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INSTRUMENTAL SURFACE AIR TEMPERATURE VARIATIONS SINCE THE 1850S IN THE CONTEXT OF THE LAST MILLENNIUM, AND FUTURE PROJECTIONS PHIL JONES The surface air temperature record has been extensively reviewed by individuals1 and by the Intergovernmental Panel on Climate Change2-4.

Averaged for all available global areas, a warming of about 0·6°C has occurred between the late nineteenth century and 1998. How reliable is this estimate and what are the remaining uncertainties? This brief review illustrates seasonal and spatial patterns of change and discusses issues related to the homogeneity of the record and its accuracy.

Homogeneity The surface temperature record is composed of two components, values measured at fixed stations on land and observations over the ocean taken by merchant and naval vessels and buoys. Both components have numerous problems related to alterations in the methods of observation and in coverage which can compromise the long-term homogeneity of the basic measurements.

For land stations, observations are affected by changes over the last 150 years in instrumentation and measurement technique, movement of the instruments to new sites, observation times and the environment around the station. In an ideal world we would like all records to be homogeneous, i.e. affected solely by the vagaries of the weather and climate. All of the above problems can compromise an individual site record causing the record to be potentially biased in some way. All records, therefore, must be assessed for homogeneity. In the dataset to be used later1 each station has been checked by local inter-station comparisons. Stations indicating jumps/trends relative to their neighbours were either adjusted or discarded. Most of the problems induce random biases in the data and will be detected as part of the exercise.

The most serious problem is urbanization which can influence station records with an artificial warming trend. Comparisons with specifically designated rural stations indicate that any residual differences are an order of magnitude smaller than the 0·6°C warming evident in the record5.

Marine temperatures are taken in two forms: sea surface temperature (SST) and marine air temperature (MAT). Both types of data have homogeneity problems but it is much easier to correct SST data. Problems with SST relate principally to the switch from the use of canvas buckets before the early 1940s to engine intake measurements afterwards. The latter have been shown to be about 0·3–0·7°C warmer than bucket measurements. Canvas buckets are uninsulated and the water in the bucket cools evaporatively on average by the above amount between sampling and reading. Folland and Parker6 have devised a physical-based method that accounts for this cooling, enabling the amount of cooling to depend upon season and location.

That both sets of corrections agree is attested to by comparisons on a hemispheric basis and for regions of the world.

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Figure 1: Hemispheric and global temperature averages for the period 1856–1998, relative to 1961–90. The smooth line on these and subsequent time series plots highlights variations on the decadal timescale.

Hemispheric and Global Averages Figure 1 shows annual time series for the two hemispheres and the globe.

Both hemispheres show a century timescale warming. On a global basis the warming is 0·57°C for the 1861–1997 period (0·62°C cover 1901–1997), with the values slightly higher by 0·02–0·03 for the SH compared to the NH. The warming is highly statistically significant; incorporating the standard errors gives a range of only ±0·03, a similar value to the standard error of estimating a trend. Inspection of Figure 1 clearly shows, however, that the warming is not well approximated by a linear trend, occurring rather in two distinct phases (the early 1920s to the mid-1940s and since the mid-1970s). In the global series 11 of the 13 warmest years occur between 1986 and 1998. The warmest year of the entire series occurred in 1998, 0·57°C above the 1961–90 average.

Trends in maximum and minimum temperatures Most studies of temperature changes this century have tended to consider only changes in the mean, principally because this variable is the most widely

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available. Greater understanding of the causes of some of the changes can be gained by considering other variables. Maximum and minimum temperatures, although less widely available have been examined7, and for data covering 37% of the global land area, minimum temperatures warmed relative to maximum temperatures over 1951–90 by 0·14°C per decade. Recently, more extensive analyses, using 54% of the global land area, confirmed these findings with data covering the period 1950–93. Minimum temperature rose, over this time, by 0·18°C per decade, maxima by 0·08°C per decade and the diurnal temperature range decreased by 0·08°C per decade.

