| Data Retrieve Data
We employ a multiproxy network
consisting of widely distributed high-quality annual resolution proxy climate indicators
individually collected and formerly analyzed by many paleoclimate researchers (details and
references are available; see Supplementary information). The network includes (see Figure
1a) the collection of annual resolution dendroclimatic, ice core, ice melt, and long
historical records used by Bradley and Jones combined with other coral, ice core,
dendroclimatic, and long instrumental records. The long instrumental records have been
formed into annual mean anomalies relative to the 1902-1980 reference period, and gridded
onto a 5 degree by 5 degree grid (yielding 11 temperature gridpoint series and 12
precipitation gridpoint series dating back to 1820 or earlier) similar to that shown in
Figure 1 (bottom). Certain densely sampled regional dendroclimatic datasets have been
represented in the network by a smaller number of leading principal components (typically
3-11 depending on the spatial extent and size of the dataset). This form of representation
insures a reasonably homogeneous spatial sampling in the multiproxy network (112
indicators back to 1820).
Potential limitations specific to
each type of proxy data series must be carefully taken into account in building an
appropriate network. Dating errors in a given record (e.g., incorrectly assigned annual
layers or rings) are particularly detrimental if mutual information is sought to describe
climate patterns on a year-by-year basis. Standardization of certain biological proxy
records relative to estimated growth trends, and the limits of constituent chronology
segment lengths, (e.g. in dendroclimatic reconstructions) |
can restrict the maximum timescale of climatic variability that is
recorded. However, the dendroclimatic data used were carefully screened for conservative
standardization and sizeable segment lengths. Moreover, the mutual information contained
in a diverse and widely distributed set of independent climatic indicators can more
faithfully capture the consistent climate signal that is present, reducing the
compromising effects of biases and weaknesses in the individual indicators. Monthly instrumental land air and sea
surface temperature gridpoint data (Figure 1--bottom) from the period 1902-1995 are used
to calibrate the proxy dataset. Although there are notable spatial gaps, this network
covers significant enough portions of the globe to form reliable estimates of Northern
Hemisphere (``NH'') mean temperature, and certain regional indices of particular
importance such as the 'NINO3' eastern tropical Pacific surface temperature index often
used to describe the El Nino phenomenon. The NINO3 eastern equatorial Pacific sea surface
temperature index is constructed from the 8 gridpoints available within the conventional
NINO3 box shown between 5S to 5N, 90W to 150W. |