Temperature Reconstructions

The reconstructions henceforth discussed are derived using all indicators available, and using the optimal eigenvector subsets determined in the calibration experiments described above (11 from 1780-1980, 9 from 1760-1779, 8 from 1750-1759, 5 from 1700-1749, 4 from 1600-1699, 2 from 1450-1599, 1 from 1400-1449). To better illustrate the workings and effectiveness of the proxy pattern reconstruction procedure, we show as an example (Figure 4) the actual, EOF-filtered, and reconstructed temperature patterns for a year (1941) during the calibration interval. This year is a known ENSO year, associated with a warm eastern tropical Pacific, and a cold central North Pacific. Pronounced cold anomalies are also found over large-parts of Eurasia. The proxy-reconstructed pattern captures these features, although in a relatively smoothed sense (describing about 30% of the full variance in that pattern), and is remarkably similar to the raw data once it has been filtered by retaining only the 11 eigenvectors (#1-5,7,9,11,14-16)  used in pattern reconstruction. It is thus visually apparent that the multiproxy network is quite capable of resolving much of the structure resolved by the eigenvectors retained in the calibration process.

It is interesting to consider the temporal variations in the first 5 reconstructed PCs (Figure 5a). The positive trend in RPC #1 during the 20th century is clearly exceptional in the context of the long-term variability in the associated eigenvector, and indeed describes much of the unprecedented warming trend evident in the NH reconstruction. Equally interesting is the negative trend in RPC # 2 during the past century which is also anomalous in the context of the longer-term evolution of the associated eigenvector. The recent negative trend is associated with a pattern of cooling in the eastern tropical Pacific (superimposed on warming associated with the pattern of eigenvector #1) which may be a modulating negative feedback on global warming. RPC #5 exhibits notable multidecadal variability throughout both the modern and pre-calibration interval, associated with the wavelike trend of warming and subsequent cooling of the North Atlantic this century discussed earlier and the longer-term multidecadal oscillations in that region detected in a previous analysis of proxy climate networks. This variability may be associated with ocean-atmosphere processes related to the North Atlantic thermohaline circulation.

The long-term trends in the reconstructed annual mean NH series (Figure 5b) are quite similar to those of the decadal Northern Hemisphere summer temperature reconstructions of Bradley and Jones, exhibiting pronounced cold periods during the mid 17th and 19th centuries, and somewhat warmer intervals during the mid 16th and late 18th centuries, with almost all years prior to the 20th century well below the 20th century climatological mean. Taking into account the uncertainties in our NH reconstruction (see 'Methods--calibration'), it appears that the years 1990, 1995 and now 1997 (this value recently calculated and not shown) each exhibit anomalies that are greater than any other year back to 1400 at 3 standard errors, or, roughly a 99.7% level of certainty. It should be kept in mind that hemispheric mean values are not associated with globally or hemispherically uniform trends. An example of the global pattern for an historically documented 'very strong' El Niņo year (1791) is shown in Figure 6a demonstrating the classic warm eastern tropical Pacific and cold central North Pacific SST patterns. Analysis of ENSO variability in these reconstructions is discussed in more detail elsewhere. The reconstructed pattern for 1816 is also shown (Figure 6b) exhibiting quite anomalous cold throughout much of the northern hemisphere (even relative to this generally cold decade) but with a quadrapole pattern of warmth near Newfoundland and the Near East, and enhanced cold in the eastern United States and Europe consistent with the anomalous atmospheric circulation associated with the North Atlantic Oscillation (NAO) pattern. Such a pattern is indeed observed in empirical and model-based studies of the atmospheric response to volcanic forcing. We infer in the 1816 temperature pattern a climatic response to the explosive Tambora eruption of April 1815 based on both the anomalous hemispheric coolness and the superimposed NAO-like pattern. Reconstructed time series RPCs #1-5, the NH series, NINO3 index and reconstructions for specific gridpoints can be obtained through the NOAA Paleoclimatology web site.
Revised:  April 28, 1998