Daily temperature and pressure series for Salem, Massachusetts (1786–1829)

The digitization and homogenization of a record with four daily thermometer and two daily barometer readings is described for the meteorological journal of Dr. E. A. Holyoke of Salem (Massachusetts). These records begin in January 1786 and span the period to March 1829 for temperature, and the period to December 1820 for pressure. The records are reconstructed and some inhomogeneities are identified and corrected for. The temperature data compares favourably with monthly temperature data from New Haven (Connecticut) and a comparison with independently reconstructed daily pressure data for nearby Cambridge (1780–1789) show that the temporal variations of the data agree very well. It is shown that the number of extremely cold days was considerably greater during 1786–1829 than recent times, while the number of warm days in the early and modern records are comparable. A probability distribution of daily winter pressure values shows a mean of the distribution which is 3 to 4 hPa lower during 1786–1820.     

Documentary evidence of climate variability during cold seasons in Lesotho, southern Africa, 1833–1900

This study presents the first 19th century cold season climate chronology for the Kingdom of Lesotho in southern Africa. The chronology is constructed using a variety of documentary sources including letters, diaries, reports, monographs and newspaper articles obtained from southern African and British archives. Information relating to cold season weather phenomena during the austral autumn, winter and early spring months were recorded verbatim. Each of the cold seasons from 1833 to 1900 was then classified as “very severe”, “severe” or “normal/mild”, with a confidence rating ranging from low (1) to high (3) awarded against each annual classification. The accuracy of the document-derived chronology was verified against temperature data for Maseru for the period 1893–1900. Excellent correspondence of the document-derived chronology with the Maseru instrumental data and also with other global proxy temperature records for the 19th century is achieved. The results indicate 12 (18% of the total) very severe, 16 (23%) severe and 40 (59%) normal/mild cold seasons between 1833 and 1900. The overall trend is for more severe and snow-rich cold seasons during the early part of the study period (1833–1854) compared with the latter half of the 19th century (with the exception of the 1880s). A reduction in the duration of the frost season by over 20 days during the 19th century is also tentatively identified. Several severe to very severe cold seasons in Lesotho follow after major tropical and SH volcanic eruptions; such years are usually characterized by early frosts, and frequent and heavy snowfalls. The blocking of solar radiation and the enhanced northward displacement of polar fronts that are directly or indirectly associated with volcanic events, may account for many of the most severe Lesotho winters during the 19th century.     

North Atlantic oscillation control of droughts in north-east Spain: evaluation since 1600 a.d.

This paper analyses the role played by the North Atlantic Oscillation (NAO) in the creation of drought conditions in a semi-arid region of north-east Spain (the middle Ebro valley), from 1600 to the year 2000. The study used documents from ecclesiastical archives for the seventeenth, eighteenth and nineteenth centuries. For the twentieth century, instrumental precipitation records were used as well. A December–August drought index from 1600 to 1900 was compiled from the historical documentary sources (rogation ceremonies). The index was validated by means of precipitation records between 1858 and 1900 and independent precipitation data from 1600 reconstructed by means of dendrochronological records. Using instrumental data a drought index was also calculated (Standardized Precipitation Index, SPI) for the 1958–2000 period. We found that the NAO was important in explaining the droughts identified in the study area from documents and instrumental data. Positive values of the winter NAO index are prone to cause droughts in the middle Ebro valley. This finding has been verified since 1600 by means of two independent reconstructions of the winter NAO index. The same behaviour has been observed during the nineteenth and twentieth centuries by means of instrumental records. The climatic and geographic factors that explain the high influence of North Atlantic Oscillation on droughts in this region are discussed in depth.     