Daily temperature extremes in long European series The last two sections consider extremes on a monthly basis, but the public (rather, media) perception of climate change is often gleaned from daily extremes. A few extreme cold days in a row in a region might question the long term warming whereas a heatwave produces the opposite reaction! In the context of the global warming issue, daily data should be relatively unimportant as detection of change and attribution to human influences is primarily concerned with underlying trends on decadal timescales. In the public and political worlds though, changing frequencies of extremes are how much of the global warming debate is perceived.

Daily temperature series present greater problems to the climatologist with respect to homogeneity than monthly data. Many of the homogeneity issues discussed earlier have a much greater influence on daily values with the effects minimising on the monthly timescales. Site and observation time changes are particularly important and may not be correctable in all cases. Few long daily series are, therefore, totally homogeneous and availability of series in digital form means most analyses are restricted to the last 50 years.

The longest daily series are available in Europe and a few have had extensive homogeneity analyses undertaken, enabling some confidence to be placed in derived results. The longest currently available daily record (1772 to the present) is that for Central England, an average of three sites 8.

The public perception of extremes might be cold winter days or hot summer days, but this definition only considers the extreme seasons and necessitates a somewhat arbitrary definition of a threshold for cold and hot.

A cold day threshold of 0°C clearly has important consequences, but a warm day threshold would clearly be somewhat arbitrary. A better and universally applicable means of defining extremes is to let the data decide the threshold.

Figure 2 shows the annual counts of warm and cold days (greater than 90th and less than 10th percentile respectively) for the central England series for 1772 to 1998. The 90th percentile is the threshold above which warm days occur 10% of the time. Similarly, the 10th percentile is the threshold below which cold days occur 10% of the time. As would be expected, there is a strong correlation with annual mean temperatures. The counts show an increase in warm days over the present century, but the largest trend is in the reduction of cold days. The method is insensitive to the choice of base period, another choice producing similar trends around a different base. Analyses for other long daily series in Europe (St. Petersburg, Stockholm, Uppsala, Brussels, Milan, Padua and Cadiz) show similar trends to the Central England series.

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Figure 2: Top: Annual cycle of daily Central England temperatures (CET), 1961–90, Middle: Counts of days each year, 1772–1998 in CET above/ below percentile thresholds (90th and 10th), Lower: Annual temperature anomalies from 1961–90.

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The last three sections have all looked at extremes in different ways, but all show similar conclusions. The warming of the present century is mostly manifest, not by increases in warm extremes, but by reductions in cold extremes. These sort of extremes often pass by unnoticed by the majority.

Only in sectors where cold temperatures are important have their effects been noticed. Pesticide use is much greater because insect killing frosts are less severe, the skiing season is shorter and expected freeze-up periods of some major rivers are reduced.

The last 150 years in a longer term context Global temperatures have risen over the last 150 years but what is the context of this over the past millennium? The last millennium is the period for which we know most about the pre-instrumental past, but it must be remembered that even this knowledge is considerably poorer compared to the post-1850 period.

Information about this period comes from proxy climatic sources (early instrumental records back to the late 17th century, historical records, tree ring densities and widths, ice core and coral records)9,10. Uncertainties are greater with proxy records because of limited coverage and all are only imperfect records of past temperature change.

Over the last few years, a number of extensive compilations have been assembled. Figure 3 shows three reconstructions for parts of the last millennium of Northern Hemisphere temperature. The reconstructions are of different seasons, two definitions of summer and one annual. The short instrumental record on an annual basis is superimposed. The most striking feature of the three series is the warming during the 20th century, for both its magnitude and duration. If these records have any validity, then the 20th century is both the warmest of the millennium and the warming in it unprecedented. Furthermore, the four recent years 1990, 1995, 1997 and 1998 are the warmest since 1400 and probably since 1000. The impending La Niña in the eastern equatorial Pacific means that 1999 and 2000 should be cooler so 1998 will probably be the warmest year of the millennium.

