limatologists around the world are trying to detect the impact of the gradually increasing man-made greenhouse gases in the atmosphere (principally CO, and methane). According to numerical climate models the globe should be warming ( I ) and, since about 1900, it has been. However, using the observed global mean surface air temperature rise as a measure of CO, climate impaci is not without problems. Most of the warming occurred in the first half of the century when the CO, rise had been relatively modest. From the 1940s to the 1970% at the time of the f a s t e s t CO, i n crease, much of the Northern Hemisphere cooled (2). According to the models, the warming should be most pronounced in the Arctic a n d least pronounced in the tropics [I). During the past 50 years it has been the other way around. The temperature gradients between the high and low latitudes are increasing (consequently the storminess in the middle latitudes is also)-not decreasing as climate models show (3).Although thex is no rlniiht that ~.~ the thermal structure of the atmosphere changed with the addition of CO, and CH,, we still do not know to what degree the changes in global mean temperature are in response to the greenhouse gas buildup. Can t h e global t e m p e r a t u r e change have a natural cause? It is possible. Look at the mean annual surface air temperature in central England between 1886 and 1986 (Figure 1). As in most other places in the Northern Hemisphere, it rose in the first half of the century and declined in the second halt a similar change happened 200 years earlier. Many people believe that hu1486 Envimn. Sci. Technol., Vol. 27, No. 8. 1993
m a n i t y i s responsible for t h e current temperature rise (I),but the credit for the much more pronounced warming in the first half of the pre-industrial 18th century must be given to nature. Is there anything currently happening to our climate that would reflect the increase in greenhouse gases? At first glance, it appears that the pronounced post-World War n rise in nighttime and early morning temperatures observed in many parts of the world may be an indication. Since the 1950% when suffi-
cient data became available for an analysis, the minimum daily temperatures over many of the continents rose almost three times as fast as the maximum daily temperatures. As a result, the daily temperature range dropped noticeably. Almost all of the observed increases of mean daily temperature over land during the past 40 years appear to be a result of the increase in early morning minimum temperatures. One is tempted to reason that the efficient blanket of the C0,-rich atmosphere keeps the nighttime temperatures high. Data made available internation-
ally are usually the average temperatures for a day or a month. They do not include the daily maximum and minimum temperatures, which is why, until recently, most studies of the ongoing climate change were restricted to monthly means. The first indication that there might be an important large-scale change in the daily temperature structure was reported from a set of rural stations in North America. Researchers found that the mean diurnal (daily) temperature range (DTR) decreased noticeably during the past several decades (5).The mean monthly DTR was defined as the difference between the mean monthly maximum a n d minimum temperatures. A bilateral agreement between the U.S. Department of Energy and the People's Republic of China Academy of S c i e n c e s i n 1990 provided an opportunity to extend the analysis to eastern Asia (6). The Intergovernmental Panel on Climate Change (IPCC) made corr e s p o n d i n g arrangements with the Australian National Climate Center, and a bilateral exchange agreement between the United States and the former Soviet Union vrovided maximum and minimum temperatures at rural stations of the Commonwealth of Independent States (CIS). Data recently have been analyzed from Australia, Sudan, Japan, Denmark, northern Finland, several Pacific islands, Pakistan, South Africa, and Europe. The analyzed area now covers more than 50% of the land in the Northern Hemisphere and about 10% of laud in the Southern Hemisphere (Figure 2). The average monthly maximum and minimum temperatures as well as the mean DTR were calculated for various regions from data supplied by 1000
0013-936X/93/0927-1468$04.00/0Q 1993 American Chemical Society
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Annual mean surface air temperature in rural central England
stations from 1951 to 1990,and departures from the 1951-1990 average were calculated (4). To ensure the large-scale representativeness of data, a fixed network of Historical Climate Network stations was used in the United States. Most 1470 Environ. Sci. Technol.. Vol. 27, No. 8, 1993
stations are located in rural mas that are free of development and have populations of less than 10,000.Data from the stations were adjusted for differences in instrument height and hardware, observation times, and urban growth. Data for the CIS were not
adjusted for any random or systematic inconsistencies: moreover, station histories showed few systematic changes in the network operation during the past 50 years. The Canadian results were derived from a set of 35 rural stations. A significant number of the 58 available Chinese stations are located within or near cities. However, a comparison of urban and rural stations revealed that the actual decrease in the DTR is at least six times larger than if the decrease were caused only by urban warming. Stations in Australia were not subjected to homogeneity analysis, hut they were selected to minimize differences in instrument types, heights, and other factors. They are located in villages or rural areas free of development. Data from Sudan, South Africa, and Japan were collected from rural and some urban stations. Countrywide decreases of the DTR are obvious from data collected from rural as well as urban stations.
