Past as Prologue: What have the last 21,000 years told us about present and future climate change?
Shaun Marcott, University of Wisconsin-Madison
The geologic record provides an essential perspective on the significance of current and future climate change. To provide a broader perspective, a recent global temperature reconstructions of the Holocene and last deglaciation is combined with long-term model projections of surface temperature to produce time series of global temperature extending 20,000 years into the past and 10,000 years into the future. This past interval is of particular interest because it spans the end of the most recent ice age as well as the entire timespan of human civilization. Projected anthropogenic warming represents a sharp deviation from the relatively stable climate of the Holocene and may ultimately be similar in magnitude to, but occur at rates several times faster than, global warming associated with the last deglaciation. Given that deglacial warming led to a profound transformation of earth and ecological systems, projected warming will also likely reshape the world. If the rapid increase of atmospheric greenhouse gases continues unabated, long-term model projections indicate a commitment to this new and anomalously warm epoch for at least the next 10 millennia.
This new synthesis places the 20th and 21st centuries, when most emissions are likely to occur, into the context of the last twenty mil- lennia over which time the last Ice Age ended and human civilization developed, and the next ten millennia, over which time the pro- jected impacts will occur. When viewed from this long-term perspective, current and future climate scenarios are put into a geologic framework and the anomalousness of them is more clearly realized.