Tree-ring records from New Zealand: long-term context for recent warming trend

 Distinct periods of warmth have been identified in instrumental records for New Zealand and the surrounding southwest Pacific over the past 120 years. Whether this warming is due to natural climate variability or the effects of increasing greenhouse gases is difficult to determine given the limited length of instrumental record. Longer records derived from tree rings can help reduce uncertainties in detection of possible causes of climatic change, although relatively few such records have been developed for the Southern Hemisphere. In this work, we describe five temperature-sensitive tree-ring width chronologies for New Zealand which place the recent warming trend into a long-term (pre-anthropogenic) context. Included are three pink pine (Halocarpus biformis) chronologies, two for Stewart Island and one for the North Island of New Zealand. Two silver pine (Lagarostrobus colensoi) series, one each from the North and South Islands, are updated from previous work. The length of record ranges from AD 1700 for Putara, North Island to AD 1400 for Ahaura, South Island. The pink and silver pine are different species from those used previously to reconstruct temperatures for New Zealand. All five chronologies are positively and significantly correlated with warm-season (November-April) individual station temperature records, a New Zealand-wide surface air temperature index and gridded land/marine temperatures for New Zealand and vicinity. The highest 20 and 40-year growth periods in all five tree-ring series coincide with the New Zealand temperature increase after 1950. An exception is found for the 40-year interval at Ahaura, the least temperature-sensitive of the five sites. A t-test comparison indicates that these recent growth intervals are significantly higher (0.01 to 0.0001 level) than any of those prior to the twentieth century for three of the five sites, dating as far back as AD 1500. The results suggest that the recent warming has been distinctive, although not clearly unprecedented, relative to temperature conditions inferred from tree-ring records of prior centuries. Received: 18 February 1997/Accepted: 11 September 1997     

Observed and simulated multidecadal variability in the Northern Hemisphere

 Analyses of proxy based reconstructions of surface temperatures during the past 330 years show the existence of a distinct oscillatory mode of variability with an approximate time scale of 70 years. This variability is also seen in instrumental records, although the oscillatory nature of the variability is difficult to assess due to the short length of the instrumental record. The spatial pattern of this variability is hemispheric or perhaps even global in scale, but with particular emphasis on the Atlantic region. Independent analyses of multicentury integrations of two versions of the GFDL coupled atmosphere-ocean model also show the existence of distinct multidecadal variability in the North Atlantic region which resembles the observed pattern. The model variability involves fluctuations in the intensity of the thermohaline circulation in the North Atlantic. It is our intent here to provide a direct comparison of the observed variability to that simulated in a coupled ocean-atmosphere model, making use of both existing instrumental analyses and newly available proxy based multi-century surface temperature estimates. The analyses demonstrate a substantial agreement between the simulated and observed patterns of multidecadal variability in sea surface temperature (SST) over the North Atlantic. There is much less agreement between the model and observations for sea level pressure. Seasonal analyses of the variability demonstrate that for both the model and observations SST appears to be the primary carrier of the multidecadal signal. Received: 8 June 1999 / Accepted: 11 February 2000     

Secular Variability of the Coupled Tropospheric and Stratospheric Circulation in the GCM ECHAM 3/LSG

Summary  Secular or multi-decadal variability is a widely observed phenomenon, apparent in instrumental and paleo climatic records. These long time oscillations are found in many variables of the climate system. The ocean especially experiences low frequency variations. But also atmospheric variables such as temperature, wind velocity and sea level pressure can show secular variability. The low frequency variability here is examined in the coupled atmosphere-ocean model ECHAM3/LSG T21. A coupled stratospheric and tropospheric mode is detected oscillating with a period of approximately 100 years. The atmospheric pressure system mainly involved in this oscillation is the northern hemispheric winter stratospheric polar vortex. The near surface temperature experiences variations of the same magnitude as the observed temperature trends of the last decades. Multi decadal variability is also shown in the North Atlantic Oscillation Index. A shift of the length of the oscillation period between longer and shorter time scales indicates that chaotic processes might be responsible for the variability. Received February 22, 1999/Revised August 5, 1999     

Monthly, seasonal and annual temperature reconstructions for Central Europe derived from documentary evidence and instrumental records since AD 1500

