The Decreasing Diurnal Temperature Range (DTR)

Daytime temperatures are higher than nighttime temperatures. The difference is defined as the diurnal temperature range (DTR). Observations show that in most places the DTR is decreasing in recent years. It is caused by temperatures increasing faster at night than during the day. Thomas Karl (1993) states: "Since 1950 all of the increase of temperature across the U.S.A. is due to an increase in the minimum temperature (about 0.75 degrees C/ Century or 1.5 degrees F/Century ) with no change in the daily maximum temperature. This caused a decrease in the diurnal temperature range." Subsequently, this type of behavior has been observed at other locations and is stronger as one goes towards the polar regions. It now appears most of the observed global surface warming of recent decades is occurring mostly at night.

Explanations involving Changes in the Atmosphere

A decreasing DTR presents problems for the greenhouse warming theory. In 1995, James Hansen noted: "Models show that daytime warming will be almost as great as nighttime warming" [for a greenhouse gas forcing]. In a 1997 paper in Climate Dynamics, climate modeler Watterton points out that the observed decrease in the range of diurnal temperatures "are not consistent with their being produced by the observed increase in greenhouse gases." No climate model yet devised can fully account for the observed diurnal variations and hence the recent observed warming. Watterton suggests that there are major errors in the way climate models treat clouds. A climate model experiment that comes close to explaining the results is given by Hansen et al. (1995) in Atmospheric Research. Their model experiment implies that a greenhouse gas doubling, accompanied by a 1.2% increase in low level clouds, will reduce the diurnal cycle by 0.21 C compared to an already observed decrease of about 0.5 C. This experiment implies clouds are acting as a strong negative feedback, so that when greenhouse gases double, the global warming will equal 0.67 C. Even this number may be high since they are unable to explain the full 0.5 C change, meaning even more clouds causing more cooling may be required. However, a recent study by Kaiser (1998) shows that cloud cover in China is decreasing along with the decrease in diurnal cycle amplitude. Thus, Hansen's model is not consistent with observations. Neither greenhouse gases nor changing cloud cover can account for the decreasing diurnal temperature cycle in the surface observations, which is the primary evidence that is used to claim the warming is caused by increased greenhouse gases.

More recently, Stone and Weaver (2003) have looked at the DTR using climate models and find that “the cause of the DTR trend is still poorly understood, as is its relation to anthropogenic forcing.” They argue that increasing cloud cover and increasing soil moisture may be the cause of the observed DTR variations. Increasing cloud cover suppresses warming during the day and is thus a negative feedback.

It seems that changes in aerosols, water vapor, or ozone are inadequate to explain the observed DTR trends. Changes in cloud cover are a contender, but if true, it implies clouds are a negative feedback and hence a doubling of carbon dioxide will cause a very modest warming of less than 1 C.
Explanations involving Changes in the Land Surface
Changes in land surface and the growth of urban heat islands (UHI) can also cause decreases in DTR. As early as 1996, Hughes and Balling showed there were decreases in DTR in urban locals but no change in DTR in adjacent rural locations using stations in South Africa. This study alone gives rather conclusive proof that the DTR changes are caused by UHI and not by any change in the atmosphere, such as clouds, which would be the same for nearby urban and rural stations.

Moreover, if Hansen et al. (1995) and Stone and Weaver (2003) were correct in blaming the decrease in DTR on increasing cloud cover, one would expect the albedo of the Earth to increase rather than decrease as has been shown by the measurements of Palle et al. (2004). Arguments attributing the decrease in DTR to greenhouse warming are simply not tenable.

More recently using stations in China, Zhou (2004; see press release athttp://www.zhb.gov.cn/english/news_detail.php3?id=9327) attribute most of the warming and decreases in DTR to UHI and we quote from the press release:

“To measure the variations of the DTR, Zhou and his collaborators used observational data of monthly daily maximum and minimum land surface air temperatures at 671 meteorological stations across China from January 1979 to December 1998, collected and processed by the National Meteorological Center of the China Meteorological Administration.”