Rise of nighttime minima The average and the minimum daily temperatures reveal a general rise during the analyzed interval. A decrease in the minimum temperature is uncommon: it is principally limited to the eastern coast of North America (Figure 2 ) . The more pronounced increase of nighttime temperatures as opposed to the daily maximum temperatures is almost omnipresent. As a result, the range is decreasing almost everywhere. Notable exceptions are the Pacific islands where both maximum and minimum temperatures are on the rise at an equal pace. The rate of the decrease in the DTR is close to the rate of increase in the mean daily temperature. Given the simplified procedures by which the daily means are calculated, it appears that the reported warming of the Northern Hemisphere since WWII is principally a result of the increase in nighttime temperatures. The DTR decrease varies seasonally from country to country. During the summer the decrease is not evident in Japan or substantial in China. In the United States the decrease is slight in the spring but substantial during autumn. Alaska and Canada show significant decreases in the spring, but slight ones in autumn. In the CIS the decrease in the DTR is significant throughout the year, but it is relatively weak during the winter. In the Sudan, the decrease is sizable in all seasons except summer. Summers are normally the rainy season in Sudan,
FIGURE 2
Annual trends of mean maximum and minimum temperatures and the mean diurnal temperature range, 1951 (or later) through 1990s
.. ximum annual temperature
3
cC -0
Envimn. Sci. Techmi., Vol. 27. No. 8, 1993 1471
tures at the Pic du Midi mountaintop observatory in the Pyrenees Mountains (7).The night temperatures of this pristine high-altitude air have increased significantly since the late 19th century, as has the cloud cover.
I
but the country has recently been plagued by severe droughts. In South Africa, the DTR decrease is largest in the spring (September through November), whereas in autumn it increases. In the eastern half of Australia the decrease in DTR is apparent throughout the year but it is small during the summer. The decrease in the Northern Hemisphere is quite uniform. Considering all seasons and all countries, almost 60% of the decreasing trends is statistically significant at the 95% or 99% level. The coverage in the Southern Hemisphere is still too limited to generalize about trends in that part of the world. We also do not know if the daily temperature structure changes over the oceans. In the United States a network of approximately 500 stations with high-quality data has enabled us to inspect trends since the turn of the century. In the CIS, data going back to the 1930s from 200 stations were studied. Analysis from the United States and the CIS reflects significant 1472 Environ. Sci. Technol.. Vol. 27. No. 8, 1993
T I
variations in the minimum temperatures and DTRs from decade to decade. This is particularly well expressed during the dry 1930s and 1950s in the United States. The overall decrease of the DTR did not begin until the late 1950s in the United States and until the 1970s in the CIS. At the Prague Klementinum, an urban station in the Czech Republic with ZOO-year record of minimum and maximum temperatures, the DTR had been increasing until the first half of the 20th century: since about 1950 it has dropped substantially. Its rise was accompanied by an increase in mean daily temperatures, and its decrease occurred when the mean temperature changed little overall. The DTR at Sodankyla, Finland, also has displayed a gradual decrease since 1950. There has been a general decline in the DTR over Denmark since about 1950,noticed at several long-term rural stations. Especially interesting is the long record of maximum and minimum tempera-
Possible causes The key regulator of nighttime temperatures is the radiative cooling of the ground opposed by downward thermal radiation from the atmosphere. The latter is relatively weak in clear and dry atmospheres and stronger in the humid and cloudy air. Low-altitude clouds are especially effective in keeping the ground warm at night. Carbon dioxide and methane in the atmosphere also tend to increase the nighttime temperature by trapping and reradiating the escaping longwave energy. These greenhouse gases allow short-wave solar radiation to pass through and therefore are equally efficient in boosting the maximum daytime temperature. What then is the likely explanation for the observed trends? The list of potential causes (Figure 3) is long and includes natural as well as man-made variables. Following is a list of the most probable causes. Natural cloud increase. Cloudiness may have increased over land as a result of natural changes in the circulation patterns of oceans and the atmosphere. Nighttime cooling under clouds is less severe and daytime warming less intense. Thus, the increase in cloud cover could adequately explain the rise of nighttime temperatures and the simultaneous drop in daytime highs. Cloud increase over the past several decades has indeed been reported for many parts of the United States and Europe as well as for the entire globe (8).Analysis of data from the United States and the Pyrenees found statistically significant associations between increasing cloud cover and diminishing DTRs (7,9). Natural changes in cloudiness would adequately explain regional variations in temperature over century-long intervals, such as those recorded in England (Figure 1). Anthropogenic cloud increase. Increased cloud cover density caused by industrial pollution is another potential cause. Burning of coal, oil, and wood releases CO, and many other pollutants, of which SO, is the most potent climate modulator (10).It is a factor in acid rain, and it converts into sulfates, which act as cloud condensa-
