Keith R. Briffa

High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control-run temperatures:

Palaeoclimatology provides our only means of assessing climatic variations before the beginning of instrumental records. The various proxy variables used, however, have a number of limitations which must be adequately addressed and understood. Besides their obvious spatial and seasonal limitations, different proxies are also potentially limited in their ability to represent climatic variations over a range of different timescales. Simple correlations with instrumental data over the period since ad 1881 give some guide to which are the better proxies, indicating that coral- and ice-core-based reconstructions are poorer than tree-ring and historical ones. However, the quality of many proxy time series can deteriorate during earlier times. Suggestions are made for assessing proxy quality over longer periods than the last century by intercomparing neighbouring proxies and, by comparisons with less temporally resolved proxies such as borehole temperatures. We have averaged 17 temperature reconstructions (representing various seasons of the year), all extending back at least to the mid-seventeenth century, to form two annually resolved hemispheric series (NH10 and SH7). Over the 1901–91 period, NH10 has 36% variance in common with average NH summer (June to August) temperatures and 70% on decadal timescales. SH7 has 16% variance in common with average SH summer (December to February) temperatures and 49% on decadal timescales, markedly poorer than the reconstructed NH series. The coldest year of the millennium over the NH is ad 1601, the coldest decade 1691–1700 and the seventeenth is the coldest century. A Principal Components Analysis (PCA) is performed on yearly values for the 17 reconstructions over the period ad 1660–1970. The correlation between PC1 and NH10 is 0.92, even though PC1 explains only 13.6% of the total variance of all 17 series. Similar PCA is performed on thousand-year-long General Circulation Model (GCM) data from the Geophysical Fluid Dynamics Laboratory (GFDL) and the Hadley Centre (HADCM2), sampling these for the same locations and seasons as the proxy data. For GFDL, the correlation between its PC1 and its NH10 is 0.89, while for HADCM2 the PCs group markedly differently. Cross-spectral analyses are performed on the proxy data and the GFDL model data at two different frequency bands (0.02 and 0.03 cycles per year). Both analyses suggest that there is no large-scale coherency in the series on these timescales. This implies that if the proxy data are meaningful, it should be relatively straightforward to detect a coherent near-global anthropogenic signal in surface temperature data.

Influence of volcanic eruptions on Northern Hemisphere summer temperature over the past 600 years

A network of temperature-sensitive tree-ring-density chronologies provides circum-hemisphere information on year-by-year changes in summer warmth in different regions of the northern boreal forest. Combining these data into a single time-series provides a good summer-temperature proxy for northern high latitudes and the Northern Hemisphere as a whole. Here we use this well dated, high-resolution composite time-series to suggest that large explosive volcanic eruptions produced different extents of Northern Hemisphere cooling during the past 600 years. The large effect of some recent eruptions is apparent, such as in 1816, 1884 and 1912, but the relative effects of other known, and perhaps some previously unknown, pre-nineteenth-century eruptions are also evaluated. The most severe short-term Northern Hemisphere cooling event of the past 600 years occurred in 1601, suggesting that either the effect on climate of the eruption of Huaynaputina, Peru, in 1600 has previously been greatly underestimated, or another, as yet unidentified, eruption occurred at the same time. Other strong cooling events occurred in 1453, seemingly confirming a 1452 date for the eruption of Kuwae, southwest Pacific, and in 1641/42, 1666, 1695 and 1698.

Reduced sensitivity of recent tree-growth to temperature at high northern latitudes

Tree-ring chronologies that represent annual changes in the density of wood formed during the late summer can provide a proxy for local summertime air temperature. Here we undertake an examination of large-regional-scale wood-density/air-temperature relationships using measurements from hundreds of sites at high latitudes in the Northern Hemisphere. When averaged over large areas of northern America and Eurasia, tree-ring density series display a strong coherence with summer temperature measurements averaged over the same areas, demonstrating the ability of this proxy to portray mean temperature changes over sub-continents and even the whole Northern Hemisphere. During the second half of the twentieth century, the decadal-scale trends in wood density and summer temperatures have increasingly diverged as wood density has progressively fallen. The cause of this increasing insensitivity of wood density to temperature changes is not known, but if it is not taken into account in dendroclimatic reconstructions, past temperatures could be overestimated. Moreover, the recent reduction in the response of trees to air-temperature changes would mean that estimates of future atmospheric CO2 concentrations, based on carbon-cycle models that are uniformly sensitive to high-latitude warming, could be too low.

