This record is expected to differ from the surface temperature record for several reasons. For example, over continental regions, the surface temperature shows greater variation in response to warming from solar energy than does the thick layer of air above the land; in contrast, the seasurface temperature shows even less temperature variability because of the oceans' great thermal inertia. Relatively short-term, major events such as the volcanic eruptions of Mt. Pinatubo (1991) and El Chicon (1982), as well as the oceanic temperature changes during El Niño events, explain over 60% of the monthly variation in the MSU tropospheric temperature record. Additionally, air pollutants such as aerosols tend to cool the surface and lower troposphere. Without these "cooling" effects, the global temperature record (as measured by the MSU instruments) has risen over the last 19 years by +0.06 C/decade, while GCM's have predicted a warming rate of +0.08 to +0.30 C/decade over the last few decades. The surface record shows more than 0.1 degree C per decade warming over this same period. Q5.2 What type of research is U.S. Global Change Research Program conducting to resolve this discrepancy? A5.2 The USGCRP supports a wide range of activities relating to detection of climate change and identifying human components. These activities include support of satellite observations, including the work by Spencer and Christy with the Microwave Sounding Unit (MSU) instrument record. The USGCRP also supports the creation of temperature records from surfacebased sources, such as the work by Jim Hansen and his colleagues at the Goddard Institute for Space Studies (GISS), and work to reconstruct very long-term historical records of the Earth's climate. In addition, the program supports numerous analyses and modeling efforts by teams seeking to resolve apparent inconsistencies, as well as many other activities that contribute to the overall understanding of the Earth system and how it works. The results from all these efforts are regularly published in the peerreviewed literature, discussed at scientific meetings, and used as the basis for the definition of new projects and programs to help us narrow the uncertainty of measurements and predictions and increases Our understanding of the climate system. This includes major efforts by NASA to develop and deploy improved, more accurate instruments for measuring various aspects of climate change, including temperatures, from space. As is true of all science, the USGCRP welcomes careful analyses that start from a skeptical perspective and test and reexamine all findings. Just as the Q6. A6. findings raise questions about the satellite data. Working through all of this is part of the intense self-scrutinizing process that science demands. The land-based temperature record is subject to the effects of urbanization that could raise temperatures. Because of this bias, some scientists have questioned the appropriateness of using the land-based temperature record alone to detect global warming. Because the satellites measure temperatures three-dimensionally (i.e., between 5,000 and 30,000 feet), there is no bias from urbanization or other factors. Moreover, the satellites cover the entire globe whereas ground-based thermometers do not. How reliable, then, is the ground-based temperature record? The ground-based temperature record is very reliable. The question you raise about urbanization and the heat-island effect has been carefully and methodically addressed by scientists. The potential impact of the placement of thermometers has been systematically reviewed and adjusted for the influence of urban heat islands. Some of our nation's most prominent researchers have participated in these exercises. See, for example, the recently published paper by Easterling, et al. from Science 277:364-367, July 18, 1997, copy attached. (1994)] showed a large-scale anticyclonic gyre in the general vicinity of the gyre shown in Fig. 1C. Howev er, Raid's gyre and the one depicted in Fig. 1C have substantial differences in their vertical structure and horizontal substructure. 7. Southward flow of North Atlantic Deep Water on the eastern side of the Mid-Atlantic Ridge is supported by P. M. Saunders J. Mar. Res. 40, 641 (1982)], S. Gana and C. Provost J. Mar. Syst. 4,67 (1993)), and J. Pallet and H. Mercier (Deep-Sea Res., Part I, in press). 8. Topographic sill depths of -1000 m are a barrier for flow of this density into the Nordic Seas. 9. LV. Worthington, Johnt Hopkins Oceanogr. Stud. 6. (1976), R. A. Clarko, H. W. Hill, R. F. Reininger. BA Warren, J Phys Oceanog 10, 25 (1980): M. S. McCartney, Prog. Oceanogr. 29, 283 (1992). W. J. Schmitz and M. S. McCartney, Rev. Geophys. 31,29 (1993). 10. R. S. Pickart and W M. Smethie, J. Phys. Oceanogr. 