The climate is commonly considered to be the weather averaged over a long period of time, typically 30 years. Somewhat more precisely, the concept of "climate" also includes the statistics of the weather - such as the degree of day-to-day or year-to-year variation expected. The Intergovernmental Panel on Climate Change (IPCC) glossary definition is:

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.

Climate versus weather

Climate diagram of Bangkok, ThailandIn the most succinct words, weather is the combination of events in the atmosphere and climate is the overall accumulated weather in a certain location.

The exact boundaries of what is climate and what is weather are not well defined and depend on the application. For example, in some senses an individual El Niņo event could be considered climate; in others, as weather.

When the original conception of climate as a long-term average came to be considered, perhaps towards the end of the 19th century, the idea of climate change was not current, and a 30 year average seemed reasonable.

Given the current availability of data on long-term trends in the temperature record, it is harder to give a definition of climate to suit all purposes: over a 30 year period, averages may shift; over a shorter period, the statistics are less stable.

In a given geographical region, the climate generally does not vary over time on the scale of a human life span.

However, over geological time, climate can vary considerably for a given place on the Earth. For example, Scandinavia has been through a number of ice ages over hundreds of thousands of years (the last one ending about 10,000 years ago).

Paleoclimatology is the study of these past climates, their origin, and by extension, the origin of today's climate.

Classifications and Other Information Wikipedia

Ancient climate change 'link' to CO2   BBC - June 17, 2010
A "global pattern" of change in the Earth's climate began 2.7 million years ago, say scientists.

Geologist discovers pattern in Earth's long-term climate record   PhysOrg - April 6, 2010

2009 United Nations Climate Change Conference  Wikipedia

Ethiopia's climate 27 million years ago had higher rainfall, warmer soil   PhysOrg - October 23, 2009

Seafloor Fossils Provide Clues on Climate Change   PhysOrg - October 23, 2009