Odd seasons

 R. Wassmann, G. Centeno, and S.B. Peng   |  
The effect of high night temperature associated with global warming on rice yield is being studied at IRRI using temperature-controlled and open- top chambers. (Photo: IRRI)

The effect of high night temperature associated with global warming on rice yield is being studied at IRRI using temperature-controlled and open- top chambers. (Photo: IRRI)

Climate change has often been blamed for unusual weather phenomena, especially the extreme cases. But then again, climate has never been a “fixed” condition. It constantly changes. So, any detection of genuine signs of climate change has to filter out the new but normal climate patterns that occur at the same time. In the Philippines, variations in climate are largely influenced by El Niño, which is an irregular climatic occurrence. It generally arrives every 3 to 5 years.

Rice is grown in the Philippines during two distinct seasons—the dry and the wet season. The dry season, from January to April, tends to produce higher yields than the wet season, from July to October. Climatic changes within these seasons can significantly affect rice production. Thus, farmers and researchers alike grew concerned when they experienced very wet conditions during the first months of 2008 and 2009. Those in Los Baños, Laguna, where the International Rice Research Institute (IRRI) is situated, wondered whether the weather was undergoing some fundamental changes. Given such occurrence, should we start re-defining our growing seasons? IRRI’s Climate Unit thus studied this occurrence to assess the potential changes in the supposedly dry season of Los Baños.

Needed sunshine

More sunshine translates into more energy to convert atmospheric carbon dioxide and water into carbohydrates that subsequently result in higher rice yields. If the sun becomes covered by clouds, especially heavy dark clouds that bring rains, rice plants find it harder to work. Their photosynthetic process slows down, so the plants produce lower yields. Because of this, potential yield in the dry season is normally higher. When farmers plant rice in early January, the crop’s flowering and grain-filling stages fall right on time during April, when the month’s bright sunny days bring enough solar radiation to achieve maximum plant photosynthesis.

Oddly, however, almost all the days of the 2008 and 2009 dry season received low radiation (see Fig. 1 below). The average daily radiation from January to April was 14.9 and 15.1 megajoules per square meter per day in 2008 and 2009, respectively. Significantly, these values are the lowest since 1979, when IRRI’s weather station started recording data.


Moreover, these values are about 20% below the 30- year average dry season value. They are even lower than the 30-year average wet-season value, which is 16.5 megajoules per square meter per day (from June to September of 1979-2009).

Too much rain

The wettest dry season in the past 30 years occurred in 2009. This year also had the highest daily rainfall amount ever recorded in a dry season. To note, rainfall on 21 April (104.1 mm) was extremely high compared with the average rainfall of 152 mm over the entire dry season.

El Niño effect

The interannual variability of the climate in the Philippines—as well as in all of Southeast Asia—is also largely determined by El Niño. El Niño corresponds to the wet years.

Its counterpart, La Niña, is more associated with high rainfall. La Niña conditions prevailed from early 2007 to early 2009. Its occurrence in 2007-08 was the strongest within the past 20 years. Over the past few months, however, La Niña has transitioned to El Niño. According to the World Meteorological Organization, during June and July 2009, the ocean surface in the central and eastern equatorial Pacific has been substantially warmer than normal, supporting the development of El Niño through the remainder of 2009 and probably into the first quarter of 2010. In all likelihood, the next dry season will not be as wet as the preceding ones.

Unusual dry seasons

Following these observations, it can be said that the dry seasons of 2008 and 2009 were more like a wet season for the rice crop. Is this evidence of climate change? Should we indeed start re-defining our growing seasons? At this point, it is still premature to describe this as a symptom of a persistent trend. Scattered over the past decades were years with unusually high rainfall, but none of which had a discernible trend as 2008 and 2009 did. We will remember the dry season of 2009, but we have yet to see whether this extreme case will occur again soon.

Two years of study are simply not enough to detect any statistically significant amount of climate change. We do, however, need to carefully watch future development in terms of seasonal shifts. If wet dry seasons occur more frequently, breeding programs and crop management recommendations will have to be adjusted quickly to help farms continue to yield more rice despite poor radiation and more rainfall.

Leave A Response