23. 2602 (1993) 11. J Palet, M. Arhan, M. S. McCartney, in preparation. 12. P. B. Rhines and W. R. Holland, Dyn. Atmos. Oceans 3,289 (1979). 13. W. R. Holland, Geophys. Fluid Dyn. 4, 187 (1973). 14. R. Gerdes and C. Köberle, J. Phys. Oceanogr. 25, 2624 (1995) 15. These gyres at intermediate depths are distinct from the smaller scale gyres at abyssal depths that are more clearly associated with the basin topography, 16. P. B. Rhimes and W. R. Young, J. Fluid Mech. 122. 347 (1982) 17. W. R. Holland and P. B. Rhines, J. Phys. Oceanogr. 10 1010 (1980) 18. S. McDowell, P. Phines. T. Keffer, ibid. 12, 1417 (1982), T. Keffer, ibid. 15, 509 (1985), J L. Sarmiento, C. G. H. Rooth, W. Roether, J. Geophys. Res 87, 8047 (1982). 19. J. Pedlosky. Ocean Circulation Theory (SpringerVerlag, New York, 1996). 20 Potential vorticity is calculated as fa, x do/az. where is the planetary vorticity, z is the vertical coordinate, and e, is a constant potential density. The vertical denvative is computed locally over a nominal depth of 100 m in an effort to approximate neutral surfaces (T. J. McDougall, J. Phys. Oceanog 17. 1950 (1987). potential vorticity was also calculated as th, where h is the distance between two locally referenced isopycnals. The differences in the two methods were insignificant to the results of this study, that is, the region and extent of homogenization were the same with either calculation. 21 M. S. McCartney and L. D. Talley, ibid. 12, 1169 (1982) 22 J. O'Dwyer and R. G. Williams J. Phys. Oceanogr.. in press) report, from an analysis of the Levitus data set, possible regions of homogenization at abyssal depths in the western North Atlantic. 36.95 23 An absolute flow field was calculated for a, by differentiation of a modified H.-M. Zhang and N. G. Hogg, J. Mar Res 50, 385 (1992) Montgomery stream-function field (R. 8. Montgomery, Bull Am. Meteorol Soc. 18, 210(1937)] with a 41.45 (at-3000 m) as the level of no motion. 24. DL Musgrave, J. Geophys Res 90, 7037 (1985). 25. Surfaces shallower than 2 = 36.95 show counterrotating gyres separated by this instability region in the western North Atlantic, which suggests the importance of eddy flux divergence in the forcing of the gyres. 26. J Padiony. J. Phys. Oceanogr. 13, 2121 (1983). 27 N Hogg Deep-Sea Res. Part A 30, 945 (1983). 28. P. B Phines and W. R. Young, J. Mar. Res. 40, 559 (1962) 29. P. Cesa, G. lerley, W. Young, J. Phys. Oceanogr. 17, 1640 (1987), P. Cessi, ibid. 18, 662 (1988). 30. WJ Jenkins and P. B. Rhines, Nature 286, 877 (1980); R. S. Pickart, Deep-Sea Res. Part A 39, 1553 (1992). W M. Smethie, Prog Oceanogr 31, 51 (1993). 31. I thank J. Pedlosky for his aid in the interpretation of these fields and P. Rhines for his comments on the manuscript. Support from NSF (grant OCE-9629489) is gratefully acknowledged. 20 February 1997, accepted 22 May 1997 Maximum and Minimum Temperature David R. Easterling, Briony Horton, Philip D. Jones, Thomas C. Peterson, Thomas R. Karl, David E. Parker, M. James Salinger, Vyacheslav Razuvayev, Neil Plummer, Paul Jamason, Christopher K. Folland Analysis of the global mean surface air temperature has shown that its increase is due, at least in part, to differential changes in daily maximum and minimum temperatures, resulting in a narrowing of the diumal temperature range (DTR). The analysis, using station metadata and improved areal coverage for much of the Southern Hemisphere landmass, indicates that the DTR is continuing to decrease in most parts of the world, that urban effects on globally and hemispherically averaged time series are negligible, and that circulation variations in parts of the Northern Hemisphere appear to be related to the DTR. Atmospheric aerosol loading in the Southern Hemisphere is much less than that in the Northern Hemisphere, suggesting that there are likely a number of factors, such as increases in cloudiness, contributing to the decreases in DTR. The global mean surface air temperature has risen about 0.5°C during the 20th century (1). Analysis has shown that this rise has resulted, in part, from the daily minimum temperature increasing at a faster rate or decreasing at a slower rate than the daily maximum, resulting in a decrease in the DTR for many parts of the world (2, 3). Decreases in the DTR were first identified in the United States, where large-area trends show that maximum temperatures have remained constant or have increased only slightly, whereas minimum temperatures have increased at a faster rate (4). Similar changes have been found for other parts of the world as data have become available, allowing more global analyses (2, 3). However, in some areas the pattern has been different: In parts of New Zealand (5) and alpine regions of central Europe (6), maximum