An increase in clouds may explain why climate change is shrinking the difference between the daily high temperature and the daily low in many parts of the world, according to a new study.
The gap between the two, known as the diurnal temperature range (DTR), has a significant effect on growing seasons, crop yields, residential energy consumption, and human health issues related to heat stress.
Why and where the DTR shrinks with climate change has been something of a mystery. Researchers who examined the DTR at the end of the 21st century believe they have found the answer: The increase in clouds, which blocks incoming-shortwave radiation from the sun during the day.
“Clouds are one of the big uncertainties in terms of climate projections.”
This means that while both the daily maximum temperature and the daily minimum are expected to continue to increase with climate change, the daily maximum temperature will increase at a slower rate. The end result is that the DTR will continue to shrink in many parts of the world, but that the changes will vary depending on a variety of local conditions, the researchers say.
The study, published in the journal AGU Geophysical Research Letters, is the first to use high-resolution computer modeling to delve into the issue of the Earth’s shrinking DTR, particularly how it is related to cloud cover.
“Clouds are one of the big uncertainties in terms of climate projections,” says coauthor Dev Niyogi, a professor at the University of Texas at Austin Jackson School of Geosciences. “When we do this with a very high spatial resolution modeling framework, it allows us to explicitly simulate clouds.”
This is vital for understanding the future of the DTR, says lead author Doan Quang Van, an assistant professor at the University of Tsukuba Center for Computational Sciences in Japan.
“Clouds play a vital role in the diurnal temperature variation by modulating solar radiative processes, which consequently affect the heat exchange at the land surface,” he says.
Using supercomputers at the University of Tsukuba Center for Computational Sciences to do the modeling work, the team was able to model the complicated interplay of land-surface processes on climate change.
These include changes in land use (such as deforestation), soil moisture, precipitation, cloud cover, and other factors that can affect the temperature in a local region. By creating a model with a finer resolution grid—2 square kilometer grids rather than the 100-kilometer grids used in most climate models—the researchers were able to more closely analyze the impacts of climate change.
The team focused on two areas: the Kanto region of Japan and the Malaysian peninsula. They used the 10-year period from 2005-2014 as a baseline and then ran different climate scenarios to project what will happen to the DTR in the two regions at the end of the century.
They found that the temperature gap closes by about 0.5 degrees Celsius (about 0.9 degrees Fahrenheit) in the temperate Kanto region and 0.25 Celsius in the more tropical Malaysian peninsula. The researchers attribute these changes in large part to increased daytime cloud coverage that would be expected to develop under these climate conditions.
The researchers say the study can help scientists improve current global climate models and aid in understanding how the shrinking DTR will affect society and the environment as the climate continues to warm.
“It is very important to know how DTR will change in the future because it modulates human, animal, and plant metabolisms,” says Quang Van. “It also modulates the local atmospheric circulation such as the land-sea breeze.”
Additional coauthors are from the National Center for Atmospheric Research, Shanghai University of Engineering Science, the National Defense Academy of Japan, the University of Tsukuba, and UT Austin.
The Japan Society for the Promotion of Science Grants-in-Aid for Scientific Research (KAKENHI), NASA Interdisciplinary Research in Earth Science, and the US Department of Agriculture National Institute for Food and Agriculture funded the work.
Source: UT Austin