June 1993 T. Nitta and J. Yoshimura 367 Trends and Interannual and Interdecadal Variations of Global Land Surface Air Temperature By Tsuyoshi Nitta Center for Climate System Research, University of Tokyo, Meguro-ku, Tokyo 153, Japan and Jun Yoshimura Tokyo Airport Local Meteorological Observatory, Ota-ku, Tokyo 144, Japan (Manuscript received 22 January 1993, in revised form 9 April 1990 Abstract Long-term trends and inter-annual and inter-decadal variabilities of the global land air temperature are analyzed by using the 110-year time series data over 8 sub-regions. The global mean temperature exhibits a generally increasing trend during the 100 year period with a rate of about 0.54C/(100 years). However, the temperature has inter-decadal variabilities, with the largest warming for the recent 20 years, but with a cooling trend during 1940-1970. Spectral analysis of the air temperature shows that there exist three dominant time scales with period ranges of several years, 10-20 years, and longer than 30 years. 2-7-year-period variations over the tropical sub-regions are greatly affected by El Nino and Southern Oscilation (ENSO) cycles, but those over the extra-tropical sub-regions are not so affected by ENSO. The 8-25-year band-pass-filtered variations have amplitudes of about 0.1C-0.2C. Their phases were nearly the same over the whole sub-regions before about 1920, but became different between different sub-regions after 1920. The phases of this component of the global mean temperature agree quite well with those of the sunspot numbers in recent years after about 1960, but they were nearly out of phase before about 1920. The low-pass-filtered variations with periods longer than 30 years had a positive peak around 1930-1940 and negative peaks around 1890 and 1970. This component seems to contribute to the large warming during recent 10 years. 1. Introduction Actually, the global-averaged surface air temperature did not increase constantly, but fluctuated with In recent years it has become a common concern various time scales ranging from several years to in the world that the global air temperature may increase through the enhanced greenhouse effect. Fig- several decades, as shown in Fig. 1. Also, features of temperature variations vary from different subregions in the globe, as will be shown later. It may ure 1 shows time variations of global-averaged surface air temperature over land for 1881-1990 analyzed by the Japan Meteorological Agency (JMA). be quite important to examine temporal and spatial variations of the surface air temperature not only In general, the surface air temperature has increased for the understanding of characteristics and mechanisms of the variations themselves, but also for the for the past 110 years with an average rate of about 0.5C/(100 years), which is consistent with the value detection of the greenhouse effect. estimated by the Intergovernmental Panel on Climate Change (IPCC, 1990, 1992). This general It has been reported that the global mean air temperature is affected by the El Nino and Southern Oscillation (ENSO) phenomena (Pan and Oort, trend of air temperature is suggested to be due to the greenhouse effect, but it may not be easy to detect the signals of the greenhouse effect from real 1983; Angell, 1988; IPCC, 1990). Pan and Oort observations because of the existence of other natural variabilities. (1983) have shown that global-mean temperatures have maximum values about six months after the peak of El Nino, but most warming effects by El Nino are limited to the tropical and sub-tropical re-
gions (Angell, 1988). Quite recently Ghil and Vantard (1991) applied singular spectrum analysis to the time series of global surface air temperatures for the past 135 years and determined the exitence of inter-decadal variations. Nitta and Yamada (1989) has shown that sea surface temperature (SST) in the tropics was warmer by about 0.3C-0.4C in the 1980's than in the 1970's. Trenberth (1990) has reported the existence of large inter-decadal climate changes over the North Pacific Ocean in recent decades. Although inter-decadal variations have being paid attention in recent years, their characteristics and mechanism remain unresolved. This paper will focus on inter-annual and interdecadal variations as well as long-term trends of global surface air temperature and examine their temporal and spatial characteristics. Relationships between variations of air temperature and those of ENSO and solar activity will be also investigated. 