Records Of Climate Forcing Factors

Historical observations have been very important in documenting two factors outside the climate system that may be important in causing climate to change (forcing factors). These are major explosive volcanic eruptions and solar variability. We will also consider here records of El Niño (ENSO) events,34 which are not strictly external forcings, but involve large-scale reorganizations of the ocean-atmosphere system, with global consequences.

Unusual post-sunset sky colors were commonly recorded by astute observers of the heavens, and many such observations can be linked to large explosive volcanic eruptions that lofted sulfur-rich gases and particulate matter high into the stratosphere. Scattering of solar radiation by these particles can reduce direct radiation and cause dramatic early morning or late evening sky colors (Meinel and Meinel, 1983). For example, Chinese chroniclers reported that, in the reign of Emperor Ling Ti (A.D. 168-189) "several times the sun rose in the east red as blood and lacking light . . . only when it had risen to an elevation of more than two zháng [24°] was there any brightness. . . .". At the same time, Roman observers noted, "before the war of the deserter [A.D. 186] the heavens were ablaze. . . . stars were seen all the

34 El Ninos (EN) are quasi-periodic changes in oceanographic conditions characterized by unusually warm water off the coasts of Ecuador and northern Peru, especially in December. They are associated with atmospheric anomalies that involved the redistribution of atmospheric mass across the South Pacific, a phenomenon termed the Southern Oscillation (SO). Together, oscillations of the coupled ocean-atmosphere system in the Pacific are termed ENSO events.

day long . . . hanging in the air which was a token of a cloud. ..." (Wilson et al., 1980). These are typical descriptions of the sky following major explosive volcanic eruptions; it seems likely that this particular event was related to eruptions in Alaska (White River ash), which have been radiocarbon-dated to around that time. Lamb (1970, 1977, 1983) used these kinds of observations as the basis for constructing a chronology of explosive volcanism over the past -500 yr, which he termed the Dust Veil Index (DVI) (Fig. 11.16). This proved to be invaluable in interpreting the acidity record in ice cores, which is also a register of explosive volcanic events (see Section 5.4.4). Many studies have used the DVI to assess the impact of explosive volcanism on temperature variations (Sear et al., 1987). Historical records have also been useful in documenting the global effects of one of the largest eruptions in the late Holocene (that of Tambora, Indonesia, in April, 1815). This event led to cold conditions in many areas the following year, which became known as "the year without a summer" (Harington, 1992).

Early astronomical observations of the sun noted dark spots on the photosphere, and records of these sunspots extend back to the early seventeenth century (Hoyt and Schatten, 1997). Long-term sunspot observations demonstrated a periodic variation in solar activity with a mean cycle length of -11 yr. Satellite observations have now shown that these variations involve changes in the solar constant of -0.1%. There is also documentary evidence that a prolonged episode of little or no

FIGURE 11.16 A dust veil index (DVI) for the northern hemisphere, assuming dust from an individual eruption is apportioned over four yr, with 40% of each DVI assigned to year 1,30% to year 2,20% to year 3, and 10% to year 4.Thus, the 1883 eruption of Krakatau (DVI = 1000) results in values of 400 in 1883, declining to 100 in 1886. It is further assumed that all dust from eruptions poleward of 20° N remained in the northern hemisphere. For eruptions equatorward of 15° N, the dust was assigned equally between the two hemispheres and for eruptions between 15° and 20° N and 15° and 20° S, it was assumed that two-thirds of the material remained in the hemisphere of the eruption, and one-third was dispersed to the other hemisphere (Bradley and Jones, 1992b; DVI values from Lamb, 1970, 1977, 1983).

FIGURE 11.16 A dust veil index (DVI) for the northern hemisphere, assuming dust from an individual eruption is apportioned over four yr, with 40% of each DVI assigned to year 1,30% to year 2,20% to year 3, and 10% to year 4.Thus, the 1883 eruption of Krakatau (DVI = 1000) results in values of 400 in 1883, declining to 100 in 1886. It is further assumed that all dust from eruptions poleward of 20° N remained in the northern hemisphere. For eruptions equatorward of 15° N, the dust was assigned equally between the two hemispheres and for eruptions between 15° and 20° N and 15° and 20° S, it was assumed that two-thirds of the material remained in the hemisphere of the eruption, and one-third was dispersed to the other hemisphere (Bradley and Jones, 1992b; DVI values from Lamb, 1970, 1977, 1983).

■ 11111111111111111 [ 1111111111111111111 Solar total irradiance

FIGURE 11.17 Reconstruction of solar total irradiance from 1610 to the present.The thin line is the irradiance variability of the Schwabe cycle, and the thick line is the Schwabe cycle plus a longer-term component that accounts for the amplitude of irradiance reductions since the Maunder Minimum (1645-1715) estimated independently from observations of sun-like stars (Lean et al„ 1995).

1364 1600

1700

1800

1900

2000

FIGURE 11.17 Reconstruction of solar total irradiance from 1610 to the present.The thin line is the irradiance variability of the Schwabe cycle, and the thick line is the Schwabe cycle plus a longer-term component that accounts for the amplitude of irradiance reductions since the Maunder Minimum (1645-1715) estimated independently from observations of sun-like stars (Lean et al„ 1995).

1700 -I VS

1800 VS

FIGURE 11.18 A reconstruction of the most important El Niño events since 1525, based largely on historical documentary sources. M = Medium, S = Significant,VS = Very Significant events (Quinn and Neale, 1992).

solar activity (the Maunder Minimum) occurred from -1675-1715 (Eddy, 1976). Lean et al. (1992, 1995) estimate that the long-term variability in solar activity from the Maunder Minimum to the present represents an overall increase in solar output of -0.24% (Fig. 11.17) that is significant enough to have had an effect on global climate (Rind and Overpeck, 1993; Lean and Rind, 1994). Observations of solar variability have also been valuable in showing that both 14C and 10Be records (from tree rings and ice cores, respectively) contain a solar-modulated signal. This may allow these isotopes to be used to reconstruct the history of solar variability, and its influence on climate, long before the beginning of historical sunspot observations (Beer et al., 1996; Stuiver and Braziunas, 1991).

Historical records of unusual weather events in key parts of the world have also been used to reconstruct the history of ENSO events (Quinn, 1993a, b; Quinn and Neal, 1992; Mabres et al., 1993). The ENSO events result in worldwide disruptions of the climate system, with certain regions being particularly affected. Thus, heavy rains in coastal Ecuador and northern Peru are common, often causing floods and landslides. Across the Pacific, ENSOs are associated with droughts in Indonesia and northeastern Australia, where brush fires are also likely. Historical documents have been used to piece together these events in the past and to rank them in terms of their overall magnitude (Fig. 11.18). Once again, this has proven useful in interpreting and verifying other proxy records of ENSO events, such as those derived from corals (see Section 6.8).

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