Natural archives, changing climates
Climatic changes have occurred throughout human history, but instrumental measurements do not provide us with a very long perspective on climate variations. In many regions, instrumental records only extend back a century or two. To understand the longer-term variability of the climate system, we rely on natural archives- sediments, ice caps, peat bogs, cave deposits, banded corals and tree rings-in which a record of past changes in climate has been preserved. They are a treasure trove of the climatic and environmental history of the planet and provide information about factors that may have caused the climate to change, such as major explosive volcanic eruptions, changes in solar irradiance and human effects on the atmosphere. Paleoclimate archives show that the world has experienced very different conditions from today, even in the recent past, and they provide a framework for us to assessthe magnitude of future changes that we are likely to experience as greenhouse gases continue to accumulate in the atmosphere. Societies in the past have been disrupted by abrupt and unexpected climate changes, and the paleoclimatic evidence demonstrates our vulnerability to rapid shifts in climatic patterns. Unfortunately, many of the natural archives that provide this unique perspective on past climate are now under threat by human activities, and the very climatic changes that we seek to understand.