Volcanism and the Little Ice Age THOM A S J. CRO wl E Y 1*, G. ZIELINSK I 2, B. VINTHER 3, R. UD ISTI 4, K. KREUT Z 5, J. CO L E -DA I 6 A N D E. CA STE lla NO 4 1School of Geosciences, University of Edinburgh, UK; [email protected] 2Center for Marine and Wetland Studies, Coastal Carolina University, Conway, USA 3Niels Bohr Institute, University of Copenhagen, Denmark 4Department of Chemistry, University of Florence, Italy 5Department of Earth Sciences, University of Maine, Orono, USA 6Department of Chemistry and Biochemistry, South Dakota State University, Brookings, USA *Collaborating authors listed in reverse alphabetical order. The Little Ice Age (LIA; ca. 1250-1850) has long been considered the coldest interval of the Holocene. Because of its proximity to the present, there are many types of valuable resources for reconstructing tem- peratures from this time interval. Although reconstructions differ in the amplitude Special Section: Comparison Data-Model of cooling during the LIA, they almost all agree that maximum cooling occurred in the mid-15th, 17th and early 19th centuries. The LIA period also provides climate scientists with an opportunity to test their models against a time interval that experienced both significant volcanism and (perhaps) solar insolation variations. Such studies provide information on the ability of models to simulate climates and also provide a valuable backdrop to the subsequent 20th century warming that was driven primarily from anthropogenic greenhouse gas increases. Although solar variability has often been considered the primary agent for LIA cooling, the most comprehensive test of this explanation (Hegerl et al., 2003) points instead to volcanism being substantially more important, explaining as much as 40% of the decadal-scale variance during the LIA. Yet, one problem that has continu- Figure 1: Comparison of new ice core reconstruction with various instrumental-based reconstructions of strato- ally plagued climate researchers is that the spheric aerosol forcing. The asterisks refer to some modification to the instrumental data; for Sato et al. (1993) and the Lunar AOD, the asterisk refers to the background AOD being removed for the last 40 years. For Stothers paleo-volcanic record, reconstructed from (1996), it refers to the fact that instrumental observations for Krakatau (1883) and the 1902 Caribbean eruptions Antarctic and Greenland ice cores, cannot were only for the northern hemisphere. To obtain a global AOD for these estimates we used Stothers (1996) data be well calibrated against the instrumen- for the northern hemisphere and our data for the southern hemisphere. The recon,struction for Agung eruption (1963) employed Stothers (1996) results from 90°N-30°S and the Antarctic ice core data for 30-90°S. tal record. This is because the primary in- strumental volcano reconstruction used marily Santa Maria), and the 1912 erup- Although the above analysis may not per- by the climate community is that of Sato tion of Novarupta/Katmai (Alaska) against manently put to rest uncertainties about et al. (1993), which is relatively poorly con- a reanalysis (Stothers, 1996) of the original calibration of ice core sulfate data, it does strained by observations prior to 1960 (es- AOD data and lunar eclipse estimates of represent an encouraging advance over pecially in the southern hemisphere). AOD for Krakatau (Keen, 1983). The agree- present reconstructions. We therefore ap- Here, we report on a new study that ment (Fig. 1) is very good—essentially plied the methodology to part of the LIA has successfully calibrated the Antarctic within the uncertainty of the independent record that had some of the largest tem- sulfate record of volcanism from the 1991 data. Our new ice core reconstruction perature changes over the last millenni- eruptions of Pinatubo (Philippines) and shows several significant disagreements um. A total of 13 Greenland and Antarctic Hudson (Chile) against satellite aerosol op- with the Sato et al. (1993) reconstruction ice cores were used as the main database, tical depth (AOD) data (AOD is a measure in the late 19th and early 20th centuries. As with spot data included from other cores. of stratospheric transparency to incoming we have essentially the same number of The ice core reconstruction was then com- solar radiation). A total of 22 cores yield records over the entire interval of the Sato pared (Fig. 2) against the temperature re- an area-weighted sulfate accumulation et al. reconstruction, and their database construction of Jones et al. (1998), which rate of 10.5 kg/km2, which translates into decreases significantly prior to 1960, we indicates the standard cooling patterns a conversion rate for AOD of 0.011 AOD/ contend that our reconstruction for the discussed above, including the well known kg/km2 sulfate. We validated our time se- earlier intervals is at least as good as that very severe cooling of the mid-1690s in ries by comparing a canonical