on the climate system, through the enhancement of the natural greenhouse effect.

The overall state of the global climate is determined by the amount of energy stored by the climate system, and in particular the balance between energy the Earth receives from the Sun and the energy which the Earth releases back to space, called the global energy balance. How this energy balance is regulated depends upon the flows of energy within the global climate system. Major causes of climate change involve any process that can alter the global energy balance, and the energy flows within the climate system. Causes of climate change include changes in the Earths orbit around the Sun, changes in the amount of energy coming from the Sun, changes in ocean circulation or changes in the composition of the atmosphere. Large volcanic eruptions can affect the global climate over only a few years. By contrast, the movement of continents around the world over hundreds of millions of years can also affect global climate, but only over these much longer time scales.

Throughout the Earths history climate has fluctuated between periods of relative warmth and relative cold. Palaeoclimatology is the study of climate and climate change prior to the period of direct measurements. Direct records of temperature and other climatic elements span only a tiny fraction of the Earth's climatic history, and so provide an inadequate perspective on climatic change and the evolution of the climate today and in the future. A longer perspective on climate variability can be obtained by the study of natural phenomena which are climate-dependent. Such phenomena provide a record of past climates, and are revealed through the study of, amongst other techniques, tree rings, ice cores and sea floor sediments.

In the last 100 years or so, the Earths surface and lowest part of the atmosphere have warmed up on average by about 0.6^oC. During this period, the amount of greenhouse gases in the atmosphere has increased, largely as a result of the burning of fossil fuels for energy and transportation, and land use changes, for food by mankind. In the last 20 years, concern has grown that these two phenomena are, at least in part, associated with each other. That is to say, global warming is now considered most probably to be due to the man-made increases in greenhouse gas emissions. Whilst other natural causes of climate change, including changes in the amount of energy coming from the Sun and shifting patterns of ocean circulation, can cause global climate to change over similar periods of time, the balance of evidence now indicates that there is a discernible human influence on the global climate.