Quaternary Science Reviews 19 (2000) 391 } 402 Past global changes and their signi"cance for the future Raymond S. Bradley* Climate System Research Center, Department of Geosciences, University of Massachusetts, Amherst, MA 01003-5280, USA Abstract Placing the short instrumental record of climate into a longer-term perspective provides valuable insights into the envelope of climatevariabilityontimescalesofsigni"cancetosocietytoday.Numerouspaleotemperaturerecordsrevealthatthe20thcenturyhas beenexceptionallywarminthecontextofthelastmillennium,andperhapsmanymillennia.Furthermore,thecoldestdecadesofthe lastcentury(thenadirofthe `LittleIceAgea)wereamongthecoldesttimesinthelateHolocene.Thus,theworldhasexperiencedboth thewarmestandthecoldestextremesofthelateHolocenewithinabriefintervaloflessthan200years.Extendingtheclimaterecord backintimeenablestheunderlyingforcingfactors(priortoglobal-scaleanthropogenice!ectsontheclimatesystem)tobeidenti"ed. Paleoclimaticdataareessentialtoobtainacomprehensiveunderstandingoftheclimatesystem,withoutwhichreliableforecastingof future conditions will not be possible.  1999 Elsevier Science Ltd. All rights reserved. 1. Introduction The overall goal of the International Geosphere Biosphere Programme (IGBP) is: **To describe and understand the interactive physical, chemical and biolo- gical processes that regulate the total earth system, the unique environment that it provides for life, the changes that are occurring in this system, and the manner in which they are inyuenced by human actions 2 .++. The key interac- tionsandsigni"cantchangesthatareofprimaryinterest arethosethatoccurontimescalesofdecadestocenturies, that a!ect the biosphere, that are most susceptible to human perturbations, and that (most likely) will lead to a practical predictive capability. These objectives are of course driven by concerns over the ever-increasing im- pactofhumanactivitiesontheclimatesystem, sensu lato. However, whatever anthropogenic e!ects there are on climate in the future, they will be superimposed on the underlying background of `naturala climate variability which varies on all timescales in response to di!erent forcing factors. Paleoclimatic research provides the essential perspective on climate system variability, its relationshiptoforcingmechanismsandtofeedbacksthat may amplify or reduce the direct consequences of par- ticular forcings. Such a perspective cannot be provided * Corresponding author. Tel.: 001-413-545-2286; fax: 001-413-545- 2328. E-mail address: rbradley@geo.umast.edu (R.S. Bradley) by the very limited set of instrumental data at our dis- posalandwemustthereforeexaminerecordsofthepast inordertocomprehendthefullrangeofnaturalvariabil- ityandprovidecon"denceinourabilitytopredictfuture global changes. Here I present a few examples to illus- tratethesepoints,focusingonpaleotemperaturerecords from the late Holocene. Elsewhere in this volume, the valueofamuchlongerpaleoclimaticperspectiveisdem- onstrated,andtheimportanceofpaleorecordsofvarious forcing factors is also illustrated (Zielinski, 1999; Beer et al., 1999). 2. An essential perspective Althoughafewinstrumentalmeteorologicalmeasure- ments extend back into the 17th century, no extensive network of data exists prior to the mid-19th century. Hence, although we may have a 300 year record of climateatafewspeci"clocations,wehaveonlya &150 yearperspectiveonthespatio-temporalvariabilityofthe earth's climate system as a whole. Unfortunately, the period for which we have detailed instrumental records happenstocoincidewiththetimewhenhumanactivities haveincreasinglycontaminatedtheatmosphereonaglo- bal scale. If we want to understand the relationship between any underlying `natural forcinga and climate variability we must probe further back in time, and for this we must rely on paleoclimate archives (climate proxies). Some archives also contain records related to forcing factors, such as non-sea salt sulfate, or Be in 0277-3791/99/$-see front matter  1999 Elsevier Science Ltd. All rights reserved. PII:S0277-3791(99)00071-2