2. Data Fig. 1. Global mean land air temperature anomalies relative to 1951-1980 constructed from the data by JMA. Annual values and 7-year running means are plotted. Monthly mean land air temperature data averaged over 8 sub-regions as shown in Fig. 2, Northern and Southern Hemispheres and the whole globe for 1881-1990 are used in the analysis. These data are provided by the JMA based on the following procedures. (1) Mean-temperature anomalies in each grid as shown in Fig. 2 are computed by averaging temperature anomalies at individual stations (deviations from the 1951-1980 averages) inside a grid weighted by the distance from the center of a grid. (2) Mean-temperature anomalies over each subregion, each hemisphere and the whole globe are obtained by using weighted averages of individual grids inside the region. These data include only observations over land and islands but not over oceans, as shown in Fig. 2. Therefore, "global air temperatue" in this study actually means global land air temperature. Since urbanization effects are not considered in the JMA data, warming trends obtained in this study might be overestimated. However, Jones et al. (1990) indicated that urbanization effects have resulted in a warming of less than 0.05C during this century over the global land which is about 10 % of the actual warming. It is suggested from their results that the neglect of urbanization effects may not influence conclusions in this study. Monthly values of the Southern Oscillation Index (SOT; Tahiti-Darwin) for 1881-1990 and yearly values of Wolf Number of sunspots for 1881-1990 are used to examine relationships with temperature variations. 3. Trends Before discussing the inter-annual and interdecadal variations, we analyzed general trends of surface temperature. Figure 3 shows rates of temperature changes during the 110 years from 1881 to 1990 over 8 sub-regions, both hemispheres and the globe, which were estimated by linear fitting to the time series data. Trends during each 21-year sub-period are also computed at 10-year intervals to understand the variability of the trend. The trends for the 110 years are 0.54C/(100 years) in the global average and 0.58 C/(100 years) in the Northern Hemisphere, respectively, which are consistent with those estimated by IPPC. However, the trend in the Southern Hemisphere is 0.36C/(100 years) which is smaller than that estimated by IPC, probably due to different use of observation stations and analysis methods. The warming trends are observed for the 110 years over all subregions except for North and South Africa. The rate of the warming is generally larger in the middle-high latitude sub-regions than in the low latitude subregions. However, the warming in Europe is not so significant and that in South America is quite large compared with other tropical sub-regions. The results of the trends for the different 21-year sub-periods indicate that the largest warming occurred during the most recent 21 years (1970-1990) in the past 110 years in the global and hemispheric averages. This large warming trend in recent years was observed in almost all sub-regions except in Oceania and South America. The moderate warming was observed during the 1910-1940 period in the global average. However, the warming trend did not
Fig. 2. 8 sub-regions used for temperature analysis. 1: Europe, 2: North Asia, 3: North America, 4: North Africa, 5: South Asia, 6: South Africa, 7: Oceania and 8: South America. Small rectangles correspond to individual grids. (Provided by JMA). Fig. 3. Linear trends of land air temperature during the whole period (1881-1990) (right) and during sub-periods of 21 years for 8 sub-regions, both hemispheres and the globe. Units are C/(100 years) and asterisks denote the results with a confidence level higher than 95 %. occur in all the sub-regions during this period and some cooling trends took place in the tropical subregions. There existed cooling trends during the 1940-1970 in the global average, but their magnitudes and timing were different for different subregions. The cooling seems to have begun earlier in the tropical sub-regions such as North Africa around 1930, but late in Europe around 1960. 4. Spectral analysis and time filtering Spectral analysis was applied to temperature data to detect dominant periods of the variations. Figure 4 presents the spectral results for the globe and three different sub-regions. There exists large power in the period ranges around 10 years and longer than about 30 years in the global temperature. In Europe, both variations with periods of 2-3 years and around 10 years are predominant. 3-7-yearperiod variations dominate over the tropical subregions such as South Asia and South Africa, probably due to ENSO effects. Since the normalized standard deviation of the spectral estimate in this study is rather large (50%) due to the small number of degrees of freedom (8) (Hino, 1977), individual spectral peaks in Fig. 4 may not satisfy the statistical significance. However, the spectral results in Fig. 4 and those over other sub-regions (not shown) generally indicate that a large power of temperature variations is mostly found in the three period ranges, such as several years, 10-20 years and longer than 30 years.
