Natural Climate Variation
Natural climate variations have two main causes. First, the climate system can react to “radiative forcing,” an imbalance between incoming solar radiation (external) and outgoing infrared radiation. Second, natural climate variation can come from internal interactions among components of the climate system. Therefore a distinction is made between externally and internally induced natural climate variability and change. One example of internal climate variation is the El Niño-Southern Oscillation (ENSO), resulting from the interaction between atmosphere and ocean in the tropical Pacific. Another example is the North Atlantic Oscillation (NAO), which has a strong influence on the climate of Europe and part of Asia.
Climate varies naturally over short and long timescales and over large and small spatial scales.
- During the last million years or so, glacial periods have alternated with interglacial periods as a result of variations in the Earth’s orbit. From studying Antarctic ice core samples and ocean sediments, we know that the period of this cycle is about 100,000 years.
- We also know that during the last glacial period large and very rapid temperature variations took place over large parts of the globe, in particular in the higher latitudes of the Northern Hemisphere. These abrupt events saw average global temperature changes of many degrees within a human lifetime.
- In contrast, the last 10,000 years have been relatively stable, although quite large changes have occurred locally.
Regional or local climate is generally much more variable than climate on a hemispheric or global scale. This is because when we look at the global “average,” regional or local variations in one place tend to balance opposite variations elsewhere.
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