Fennoscandian summers from AD 500: Temperature changes on short and long timescales

Quantitative estimates of 1480 years of summer temperatures in northern Fennoscandia have previously been derived from continuous treering records from northern Sweden. Here we show the results of spectral analyses of these data. Only a few peaks in the spectra are consistently significant when the data are analyzed over a number of sub-periods. Relatively timestable peaks are apparent at periods of 2.1, 2.5, 3.1, 3.6, 4.8, ∼ 32–33 and for a range between ∼ 55–100 years. These results offer no strong evidence for solar-related forcing of summer temperatures in these regions. Our previously published reconstruction was limited in its ability to represent long-timescale temperature change because of the method used to standardize the original tree-ring data. Here we employ an alternative standardization technique which enables us to capture temperature change on longer timescales. Considerable variance is now reconstructed on timescales of several centuries. In comparison with modern normals (1951–70) generally extended periods when cool conditions prevailed, prior to the start of the instrumental record, include 500–700, 790–870, 1110–1150, 1190–1360, 1570–1750 (A.D.) with the most significant cold troughs centred on about 660, 800, 1140, 1580–1620 and 1640. Predominantly warm conditions occurred in 720–790, 870–1110 and 1360–1570 with peaks of warmth around 750, 930, 990, 1060, 1090, 1160, 1410, 1430, 1760 and 1820.

Low-frequency temperature variations from a northern tree ring density network

We describe new reconstructions of northern extratropical summer temperatures for nine subcontinental-scale regions and a composite series representing quasi “Northern Hemisphere” temperature change over the last 600 years. These series are based on tree ring density data that have been processed using a novel statistical technique (age band decomposition) designed to preserve greater long-timescale variability than in previous analyses. We provide time-dependent and timescale-dependent uncertainty estimates for all of the reconstructions. The new regional estimates are generally cooler in almost all precalibration periods, compared to estimates obtained using earlier processing methods, particularly during the 17th century. One exception is the reconstruction for northern Siberia, where 15th century summers are now estimated to be warmer than those observed in the 20th century. In producing a new Northern Hemisphere series we demonstrate the sensitivity of the results to the methodology used once the number of regions with data, and the reliability of each regional series, begins to decrease. We compare our new hemisphere series to other published large-regional temperature histories, most of which lie within the 1σ confidence band of our estimates over most of the last 600 years. The 20th century is clearly shown by all of the palaeoseries composites to be the warmest during this period.

Recent warming reverses long-term arctic cooling.

Climate Reversal The climate and environment of the Arctic have changed drastically over the short course of modern observation. Kaufman et al. (p. 1236) synthesized 2000 years of proxy data from lakes above 60° N latitude with complementary ice core and tree ring records, to create a paleoclimate reconstruction for the Arctic with a 10-year resolution. A gradual cooling trend at the start of the record had reversed by the beginning of the 20th century, when temperatures began to increase rapidly. The long-term cooling of the Arctic is consistent with a reduction in summer solar insolation caused by changes in Earths orbit, while the rapid and large warming of the past century is consistent with the human-caused warming. A 2000-year-long Arctic cooling trend seen in a surface air temperature reconstruction was reversed during the last century. The temperature history of the first millennium C.E. is sparsely documented, especially in the Arctic. We present a synthesis of decadally resolved proxy temperature records from poleward of 60°N covering the past 2000 years, which indicates that a pervasive cooling in progress 2000 years ago continued through the Middle Ages and into the Little Ice Age. A 2000-year transient climate simulation with the Community Climate System Model shows the same temperature sensitivity to changes in insolation as does our proxy reconstruction, supporting the inference that this long-term trend was caused by the steady orbitally driven reduction in summer insolation. The cooling trend was reversed during the 20th century, with four of the five warmest decades of our 2000-year-long reconstruction occurring between 1950 and 2000.

Annual climate variability in the Holocene: interpreting the message of ancient trees

Abstract Over vast areas of the worlds landmasses, where climate beats out a strong seasonal rhythm, tree growth keeps unerring time. In their rings, trees record many climate melodies, played in different places and different eras. Recent years have seen a consolidation and expansion of tree-ring sample collections across the traditional research areas of North America and Europe, and the start of major developments in many new areas of Eurasia, South America and Australasia. From such collections are produced networks of precisely dated chronologies; records of various aspects of tree growth, registered continuously, year by year across many centuries. Their sensitivities to different climate parameters are now translated into ever more detailed histories of temperature and moisture variability across expanding dimensions of time and space. With their extensive coverage, high temporal resolution and rigid dating control, dendroclimatic reconstructions contribute significantly to our knowledge of late Holocene climates, most importantly on timescales ranging from 1 to 100 years. In special areas of the world, where trees live for thousands of years or where subfossil remnants of long dead specimens are preserved, work building chronologies covering many millennia continues apace. Very recently, trees have provided important new information about major modes of general circulation dynamics linked to the El Nino/Southern Oscillation and the North Atlantic Oscillation, and about the effect of large volcanic eruptions. As for assessing the significance of 20th century global warming, the evidence from dendroclimatology in general, supports the notion that the last 100 years have been unusually warm, at least within a context of the last two millennia. However, this evidence should not be considered equivocal. The activities of humans may well be impacting on the `natural’ growth of trees in different ways, making the task of isolating a clear climate message subtly difficult.