Monthly temperature series for Central Europe back to AD 1500 are developed from documentary index series from Germany, Switzerland and the Czech Republic (1500–1854) and 11 instrumental temperature records (1760–2007). Documentary evidence from the Low Countries, the Carpathian Basin and Poland are used for cross-checking for earlier centuries. The instrumental station records are corrected for inhomogeneities, including insufficient radiation protection of early thermometers and the urban heat island effect. For overlapping period (1760–1854), the documentary data series correlate with instrumental temperatures, most strongly in winter (86% explained variance in January) and least in autumn (56% in September). For annual average temperatures, 81% of the variance is explained. Verification statistics indicate high reconstruction skill for most months and seasons. The last 20 years (since 1988) stand out as very likely the warmest 20-year period, accounting for the calibration uncertainty and decreases in proxy data quality before the calibration period. The new reconstruction displays a previously unobserved long-term decrease in DJF, MAM and JJA temperature variability over last five centuries. Compiled monthly, seasonal and annual series can be used to improve the robustness of gridded large-scale European temperature reconstructions and possible impact studies. Further improvement of the reconstruction would be achieved if documentary data from other European countries are further developed.     

Calibration and Instrumental Errors in Early Measurements of Air Temperature

Calibration and instrumental errors of early thermometers are analysed. The first concepts, the development of scientific ideas, the main problems linked to the construction of early thermometers and the calibration are considered in order to evaluate errors. The operating principle of the `constant-volume' air thermometer is presented andits limits are discussed. The theory shows that for an ideal Amontons' thermometer, only one calibration point is sufficient from which the other can be calculated; it is therefore possible to determine the difference from an ideal instrument. A comparison is made between calibrations and instruments made by G. Amontons (1699), G. Poleni (Venice, 1709; Padova 1725), and J.H. Lambert (1779). Amontons' thermometer needs to be integrated with a barometric reading; an important error arises from the different density of mercury during calibration and usage. The calibration was made in winter and at the upper point the thermometer was at 100 ° C, while the barometer remained near 0 ° C. However, field observations were made with both instruments at the same temperature and this caused an error that in the cold season is negligible, but in the hot season reaches 1 ° C. Problems connected with the calibration and scale linearity are discussed in view of the beliefs of the time and the operative methodologies used in early meteorology. Emphasis is given to comparability of different instruments, thermometric scales and calibration methodologies used in Padova in the 18th century for Amontons' Poleni's, Fahrenheit's, Réaumur's and Delisle's thermometers. The instrument supports and the incision of the scale were subject to expand or contract depending on temperature or humidity changes, and this was a source of error that can be corrected with the help of observed or estimated data. Problems linked to the construction of thermometers have been evaluated, as well as the linearity of displacements of the thermometric liquid, or the drift due to ageing and transformation of the thermometric liquid, the glass or the support. All these errors have been evaluated and some of them have been found negligible, but some are of the order of 0.5 ° C. The quantitative resultsobtained here can also be usefully applied to correct and validate other long series.     

The earliest temperature observations in the world: the Medici Network (1654–1670)

This paper presents the earliest temperature observations, scheduled every 3–4 h in the 1654–1670 period, which have been recovered and analysed for the first time. The observations belong to the Medici Network, the first international network of meteorological observations, based on eleven stations, the two main ones being Florence and Vallombrosa, Italy. All observations were made with identical thermometers and operational methodology, including outdoor exposure in the shade and in the sunshine to evaluate solar heating, state of the sky, wind direction and precipitation frequency. This paper will consider only the regular temperature series taken in the shade. The observations were made with the newly invented spirit-in-glass thermometer, also known as Little Florentine Thermometer (LFT). The readings have been transformed into modern units of temperature (°C) and time (TMEC). The LFT has been analysed in detail: how it was made, its linearity, calibration and performances. Since the middle of the LIA, the climate in Florence has shown less than 0.18°C warming. However, although the yearly average showed little change, the seasonal departures are greater, i.e. warmer summers, colder winters and unstable mid seasons. The temperature in the Vallombrosa mountain station, 1,000 m a.m.s.l, apparently rose more, i.e. 1.41°C. A discussion is made on the interpretation of this finding: how much it is affected by climate change or bias. A continuous swinging of the temperature was observed in the Mediterranean area, as documented by the long instrumental observations over the 1654–2009 period. However, changes in vegetation, or exposure bias might have contributed to reduce the homogeneity of the series over the centuries.     

