Because widespread, reliable instrumental records are available only for the last 150
years or so, scientists estimate climatic conditions in the more distant past by analyzing
proxy evidence from sources such as tree rings, corals, ocean and lake sediments, cave
deposits, ice cores, boreholes, glaciers, and documentary evidence. For example, records of
Alpine glacier length, some of which are derived from paintings and other documentary
sources, have been used to reconstruct the time series of surface temperature variations in
south-central Europe for the last several centuries. Studying past climates can help us put the
20th century warming into a broader context, better understand the climate system, and
improve projections of future climate.
http://www.pnas.org/content/early/2014/08/07/1407229111.full.pdf+html – original link
A recent temperature reconstruction of global annual temperature shows Early Holocene warmth followed by a cooling trend through the Middle to Late Holocene [Marcott SA, et al., 2013, Science 339(6124):1198-1201]. This global cooling is puzzling because it is opposite from the expected and simulated global warming trend due to the retreating ice sheets and rising atmospheric greenhouse gases. Our critical reexamination of this contradiction between the reconstructed cooling and the simulated warming points to potentially significant biases in both the seasonality of the proxy reconstruction and the climate sensitivity of current climate models.
[This builds on the piece from Science last year that reconstructed the temperature record for the last 11.5K years of the Holocene. The physical record – tree rings, pollen, etc. – shows overall slight cooling, which would be consistent with the standstill in sea level. But the climate models show warming when they hindcast this period. If the physical record is correct, then the current warming is much more anomalous than previously thought, and the models might be underestimating future warming. The article does not rule out that there might be problems with the physical record, but points out that until the question is resolved, there is a significant question about the predictions about sensitivity to CO2 in the current models.]
Starting in the late 1990s, scientists began combining proxy evidence from many different
locations in an effort to estimate surface temperature changes averaged over broad
geographic regions during the last few hundred to few thousand years. These large-scale
surface temperature reconstructions have enabled researchers to estimate past temperature
variations over the Northern Hemisphere or even the entire globe, often with time resolution
as fine as decades or even individual years. This research, and especially the first of these
reconstructions published in 1998 and 1999 by Michael Mann, Raymond Bradley, and
Malcolm Hughes, attracted considerable attention because the authors concluded that the
Northern Hemisphere was warmer during the late 20th century than at any other time
during the past millennium. Controversy arose because many people interpreted this result
as definitive evidence of anthropogenic causes of recent climate change, while others criticized
the methodologies and data that were used.
In response to a request from Congress, this committee was assembled by the National
Research Council to describe and assess the state of scientific efforts to reconstruct surface
temperature records for the Earth over approximately the last 2,000 years and the implications
of these efforts for our understanding of global climate change.