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UW ATMS 211 - Lecture Notes

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On Past Temperatures and Anomalous late-20th Century Warmth Mann, M.E., Ammann, C.M., Bradley, R.S., Briffa, K.R., Crowley, T.J., Hughes, M.K., Jones, P.D., Oppenheimer, M., Osborn, T.J., Overpeck, J.T., Rutherford, S., Trenberth, K.E., Wigley, T.M.L. (author affiliations at end) Evidence from paleoclimatic sources and modeling studies support AGU's official position statement on "Climate Change and Greenhouse Gases," that there is a compelling basis for concern over future climate changes, including increases in global-mean surface temperatures, due to increased concentrations of greenhouse gases, primarily from fossil-fuel burning. More specifically, a number of reconstructions of large-scale temperature changes over the past millennium support the conclusion that late-20th century warmth was unprecedented over at least the past millennium. Modeling and statistical studies indicate that such anomalous warmth cannot be fully explained by natural factors but, instead, require a significant anthropogenic forcing of climate that emerged during the 19th and 20th centuries. Two (nearly identical) recent papers [Soon and Baliunas, 2003 and Soon et al., 2003--henceforth both referred to as 'SB03'] challenge this view, and have been used to support the claim that recent hemispheric-scale warmth is not unprecedented in the context of the past millennium (see e.g. "20th Century Climate Not So Hot", press-release, Harvard-Smithsonian Center for Astrophysics, March 31, 2003: http://cfa-www.harvard.edu/press/pr0310.html). Such claims are inconsistent with the preponderance of scientific evidence. We therefore review these claims in the light of the fact that they have found their way into the media and have been read into the U.S. Senate record. Instrumental data for use in computing global mean surface temperatures are only available for about the past 150 years [Jones et al., 1999]. Estimates of surface temperature changes further back in time must make use of historical documents and natural archives or "proxy" indicators, such as tree rings, corals, ice cores and lake sediments, to reconstruct the patterns of past climate change. Due to the paucity of data in the Southern Hemisphere, recent studies have emphasized the reconstruction of Northern Hemisphere (NH), rather than global mean temperatures over roughly the past 1000 years. A large number of such reconstructions [Mann et al., 1999; Jones et al., 1998; Crowley and Lowery, 2000] now support the conclusion that the hemispheric-mean warmth of the late 20th century (i.e., the past few decades) is likely unprecedented in the last 1000 years [Jones et al., 2001; Folland et al., 2001]. Preliminary evidence [Mann and Jones, 2003] suggests that such a conclusion may well hold for at least the past two millennia (Figure 1). Climate model simulations employing estimates of natural and anthropogenic radiative forcing changes [Crowley, 2000; Gerber et al., 2002; Bauer et al., 2003] agree well, for the most part, with the proxy-based reconstructions (Figure 1). The simulations, furthermore, show that it is not possible to explain the anomalous late 20th century warmth without the contribution from anthropogenic forcing factors [e.g., Crowley, 2000] and that the role of anthropogenic forcing can clearly be detected in the proxy-based temperature reconstructions [Hegerl et al., 2003]. Here, we raise the following key points regarding recent assertions (SB03) challenging these findings: (1) In drawing inferences regarding past regional temperature changes from proxy records, it is essential to assess proxy data for actual sensitivity to past temperature variability. Seminal work in the reconstruction of past climate [Lamb, 1965] examined a number of different variables, including hydrological indicators, for insights into past climate change, but only in a particular region (Europe) where the synoptic-scale relationship between temperature and hydrological variability was fairly well established and understood. The existence of possible underlying dynamical relationships between temperature and hydrological variability should not be confused with the patently invalid assumption that hydrological influences can literally be equated withtemperature influences in assessing past climate (e.g. during Medieval times). Such a criterion is implicit, for example, in the SB03 approach that defines a global 'warm anomaly' as a period during which various regions appear to indicate climate anomalies that can be classified as being either 'warm', 'wet', or 'dry' relative to '20th century' conditions. Such a criterion, ad absurdum, could be used to define any period of climate as 'warm' or 'cold' and thus makes no meaningful contribution to discussions of past climate change. (2) It is essential to distinguish [e.g. by compositing or otherwise assimilating different proxy information in a consistent manner—e.g., Jones et al., 1998; Mann et al., 1998, 1999; Briffa et al., 2001] between regional temperature anomalies and anomalies in hemispheric mean temperature which must represent an average of temperature estimates over a sufficiently large number of distinct regions [see e.g. Folland et al., 2001; Trenberth and Otto-Bliesner, 2003]. It is well known that weather patterns have a wave-like character to them. This character ensures that certain regions tend to warm (due, for example, to a southerly flow in the Northern Hemisphere winter mid-latitudes) when other regions cool (due to the corresponding northerly flow that must occur elsewhere). In a similar vein, the specification of a warm period, requires that warm anomalies in different regions should be synchronous and not merely required to occur during any 50 year period within a very broad interval in time, such as AD 800-1300, as in SB03. Figure 2 demonstrates the considerable spatial variability in temperature variations of the past millennium, and the false impression one might gain regarding hemispheric-scale temperature changes from the apparent temperature changes in any particular region. The specific notions of the 'Little Ice Age' and 'Medieval Warm Period' arose, understandably, from the Eurocentric origins of historical climatology [e.g. Lamb, 1965]. While


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