Tree-ring estimates of Pacific decadal climate variability

 Decadal-scale oscillatory modes of atmosphere-ocean variability have recently been identified in instrumental studies of the Pacific sector. The regime shift around 1976 is one example of such a fluctuation, which has been shown to have significantly impacted climate and the environment along the coastline of the western N and S Americas. The length of meteorological data for the Pacific and western Americas critically limits analyses of such decadal-scale climate variability. Here we present reconstructions of the annual Pacific Decadal Oscillation (PDO) index based on western North American tree-ring records which account for up to 53% of the instrumental variance and extend as far back as AD 1700. The PDO reconstructions indicate that decadal-scale climatic shifts have occurred prior to the period of instrumental record. Evaluation of temperature and precipitation-sensitive tree-ring series from the northeast Pacific as well as these reconstructions reveals evidence for a shift towards less pronounced interdecadal variability after about the middle 1800s. Our analyses also suggest that sites from both the northeast Pacific coast as well as the subtropical Americas need to be included in proxy data sets used to reconstruct the PDO. Received: 15 September 2000 / Accepted: 30 March 2001     

Analysis of Durban rainfall and Nile river flow 1871–1999

Summary  Climatic fluctuations across Africa are analysed from two century+ records of rainfall at Durban, South Africa and the Nile River flow in southern Egypt. A wavelet transform analysis is applied to the rainfall record to determine the strength of intra-seasonal to decadal rhythms. The annual cycle constitutes 33% of variability, whilst 2.3–4 year cycles account for 10% of the variance. A contingency analysis of flood events reveals a bimodal character with peaks in November and March. The Durban rainfall time series is compared with remote environmental variables. Close relationships are found with the meridional gradient of sea surface temperature in the Atlantic and the southern oscillation index. Comparisons are made between the southern summer rainfall at Durban (November–March) and northern summer Nile River flow (July–October). Cross-wavelet analysis of the two records indicates a matching of frequency in quasi-biennial and El Ni?o frequency bands. This suggests that the uptake of ‘teleconnections’ governing African climate occurs in a widespread manner. Received February 25, 2000     

A comparison of the Vostok ice deuterium record and series from Southern Ocean core MD 88-770 over the last two glacial-interglacial cycles

Taking advantage of the fact that the Vostok deuterium (δD) record now covers almost two entire climatic cycles, we have applied the orbital tuning approach to derive an age-depth relation for the Vostok ice core, which is consistent with the SPECMAP marine time scale. A second age-depth relation for Vostok was obtained by correlating the ice isotope content with estimates of sea surface temperature from Southern Ocean core MD 88-770. Both methods lead to a close correspondence between Vostok and MD 88-770 time series. However, the coherence between the correlated δD and insolation is much lower than between the orbitally tuned δD and insolation. This reflects the lower accuracy of the correlation method with respect to direct orbital tuning. We compared the ice and marine records, set in a common temporal framework, in the time and frequency domains. Our results indicate that changes in the Antarctic air temperature quite clearly lead variations in global ice volume in the obliquity and precession frequency bands. Moreover, the average phase we estimated between the filtered δD and insolation signals at precessional frequencies indicates that variations in the southern high latitude surface temperature could be induced by changes in insolation taking place during a large period of the summer in northern low latitudes or winter in southern low latitudes. The relatively large lag found between Vostok δD variations and obliquity-driven changes in insolation suggests that variations in the local radiative balance are not the only mechanism responsible for the variability in surface temperature at those frequencies. Finally, in contrast to the cross-spectral analysis method used in previous studies, the method we use here to estimate the phases can reveal errors in cross-correlations with orbitally tuned chronologies. Received: 11 April 1995 / Accepted: 19 July 1995     

