However, large-scale surface tempera-
ture reconstructions for the last 2,000 years are not the primary evidence for the widely
accepted views that global warming is occurring, that human activities are contribut-
ing, at least in part, to this warming, and that the Earth will continue to warm over the
next century. The primary evidence for these conclusions (see, e.g., NRC 2001) in-
cludes:
• measurements showing large increases in carbon dioxide and other greenhouse
gases beginning in the middle of the 19th century,
• instrumental measurements of upward temperature trends and concomitant
changes in a host of proxy indicators over the last century,
• simple radiative transfer calculations of the forcing associated with increasing
greenhouse gas concentrations together with reasonable assumptions about the sign
and magnitude of climate feedbacks, and
• numerical experiments performed with state-of-the-art climate models.
Supporting evidence includes:
• The observed global cooling in response to volcanic eruptions is consistent with
sensitivity estimates based on climate models.
• Proxy evidence concerning the atmospheric cooling in response to the increased
ice cover and the decreased atmospheric carbon dioxide concentrations at the time of
the last glacial maximum is consistent with sensitivity estimates based on climate
models.
• Documentation that the recent warming has been a nearly worldwide phenom-
enon.
• The stratosphere has cooled and the oceans have warmed in a manner that is
consistent with the predicted spatial and temporal pattern of greenhouse warming.
Surface temperature reconstructions for the last 2,000 years are consistent with
other evidence of global climate change and can be considered as additional supporting
evidence. In particular, the numerous indications that recent warmth is unprecedented
for at least the last 400 years and potentially the last several millennia, in combination
with estimates of external climate forcing variations over the same period, support the
conclusion that human activities are responsible for much of the recent warming.
However, the uncertainties in the reconstructions of surface temperature and external
forcings for the period prior to the instrumental record render this evidence less con-
clusive than the other lines of evidence cited above. It should also be noted that the
scientific consensus regarding human-induced global warming would not be substan-
tively altered if, for example, the global mean surface temperature 1,000 years ago was
found to be as warm as it is today.