and minimum temperature have increased at similar rates, and in India, the DTR has increased as a result of a decrease in the minimum temperature (7). To evaluate these varying results, we conducted an expanded analysis on global and regional scales. Local effects such as urban growth, ir D. R. Eastering, T. C. Peterson, T. R. Karl, National Ci matic Data Center, Asheville, NC 28801, USA. 8 Horton, D. E. Parker, C. K. Folland, Hadley Center, Meteorological Office, Bracknell, Berkshire, UK. P. D. Jones, Climac Research Unit, University of East Anglia, Norwich, UK. M. J. Salinger, National Institute of Water and Atmospheric Research, Auckland, New Zealand V. Razuvayev, All-Russia Research Institute of Hydrometeorological Information, Obrinsk, Russia. N. Plummer, National Climate Center, Bureau of Meteorology, Melbourne, Australia. P. Jamason, DynTel Inc., National Climatic Data Center, Asheville, NC 28801, USA. *To whom correspondence should be addressed: E-mail: deaster@ncdc.noaa.gov rigation, desertification, and variations in local land use can all affect the DTR (3); in particular, urbanized areas often show a narrower DTR than nearby rural areas (8). Large-scale climatic effects on the DTR include increases in cloud cover, surface evaporative cooling from precipitation, greenhouse gases, and tropospheric aerosols (9, 10). Recent studies have demonstrated a strong relation between trends of the DTR and decreases in pan evaporation over the former Soviet Union and the United States (11), suggesting that the DTR decrease in these areas is influenced by increases of cloud amount and reduced insolation (1). Furthermore, recent modeling studies have suggested that the decrease in the DTR may be a result of a combination of direct absorption of infrared portions of incoming solar radiation, aerosols, and water-vapor feedbacks, including surface evaporative effects (12). We analyzed monthly averaged maximum and minimum temperatures and the DTR at 5400 observing stations around the world. Each time series from each station was subjected independently to homogeneity analyses and adjustments according to recently developed techniques (13). In general, these homogeneity adjustments have little effect on large-area averages (global or hemispheric), but they can have a noticeable effect on smaller regions (14), particularly when comparing trends at individual or adjacent grid boxes. Our data covers 54% of the total global land area, 17% more than in previous studies (3). Most of the increases are in the Southern Hemisphere, with the addition of data for South America, New Zealand, all of Australia, a number of Pacific islands, and Indonesia. Data were also in cluded for tropical and subtropical areas, such as the Caribbean, parts of Africa, Iran, Pakistan, and Southeast Asia. However, there are still large parts of the world that remain unanalyzed because of a lack of data, particularly in the tropics, and updating these data remains a problem, as shown by our analysis ending in 1993 (15). In our analysis, we first calculated anomalies from the mean of the base period of 1961 to 1985 for all stations in each 5° by 5° latitude-longitude grid box. The global or hemispheric value for each year was then determined by area-weighting each grid box and averaging all the weighted grid-box values. The overall global trend for the maximum temperature is +0.88°C per 100 years, which is consistent with an earlier finding (3). However, the trend for the minimum temperature is +1.86°C per 100 years, which is considerably less than that found in previous analyses. This reduction in the trend for minimum temperature results in a smaller trend in the DTR of -0.84°C per 100 years. This finding is not surprising, because most of the data added here are for tropical and subtropical regions, where temperature changes are not expected to be as large as in regions of higher latitude (1), and because of the effects of the Mount Pinatubo eruption. The temperature increases for these 25 years are greater than those over the rest of the 20th century, reflecting the stronger warming during the latter half of this century (1). We examined urban effects on global and hemispheric trends using a metadata set developed at the U.S. National Climatic Data Center. These data indicate whether a station is in an urban or nonurban environment, where urban is defined as a city of 50,000 or greater popu lation (16). Approximately 1300 of the original 5400 stations were determined to be urban by this rough measure. The globally averaged time series for the annual maximum and minimum temperature and DTR calculated using only nonurban stations show only slight differences from those calculated using all available stations (Fig. 1). The trend for the maximum temperature