|Reconstructions of annual global surface temperature patterns over several
centuries are now possible, based on the multivariate calibration of widely distributed
high-resolution proxy climate indicators. These reconstructions provide insight into both
the spatial and temporal nature of climatic variations during the past six centuries.
Time-dependent correlations of these temperature reconstructions with time series
representing greenhouse gases, solar irradiance, and volcanic aerosols, suggest that each
of these forcings has played a role in the climatic variability of the past several
centuries, with greenhouse gases appearing to emerge as the dominant forcing during the
20th century. Northern hemisphere mean annual temperatures for three of the past eight
years are warmer than any other year since (at least) 1400 AD, at a greater than 99.5%
level of confidence.
Knowing both the spatial and temporal patterns of climatic change over the
past several centuries remains a key to assessing a possible anthropogenic impact on
post-industrial climate. In addition to the possibility of warming due to enhanced
greenhouse gases during the past century, there is evidence that both solar irradiance and
explosive volcanism have played an important part in forcing climate variations over the
past several centuries. The unforced 'natural variability' of the climate system may also
be quite important on multidecadal and century timescales. If a faithful empirical
description of climate variability can be obtained for the past several centuries, a more
confident estimation can be made of the roles of different external forcings and internal
sources of variability on past and recent climate.
|Because widespread instrumental climate data are available for only about
one century, we must use proxy climate indicators combined with any very long instrumental
records that are available to obtain such an empirical description of large-scale climate
variability during past centuries. A variety of studies have sought to employ a
'multiproxy' approach to study long-term climatic variations, in which a
widely-distributed set of proxy and instrumental climate indicators are analyzed to yield
insights into long-term global climatic variations. Building on such past studies, we take
a new statistical approach to reconstructing global patterns of annual temperature back
through the 15th century, based on the calibration of multiproxy data networks by the
dominant patterns of temperature variability in the instrumental record.
Using these statistically-verifiable
yearly global temperature reconstructions, we analyze the spatiotemporal patterns of
climatic change over the past half-millenium, and then take an empirical approach to
estimating the relationship between global temperature changes, variations in volcanic
aerosols, solar irradiance, and greenhouse gas concentrations during the same period.