HYPOTHESIZED CAUSES OF DECADE-TO-CENTURY-SCALE CLIMATE VARIABILITY - CLIMATE MODEL RESULTS

There are at least five hypothesized causes of decadal to century-scal e climate variability, both 'natural' and anthropogenically-induced: ( 1) inherent ('random') variability in the atmosphere (i.e. no external forcing); (2) inherent or forced variability in the ocean dynamical s ystem (e.g. North Atlantic Deep Water fluctuations); (3) solar variabi lity (e.g. the Maunder Minimum); (4) variability in volcanic aerosol l oading of the atmosphere (e.g. Tambora); and (5) atmospheric trace gas variability (e.g. CO2 methane). Modeling experiments conducted for ea ch of these potential mechanisms show that they have different signatu res in time and space which may allow for discrimination in the climat e record. The effects of variations in ocean thermohaline circulation are likely to be largest in those regions adjacent to and downwind of areas of deepwater formation. Decreased insolation and large low latit ude volcanic eruptions affect all latitudes, including changes in Hadl ey cell-dominated low-latitude systems. In contrast, trace-gas induced warming is likely to be amplified at high latitudes, although still c ontaining a significant low latitude effect. Each of the hypothesized causes may have been important, and it is likely that several may have acted in concert to produce the observed climate record. During the H olocene, natural global mean decade-to-century-scale variability is li kely to have been limited to 0.5 to 1.0-degrees-C, and to have been no n-uniformly distributed over the globe. These natural sources of clima te variability are thus unlikely to counter future greenhouse warning in any long-term way.