date: Thu, 12 Oct 2000 12:12:38 -0400 from: "Raymond S. Bradley" subject: synthesis section 1-revised to: k.briffa@uea.ac.uk, jcole@geo.Arizona.EDU, mhughes@ltrr.arizona.edu Can you take a quick glance through this. I've revised it & reorganised things a bit--regional sections folow this Ray Chapter 6. The Climate of the Last Millennium Raymond S. Bradley, Keith R. Briffa, Julia E. Cole and Malcolm K. Hughes 6.1 Introduction We are living in unusual times. Twentieth century climate was dominated by near universal warming; almost all parts of the globe had temperatures at the end of the century that were higher than when it began, and in most areas temperatures were significantly higher (Parker et al., 1998; Jones et al., 1999; Wallace et al., 2000). [Do we want a figure here Figure 6.1say a global map of (1900-1909)-(1990-1999) MAT from instrumental data?] However the instrumental data provide only a limited temporal perspective on present climate. How unusual was the last century when placed in the longer-term context of climate in the centuries and millennia leading up to the 20th century? Such a perspective encompasses the period before large-scale contamination of the global atmosphere and global-scale changes in land-surface conditions. By studying the records of climate variability and forcing mechanisms in the recent past, it is possible to establish how the climate system varied under “natural” conditions, before anthropogenic forcing became significant. Natural forcing mechanisms will continue to operate in the 21st century, and will play a role in future climate variations, so regardless of how anthropogenic effects develop it is essential to understand the underlying background record of forcing and climate system response. Sources of information on the climate of the last millennium include: historical documentary records, tree rings (width, density); ice cores (isotopes, melt layers, net accumulation, glaciochemistry); corals (isotopes and other geochemistry, growth rate); varved lake sediments (varve thickness, sedimentology, geochemistry, diatom and pollen content); banded speleothems (isotopes). These are all paleoclimatic proxies that can provide continuous records with annual resolution. Other information may be obtained from sources that are not continuous in time, and that have less rigorous chronological control. Such sources include geomorphological evidence (e.g. from former lake shorelines and glacier moraines) and sub-fossil macrofossils that indicate the range of plant or animal species in the recent past. In addition, ground temperature measurements in boreholes reflect the past history of surface temperatures, with temporal resolution decreasing with depth. These provide estimates of overall temperature changes from one century to the next (Pollack et al. 19XX). Proxies of past climate are generally controlled by a particular aspect of the system that causes a climate-related signal to be recorded. For some biological proxies, such as tree ring density or coral growth rate, the main factor might be temperature or more specifically, the temperature of a particular season (or even just part of a season). Density and growth rate might also be influenced by antecedent climatic conditions, or by other non-climatic factors. Similar issues are important in other proxies, such as the timing of snowfall events that make up an ice core, or the rate and timing of sediment transport to a lake. Though we recognize that the details of such relationships are important, proxies are rarely interpreted directly in terms of very specific controls, but rather in terms of temperature or precipitation in a particular season. In many cases the main climatic signal in a proxy record is not temperature alone. For example, evidence of a formerly high lake level may indicate higher rainfall amounts and/or a decrease in evaporation related to cooler temperatures. Such issues are grist to the paleoclimatologists’ mill and are the subject of numerous studies. Suffice it to say that proxies are generally selected to optimize a reconstruction of either temperature or precipitation and it is these studies that provide the basis for our review. Changes in temperature have large-scale spatial coherence, making it easier to identify major variations with relatively few records. Precipitation changes are more local or regional in extent, but they often reflect circulation changes that may have large-scale significance (as, for example, in ENSO-related rainfall increases that commonly occur in the southeastern U.S. during strong El Niño events; Stahle et al., 1998). In this chapter, we focus mainly on temperature variations, but precipitation and hydrological variability are examined where there is good evidence for important changes at the regional scale. In particular, we ask two questions regarding each attribute: · does the 20th century record indicate unique or unprecedented conditions? · do 20th century instrumental data provide a reasonable estimate of the range of natural variability that could occur in the near future? First we deal with the overall pattern of temperature change at the largest (hemispheric) scale. Then, we examine variability over four large sub-regions of the globe. These results are placed in the longer-term perspective of Holocene climatic changes, and finally we examine forcing factors that may have played a role in the variations that have been identified. 