SINGAPORE — Generations of Australians have learned that their island-continent is a land of alternating droughts and floods. Recent prolonged rain and devastating flooding across eastern Australia, particularly in the state of Queensland, underscore this heartbreaking cycle.

Weather experts say that the immediate cause is natural: a periodic fluctuation in sea surface temperature of the central Pacific Ocean along the equator and in air pressure of the atmosphere above. Known as the El Nino-Southern Oscillation (ENSO), it affects weather patterns in many parts of the Pacific, including Australia and Southeast Asia.

However, there are also signs that that ENSO itself may be affected by man-made climate change. As Queensland counts the heavy financial cost, some fear that the economic disruption and damage to industry and infrastructure may be repeated more often and more intensely in Australia in future. Losses to the Queensland economy and the cleanup bill are expected to exceed $10 billion. The national growth rate may also take a hit. Queensland is Australia’s largest coal exporter — mainly to Asia — and accounts for about 20 percent of the country’s $1.2 trillion economy, which relies heavily on mineral, energy and farm exports.

ENSO has two extreme phases in its typical seesaw every three to eight years. One, El Nino, is associated with hotter than normal temperatures and diminished rainfall. The other, La Nina, usually brings above-average wet weather and lower temperatures.

The Australian government’s Bureau of Meteorology said earlier this month that the La Nina phase bringing the deluge to eastern Australia was the strongest since at least the mid-1970s. As a result, Australia had its third wettest year on record in 2010.

Indonesia’s Met Office reported recently that rain across the far-flung island-nation would continue until June. It said the dry season, which normally starts in April and lasts until October, would only start in July. Meanwhile, Brazil and Sri Lanka have been hit by unusually heavy and damaging downpours, just as northern Europe and much of the United States feel the bite of abnormally frigid winter weather.

Despite these bursts of wet and cold weather, two leading U.S. climate agencies said on Jan. 12 that the average land and sea surface temperature last year tied with 2005 as the warmest on record since data collection started in 1880. The global temperature was 0.62 degree Celsius above the 20th-century average.

Attributed by many scientists to the growing release of carbon dioxide, methane and other global-warming gases from human activity into the atmosphere, this temperature rise is happening at the same as the natural ENSO cycle. James Hansen, director of one of the U.S. climate agencies, said that the average global temperature increased as fast in the past decade as in the prior two decades, despite year-to-year fluctuations associated with ENSO.

A summary on the state of the Australian climate published last year by the Met Bureau and the CSIRO, Australia’s leading scientific research organization, said that in the past 50 years the mean temperature in Australia had risen by about 0.7 C and was projected to increase further by 0.6 to 1.5 C by 2030. It said that if global greenhouse gas emissions continued to grow at business-as-usual rates, Australia could be 2.2 to 5.0 C hotter by 2070.

Scientists say that the warming trend increases the likelihood of extreme weather events such as heat waves, droughts and floods. In addition to being the equal hottest year ever, 2010 was also the wettest on record. A hotter world causes more evaporation from land and oceans. A warmer atmosphere holds and releases more water, which can mean more violent storms and bigger floods.

The equatorial expanse of the Pacific, which is far larger than the Indian and Atlantic Oceans, is critical to the development of ENSO. During La Nina, trade winds blowing west bring moist air to northern Australia and Indonesia. Heated by the tropical sun and warm water, the air rises to create towering bulbous clouds and heavy rainfall.

A question that must concern Australia and Southeast Asia is whether man-made global warming from burning fossil fuels and clearing forests is intensifying natural weather patterns like ENSO and, if so, how?

It is clear that if an exceptionally dry El Nino phase occurs against the backdrop of long-term man-made global warming, one will make the other hotter. This happened in Indonesia in 1997-98 during the Asian financial crisis when forest fires spread haze pollution across Southeast Asia.

Some scientists also think that there is a link between rising global sea temperature and the strength of ENSO cycles. The annual climate statement by the Australian Met Bureau, issued on Jan. 5, noted that sea-surface temperatures in the Australian region last year were the warmest on record, 0.54 C above the 1961 to 1990 average. The last decade was also the warmest on record for sea surface temperatures. The statement added that “very warm sea surface temperatures contributed to the record rainfall and very high humidity across eastern Australia during winter and spring.”

Echoing the scientific panel advising the United Nations on climate change, the Australian Met Bureau-CSIRO assessment for 2010 said that there was a greater than 90 percent certainty that increases in greenhouse gas emissions have caused most of the global warming since the mid-20th century.

If those who believe that man-made global warming gases are intensifying extreme ENSO weather are right, the flood devastation in Australia is a warning that we upset the complex climate system at our own peril.

Michael Richardson is a visiting senior research fellow at the Institute of South East Asian Studies in Singapore.

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