The studies discussed here clearly question perceived wisdom about past periods known as the Medieval Warm Epoch and the Little Ice Age. Considerable advances have been made in paleoclimatology in recent years and up to several hundred proxies are used in the compilations shown here. It is apparent from these studies that the significance of these past periods has been exaggerated in past studies. The coldest century of the millennium was the 17th, but temperatures were only 0·5–0·8°C below the 1961–90 base, only marginally (~0·1–0·2°C) cooler than those prevailing at the start of the instrumental era around 1850.

Absolute temperatures Throughout this brief review, temperature series have been quoted in °C with respect to the period 1961–90. The use of anomaly value overcomes problems related to the use of absolute temperatures, particularly elevation. Anomaly values should be all that are required but absolute values are often requested by both scientists and the media. Examples of where absolute values are needed are climate model evaluation, large scale catchment modelling and aspects where threshold values are important. They are not needed by the media!

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Figure 3: Northern Hemisphere temperature reconstructions from paleoclimatic sources. The three series are Mann et al., [1998]9 (thick), Briffa et al. [2000]11 (medium) and Jones et al. [1998]10 (thin). All three annually resolved reconstructions have been smoothed to highlight variations on the 50-year timescale. The fourth (thickest) line is the short annual instrumental record smoothed in a similar manner. All series are plotted relative to the 1961–90 average.

Recently, a climatology on a 1° basis has been assembled from a variety of sources, land stations, marine air temperatures and Arctic buoy data. Its development is discussed in detail in a number of technical publications1. In the development of the 1961–90 climatology, elevation is used as a dependent variable. The magnitude of the seasonal cycle is 13·4°C in the NH and 5·7°C in the SH. In the NH (and the globe) the warmest and coldest months of the year are the expected July and January. In the SH, January is warmest with August coldest. The average temperature of the world is 14·0°C based on the period 1961–90.

Future Climate (both globally and in the UK) Possible future climates are estimated using climate models (long-term extensions of the same computer models that make weather forecasts). The UK has one of the best of these models and a series of experiments have been performed at the Hadley Centre of the Meteorological Office. The rate of future warming globally is found to be in the range 0·16° to 0·35°C per century depending upon future emissions12. Rises towards the lower end of the range will be achieved if the world is successful implementing agreements such as the Kyoto Protocol.

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The rise in annual-mean temperature over the UK is slightly smaller than for the global average, with a pattern of larger increases in the southeast than in the northwest. By the 2080s, annual temperatures over southeast England will be between 1·5° and 3·2°C warmer than the 1961–90 average, but over Scotland only 1·2° to 2·6°C warmer. The warming is generally slightly more rapid in winter than in summer and greater during night time than daytime.

Changes in mean annual precipitation are quite modest. By the 2080s, annual precipitation increases by between 0 and 10% over England and Wales and between 5 and 20% over Scotland. There will be large seasonal differences, however, with winters and autumns becoming wetter over the whole UK, up to 20% in some scenarios. Spring, and especially summer, experience a contrast between the southeast which gets drier and the northwest of the UK which gets wetter. Reductions in summer precipitation over large parts of England reach 10 to 20% by the 2080s. Precipitation that does fall will tend to occur on fewer days (i.e. heavier rainfall when it does fall) with longer drier spells in between. With the higher temperature rates potential evapotranspiration will increase, making water scarce in summer, particularly in the southeast. The higher temperatures will enable more exotic crops to be grown provided water is available. One of the big challenges in the future, therefore, will be managing water resources through storing more of the excess rains from the winter half of the year.

Globally, mean sea level rises in all the scenarios, the rate of increase ranging from 2·4 to 10cm per decade in different parts of the UK. The rises will be greatest in the region from Lincolnshire to Kent, as this is the part of the country where the land is sinking relative to the present sea level.