6.2 Temperatures over the last millennium Most high resolution paleoclimate records (i.e. those with annual resolution and a strong climate signal) do not extend back in time more than a few centuries. Consequently, while there are numerous paleoclimate reconstructions covering the period from the 17th century to the present, the number of high resolution millennium scale records is very limited. Continuous records are restricted to ice cores and laminated lake sediments, where the climatic signal is often poorly calibrated, and to a few long tree ring records, generally from high latitudes. Inevitably, this leads to large uncertainties in long-term climate reconstructions that attempt to provide a global or hemispheric-scale perspective. Bearing this in mind, what do current reconstructions tell us about the last millennium? Figure 6.2 shows a reconstruction of northern hemisphere mean annual temperature for the last 1000 years, based on a set of over [100?] well-distributed paleoclimatic records spanning the past 600 years but a much smaller number of data sets (12) for the period prior to A.D. 1400 (Mann et al., 1998, 1999). The paleoclimatic proxies were calibrated in terms of the main modes of temperature variations (eigenvectors) represented in the instrumental records for 1902-1980. Variations across the network of proxies, for the period before instrumental records, were then used to reconstruct how the main temperature patterns (i.e. their principal components) varied over time. By combining these patterns, regional, hemispheric or global mean temperature changes, as well as spatial patterns over time were reconstructed (Mann et al., 2000). To accurately reproduce the spatial pattern requires that the proxy data network is extensive enough to capture many of the principal eigenvector patterns. With the data available, regional patterns of temperature variation could only be meaningfully reconstructed for 250 years, although the large-scale (hemispheric) mean temperature could be reconstructed for a longer period. This is possible because the proxy data network, even at its sparsest, exhibits a coherent response to variability at the largest scale. Thus a reconstruction of hemispheric mean temperature back 1000 years is possible, using a quite limited network of data, albeit with ever-increasing uncertainty (i.e. expanding confidence limits) the further back in time one goes (Figure 6.2Mann et al 1000yr reconstruction with errors). This reconstruction shows an overall decline in temperature of ~0.2ºCfrom A.D. 1000 until the early 1900s when temperatures rose sharply. Superimposed on this decline were periods of several decades in length when temperatures were warmer or colder than the overall trend. Mild episodes, lasting a few decades, occurred around the late 10th/early 11th century and in the late 13th century, but there was no period when mean temperature was comparable with levels in the late 20th century. Coldest conditions occurred in the 15th century, the late 17th century and in the entire 19th century. Other attempts to assess northern hemisphere temperatures have taken a simpler approach, either averaging together normalized paleo-data of various types (Bradley and Jones, 1993; Jones et al., 1998) or averaging data scaled to a similar range (Crowley and Lowery 2000). Such approaches do not provide an estimation of uncertainty, and indeed may lead to rather arbitrary combinations of very diverse data (often having different temporal precision). Nevertheless, the resulting time series from all of these studies are similar, at least for the first 400-500 years. Thereafter, some series indicate especially cold conditions, from the late 16th century until the 19th century, but still generally within the 2 standard error confidence limits of Mann et al. (1999) (Figure 6.3composite showing low frequency records from Mann et al, 1999; Jones et al. * Crowley & Lowery + Mann et al uncertainties--from Briffa et al 2000 Fig. 10, without green line[Briffa 2000] and including both sets of uncertainty “curtains”). The differences between them may be explained by the different seasons and spatial coverage of data used. Each reconstruction represents a somewhat different spatial domain. In the Mann et al. studies, the “northern hemisphere mean” series is the same geographical domain as the gridded instrumental data set available for the period 1902-80. This means that some regions within the northern hemisphere (in the central Pacific, central Eurasia and regions beyond 70ºN) were not represented. However, the global eigenvector patterns that were reconstructed are based on data from low latitudes and parts of the southern hemisphere. Other reconstructions generally do not include data from sub-tropical or tropical regions, and this may explain the colder periods in the latter half of the Jones et al. and Crowley and Lowery records, if higher latitudes were particularly cold at that time compared to the Tropics. Another reason for the differences in Figure 6.3 may be because each reconstruction represents a somewhat different season. In the Mann et al. (1998, 1999) reconstruction, mean annual temperature data were used for calibration, since data from both hemispheres were used to constrain the eigenvector patterns and data from different regions may have had stronger signals in one season than in another. For example, some data from western Europe might contain a strong NAO (winter) signal, whereas data from elsewhere might carry a strong summer precipitation signal related to ENSO. Both data sets nevertheless help to define important modes of climate anomalies that themselves capture large scale annual temperature patterns (Bradley et al., 2000). Other reconstructions are for summer months (April-September) and this may also explain some of the differences between the series, for example, if summers were particularly cool in extra-tropical regions, in the 17th-19th centuries. Another critical question in any long-term reconstruction is to what extent does the proxy adequately capture the true low frequency nature of the climate record. Given that most of the long-term data used in all of these paleotemperature reconstructions are from tree-rings, it is important to establish that they are not contaminated by biological growth trends. This matter is especially critical when individual tree ring records, of differing record lengths (often limited to a few hundred years) are patched together to assess long-term climate changes. Briffa et al. (2000) have carefully evaluated this problem, using a maximum ring width density data set that is largely independent of that used by Mann et al. (1998, 1999). By combining sets of tree ring density data grouped by the number of years since growth began at each site, Briffa et al. provide a methodology that largely eliminates the biological growth function problem. They also estimate confidence limits through time (Figure 6.4Fig 10 from Briffa et al 2000 on its own, 2SE limits). The Briffa et al series shows similar temperature anomalies as Mann et al. in the 15th century (though no sharp decline in temperatures around A.D.1450) but markedly colder