Fig. 4. Power spectra of temperature averaged over the globe, Europe, South Asia and South Africa. Units are (C) 2. year. Fig. 5. 13-month running means and filtered data of 2-7-year band-pass, 8-25-year band-pass and low-pass longer than 30 years of the global mean land air temperature. Units are C. In order to pick up different time scale variations, we applied three types of time filters to the original data, which pass 2-7-year period, 8-25-year period and longer period than 30 years, respectively. Linear trends for the whole period are removed and then 13- months running means are taken to remove shortperiod variations less than about one year before filterings are applied. Figure 5 shows 13-months running-mean data and three different filtered data of the global temperature. 13-months running-mean data appear to include inter-annual and inter-decadal variations. The two shorter period variations (2-7 years and 8-25 years) have amplitudes of about 0.1C-0.2C on average, but their amplitudes and periods seem to vary in time. The longest-period variation has a positive peak around 1930-1940 and negative peaks around 1890 and 1970, which can be identified even in the original data, as shown in Fig. 1. The low-passfiltered temperature has been increasing since about 1970 and the 8-25-year band-pass-filtered temperature was in an increasing phase after the mid 1980's. These results suggest that severe warming in recent several years as shown in Fig. 1 may be largely contibuted by these two components. In the following sections we will describe more detailed features of these three different period variations. Fig. 6. 2-7-year band-pass-filtered data of land air temperatures averaged for 8 subregions, both hemispheres and the globe. Units are C. 5. 2-7-year-period variations Figure 6 shows the filtered temperature variations in the period range of 2-7 years over the globe, both hemispheres and the individual sub-regions. The
Fig. 7. Lag correlations of the 2-7-year band-pass-filtered temperatures between different sub-regions. Positive (negative) lags represent that the right sub-region lags (leads) the left sub-region. global and hemispheric variations have amplitudes of about 0.1C-0.2C, but their amplitudes appear to have changed for different sub-periods. Relatively large amplitudes were observed before about 1920 and after about 1950, but relatively small amplitudes existed during the intermediate period. The amplitudes over the extra-tropical sub-regions are larger than those over the tropical sub-regions. Dominant periodicities over the former sub-regions seem to be a little shorter than those over the latter. In order to examine relationships between variations over different. sub-regions, we compute lag correlations of the filtered data between all pairs of 8 sub-regions. Figure 7 shows the lag correlation results between different sub-regions. Only results for 9 pairs among 28 pairs are shown in the figure as representatives. It is roughly estimated that correlation coefficients larger than 0.2 exceed the significance level of 99%. There exist large positive correlations among tropical sub-regions (upper figures). Among the tropical sub-regions, variations in South America lead in phase and those in Oceania are the last, lagged by 6 months from South America. Although North America has a moderate positive correlation with South Asia, correlations between the tropical sub-regions and the extra-tropical sub-regions are generally small (middle figures), suggesting little link between the tropics and extra-tropics. Although there exist some correlations between North Asia and the other two extra-tropical sub-regions, the correlation between Europe and North America is quite small. These results suggest the possibility of the existence of different types of variations in the extra-tropics in this period range. Next the relationship between the 2-7-year-period variations and ENSO is examined. Figure 8 describes lag correlations between the 2-7-year-period variations over the whole globe and three subregions and the Southern Oscillation Index (SOT), which is a measure of ENSO cycles. The correlation coefficient corresponding to a 99% significance level is about 0.2. There is a significant negative correlation between the global mean temperature and SOT with a maximum correlation at about 5 months lag, indicating that the global temperature reaches the warm peak about a half year after the negative peak of SOT (maximum activity of El Nino). These results are generally consistent with those of earlier works such as Angell (1988) and IPCC (1990). However, this does not mean that the temperature over the whole region of the globe is affected by ENSO. Large negative correlations are also found between SOT and the temperature over South Asia. Similar large correlations with SOT are obtained over the other tropical sub-regions (not shown). There exist similar negative correlations over North America, but their magnitude is not so large. Correlations over Europe and North Asia are rather small (not
