Climate Change and the Southeast Asian Monsoon
by Loo Yen Yi
Climate change due to human activity is happening right now. We do not have time to debate about it. Whether we will be able to cope with the consequence of rising temperature depends on our ability to predict the weather systems and cope with the damages.
There is a difference between climate and weather. Weather is the day-to-day condition forecast, i.e. rainy, sunny, cloudy, hot or cold for the next few hours or days. Climate is the average atmospheric conditions over a period of several decades. It is usually measured in temperature, rainfall, or it can be indicated as carbon dioxide concentration, which contributes to temperature changes because it acts as a greenhouse gas that warms up the air within the atmosphere.
Seeing the temperature difference in this way allows us to understand the larger scale differences and how it affects the global atmospheric processes. Figure 1 shows how the climate has been changing since the 1850s and there is a clear pattern of increasing temperature, which has been linked to increased rainfall .
Climate change deniers would argue that the climate had changed in the past, and that we are only passing through another phase of a warm spell. However, the rate of temperature increase in our period has no historical parallel. In fact, there was a gradual cooling trend for the past 1,000 years until the start of the Industrial Revolution in the 1760s .
This rapid rise in temperature in such a short time presents many abrupt changes that life on Earth would struggle to adapt. One we can relate to, closer to home is the monsoon season of Southeast Asia (SEA). The recent unpredictable flooding has damaged lives, and farmlands with a huge economic burden.
The monsoon seasons are governed by global weather systems, so it is important to understand the fluctuations in the monsoon season and rainfall (variability), and how climate change has an effect on monsoon seasons.
Tilting of the Earth and the difference in solar radiation received between land and sea creates the monsoon weather systems (Figure 2). Furthermore, the northern hemisphere is warmed more than the southern hemisphere as it has more land mass. This causes the opposite seasonality in the north and the south.
This seasonality is important in regulating the monsoon rainfall regime. The dry season in southeast Asia during the east Asian (EA) winter monsoon is caused by both the decrease in temperature in the north during winter resulting from the cooling of the northern hemisphere and the anti-cyclones caused by the reduced atmospheric pressure. During the EA summer monsoon, the landmass in the north warms up and the monsoon winds transports the rising warm air to the southern hemisphere .
Several other weather systems affect and are affected by the monsoon seasons. The Siberian High, or Siberian Anticyclone, collects cold and dry air and accounts for the lowest temperatures and highest pressures from September to April each year. The Siberian High influences the EA winter monsoon – a strong Siberian High leads to a strong EA winter monsoon.
There is also another weather system playing in this orchestra and that is the Arctic Oscillation. Generally, a high atmospheric pressure in the Arctic is usually coupled with a low atmospheric pressure in the northern mid-latitudes . The weather and climate of the Arctic influence the intensity of Siberian High to some extent, which in turn affects how strong its winter monsoon will be.
The EA summer monsoon, however, is dominated by the western Pacific Sub-tropical High (WPSH), which in turn is affected by the atmospheric temperature. An interesting study showed that the WPSH had shifted westward from 133.5°E in the 1970s to 119.5°E between 1980 – 1999 . That is a shift of 14° or about 1556 kilometres! Scientists believe that this shift was due to the warming of the Indian Ocean and the West Pacific [6, 7].
It is also important to note that the monsoon season of Southeast Asia will be delayed up to 15 days according to scientists’ predictions . What’s more, it is predicted so because of global warming from the late 21st to early 22nd century, and we will receive about 70% less rain during the monsoon seasons in India and Southeast Asia . This will cause major water availability issues especially for the Indian population because they rely heavily on these monsoon rains. In the past, north-eastern India and Bangladesh received an early monsoon season and caused major flooding events on the 24th of July 2004 that claimed 1,000 lives (Figure 3).
The monsoon season is also becoming less predictable. We might be receiving bouts of heavy rain in one year that causes these floods and then dry spells in the next. From statistical models, scientists found that in countries closer to the equator, such as Malaysia and Singapore, experience rain and drought in a 10-year cycle. For Philippines and Thailand, which are slightly further north, it is a 30-year cycle .
Flooding events in the 20th century proved that too much rain is bad news. Often we hear stories of infrastructure damage, resident eviction, and the cost of human lives. In addition to that, it can trigger landslides that consequently damage roads and vehicles as well as claiming lives .
Many of us would have heard about or seen devastating flooding events happening in Malaysia. Many of the flooding events in SEA are associated with the summer monsoon downpour. These abnormal flooding patterns tell us that the monsoon rainfall is changing, most possibly due to global warming.
The change in monsoon patterns might not have been obvious before, because it changes seasonally, and perhaps we have not lived long enough to feel the changes since the pre-industrial ages. Nevertheless, monsoon changes are happening, and the monsoon systems will continue to be a destabilising factor over the years if we do not act now.
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About the author
LOO YEN YI is an aspiring ecologist who took the path less travelled by Malaysian students. During her bachelor’s degree in Environmental Science in the University of Nottingham Malaysia Campus, she published a paper on the monsoon changes in SEA due to global warming. Today she is studying abroad in UK at Manchester Metropolitan University, pursuing a newfound passion – Bird Conservation. She is optimistic about the exponential growth in numbers of Malaysian students taking up the path to unite lawyers, businessmen, politicians and academics in mitigating climate change. To find out more about Yen Yi, visit her SciMy profile at http://www.scientificmalaysian.com/members/looyenyi/