tion nuclei and thus as catalysts for cloud formation. In polluted air, the c l o u d s t e n d t o be d e n s e r a n d brighter ( 1 1 ) . In the pre-WWII era most smokestacks were low, and the aerosols were deposited near industrial centers or washed away by rain. Today, smokestacks are tall a n d the immediate vicinity of plants is cleaner, but the contaminants are spread over rural areas and into the higher levels of the atmomhere. where thev can readily interact with clouds. ’ As a r e s u l t , t h e ground monitoring stations in industrial hubs report decreasing pollution, while ever-larger regions downwind become contaminated (12). Residence time of industrial emissions and their derivatives in the atmosphere is increasing. Some p o l l u t a n t s reach the upper troposphere, where they reside for many days: eventually they even reach the Arctic. Satellite images show plumes of tropospheric aerosols downwind from the North American coasts and penetrating deep over the Atlantic (13).Dark, low-level haze is now commonplace not only above cities and towns, but also over rural areas of Europe, eastern Asia, South America, and Africa. Long-range commercial aircraft flying near the tropopause (the transition zone between the troposphere and stratosphere) release water vapor (called contrails), which contributes to high-level cloudiness in the main air traffic corridors of the United States and Europe. Denser clouds are likely to keep night temperatures higher and, because they screen sunlight, the daily maximum temperatures are lower. These effects agree with the observed trends. Urban heat islands. Cities are warmer than the countryside. The so-called urban heat island is strongest during the night. In mid-latitudes, the urban-rural temperature difference commonly peaks at night shortly after sunset and decreases after sunrise (14). The urban heat islands are caused by efficient heat retention of buildings and pavements. It may also be caused by the lack of evaporative cooling from bare urban
surfaces with little, if any, vegetation. Thus, increased urbanization leads to a preferential increase in daily minimum temperatures. Irrigation. The DTR over irrigated land is less extreme than that over natural fields. This is because the moist soil is warmer at night and, because of evaporative cooling, it is colder in the afternoon. The importance of this effect was tested in the United States where the recent expansion of irrigated land is one of the largest. It was found that irrigation has a relatively small impact--only locally significant-and does not explain t h e reported hemispheric trends ( 4 ) . Desertification, drying of wetlands, and deforestation would have the opposite effect, depressing nighttime temperatures and increasing davtime temperatures and the DTR. Anthropogenic greenhouse gases. The direct radiative effect of increasing CO, alone is un: likely to explain the current trends, because although the nighttime temperatures would rise in a C0,rich atmosphere, the daytime highs would increase as well if other variables were unchanged. This has been shown by models that analyze changes in the mean minimum and maximum daily temperatures when atmospheric CO, is doubled. The models show an increase in the minimum daily temperatures and an almost equally large rise in maximum daily temperatures (15, 16). Thus the resulting drop in the DTR was calculated to he small, representing only a fraction of the projected overall increase of the mean global temperature. In contrast, the observations show almost a one-toone correlation between warming and the DTR drop. The DTR decrease shown in the models is caused mainly by higher sensible heat exchange of energy at the ground-air interface and higher evaporation losses in the C0,-rich atmosphere: it is not caused by clouds. The models show increased and decreased cloudiness as a result of the CO, doubling. Decreases occur predominantly in the low and
Uhe direct
radiative effect
of increasing
CO2 alone is unlikely to
explain the
- .
middle troposphere: increases occur in the upper troposphere. An important question remains: Can the DTR change result from a combination of greenhouse warming and aerosol cooling? Although g r e e n h o u s e warming operates throughout the day and night, industrial haze affects only daytime temperatures by cutting down the a m o u n t of s u n l i g h t r e a c h i n g ground. Nighttime temperatures remain principally unchanged. What conclusions can be drawn? Perhaps the simultaneous CO, and SO, emissions are causing the observed trends, and properly constructed models in the future will consider changes of both variables and be able to explain the data. Maybe the climate models do not accurately reproduce the effects of clouds. Perhaps the CO, rise causes more evaporation from the oceans than the models show, leading to a higher than expected increase in cloud cover over land. It is no secret that the representation of clouds is the least satisfactory element of the atmospheric general circulation
current trends.
George K u k h is the senior research sci-
entist at the Lamont-Doherty Earth Observotovof Columbia Universityin New York. He received a Doctor of Natural Sciences degree at Charles University in Prague. He is studying changes in North American weather patterns and the records of climote changes around the world.
Thomas R. Karl, the senior scientist at the NOAA-Notional Climatic Data Center, received his M.S.degree in meteorology at the University of Wisconsin. An expert on climate change, he serves on many governmental advisory and scientific panels. He has received the bronze and the gold medals from the US.Deportment of Commerce. Environ. Sci. Technol.. Vol. 27, NO. 8. 1993 1473
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models (1). Perha s the observed trends are the resu t of entirely natural causes having little to do with the anthropogenic alteration of the atmosphere. Whatever the case, future climates may be considerably different from those simulated by models in which the greenhouse gases are the only variables that change. It is important to understand the shifting diurnal temperature structure for two reasons: First, because the DTR trend appears to be inconsistent with current climate models that consider only increases of greenhouse gases, other variables may be modifying Earth's climate. Second, because the ongoing temperature change over land indicates that weather extremes will be moderated rather than amplified, it would be useful to know whether the trend is likely to continue.
f
Acknowledgment This work was supported by the Departm e n t of Energy, Environmental Sciences Division, Office of Health and Environmental Research a n d NOAA Climate and Global Change Program.
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