This development could have a far-reaching impact on the yield of rice, causing a shortfall in an important staple crop in a crowded country already grappling with food security and inflationary issues, said Krishna Kumar Kanikicharla, a scientist at the Indian Institute of Tropical Meteorology in Pune, India.

A nighttime temperature increase of 0.20 degree Celsius has been seen in the past decade, said Kanikicharla. This is higher than the measured daytime increase of 0.16 degree Celsius over the same period. His findings will be published in the journal Climate Research shortly.

This is the first time that a nighttime temperature rise has been observed in India. The Earth radiates back heat at night, so a tight blanket of greenhouse gases and clouds in the upper atmosphere acts as a block to prevent radiation back into space. As a result of this blanketing, warmer nights can be an indicator of climate change, according to some experts.

South Asia's agriculture will be hard hit by rising temperatures and irregular rainfall associated with climate change, according to experts. Since witnessing a near tripling of yields between the 1950s and 2000s due largely to the technological advances of the first green revolution, India's yields of rice have leveled off in recent years, according to data from the United Nations' Food and Agriculture Organization.

The production of food grains, which are an important source of protein in the largely vegetarian nation, has been far outstripped by population growth, said R. Rukmani, director of food security at the M.S. Swaminathan Research Foundation.

Yields drop as nighttime temperatures rise

The effect of increasingly hot nights on rice cultivation has been measured in a controlled experiment in the Philippines by the International Rice Research Institute (IRRI).

Researchers were serendipitously growing rice in a plot next to a weather station making detailed temperature measurements. The rice was given a stable amount of water and other nutrients, and the only variable factor was the sun.

After about 25 years, the researchers realized that the two data sets could be easily compared to figure out the effect of temperature on rice.

"Every 1-degree-Celsius increase in nighttime temperature led a 10 percent reduction in yield," said Kenneth Cassman, a professor of agronomy at the University of Nebraska and a co-author in the IRRI project.

That is a large amount because a major part of the projected 4-degree-Celsius increase in temperature due to climate change will happen at night, said Cassman. In a situation analogous to running a marathon on a hot summer day, the plant finds it difficult to respire and requires more energy.

The plant also releases large amounts of a reactive molecule called a "reactive oxygen species," according to Abdul Razack Mohammed and Lee Tarpley, agronomists at the Texas AgriLife Research and Extension Center. They tested rice at nighttime temperatures of 32 degrees Celsius and found that the released oxygen molecules damage the membranes of the leaf.

Ruling out other variables

The plants also have less pollen germination and can't reproduce as successfully, according to Mohammed. Their rice yields fell by 90 percent.

But the growth of rice has various inputs -- water, fertilizer, seed quality -- all of which can alter yields in a more significant manner than climate change. The correlation between climate change and rice yields is valid only if it can be shown that warmer nights are not caused by confounding factors such as an urban heat island effect, said Cassman.

The Indian temperature analysis was done using temperature measurements from 120 weather stations in the country, collected between 1901 and 2007, according to Kanikicharla. Particular cities, especially in northern India, have shown cooling trends in recent years due to aerosols, which act as coolants in the atmosphere.

But the large-scale pattern of trends is toward warming, said Kanikicharla.

All 120 weather stations were close to cities with more than 100,000 people over the time measurements were taken. This reduces variability due to the urban heat island effect, in which the temperature increase in urban areas can be attributed to city surfaces trapping heat, he said.

"There is no such thing as a village, really, in India," he said.