growth and of Sato et al. It is, at the very least, a legiti- western Europe. decay curve for eruptions for Krakatau mate alternate reconstruction to use for The ice core chronology of volcanoes (1883), the 1902 Caribbean eruptions (pri- this earlier time interval. is completely independent of the (pri- 22 PAGES News • Vol.16 • No 2 • April 2008 canic eruptions. If this uncertainty can be significantly reduced, which now seems to be the case, it will significantly narrow the range of uncertainty in the paleoclimate sensitivity estimate—a prospect we plan to explore after the initial paper has been written. Full documentation of the scaling and time series is now being prepared for pub- lication. The database for the reconstruc- tion plus the volcano time series of forc- ing, interpolated to ca. 10 day intervals for the purpose of use in climate model simu- lations, will be released upon publication. References Hegerl, G., Crowley, T., Baum, S., Kim, K.-Y. and Hyde, W., 2003: Detection of volcanic, solar, and greenhouse signals in paleo-reconstruc- tions of Northern Hemisphere temperature, Geophysical Research Special Section: Comparison Data-Model Letters, 30(5): 1242, doi:10.1029/2002GL0166335. Hegerl, G., Crowley T., Hyde, W. and Frame, D., 2006: Constraints on climate sensitivity from temperature reconstructions of the last millennium, Nature, 440: 1029-1032. Jones, P., Briffa, K., Barnett, T., and Tett, S., 1998: High resolution palaeo- climatic records for the last millennium: Interpretation, integra- tion, and comparison with General Circulation Model control-run temperatures, The Holocene, 8(4): 455-471. Keen, R., 1983: Volcanic aerosols and lunar eclipses, Science, 222: Figure 2: Comparison of 30-90°N version of ice core reconstruction with Jones et al. (1998) temperature recon- 1011-1013. struction over the interval 1630-1850. Vertical dashed lines denote levels of coincidence between eruptions and Sato, M., Hansen, J., McCormick, M. and Pollack, J., 1993: Stratospheric reconstructed cooling. AOD = Aerosol Optical Depth. aerosol optical depths, 1850-1990, Journal of Geophysical Re- search, 98: 22987-22994. marily) tree ring based temperature re- that they reoccur within the 10-year recov- construction. The volcano reconstruction ery timescale of the ocean mixed layer; i.e., For full references please consult: www.pages-igbp.org/products/newsletter/ref2008_2.html is deemed accurate to within 0 ± 1 years the ocean has not recovered from the first over this interval. There is a striking agree- eruption so the second eruption drives the ment between 16 eruptions and cooling temperatures to an even lower state. events over the interval 1630-1850. Of The new reconstruction also predicts particular note is the very large cooling in higher AOD levels for Krakatau than Sato 1641-1642, due to the concatenation of et al. (1993). This has important implica- sulfate plumes from two eruptions (one tions for prior paleo-reconstructions, as in Japan and one in the Philippines), and the Sato et al. (1993) estimate has been a a string of eruptions starting in 1667 and key calibration point for absolute scaling culminating in a large tropical eruption in of the entire paleo-volcanic reconstruc- 1694 (tentatively attributed to Long Island, tion. The new result implies that methods off New Guinea). This large tropical erup- using the Sato et al. (1993) approach could tion (inferred from ice core sulfate peaks in underestimate the true magnitude of ear- both hemispheres) occurred almost exact- lier eruptions by ca. 20%. This conclusion ly at the beginning of the coldest phase of represents just one of the many of the the LIA in Europe and represents a strong benefits of the new reconstruction. argument against the implicit link of Late A final benefit of the new reconstruc- Maunder Minimum (1640-1710) cooling tion is that the uncertainty in the absolute to solar irradiance changes. magnitude of past volcanic forcing may be During the 18th century lull in erup- reduced by about one-half, to (tentatively) tions, temperatures recovered somewhat 10-15% (1 σ). This reduction may have sig- but then cooled early in the 19th century. nificant implications for estimating climate The sequence begins with a newly postu- sensitivity from the record of the last 1000 lated unknown tropical eruption in mid- years. For example, Hegerl et al. (2006) late 1804, which deposited sulfate in both estimate that the range of climate sensi- Greenland and Antarctica. Then, there are tivity for the LIA was approx. the same as four well-documented eruptions—an un- for the instrumental record. Climate sen- known tropical eruption in 1809, Tambora sitivity is a measure of the magnitude of (1815) and a second doublet tentatively the system response to a particular forc- attributed in part to Babuyan (Philip- ing, such as CO2 changes; it is essentially pines) in 1831 and Cosiguina (Nicaragua) the Holy Grail of greenhouse gas climate in 1835.