Many aspects of the earth system vary on timescales ranging from hours to millennia. Here we review natural variability in several earth system components that, on decadal and longer timescales, have substantial influence on the global system. Patterns of solar variability, the effects of the El Niño-Southern Oscillation (ENSO), changes in the atmosphere (as revealed by isotopic studies of ice cores), variability in the extent and volume of land and sea ice, and natural variability of the biosphere illustrate both the variety of internal and external sources of variation and the range of responses caused by different earth system components. It is this variety and range of responses that cause a signal-to-noise problem and make climate change detection difficult.