Geoscience Reference
In-Depth Information
The scientists tell us the 1990s were the hottest decade of the entire millennium.
—President Bill Clinton, State of the Union Address (January 27, 2000)
Although scientific revolutions in how we see the world do occur, the bulk of our scientific
understanding comes from the cumulative impact of numerous incremental studies that together paint
an increasingly coherent picture of how nature works. The hockey stick was no different. To employ a
mixed sports metaphor, the hockey stick did not suddenly appear out of left field. Rather, it arose as a
logical consequence of decades of work by paleoclimate researchers that led to increasingly rich
networks of climate proxy data and the introduction of new ways to use such data to reconstruct past
climates. My colleagues and I were the beneficiaries of this substantial body of past work.
The Reconstruction Zone
A half-century ago, when the British climatologist Hubert Lamb set out to trace temperature trends
over the past millennium, he obviously wasn't attempting to address the issue of anthropogenic
climate change. It wasn't considered an issue at the time. Reconstructing past climate change was
interesting in its own right, however. It was an engaging piece of puzzle-solving, the sort of thing that
excites scientists. How could one deduce, from the sparse and imperfect clues available, how climate
varied in the past? Could one perhaps even explain some key historical events, such as the rise or
collapse of various civilizations in terms of climate changes? Big picture, gee-whiz stuff.
These sorts of questions, rather than the threat of human-caused climate change, drove dozens of
paleoclimate researchers around the world to attempt to piece together, over several decades, the
riddle of how Earth's climate had varied over time. By the mid-1990s, it was becoming possible to
use proxy records such as tree rings and ice cores to build year-by-year chronologies of climate
change at many locations around the globe, reaching back centuries and in some cases millennia.
These proxy records could be used to address, for example, the question of how cold it really was
during the “year without a summer” of 1816 that followed the explosive Tambora volcanic eruption of
April 1815. How did that eruption influence rainfall and atmospheric circulation patterns around the
world? Was there a relationship between the 1791-1792 El Niño and the deadly failure of the Indian
monsoon during that period, which led to drought, famine, and the death of millions?
Thanks to decades of curiosity-driven work by paleoclimatologists, extensive global networks
of long-term proxy data were available for analysis by the early 1990s. Of course, by that time,
climate change itself and what it might portend for the future were far more prominent issues than they
had been in Lamb's day. By featuring the Lamb curve in the First Assessment Report in 1990, the
