The Making of the Hockey Stick - The Hockey Stick and the Climate Wars - page 40

Lowery reconstruction discussed earlier) published findings based on the use of a theoretical climate
model simulation designed to investigate causes of past temperature change. 35 Crowley subjected the
model to estimated changes in natural factors over the past thousand years using indirect measures of
changes in solar output and explosive volcanic activity, information on both of which can be
recovered from atmospheric deposits in polar ice cores. These simulations revealed that the natural
factors could explain the extent of medieval warmth in our reconstruction; in the model, this warmth
arose from a relative lack of cooling volcanic eruptions combined with relatively high levels of solar
output. The natural factors could also explain the cooler conditions of the ensuing Little Ice Age,
which resulted from relatively low levels of solar output and more frequent explosive volcanic
eruptions. Fed the natural factors only, the model could not, however, reproduce the abrupt twentieth-
century warming. In fact, the model predicted that the climate should have cooled in recent decades,
rather than warmed, if only natural factors had been at play. It was only when Crowley added the
modern human influences—increasing greenhouse gas concentrations primarily from fossil fuel
burning and the regional cooling effect of industrial sulfate aerosols emissions—to the model
simulation that it was able to track the hockey stick all the way through to the present. The conclusion
was clear: Natural factors could explain the temperature changes of the past millennium through the
dawn of the Industrial Revolution, but only human influences could explain the unusual recent
warming. Causality was at least tentatively established now.