9.3.3.2 Role of Volcanism and Solar Irradiance

Volcanic eruptions cause rapid decreases in hemispheric and global mean temperatures followed by gradual recovery over several years (Section 9.2.2.1) in climate simulations driven by volcanic forcing (Figure 6.13; Crowley, 2000; Bertrand et al., 2002; Weber, 2005; Yoshimori et al., 2005; Tett et al., 2007). These simulated changes appear to correspond to cool episodes in proxy reconstructions (Figure 6.13). This suggestive correspondence has been confirmed in comparisons between composites of temperatures following multiple volcanic eruptions in simulations and reconstructions (Hegerl et al., 2003; Weber, 2005). In addition, changes in the frequency of large eruptions result in climate variability on decadal and possibly longer time scales (Crowley, 2000; Briffa et al., 2001; Bertrand et al., 2002; Bauer et al., 2003; Weber, 2005). Hegerl et al. (2003; 2007), using a multi-regression approach based on Energy Balance Model (EBM) simulated fingerprints of solar, volcanic and greenhouse gas forcing (Appendix 9.A.1; see also Section 9.4.1.4 for the 20th century), simultaneously detect the responses to volcanic and greenhouse gas forcing in a number of proxy reconstructions of average NH mean annual and growing season temperatures (Figure 9.4) with high significance. They find that a high percentage of decadal variance in the reconstructions used can be explained by external forcing (between 49 and 70% of decadal variance depending upon the reconstruction).

Figure 9.4. Contribution of external forcing to several high-variance reconstructions of NH temperature anomalies, (Esper et al., 2002; Briffa et al., 2001; Hegerl et al., 2007, termed CH-blend and CH-blend long; and Moberg et al., 2005). The top panel compares reconstructions to an EBM simulation (equilibrium climate sensitivity of 2.5°C) of NH 30°N to 90°N average temperature, forced with volcanic, solar and anthropogenic forcing. All timeseries are centered on the 1500-1925 average. Instrumental temperature data are shown by a green line (centered to agree with CH-blend average over the period 1880-1960). The displayed data are low-pass filtered (20-year cutoff) for clarity. The bottom panel shows the estimated contribution of the response to volcanic (blue lines with blue uncertainty shade), solar (green) and greenhouse gas (GHG) and aerosol forcing (red line with yellow shades, aerosol only in 20th century) to each reconstruction (all timeseries are centered over the analysis period). The estimates are based on multiple regression of the reconstructions on fingerprints for individual forcings. The contributions to different reconstructions are indicated by different line styles (Briffa et al.: solid, fat; Esper et al.: dotted; Moberg: dashed; CH-blend: solid, thin; with shaded 90% confidence limits around best estimates for each detectable signal). All reconstructions show a highly significant volcanic signal, and all but Moberg et al. (which ends in 1925) show a detectable greenhouse gas signal at the 5% significance level. The latter shows a detectable greenhouse gas signal with less significance. Only Moberg et al. contains a detectable solar signal (only shown for these data and CH-blend, where it is not detectable). All data are decadally averaged. The reconstructions represent slightly different regions and seasons: Esper et al. (2002) is calibrated to 30°N to 90°N land temperature, CH-blend and CH-blend long (Hegerl et al., 2007) to 30°N to 90°N mean temperature and Moberg et al. (2005) to 0° to 90°N temperature. From Hegerl et al. (2007).

There is more uncertainty regarding the influence of solar forcing. In addition to substantial uncertainty in the timing and amplitude of solar variations on time scales of several decades to centuries, which has increased since the TAR although the estimate of solar forcing has been revised downwards (Sections 9.2.1.3 and 2.7.1), uncertainty also arises because the spatial response of surface temperature to solar forcing resembles that due to greenhouse gas forcing (Section 9.2.3). Analyses that make use of differences in the temporal evolution of solar and volcanic forcings are better able to distinguish between the two (Section 9.2.3; see also Section 9.4.1.5 for the 20th century). In such an analysis, solar forcing can only be detected and distinguished from the effect of volcanic and greenhouse gas forcing over some periods in some reconstructions (Hegerl et al., 2003, 2007), although the effect of solar forcing has been detected over parts of the 20th century in some time-space analyses (Section 9.4.1.5) and there are similarities between regressions of solar forcing on model simulations and several proxy reconstructions (Weber, 2005; see also Waple, 2002). A model simulation (Shindell et al., 2003) suggests that solar forcing may play a substantial role in regional anomalies due to dynamical feedbacks. These uncertainties in the contribution of different forcings to climatic events during the last millennium reflect substantial uncertainty in knowledge about past solar and volcanic forcing, as well as differences in the way these effects are taken into account in model simulations.

Overall, modelling and detection and attribution studies confirm a role of volcanic, greenhouse gas and probably solar forcing in explaining the broad temperature evolution of the last millennium, although the role of solar forcing has recently been questioned (Foukal et al., 2006). The variability that remains in proxy reconstructions after estimates of the responses to external forcing have been removed is broadly consistent with AOGCM-simulated internal variability (e.g., Hegerl et al., 2003, 2007), providing a useful check on AOGCMs even though uncertainties are large. Such studies also help to explain episodes during the climate of the last millennium. For example, several modelling studies suggest that volcanic activity has a dominant role in explaining the cold conditions that prevailed from 1675 to 1715 (Andronova et al., 2007; Yoshimori et al., 2005). In contrast, Rind et al. (2004) estimate from model simulations that the cooling relative to today was primarily associated with reduced greenhouse gas forcing, with a substantial contribution from solar forcing.

There is also some evidence from proxy data that the response to external forcing may influence modes of climate variability. For example, Cobb et al. (2003), using fossil corals, attempt to extend the ENSO record back through the last millennium. They find that ENSO events may have been as frequent and intense during the mid-17th century as during the instrumental period, with events possibly rivalling the strong 1997–1998 event. On the other hand, there are periods during the 12th and 14th centuries when there may have been significantly less ENSO variability, a period during which there were also cooler conditions in the northeast Pacific (MacDonald and Case, 2005) and evidence of droughts in central North America (Cook et al., 2004). Cobb et al. (2003) find that fluctuations in reconstructed ENSO variability do not appear to be correlated in an obvious way with mean state changes in the tropical Pacific or global mean climate, while Adams et al. (2003) find statistical evidence for an El Niño-like anomaly during the first few years following explosive tropical volcanic eruptions. The Cane-Zebiak model simulates changes similar to those in the Cobb et al. (2003) data when volcanism and solar forcing are accounted for, supporting the link with volcanic forcing over the past millennium (Mann et al., 2005). However, additional studies with different models are needed to fully assess this relationship, since previous work was less conclusive (Robock, 2000).

Extratropical variability also appears to respond to volcanic forcing. During the winter following a large volcanic eruption, the zonal circulation may be more intense, causing a relative warming over the continents during the cold season that could partly offset the direct cooling due to the volcanic aerosols (Sections 9.2.2.1 and 8.4.1; Robock, 2000; Shindell et al., 2003). A tendency towards the negative NAO state during periods of reduced solar input is found in some reconstructions of this pattern for the NH (Shindell et al., 2001b; Luterbacher et al., 2002, 2004; Stendel et al., 2006), possibly implying a solar forcing role in some long-term regional changes, such as the cooling over the NH continents around 1700 (Shindell et al., 2001b; Section 9.2.2). Indications of changes in ENSO variability during the low solar irradiance period of the 17th to early 18th centuries are controversial (e.g., D’Arrigo et al., 2005).