Cycles and shifts: 1,300 years of multi-decadal temperature variability in the Gulf of Alaska

The Gulf of Alaska (GOA) is highly sensitive to shifts in North Pacific climate variability. Here we present an extended tree-ring record of January–September GOA coastal surface air temperatures using tree-ring width data from coniferous trees growing in the mountain ranges along the GOA. The reconstruction (1514–1999), based on living trees, explains 44% of the temperature variance, although, as the number of chronologies decreases back in time, this value decreases to, and remains around ∼30% before 1840. Verification of the calibrated models is, however, robust. Utilizing sub-fossil wood, we extend the GOA reconstruction back to the early eighth century. The GOA reconstruction correlates significantly (95% CL) with both the Pacific Decadal Oscillation Index (0.53) and North Pacific Index (−0.42) and therefore likely yields important information on past climate variability in the North Pacific region. Intervention analysis on the GOA reconstruction identifies the known twentieth century climate shifts around the 1940s and 1970s, although the mid-1920s shift is only weakly expressed. In the context of the full 1,300 years record, the well studied 1976 shift is not unique. Multi-taper method spectral analysis shows that the spectral properties of the living and sub-fossil data are similar, with both records showing significant (95% CL) spectral peaks at ∼9–11, 13–14 and 18–19 years. Singular spectrum analysis identifies (in order of importance) significant oscillatory modes at 18.7, 50.4, 38.0, 91.8, 24.4, 15.3 and 14.1 years. The amplitude of these modes varies through time. It has been suggested (Minobe in Geophys Res Lett 26:855–858, 1999) that the regime shifts during the twentieth century can be explained by the interaction between pentadecadal (50.4 years) and bidecadal (18.7 years) oscillatory modes. Removal of these two modes of variance from our GOA time series does indeed remove the twentieth century shifts, but many are still identified prior to the twentieth century. Our analysis suggests that climate variability of the GOA is very complex, and that much more work is required to understand the underlying oscillatory behavior that is observed in instrumental and proxy records from the North Pacific region.

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Indicators of Climate Warming in Minnesota: Lake ICE Covers and Snowmelt Runoff