Fig. 8. Lag correlations between SOT and 2-7-year band-pass-filtered temperatures for the globe, South Asia, North America and Europe. Positive (negative) lags represent that SOT leads (lags) air temperatures. shown). These correlation results indicated that the ENSO impacts on surface air temperatures are mostly limted in the tropical regions, but not so significant in the extra-tropical regions. The global mean temperature has large correlations with ENSO, probably due to the large contributions of temperature variations over tropical regions. The 2-7-yearperiod variations of temperature in the higher latitudes may be due to other mechanisms. Further studies will be needed to investigate these mechanisms. 6. 10-20-year-period variations Fig. 9. As in Fig. 6, except for the 8-25-year band-pass-filtered data of land air temperatures. Variations of sunspot Wolf Number are also plotted (top). Units are C. Figure 9 shows the filtered temperature variations in the period range of 8-25 years over the globe, both hemispheres and individual sub-regions. Time variations of Wolf numbers of sunspots are also plotted in the figure. This component of the global average showed pronounced variations with amplitudes larger than 0.1C before around 1920, but decrease in amplitude in 1920-1960. After 1960 it again became pronounced with a regular periodicity of about 10 years. It may be interesting to note that this component was in an increasing phase during the recent 5 years, sugggesting that this component may contribute to some extent to the extreme global warming in that period. Variations over the individual sub-regions indicated that all sub-regions appeared to exhibit nearly-in-phase variations before around 1920. However, phases of the variations became somewhat different for different sub-regions after around 1920. For example, Europe and North America varied almost out-of-phase during 1930-1950. Although the global temperature has been increasing in the recent 5 years, as discussed above, large increasing trends occur only in Europe and North Asia, and other temperatures show small variations with different tendencies. It may be interesting to examine the relationship between temperature variations of this component and those of solar activity, because there exist predominant cycles of solar activity of about 11 years which are very close to time scales of this component. Figure 9 indicates that the global temperature and sunspot number vary almost in-phase during recent 30 years, suggesting the strong links between two parameters. However, these parameters varied nearly out-of-phase before about 1920. The phase relationship became obscure during 1920-1960 because of small amplitudes of the temperature variation. The change of the phase relationship between temperature and sunspot number before and after the 1920's was previously reported by Troup (1962) and Yamamoto (1981). We can not conclude the existence of simple relationships between these two parameters based on the whole 110 years of data. Even in the recent 30 years, if we look at the phase relationship in more detail, the phases of the two parameters appear to be changed in time little by little. The relationship
Fig. 10. As in Fig. 6, except for the low-passfiltered temperatures with periods longer than 30 years. Units are C. between these parameters has been debated for a long time, but no scientific conclusions have been reached so far (IPCC, 1990, 1992). Careful examinations, not only based on observational analyses but also based on physical considerations, may be needed to conclude these relationships. 7. Longer variations than a 30 years period Figure 10 shows low-pass-filtered data over the globe, both hemispheres and individual sub-regions. These components over the globe and both the hemispheres have a positive peak around 1930-1940 and negative peaks around 1890 and 1970. Similar variation patterns are found in the northern extratropical sub-regions, South Africa and South America. Variations in the northern tropical sub-regions are somewhat different from the above and no positive peaks around 1930-1940 were found in these sub-regions. In the recent 20 years, most sub-regions showed increasing phases, indicating that the largest global warming occurring in those years as discussed in Section 3 may be largely contributed by this component. Recently, Jones (1988) and Trenberth (1990) analyzed the spatial distributions of the Northern Hemispheric surface temperature during 1976-1986 and showed that the surface temperture over the Northern Hemisphere except over the North Pacific and the North Atlantic increased. Nitta and Yamada (1989) demonstrated by using global SST data that there existed some jumps in tropical SST around 1976 and that the tropical SST tended to increase after that. They further suggested that the Pacific-North America (PNA) teleconnection pattern was intensified during the latter period, probably due to tropical convective activities enhanced by the increase tropical SST. However, impacts of long-term variations of tropical SST on global-scale atmospheric circulations and surface temperature should be further investigated both observationally and theoretically in the future. 