Climate varies naturally on timescales ranging from seasons to the tens of thousands of years between ice ages. Knowledge of the natural variability of the climate system is needed to assess the extent of human impact on the climate system. At present there are no robust estimates of natural climate variability on the decades to centuries timescale that is essential for evaluating the extent to which human activities have already affected the climate system, and to provide the baseline of knowledge needed to assess how they might affect it in the future.
The natural variability of the climate system is the result of four factors:
- mathematically, the climate system exhibits “chaotic” (i.e., complex and non-linear) behavior, which means that it has limited predictability;
- important parts of the climate system exhibit oscillating behavior, e.g., the El Niño-Southern Oscillation (ENSO) cycle that repeats every 2-8 years in the tropical Pacific, and the North Atlantic Oscillation that has a cycle length of 60-80 years;
- variability in solar intensity, a key natural driver of climate, which occur in cycles which vary in length from familiar 11-year sunspot cycle to shifts in the Earth’s orbit that occur in cycles of 100,000 years; and
- the random nature of volcanic eruptions, which are also a natural driver of climate.