Large-scale temperature inferences from tree rings: a review

This paper is concerned with dendroclimatic research aimed at representing the history of very large-scale temperature changes. It describes recent analyses of the data from a widespread network of tree-ring chronologies, made up of ring width and densitometric measurement data spanning three to six centuries. The network was built over many years from trees selected to maximise their sensitivity to changing temperature. This strategy was adopted so that temperature reconstructions might be achieved at both regional and very large spatial scales. The focus here is on the use of one growth parameter: maximum latewood density (MXD). The detailed nature of the temperature sensitivity of MXD across the whole network has been explored and the dominant common influence of mean April–September temperature on MXD variability is demonstrated. Different approaches to reconstructing past temperature for this season include the production of detailed year-by-year gridded maps and wider regional integrations in the form of subcontinental and quasi-hemispheric-scale histories of temperature variability spanning some six centuries. These ‘hemispheric’ summer series can be compared with other reconstructions of temperature changes for the Northern Hemisphere over the last millennium. The tree-ring-based temperature reconstructions show the clear cooling effect of large explosive volcanic eruptions. They also exhibit greater century-timescale variability than is apparent in the other hemispheric series and suggest that the late 15th and the 16th centuries were cooler than indicated by some other data. However, in many tree-ring chronologies, we do not observe the expected rate of ring density increases that would be compatible with observed late 20th century warming. This changing climate sensitivity may be the result of other environmental factors that have, since the 1950s, increasingly acted to reduce tree-ring density below the level expected on the basis of summer temperature changes. This prevents us from claiming unprecedented hemispheric warming during recent decades on the basis of these tree-ring density data alone. Here we show very preliminary results of an investigation of the links between recent changes in MXD and ozone (the latter assumed to be associated with the incidence of UV radiation at the ground). D 2003 Elsevier B.V. All rights reserved.

Estimating sampling errors in large-scale temperature averages

A method is developed for estimating the uncertainty (standard error) of observed regional, hemispheric, and global-mean surface temperature series due to incomplete spatial sampling. Standard errors estimated at the grid-box level [SE2 5 S2(1 2 r)/(1 1 (n 2 1)r)] depend upon three parameters: the number of site records (n) within each box, the average interrecord correlation (r) between these sites, and the temporal variability (S2 )o f each grid-box temperature time series. For boxes without data (n 5 0), estimates are made using values of S2 interpolated from neighboring grid boxes. Due to spatial correlation, large-scale standard errors in a regionalmean time series are not simply the average of the grid-box standard errors, but depend upon the effective number of independent sites (Neff) over the region. A number of assumptions must be made in estimating the various parameters, and these are tested with observational data and complementary results from multicentury control integrations of three coupled general circulation models (GCMs). The globally complete GCMs enable some assumptions to be tested in a situation where there are no missing data; comparison of parameters computed from the observed and model datasets are also useful for assessing the performance of GCMs. As most of the parameters are timescale dependent, the resulting errors are likewise timescale dependent and must be calculated for each timescale of interest. The length of the observed record enables uncertainties to be estimated on the interannual and interdecadal timescales, with the longer GCM runs providing inferences about longer timescales. For mean annual observed data on the interannual timescale, the 95% confidence interval for estimates of the global-mean surface temperature since 1951 is 60.128C. Prior to 1900, the confidence interval widens to 60.188C. Equivalent values on the decadal timescale are smaller: 60.108C (1951‐95) and 60.168C (1851‐1900).

THE 'LITTLE ICE AGE': RE-EVALUATION OF AN EVOLVING CONCEPT

ABSTRACT. This review focuses on the development of the ‘Little Ice Age’ as a glaciological and climatic concept, and evaluates its current usefulness in the light of new data on the glacier and climatic variations of the last millennium and of the Holocene. ‘Little Ice Age’ glacierization occurred over about 650 years and can be defined most precisely in the European Alps (c. AD 1300–1950) when extended glaciers were larger than before or since. ‘Little Ice Age’ climate is defined as a shorter time interval of about 330 years (c. AD 1570–1900) when Northern Hemisphere summer temperatures (land areas north of 20°N) fell significantly below the AD 1961–1990 mean. This climatic definition overlaps the times when the Alpine glaciers attained their latest two highstands (AD 1650 and 1850). It is emphasized, however, that ‘Little Ice Age’ glacierization was highly dependent on winter precipitation and that ‘Little Ice Age’ climate was not simply a matter of summer temperatures. Both the glacier‐centred and the climate‐centred concepts necessarily encompass considerable spatial and temporal variability, which are investigated using maps of mean summer temperature variations over the Northern Hemisphere at 30‐year intervals from AD 1571 to 1900. ‘Little Ice Age’‐type events occurred earlier in the Holocene as exemplified by at least seven glacier expansion episodes that have been identified in southern Norway. Such events provide a broader context and renewed relevance for the ‘Little Ice Age’, which may be viewed as a ‘modern analogue’ for the earlier events; and the likelihood that similar events will occur in the future has implications for climatic change in the twenty‐first century. It is concluded that the concept of a ‘Little Ice Age’ will remain useful only by (1) continuing to incorporate the temporal and spatial complexities of glacier and climatic variations as they become better known, and (2) by reflecting improved understanding of the Earth‐atmosphere‐ocean system and its forcing factors through the interaction of palaeoclimatic reconstruction with climate modelling.