Frequently Asked Questions (FAQs)
Climate in a narrow sense is usually defined as the "average weather," or more rigorously, as the statistical description in terms of the mean and variability of relevant quantities over a period of time ranging from months to thousands or millions of years. The classical period is 30 years, as defined by the World Meteorological Organization (WMO). These quantities are most often surface variables such as temperature, precipitation, and wind. Climate in a wider sense is the state, including a statistical description, of the climate system.
The climate system is the highly complex system consisting of five major components: the atmosphere, the hydrosphere, the cryosphere, the land surface and the biosphere, and the interactions between them. The climate system evolves in time under the influence of its own internal dynamics and because of external forcings such as volcanic eruptions, solar variations and human-induced forcings such as the changing composition of the atmosphere and land-use change.
Variations in the mean state and other statistics (such as standard deviations, the occurrence of extremes, etc.) of the climate on all temporal and spatial scales beyond that of individual weather events. The term is often used to denote deviations of climatic statistics over a given period of time (e.g. a month, season or year) from the long-term statistics relating to the corresponding calendar period. In this sense, climate variability is measured by those deviations, which are usually termed anomalies. Variability may be due to natural internal processes within the climate system (internal variability), or to variations in natural or anthropogenic external forcing (external variability).
Climate change refers to a statistically significant variation in either the mean state of the climate or in its variability, persisting for an extended period (typically decades or longer). Climate change may be due to natural internal processes or external forcings, or to persistent anthropogenic changes in the composition of the atmosphere or in land use. Note that the Framework Convention on Climate Change (UNFCCC) in its Article 1, defines "climate change" as: "a change of climate which is attributed directly or indirectly to human activity that alters the composition of the global atmosphere and which is in addition to natural climate variability observed over comparable time periods". The UNFCCC thus makes a distinction between "climate change" attributable to human activities altering the atmospheric composition, and "climate variability" attributable to natural causes.
A key difference between climate variability and change is in persistence of "anomalous" conditions. In other words, events that used to be rare occur more frequently (summertime maximum air temperatures increasingly break records each year), or vice-versa (duration and thickness of seasonal lake ice decreasing with time). In statistical terminology, the curve of the frequency distribution representing the probability of specific meteorological events occurring is changed. The curve may be modified either in amplitude, or shifted about a new mean, or both.
Care must be taken not to confuse variability with change. Many regions of the world experience greater variability, climatologically speaking, than do others. In some parts of the world, or in any region for certain time periods or parts of the year, the variability can be weak (i.e. there is not much difference in the conditions within that time period). In other places or time periods, the conditions can swing across a large range, from freezing to very warm, or from very wet to very dry and exhibit strong variability. A certain amount of this is understood and accepted, instinctively, by the people in a region. What is "normal" for Denver, Colorado in terms of the frequency of precipitation events (high variability) would be "abnormal" for London, England (low variability). Thus, any single event, such as a severe tropical cyclone, cannot be attributed to human-induced climate change, given the current status of scientific understanding.
Occasionally, an event or sequence of events occurs that has never been witnessed before (or recorded before), such as the exceptional hurricane season in the Atlantic in 2005. Yet even that could be part of natural climate variability. If such a season does not recur within the next 30 years, we would look back and call it an exceptional year, but not a harbinger of change. Only a persistent series of unusual events taken in the context of regional climate parameters can suggest a potential change in climate behaviour has occurred.
The IPCC is conducting considerable efforts in trying to determine, for various hydrometeorological hazards (e.g. tropical cyclones or tornadoes) and for related events (e.g. flash floods), whether there is substantial and credible evidence of human-induced climate change. The upcoming fourth assessment report should provide more clarity on this.
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