In this section, the reconstructions of MBH98, the associated uncertainties, and raw data used in calibration and verification are available in a variety of formats.

In part due to the especially strong El Nino of 1997/1998, there has been renewed interest in past variations in the El Nino phenomenon, and the context in which they place prominent recent (1997/1998 and 1982/1983) events. Two excellent examples of past very strong El Ninos in the reconstructions are those evident in the temperature patterns for 1791 and 1878 (Figure 13). Independent corroboration of these events is provided by the historical chronology of Quinn and Neal, 1992 [see MBH98]. The reader may note that an alternative historical El Nino chronology has recently been produced by Ortlieb (2000), with conclusions that sometimes differ from those of Quinn and Neal. We note however that, to the extent that the Quinn chronology used here is imperfect, it will provide a very conservative corroboration of our own chronology, as mismatch may be due to uncertainties in the chronology as well as in our reconstruction]. Both events shown exhibit the classic eastern tropical Pacific warming and horseshoe pattern of warming and cooling in the North Pacific. The details of ENSO-related patterns of variation in the annual-mean temperature reconstructions is discussed by Mann et al (2000).

Figure 14:
Reconstructed boreal cold-season NINO3 index back to 1650. Shown for comparison is a partially independent (dendroclimatic rather than multiproxy) winter (DFJ) reconstruction of the Southern Oscillation Index (SOI-Stahle et al, 1998). Yellow shaded region indicates the 95% confidence bounds for the NINO3 reconstruction.

As commented upon earlier, there are important distinctions between regional and hemispheric trends, with regional trends exhibiting considerably greater variability and heterogeneity. Cold and warm periods in different parts of the globe, for example, are not in general synchronous. Even during the "Little Ice Age" (Bradley and Jones, 1992) not all areas were uniformly cold; geographical and temporal variations were apparent, as highlighted by an examination of the reconstructions presented here. The "Medieval Warm Period" or "Medieval Optimum" (Hughes and Diaz, 1994) is even more enigmatic.

To illustrate some of the problems inherent in estimating hemispheric mean temperature from limited regional information, consider (Figure 15) the warmest (1834), second warmest (1822), and coldest (1838) years in Europe prior to the 20th century, based on the MBH98 temperature pattern reconstructions.

Figure 15:
Global annual-mean temperature pattern reconstructions for three years associated with unusually warm or cold anomalies in the European sector during 1834 (top left), 1822 (top right), and 1838 (left).

While 1834 was the warmest year in Europe, it was colder than typical conditions (by 20th century standards) over large parts of the Northern hemisphere. This is especially true for 1822, the 2nd warmest year in Europe, but a cold year over most of the Northern Hemisphere. In contrast, the coldest year in Europe (1838) was indeed one of the coldest over much of the Northern hemisphere (see discussion below), but in fact, temperatures were nonetheless warm, relative to typical 20th century conditions, over significant portions of Greenland and Alaska. The coldest years in Europe might, by analogy with this example, have been quite unusually mild in Greenland, and a favorable opportunity for its colonization. It becomes readily evident from such examples (let alone, more careful statistical diagnostics) that inferences into hemispheric or global-scale temperature variations based on limited regional (e.g., European) information is perilous. In fact, the considerable low-frequency "noise" in the Atlantic and neighboring regions, due in large part to modes of ocean circulation variability (see e.g., Delworth and Mann, 2000), and the substantial overprint of the North Atlantic Oscillation on climate variations in this region during past centuries (see e.g., Cullen et al, 2000, Mann, 2000) particularly obscures hemispheric trends in this region. There is modeling evidence, in fact, that suggest that Medieval warmth was restricted to regions influenced by the North Atlantic (Overpeck, 1998). Statistically speaking, estimates of European temperature variability provide a very poor indication of large-scale temperature trends in past centuries, and should be strictly avoided for hemispheric, let-alone global-scale climate inferences.

Note that 1816 (the so-called "year without a summer"--Figure 16) in addition to appearing to have indeed been an especially cold summer (see Briffa et al, 1998) and a cold year for the NH temperature as a whole (though not anomalous relative to other years during that very cold decade), was an anomalously cold year only in Europe and parts of North America. In fact, conditions in the Middle and Near east were warmer than normal by 20^th century standards.

A number of other years (1870, 1864, 1838, 1820, 1700, 1642, and many years during the 1450s and 1460s decades) appear to have been substantially colder than 1816 for the hemisphere as a whole in the temperature reconstructions presented here. Our notions of this year as a particularly cold one may thus arise in large part from the fact that the coldness was most pronounced in those regions--Europe and North America--which figure most prominently in the western anecdotal and historical framework. The regional overprints of warming (e.g., in the Middle East) and extreme cold (e.g., Europe) which are superimposed on generally cold hemispheric conditions, in regions neighboring the North Atlantic may be attributed to the North Atlantic Oscillation or "NAO" (see Luterbacher et al, 1999; Cullen et al, 2000, Mann, 2000 for a discussion of inferences into past NAO-related climate variability). We believe that both the cold hemispheric conditions, and a strong NAO-like atmospheric circulation anomaly, were due to the explosive Tambora eruption in Indonesia during spring of 1815 (see MBH98). Seasonally-specific reconstructions of 1816 for the cold (Oct-Mar) and warm-half (Apr-Sep) years (see Figure 16) indicate that this pattern is clearer and more dominant during the cold-season, wherein the quadrapole pattern of warm and cold anomalies in continental regions bordering the North Atlantic is quite distinct. This is as expected, since the NAO is primarily, though not exclusively, a cold-season mode of atmospheric circulation variability. There is some evidence of the persistence of this pattern, albeit more weakly, into the warm-season. In particular, distinct cooling in the eastern United States and over much of Europe is clearly expressed during the warm-season, consistent with the notion of 1816 having been a "year without a summer" in those regions. Some of the coldness of the early 19th century might also be due to weakened solar irradiance forcing at that time. The possible influences of external climate forcings on hemispheric temperatures are discussed below.