There is little doubt that environmental (e.g., climatic, hydrologic and
oceanographic) changes are taking place. The key question is whether these
changes are due to human activity (e.g., elevated atmospheric
concentrations of trace-gases), or related to natural variability and
processes. Decision-makers need a clear answer to this question before they
can respond in an informed manner. Unfortunately, the period of
instrumental observations is too short to characterize natural variability,
and to determine if current environmental trends are merely extensions of
longer-period natural variations and responses to natural forcing (e.g., solar
or volcanic). As discussed previously, multi-century paleoclimatic time
series from many parts of the world (not all) indicate that the greatest
warming of the last 500 years was the one spanning the last 100 years.
The most
[4]
recent 500-year compilation of paleoclimatic records suggests that the northern hemisphere as a whole experienced unprecedented warming during the last 100 years [ Bradley and Jones, 1993], but this study also illustrates the need to attribute observed changes to specific natural and non-natural changes before confident conclusions can be reached regarding the role of humans in causing observed environmental change. The hope is that an improved global array of multi-century time series can be generated so that paleoclimatologists, climatologists and modelers can collaborate on detecting human-induced environmental change [ Schneider, 1993; Karl, 1994].