Climate vs weather

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 Niño 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 (but see note 1). Given the current availability of long-term trends in the temperature record, it is harder to give a precise contradiction-free definition of climate: 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.