Records of hydrologic parameters, especially those parameters that are directly linked to air temperature, were analyzed to find indicators of recent climate warming in Minnesota, USA. Minnesota is projected to be vulnerable to climate change because of its location in the northern temperate zone of the globe. Ice-out and ice-in dates on lakes, spring (snowmelt) runoff timing, spring discharge values in streams, and stream water temperatures recorded up to the year 2002 were selected for study. The analysis was conducted by inspection of 10-year moving averages, linear regression on complete and on partial records, and by ranking and sorting of events. Moving averages were used for illustrative purposes only. All statistics were computed on annual data. All parameters examined show trends, and sometimes quite variable trends, over different periods of the record. With the exception of spring stream flow rates the trends of all parameters examined point toward a warming climate in Minnesota over the last two or three decades. Although hidden among strong variability from year to year, ice-out dates on 73 lakes have been shifting to an earlier date at a rate of −0.13 days/year from 1965 to 2002, while ice-in dates on 34 lakes have been delayed by 0.75 days/year from 1979 to 2002. From 1990 to 2002 the rates of change increased to −0.25 days/year for ice-out and 1.44 days/year for ice-in. Trend analyses also show that spring runoff at 21 stream gaging sites examined occurs earlier. From 1964 to 2002 the first spring runoff (due to snowmelt) has occurred −0.30 days/year earlier and the first spring peak runoff −0.23 days/year earlier. The stream water temperature records from 15 sites in the Minneapolis/St Paul metropolitan area shows warming by 0.11^∘C/year, on the average, from 1977 to 2002. Urban development may have had a strong influence. The analysis of spring stream flow rates was inconclusive, probably because runoff is linked as much to precipitation and land use as to air temperature. Ranking and sorting of annual data shows that a disproportionately large number of early lake ice-out dates has occurred after 1985, but also between 1940 and 1950; similarly late lake ice-in has occurred more frequently since about 1990. Ranking and sorting of first spring runoff dates also gave evidence of earlier occurrences, i.e. climate warming in late winter. A relationship of changes in hydrologic parameters with trends in air temperature records was demonstrated. Ice-out dates were shown to correlate most strongly with average March air temperatures shifting by −2.0 days for a 1^°C increase in March air temperature. Spring runoff dates also show a relationship with March air temperatures; spring runoff dates shift at a rate of −2.5 days/1^°C minimum March air temperature change. Water temperatures at seven river sites in the Minneapolis/St Paul metropolitan area show an average rise of 0.46^°C in river temperature/1^°C mean annual air temperature change, but this rate of change probably includes effects of urban development. In conclusion, records of five hydrologic parameters that are closely linked to air temperature show a trend that suggests recent climate warming in Minnesota, and especially from 1990 to 2002. The recent rates of change calculated from the records are very noteworthy, but must not be used to project future parameter values, since trends cannot continue indefinitely, and trend reversals can be seen in some of the long-term records.     

Propagating decadal sea surface temperature signal identified in modern proxy records of the tropical Pacific

Analysis of 86 years of multiple modern coral δ¹⁸O records in the tropical Pacific reveals a basin-scale decadal pattern of variability. Although coral δ¹⁸O records the effects of both temperature and seawater δ¹⁸O variability due to salinity effects, in practice, most of the records used here agree well with observations of sea surface temperature on longer timescales. These coral proxy records reveal strong variability near a 12-year period. Their relative phasing suggests a signal propagating from the southwestern subtropical Pacific to other regions. The results are consistent with recent studies based on instrumental data and with coupled climate model studies, in which advection of thermal anomalies leads to El Niño/Southern Oscillation-like variability on decadal timescales. Additionally, there is evidence for a significant shift in many of the time series, along with a decrease in the decadal variability, occurring in the early 1940s. Finally, the proxy records indicate the presence of strong teleconnections between the eastern tropical Pacific and high latitude climate.     

The Changing Incidence of Extremes in Worldwide and Central England Temperatures to the End of the Twentieth Century

Annual and seasonal gridded ocean surface temperature anomalies show an increase in warm extremes and a decrease in cold extremes since the late 19th century attributable entirely to the overall warming trend. Over land, however, a reduction in the total incidence of extremes may reflect improved instrumental exposures. Our estimates of extremes are made by deriving percentiles from fits of anomalies on 5° latitude ×5° longitude resolution to modified 2-parameter gamma distributions. A non-parametric method is used to check the validity of the results. Fields of percentiles created using this technique can be used to map the distribution of unusual temperature anomalies across the globe on any time scale from a month to about a decade, from 1870 onwards. We apply a similar technique to assess changes in the incidence of extreme daily Central England temperature anomalies. The incidence of these extremes, relative to individual monthly average temperatures, has declined.     

Multi-century variability in the Pacific North American circulation pattern reconstructed from tree rings

We here present a reconstruction (1725–1999) of the winter Pacific North American (PNA) pattern based on three winter climate sensitive tree ring records from the western USA. Positive PNA phases in our record are associated with warm phases of ENSO and PDO and the reorganization of the PNA pattern towards a positive mode is strongest when ENSO and PDO are in phase. Regime shifts in our PNA record correspond to climatic shifts in other proxies of Pacific climate variability, including two well-documented shifts in the instrumental period (1976 and 1923). The correspondence breaks down in the early 19th century, when our record shows a prolonged period of positive PNA, with a peak in 1800–1820. This period corresponds to a period of low solar activity (Dalton Minimum), suggesting a ‘positive PNA like’ response to decreased solar irradiance. The distinct 30-year periodicity that dominates the PNA reconstruction in the 18th century and again from 1875 onwards is disrupted during this period.     

