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.

Revised:  April 28, 1998