Conclusions • annual global surface temperatures have warmed by 0·57°C over the period 1861–1997 and by 0·62°C over 1901–97. For both periods, warming is slightly greater in the SH rather than in the NH. • 10 of the 12 warmest years of the record have occurred between 1986 and 1997. The four warmest years have all occurred in the 1990s, 1998 (0·57°C above the 1961–90 average, with 1997 (0·43), 1995 (0·39) and 1990 (0·35) the next three.

• night time (minimum) temperatures are warming (0·18°C per decade over 1950–93) faster than daytime (maximum) temperatures, 0·08°C per decade.

• proxy evidence for periods before 1850 indicates that the 20th century is the warmest of the millennium and the warming during it, unprecedented since 1000. The 17th century was the coldest of the millennium, but only 0·5–0·8°C cooler than the 1961–90 average. 1998 is likely to have been the warmest year of the millennium.

• the average temperature of the world for the 1961–90 period, based on our 1° x 1° climatology is 14·0°C (14·6 in the NH and 13·4 in the SH).

• global average temperature will rise by between 1·6° and 3·5°C by the end of the 21st century.

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Suffolk Natural History, Vol. 36 rises over the UK will be slightly lower (1·2° to 3·2°C), but will be greatest over the southeast and larger in winter than in summer. annually, precipitation will increase all over the UK, but in spring, and especially summer, decreases will occur particularly over the southeast of England. globally, sea level will continue to rise and will be most felt in the UK in the southeast of England.

References 1 Jones, P. D., et al. (1999). Surface air temperature and its changes over the past 150 years. Reviews of Geophysics 37: 173–199.

2 Folland, C. K, Karl, T. R. & Vinnikov, K. Ya. Observed climate variations and change, in Houghton, J. T.; Jenkins, G. J.; Ephraums, J. J., eds. (1990).

Climate change. The IPCC scientific assessment, pp. 195–238. Cambridge University Press, Cambridge.

3 Folland, C. K., et al. Observed climate variability and change, in Houghton, J. T, Callander, B. A. & Varney, S. K., eds. (1992). Climate change 1992.

The supplementary report to the IPCC scientific assessment; pp. 135–170.

Cambridge University Press, Cambridge.

4 Nicholls, N., et al. Observed climate variability and change in Houghton, J. T.; et al. eds. (1996). The IPCC second scientific assessment, pp. 133–192.

Cambridge University Press: Cambridge.

5 Jones, P. D., et al. (1990). Assessment of urbanization effects in time series of surface air temperatures over land. Nature 347: 169–172.

6 Folland, C. K. & Parker, D. E. (1995). Corrections of instrumental biases in historical sea surface temperature data. Q. J. Royal Meteorol. Soc., 121: 319–367.

7 Karl, T. R., et al. (1993).A new perspective on recent global warming asymmetric trends of daily maximum and minimum temperatures Bull.

Amer. Meteorol. Soc., 74: 1007–1023.

8 Parker, D. E., Legg, T. P. and Folland, C. K. (1992). A new daily Central England temperature series, 1772–1991. Int. J. Climatol., 12: 317–342.

9 Mann, M. E., Bradley, R. S. & Hughes, M. K. (1998). Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779–787.

10 Jones, P. D., et al. (1998). High-resolution palaeoclimatic records for the last millennium: interpretation, integration and comparison with General Circulation Model control run temperatures. The Holocene 8: 455–471.

11 Briffa, K. R., et al. (2000) Extracting low-frequency temperature variations from a northern tree-ring density network. J. Geophys. Res. (submitted).

12 Hulme, M. & Jenkins, G. (1998). Climate change scenarios for the United Kingdom. UKCIP Technical Report No. 1. Climatic Research Unit, University of East Anglia, Norwich (copies of the full report and/or the summary are available from ukcip@ecu.ox.ac.uk).

P. D. Jones Climatic Research Unit, University of East Anglia, Norwich NR4 7TJ

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