“They then focused their study on 13 provinces and cities in southeast China that consist of 194 well distributed observation stations, in the area where most of China's urbanization has occurred.”

“According to them, this region has the highest meteorological station density; the most uniform station distribution, the minimal non-climatic effects; and most consistent observation data in China.”

“They found a decline in the DTR at most stations, with the largest decrease in the eastern and southern coastal areas where rapid urbanization has occurred, suggesting a rise in near surface temperature.”

“The decrease of DTR is greatest in the Yangtze and Pearl River deltas and is generally larger at coastal stations.”

“They also found the DTR change is generally consistent with several indicators of urbanization such as the number of towns and cities, urban population, and so on.”

“"If urbanization is responsible for the reduction in the DTR, changes should be correlated with factors known to affect urbanization." Zhou says in the paper.”

And we emphasize the statement “the DTR change is generally consistent with several indicators of urbanization”. A review of the Zhou paper can be found here.

On the other hand, Collatz et al. (2000) conclude that changes in land cover cause the decreased DTR. They "suggest that reported increases in vegetation cover in the Northern Hemisphere during the 1980s could have contributed to the lowered DTR."

Since land cover changes and UHI are not included in the climate models used by Hansen et al. and Stone et al., it implies that changes in the surface are a strong contender to explain the observed DTR changes. Kaiser showed that changes in cloud cover cannot always explain the observed DTR decrease. In addition, the DTR has a weekly cycle and clouds do not (although fossil fuel burning in cities does have a weekly cycle), providing further evidence that the decreasing DTRs are an effect of changing surface and not an effect of changing atmosphere (Forster et al., 2003).

What Does it All Mean?

If changes in the atmosphere are ruled out as a cause for changing DTR, then it implies UHI and land use changes dominate. Since the nighttime temperatures are rising three times as fast as the daytime temperatures (Karl et al., 1993), it implies a non-climatic signal in the nighttime data equal to about one half of the total warming. It implies the reported global warming of 0.6 C in the twentieth century should be reduced to about 0.3 C.

References:

Collatz G. J., Bounoua L., Los S. O., Randall D. A., Fung I. Y., Sellers P. J. (2000) A mechanism for the influence of vegetation on the response of the diurnal temperature range to changing climate. Geophysical Research Letters, 27, 3381–3384.

Forster, P. M. De F., and S. Solomon, 2003. Observations of a “weekend effect” in diurnal temperature range. Proc. Natl. Acad. Sci. U. S. A., 100, 11225–11230

Hansen, J., M.. Sato, and R. Ruedy, 1995. Long-term changes of the diurnal temperature cycle: Implications about mechanisms of global climate change. Atmos. Res. 37, 175-209.

Hughes W. E., Balling R. C., 1996. Urban influences on South African temperature trends. Int. J. Climatology, 16, 935-940.

Kaiser, D. P., 1998. Analysis of cloud amount over China, 1951-1994, Geophysical Research Letters, 25, 3599-3602.

Karl, T. R., Jones, P. D., Knight, R.W., Kukla, G., Plummer, N., Razuvayev, V., Gallo, K. P., Lindseay, J., Charlson, R. J., and Peterson, T. C., 1993. A new perspective on recent global warming. Bull. Amer. Meteorol. Soc., 74: 1007-1023.

Palle, E., P. R. Goode, P. Montanes-Rodriguez, and S.E. Koonin, 2004.Changes in Earth's reflectance over the last two decades. J. Climate (http://solar.njit.edu/preprints/palle1210.pdf).

Stone, D. A. and A. J. Weaver, 2003. Factors contributing to diurnal temperature range trends in twentieth and twenty-first century simulations of the CCCma coupled model, Climate Dynamics, 20, 435-445.

Watterton, I. G., 1997. The diurnal cycle of surface air temperature in simulated present and doubled CO2 climates. Climate Dynamics, 13, 533-545.