Long-term summer temperature variations in the Pyrenees

Two hundred and sixty one newly measured tree-ring width and density series from living and dry-dead conifers from two timberline sites in the Spanish Pyrenees were compiled. Application of the regional curve standardization method for tree-ring detrending allowed the preservation of inter-annual to multi-centennial scale variability. The new density record correlates at 0.53 (0.68 in the higher frequency domain) with May–September maximum temperatures over the 1944–2005 period. Reconstructed warmth in the fourteenth to fifteenth and twentieth century is separated by a prolonged cooling from ∼1450 to 1850. Six of the ten warmest decades fall into the twentieth century, whereas the remaining four are reconstructed for the 1360–1440 interval. Comparison with novel density-based summer temperature reconstructions from the Swiss Alps and northern Sweden indicates decadal to longer-term similarity between the Pyrenees and Alps, but disagreement with northern Sweden. Spatial field correlations with instrumental data support the regional differentiation of the proxy records. While twentieth century warmth is evident in the Alps and Pyrenees, recent temperatures in Scandinavia are relatively cold in comparison to earlier warmth centered around medieval times, ∼1450, and the late eighteenth century. While coldest summers in the Alps and Pyrenees were in-phase with the Maunder and Dalton solar minima, lowest temperatures in Scandinavia occurred later at the onset of the twentieth century. However, fairly cold summers at the end of the fifteenth century, between ∼1600–1700, and ∼1820 were synchronized over Europe, and larger areas of the Northern Hemisphere.     

Estimating present climate in a warming world: a model-based approach

Weather services base their operational definitions of “present” climate on past observations, using a 30-year normal period such as 1961–1990 or 1971–2000. In a world with ongoing global warming, however, past data give a biased estimate of the actual present-day climate. Here we propose to correct this bias with a “delta change” method, in which model-simulated climate changes and observed global mean temperature changes are used to extrapolate past observations forward in time, to make them representative of present or future climate conditions. In a hindcast test for the years 1991–2002, the method works well for temperature, with a clear improvement in verification statistics compared to the case in which the hindcast is formed directly from the observations for 1961–1990. However, no improvement is found for precipitation, for which the signal-to-noise ratio between expected anthropogenic changes and interannual variability is much lower than for temperature. An application of the method to the present (around the year 2007) climate suggests that, as a geographical average over land areas excluding Antarctica, 8–9 months per year and 8–9 years per decade can be expected to be warmer than the median for 1971–2000. Along with the overall warming, a substantial increase in the frequency of warm extremes at the expense of cold extremes of monthly-to-annual temperature is expected.     

Unusual Climate in Northwest Europe During the Period 1730 to 1745 Based on Instrumental and Documentary Data

This study focuses on one of the most interesting times of the early instrumental period in northwest Europe (from 1730–1745) attempting to place the extremely cold year of 1740 and the unusual warmth of the 1730s decade in a longer context. The similarity of the features in the few long (and independent) instrumental records together with extensive documentary evidence clearly indicates that remarkable climatic changes occurred rapidly in this period. We use unpublished subjective circulation charts developed by the late Hubert Lamb, to assist in understanding the course of events, particularly during the extreme year of 1740 and the four subsequent years. We also compare these subjective charts with others recently developed using more objective modern reconstruction techniques. Apart from evidence of a reduction in the number of explosive volcanic eruptions following the 1690s, it is difficult to explain the changes in terms of our knowledge of the possible factors that have influenced this region during the 19th and 20th centuries. The study, therefore, highlights how estimates of natural climatic variability in this region based on more recent data may not fully encompass the possible known range.