US National Oceanic and Atmospheric Administration
Climate Test Bed Joint Seminar Series
NASA Goddard Visitor Center, Greenbelt, Maryland, 25 May 2011                                                                              [Print Version]

Asymmetric Global Warming: Day versus Night

Liming Zhou

School of Earth and Atmospheric Sciences

Georgia Institute of Technology, Atlanta, Georgia

One distinct climate feature associated with global warming is the widespread decrease of diurnal temperature range (DTR) that has been observed over land since 1950 due to a larger warming in minimum air temperature (T_min) than in maximum air temperature (T_max). Although the warming trend of mean surface air temperature and the decreasing trend of DTR are closely related, the former has been largely attributed to anthropogenic forcing while the latter to increased cloud cover. The question remains as to what is primarily responsible for the observed DTR decrease and whether this decrease is attributable to human activities. This seminar will try to address this question with three case studies.

The first case study will be focused on analyzing spatial patterns of observed annual T_max, T_min and DTR trends from 1950-2004 and their association with precipitation and cloud cover. It presents observational evidence for a larger DTR decreasing trend and a stronger T_min warming trend over drier regions. The grid boxes at spatial resolution of 5° by 5° degrees were classified into a number of large-scale climate regions in terms of the climatological annual precipitation amount at each grid box. The regional average trends of annual T_min and DTR exhibit significant spatial correlations with the regional averaged anual precipitation, while such correlation for T_max is very weak (Fig. 1). In general, the magnitude of the downward trend of DTR and the warming trend of T_min decreases with increasing precipitation amount, i.e., stronger DTR decreasing trends over drier regions. Such spatial dependence of T_min and DTR trends on the climatological precipitation possibly reflects large-scale effects of increased global greenhouse gases and aerosols (and associated changes in cloudiness, soil moisture, and water vapor) during the later half of the 20th century.

The second case study will be focused on comparing the trends and variability in T_max, T_min, and DTR over land in observations with 48 simulations from 12 global coupled atmosphere-ocean GCMs for the later half of the 20th century. When anthropogenic and natural forcings (referred to as ALL) are included, the models generally reproduce observed major features of the warming of T_max and T_min and the reduction of DTR (Fig. 2). The greenhouse gases enhanced surface downward longwave radiation (DLW) explains most of the warming of T_max and T_min while decreased surface downward shortwave radiation (DSW) due to increasing aerosols and water vapor contributes most to the decreases in DTR in the models. When only natural forcings (referred to as NAT) are used, none of the observed trends are simulated (Fig. 2). The simulated DTR decreases are much smaller than the observed (mainly due to the small simulated T_min trend) but still outside the range of natural internal variability estimated from NAT. The much larger observed decrease in DTR suggests the possibility of additional regional effects of anthropogenic forcing that the models cannot realistically simulate, likely connected to changes in cloud cover, precipitation, and soil moisture. The small magnitude of the simulated DTR trends may be attributed to the lack of an increasing trend in cloud cover and deficiencies in charactering aerosols and important surface and boundary-layer processes in the models.

Our results also indicate that the models generally reproduce the spatial dependence of T_min and DTR trends on the precipitation (Fig. 3) in response to anthropogenic forcings in ALL, but not in NAT (also see the observations in Fig. 1).

The third case study will be focused on quantifying feedbacks of changing land surface properties on DTR in a climate model. Observations show that the DTR was reduced most in dry regions and especially in the West African Sahel during a period of unprecedented drought. Furthermore, the negative trend of DTR in the Sahel appears to have stopped and may have reversed after the rainfall began to recover. This study develops a new hypothesis with climate model sensitivity studies showing that either a reduction in vegetation cover or a reduction in soil emissivity would reduce the DTR by increasing T_min through increased soil heating and reduced outgoing longwave radiation (Fig. 4). Consistent with empirical analyses of observational data, our results suggest that vegetation removal and soil aridation would act to reduce the DTR during periods of drought and human mismanagement over semiarid regions such as the Sahel and to increase the DTR with more rainfall and better human management. Other mechanisms with similar effects on surface energy balance, such as increased nighttime downward longwave radiation due to increased greenhouse gases, aerosols, and clouds, would also be expected to have a larger impact on DTR over drier regions.

Figure 1

Figure 2

Figure 3

Figure 4

References

Zhou, L., R.E. Dickinson, A. Dai, P. Dirmeyer, 2010: Detection and attribution of anthropogenic forcing to diurnal temperature range changes from 1950 to 1999: Comparing multi-model simulations with observations. Clim Dyn, 35, 1289–1307.  doi 10.1007/s00382-009-0644-2.

Contact  Liming Zhou