Changes in temperature, sea level, and
Northern Hemisphere snow cover.
Source: IPCC, Climate change 2007, syntheses
report, summary for policy makers.
In southern Spain you expect good weather in summer. In the Alps you expect snow in winter. Yet you can come back from a wet holiday in southern Spain in summer or your skiing vacation can be spoiled by not having enough snow. About the weather you can say nothing long in advance, but the climate is relatively stable. The climate says something about the average weather conditions seen over many years. Usually it is hot and dry in southern Spain in summer and in the Alps in most years there is snow in winter.
The climate seems to be fixed, but there have always been changes. Past climate changes were caused by, for example, very slow changes in the relative position of the earth to the sun or on a somewhat shorter time scale by volcanic eruptions. The current climate change is generally ascribed to the enhanced greenhouse effect, due to greenhouse gases like CO2 (carbon dioxide). These gases are produced by people and emitted into the atmosphere.
Several models can be used to predict climate changes. All climate models, however, contain uncertainties. Some uncertainties are related to the models: it is for example hard to model the effect of water in the atmosphere. Other uncertainties are independent of the models: like population growth and economic activities that influence land use and emissions. The results of different models are somewhat different, but also within one model there will be a bandwidth of results depending on the assumptions fed into the model. Now this doesn’t make the models useless. The bandwidth tells you the maximum and the minimum values and scientists have ways to express the level of certainty of an outcome.
The Intergovernmental Panel on Climate Change, IPCC, has studied climate change research in detail. Thousands of scientists have contributed to the conclusions of the IPCC. Through a process of review and discussion of results a consensus on climate change was reached.
The IPCC looked, among other things, at observed changes in the climate, for instance in global average surface temperature, global average sea level and snow cover on the Northern hemisphere like in this diagram. Global temperature has increased in the shown period as has the global sea level. The snow cover has decreased.