conditions from A.D. 1500 to ~A.D. 1800 (cf. Figures 6.2 and 6.4). The early 19th century is also colder in the Briffa et al. series. The Mann et al. and Briffa et al. series (Figure 6.3) bracket all other paleotemperature estimates for the northern hemisphere, such as those by Bradley and Jones (1993), Jones et al. (1998) Briffa et al., (1998), Overpeck et al. (1998) and Crowley and Lowery (2000). The Briffa et al reconstruction describes a well-defined minimum in temperatures from ~A.D. 1550-1850 that conforms with the consensus view of a “Little Ice Age” (Bradley and Jones,1992). Though this period was not uniformly cold and temperature anomalies differed regionally, overall it was significantly below the 1881-1960 mean (by as much as 0.5ºC for most of the 17th century) in the regions studied by Briffa et al. (2000). Independent reconstructions derived from borehole temperatures suggest even colder temperatures about 400 to 500 years ago, and/or even greater warming in the 20th century.[elaborate] To what extent these differences in reconstructed temperatures are related to the effect of land use change on borehole temperatures remains to be resolved, but it suggests that land use change may be another factor, in addition to changes in atmospheric trace gases and aerosols, that may have to be taken into account to realistically simulate past (and future) climate change. Although the Mann et al. and Briffa et al reconstructions have much in common, they are clearly not identical. One explanation for the differences may again lie in the geographical distribution of data used in each analysis. The study of Briffa et al. is strongly weighted towards the northern treeline (60-75ºN) where temperatures were particularly low in the 17th century; the study by Mann et al. includes data from lower latitudes, and incorporates temperature reconstructions from both marine and terrestrial regions in calculating a hemispheric mean. If this is the explanation for the differences, it suggests that low latitude regions (equatorward of ~35ºN) did not experience a drop in temperatures in the latter half of the last millennium comparable to higher northern latitudes. This further suggests an increase in the Equator-Pole temperature gradient during that time. One thing that all reconstructions clearly agree on is that northern hemisphere mean temperature in the 20th century is unique, both in its overall average and in the rate of temperature increase. In particular the 1990s were exceptionally warm -- probably the warmest decade for at least 1000 years (even taking the estimated uncertainties of earlier years into account). The last ~50 years also appear to have been the warmest period by far (Table 1??). A caveat to this conclusion is that the current proxy-based reconstructions do not extend to the end of the 20th century, but are patched on to the instrumental record of the last 2-3 decades. This is necessary because many paleo data sets were collected in the 1960s and 1970s, and have not been up-dated. Furthermore, in the case of tree rings from some areas (especially at high latitudes) the climatic relationships prevalent for most of the century appear to have changed in recent decades, possibly because of increasing aridity &/or snowcover changes at high latitudes that have altered the ecological responses of trees to climate (cf. Jacoby et al; Briffa et al; Vaganov et al., 1999). Consequently, it must be recognized that an assessment of the unusual nature of the 1990s is necessarily based on a direct comparison of instrumental data with long-term proxy-based reconstructions. Nevertheless, the conclusion that temperatures rose at unprecedented rates in the 20th century, reaching levels by the end of the century that were unprecedented within the last millennium seems to be an extremely robust result from these studies. (cf. Pollack et al. 1998??). Confidence that an accurate reproduction of the recent instrumental record would be possible if all the available paleoclimatic data were updated to the present is provided by Figure 6.5 (Mann et al. v. instrumental, 1850-1980). This shows that a set of proxy data calibrated against the 1902-1980 period of instrumental data captured mean annual temperatures well both during this period and during the preceding 50 years for which an independent set of instrumental data is available. The excellent fit over the late 19th century test period provides confidence that an updated set of proxy data would also accurately reproduce recent changes. Figure 6.3 also shows that the overall range in temperature over the last 1000 years has been quite small. For example, the range in 50-year means has only been ~0.5ºC [?cf. both records] (from the coldest period in the 15th , 16th and 19th centuries, to the warmest period of the last 50 years: Table 1??). Within that narrow envelope of variability, all of the significant environmental changes associated with the onset and demise of the “Little Ice Age” (~1450-1850) took place. This puts into vivid context the magnitude of projected future changes resulting from greenhouse-gas increases and associated feedbacks (Figure 6.6 Mann et al + others? + IPCC projections). Even the low end of model estimates suggest additional temperature increases on the order of 1-2ºC by the end of the 21st century (Intergovernmental Panel on Climate Change 1996 or 2000?). The discussion so far has focused exclusively on the northern hemisphere record because there are insufficient data currently available to produce a very reliable series for the southern hemisphere. Data from the Mann et al. (1998) reconstruction (back to A.D. 1700) averaged for those parts of the southern hemisphere that were represented in the instrumental calibration period, show a similar temporal pattern to that of the northern hemisphere, but generally warmer (less negative anomalies). However, much more work is needed on southern hemisphere proxy records to extend and verify that result. Raymond S. Bradley Professor and Head of Department Department of Geosciences University of Massachusetts Amherst, MA 01003-5820 Tel: 413-545-2120 Fax: 413-545-1200 Climate System Research Center: 413-545-0659 Climate System Research Center Web Page: Paleoclimatology Book Web Site (1999): http://www.geo.umass.edu/climate/paleo/html