Natural variability of the climate system and detection of the greenhouse effect

Abstract

GLOBAL mean temperatures show considerable variability on all timescales. The causes of this variability are usually classified as external or internal¹, and the variations themselves may be usefully subdivided into low-frequency variability (timescale ≳= 10 years) and high-frequency variability (≲=10 years). Virtually nothing is known about the nature or magnitude of internally generated, low-frequency variability. There is some evidence from models, however, that this variability may be quite large^1,2, possibly causing fluctuations in global mean temperature of up to 0.4 ^°C over periods of thirty years or more (see ref. 2, Fig. 1). Here we show how the ocean may produce low-frequency climate variability by passive modulation of natural forcing, to produce substantial trends in global mean temperature on the century timescale. Simulations with a simple climate model are used to determine the main controls on internally generated low-frequency variability, and show that natural trends of up to 0.3 ^°C may occur over intervals of up to 100 years. Although the magnitude of such trends is unexpectedly large, it is insufficient to explain the observed global warming during the twentieth century.