excluding the effects of large urban areas is +0.82°C per 100 years, and for the minimum temperature is +1.79°C per 100 years; the DTR trend is -0.79°C per 100 years. The difference in the trends for the maximum and minimum temperatures is about 0.1°C per 100 years, which is consistent with other estimated urban effects on global mean temperature time series (16, 17). The likely effects of the Mount Pinatubo eruption are seen in both the maximum and minimum temperature time series, which show a distinct drop in 1992. The maximum temperature continued to drop in 1993, whereas the minimum stabilized, which resulted in a continued decrease in the DTR. Maximum temperatures have increased over most areas with the notable exception of eastern Canada, the southern United States, portions of eastern Europe, southern China, and parts of southern South America (Fig. 2). The minimum temperatures, however, increased almost everywhere except eastern Canada and small areas of eastern Europe and the Middle East. The DTR decreased in most areas, except over middle Canada and parts of southern Africa, southwest Asia, Eu Table 1. Annual and seasonal trends from 1950 to 1993 for maximum temperature, minimum temperature, and DTR for the globe and the Northern and Southern hemispheres. Trends calculated using only nonurban stations are given, with the trends using all stations given in parentheses. aerosol load in this hemisphere is much less than that in the Northern Hemisphere, additional factors, such as increases in cloudiness (9), are likely contributing to observed increases in nighttime temperature. The minimum temperature for both areas increased abruptly in the late 1970s, which is also evident in the global time series and coincides with what has been described as a fundamental shift in the El Niño-Southern Oscillation phenomenon (1). Mean annual temperatures rope, interior Australia, and the western tropical Pacific islands. It should be kept in mind that each grid-box value is the average of 1 to 20 or more stations within that grid box, and that the value for any one grid box is subject to any problems inherent in the station data. Because the DTR is the maximum temperature minus the minimum temperature, the DTR can decrease when the trend in the maximum or minimum temperature is down, up, or unchanging. This relation contributes to the appearance of less spatial coherence on the DTR map than on the other two. Seasonally, the strongest changes in the DTR were in the boreal winter season, and the smallest changes were during the bo- § real summer (Table 1), suggesting that there is an element of a seasonal cycle in the changes. Maximum temperatures in southern South America and in Southeast Asia (Fig. 1), two areas not previously analyzed, did not change significantly, although the data for Southeast Asia suggest that temperatures there decreased slightly. In both regions, minimum temperatures increased significantly, resulting in a significant decrease in DTR. Furthermore, minimum temperatures in the Southern Hemisphere increased, and because the tropospheric Fig. 2 Trends (in degrees Celsius per 100 years) for each 5° by 5° latitude-longitude grid box using only nonurban stations for annual maximum temperature, annual minimum temperatue, and diumal temper ature range. MAX MIN Trend (C per 100 years) 1.5.0 5.0 10 -1.0 DTR Mean annual anomaly (°C) Fig. 3. Mean annual temperature anomalies (from the 1961-90 mean) for the (A) Northern Hemisphere and (B) Southern Hemisphere. for the Northern and Southern hemispheres (Fig. 3) show the same abrupt increase in the late 1970s and the effect of the Mount Pinatubo eruption in 1992 (18). However, the temperatures for both hemispheres show a recovery to warmer temperatures, such that 1995 was the warmest year since 1950. Because maximum and minimum temperature changes are reflected in the mean, it is likely that the maximum and minimum temperature and DTR changes presented here continued to occur through 1995. Circulation changes during the Northern Hemisphere winter were examined for a relation to the winter DTR. A westerly index (WI) was calculated from the cold ocean-warm land (COWL) pattern (19) and regressed against the DTR. Using yearly values, we found the correlation between the WI and DTR over the region 60°W to 90°E, 30°N to 80°N to be -0.37, significant at the 95% confidence level. There is a bipolar pattern in the relation. with positive correlations occurring over the Iberian peninsula, and negative correlations over northern Europe and into Russia. This pattern suggests that strong westerly flow is associated with increased DTR over the Iberian peninsula and decreased DTR over northern Europe and into Russia. 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