Past global changes and their significance for the future

Placing the short instrumental record of climate into a longer-term perspec tive provides valuable insights into the envelope of climate variability on timescales of significance to society today. Numerous paleotemperature rec ords reveal that the 20th century has been exceptionally warm in the contex t of the last millennium, and perhaps many millennia. Furthermore, the cold est decades of the last century (the nadir of the "Little Ice Age") were am ong the coldest times in the late Holocene. Thus, the world has experienced both the warmest and the coldest extremes of the late Holocene within a br ief interval of less than 200 years. Extending the climate record back in t ime enables the underlying forcing factors (prior to global-scale anthropog enic effects on the climate system) to be identified. Paleoclimatic data ar e essential to obtain a comprehensive understanding of the climate system, without which reliable forecasting of future conditions will not be possibl e. (C) 1999 Elsevier Science Ltd. All rights reserved.