8. Summary and conclusions Global land air temperatures for the 110 years from 1881 to 1990 over the 8 sub-regions are analyzed to examine long-term trends and inter-annual and inter-decadal variabilities. The warming trends are observed for the 110 years over all sub-regions except for North and South America, and the rate of warming is 0.54C/(100 years) for the global average. However, the results of trends for individual 21-year sub-periods reveal that the trends are not so constant in time. The largest warming of the global mean temperature occurred during the recent 21 years (1970-1990) when a large warming was also observed in all sub-regions. There occurred a moderate warming during 1910-1940, but there were coolong trends during 1940-1970 in the global average. Spectral analyses of the air temperature showed that there exists large spectral power in three different time scales, such as several years, 10-20 years, and longer than 30 years. Band-pass and low-pass filters are applied to the temperature data to pick up these three different time scales. The filtered 2-7-year-period variations of the global mean temperature have amplitudes of about 0.1C-0.2C. Lag correlation computations of these period range variations between different subregions showed that there exist large positive correlations among tropical sub-regions, but small correlations between tropical sub-regions and extratropical sub-regions. It was found that the 2-7-year-period variation of the global mean temperature correlates quite well with the ENSO cycle with about a half year time lag. However, the impacts of ENSO on surface air temperature are mostly limited to the tropical regions. The short-period temperature variations in the northern higher latitudes may be caused by other mechanisms. The filtered 8-25-year-period variations have an amplitude of about 0.1C for the global average. These variations showed generally in-phase relationships between different sub-regions before around 1920. However, in-phase relationships became less significant after this period. Especially, temperatures in Europe and North America exhibit nearly out-of-phase variations during 1930-1950. Comparison between 8-25-year-period variations
and sunspot numbers shows that both variations are almost in-phase during the recent 30 years, but nearly out-of-phase before about 1920. Further studies including the considerations of physical processes linking these two parameters will be needed to conclude the mutual relationship. The low-pass-filtered temperature variations indicated that there exited a positive peak around 1930-1940 and negative peaks around 1890 and 1970. This component turned to a warming trend after about 1970 and appeared to contribute to the large warming trend during recent decades. Although the characteristic features of the interannual and inter-decadal variations of the air temperature were demonstrated in this study, the mechanism generating these variations are not clear, especially for variations longer than a decade. Much work will be needed to clarify these mechanisms in the future. Acknowledgments The authors wish to extend their thanks to the Japan Climate Data Center and the Longrange Forecast Division of the Japan Meteorological Agency for providing the global land air temperature data and SOT data, respectively. References Angell, J.K., 1988: Variations and trends in tropospheric and stratospheric global temperatures, 1958-87. J. Gum., 1, 1296-1313. Ghil, M. and R. Vantard,1991: Interdecadal oscillations and the warming trend in global temperature time series. Nature, 350, 324-327. Hino, M., 1977: Spectral Analysis (in Japanese). Asakura Publishing Company, 300pp. IPPC, 1990: Climate Change: The IPCC Scientific Assessment. J.H. Houghton, G.J. Jenkins and J.J. Ephraums, Eds.. Cambridge University Press, Cambridge, UK, 365 pp. IPCC, 1992: Climate Change 1992: The Supplementary Report to the IPCC Scientific Assessment. J.H. Houghton, BA. Callander and S.K. Varkey, Eds.. Cambridge University Press, Cambridge, UK, 200 pp. Jones, PD., 1988: Hemispheric surface air temperature variations: Recent trends and an update to 1987. J. Clim., 1, 654-660. Jones, PD., P.Ya. Groisman, M. Coughlan, N. Plummer, W.-C. Wang and T.R. Karl, 1990: Assessment of urbanization effects in time series of surface air temperature over land. Nature, 347, 169-172. Nitta, T. and S. Yamada, 1989: Recent warming of tropical sea surface temperature and its relationship to the Northern-Hemisphere circulation. J. Meteor. Soc. Japan, 67, 375-383. Pan, Y.H. and A.H. Oort, 1983: Global climate variations connected with sea surface temperature anomalies in the eastern equatorial Pacific Ocean for the 1958-1973 period. Mon. Wea. Rev., 111, 1244-1258. Trenberth, K.E., 1990: Recent observed interdecadal climate changes in the Northern Hemisphere. Bull. Amer. Met. Soc., 71, 988-993. Troup, A.J., 1962: A secular change in the relation between the sunspot cycle and temperature in the tropics. Geofisca Pua e Appl., 51, 184-198. Yamamoto, R., 1981: Change of global climate during recent 100 years. Proceedings o f the Technical Conference on Climate-Asia and Western Pacific, Guangzhou, China, 15-20 December 1980, WMO- No. 578, 360-375.