Increase in Monsoon Rainfall Largely due to Natural Climate Fluctuations, not Global Warming

According to a new study, the natural ups and down of climates are more important in predicting rainfall distribution than the effects of global warming. Researchers say that the findings of the study must be taken into account while predicting future climate change.

Knowing when and how much rainfall will occur in the Northern hemisphere is important, as it affects 60 percent of the population in Southeast Asia, West Africa and North America.

In the U.S., the state of Arizona happens to be in the region that experiences this monsoonal circulation. During summer months, winds from either west or northwest shift to south or southeast. The moisture from Gulf of Mexico along with Gulf of California then streams into the state, causing rains. The effects of monsoonal rains affect southwestern U.S. and parts of Mexico, according to the National Weather Service Forecast Office. Recently, a related study had said that there would be a decrease in the amount of rainfall over parts of the U.S. and Mexico.

Rainfall predictions help infrastructure planners make informed decisions about having sustainable economic development in the region. These predictions, especially for many decades in the future, are difficult to make, as it requires knowledge about the rise in greenhouse gases along with the pattern of natural climate fluctuations.

For the study, researchers analyzed data on monsoon rainfall over the past 30 years. During the past three decades global-mean surface-air temperature has increased by about 0.4°C. Theoretically, the monsoon rainfall should weaken with the rise of temperature. But, researchers found that rainfall actually increased in the past three decades.

Researchers say that the increase in the monsoon rainfall is due to the cooling of eastern Pacific that started in the year 1998. There have been other changes in climatic conditions that can't be explained with the rise of greenhouse gases.

Lately, there has been a mega-La Niña or cool phase. Another natural fluctuation is called the Atlantic Multidecadal Oscillation, which is responsible for higher rainfall, according to a news release.

"These natural swings in the climate system must be understood in order to make realistic predictions of monsoon rainfall and of other climate features in the coming decades. We must be able to determine the relative contributions of greenhouse-gas emissions and of long-term natural swings to future climate change," said professor Bin Wang at the International Pacific Research Center, University of Hawaii at Manoa.

The study is published in the Proceedings of the National Academy of Sciences.