Taiwan sinking: Subsidence or Global Warming Induced Sea Level Rise?

This news story about Taiwan has been making the rounds with the usual alarming news outlets. My view is clearly on subsidence, caused by poor land use practice. See below the Continue Reading line for the easily found reasons.

Excerpts: from AFP via Yahoo News

Rising sea levels threaten Taiwan

TUNGSHIH, Taiwan (AFP) – When worshippers built a temple for the goddess Matsu in south Taiwan 300 years ago, they chose a spot they thought would be at a safe remove from the ocean. They did not count on global warming.

Now, as the island faces rising sea levels, the Tungshih township is forced to set up a new temple nearby, elevated by three metres (10 feet) compared with the original site.

“Right now, the temple is flooded pretty much every year,” said Tsai Chu-wu, the temple’s chief secretary, explaining why the 63-million-dollar project is necessary.

“Once the new temple is completed, we should be able to avoid floods and the threat of the rising sea, at least for many, many years,” he said.

The temple of Matsu, ironically often described as the Goddess of the Sea, is only one example of how global warming is slowly, almost imperceptibly piling pressure on Taiwan.

…

And unlike the temple, none of these crucial economic establishments can possibly be lifted, leaving them exposed to the elements.

“If the sea levels keep rising, part of Taiwan’s low-lying western part could be submerged,” said Wang Chung-ho, an earth scientist at Taiwan’s top academic body Academia Sinica.

…

Still, environmentalists consider the risk too high to ignore, and they point out that it is compounded by the overpumping of groundwater both for traditional agriculture and for fish farming.

This has caused the groundwater level to fall and land to subside below sea level in some coastal areas, experts warn.

The greatest extent of seawater encroachment has been estimated to be as far as 8.5 kilometres inland with an affected area of about 104 square kilometres (40 square miles) in southern Taiwan’s Pingtung county, according to a study co-written by Wang.

Once low-lying areas are routinely invaded by sea water, it is very hard to turn back the tide, analysts warned.

…

In its 2007 assessment report, the Intergovernmental Panel on Climate Change of the United Nations said that due to the global warming, the world’s sea level is projected to rise by up to 0.59 metres before the end of this century.

However, Wang was more pessimistic, citing recent findings that greenhouse gas emissions are growing faster than previously believed.

Read the rest of the story here: AFP via Yahoo News

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And where is Pingtung County in Taiwain?

But that is not where the Matsu temple that is the focus of the story is, it is a misdirection. Read on.

Now consider this news story about a hi-speed rail system in Taiwan from China Daily that says:

Safety concerns were raised after according to the Taiwan High-Speed Rail Corp. (THSRC) figures revealed that at its worst, the land at one site along the stretch in Yunlin County has sunk 55 centimeters over the past seven years.

Over-pumping of underground water for irrigation has been blamed for the subsidence, and the Water Resources Agency (WRA) has identified 1,115 wells in the area that need to be sealed to stop the sinking.

Seems pretty clear that subsidence is happening quickly in that county. Here’s a paper studying the Yuanlin area, Changhua County. PDF here. Note the mention of Yunlin County, save that for later.

Using Radar Interferometry to Observe Land Subsidence in Yuanlin area, Changhua County, Taiwan

Abstract: The behavior of land subsidence in Yuanlin area, Changhua County, Taiwan has been monitored by the two-pass method of Differential Interferometric Synthetic Aperture Radar (DInSAR) during the period from 1995 to 2002. Our interferometric result has shown that the subsidence behavior is unusual right before and after the Chi-Chi earthquake. Two-month before the earthquake, the pre-seismic differential interferogram detects a substantial increase in land subsidence with a prominent U-shaped pattern of groundwater level change. Two days after the devastating earthquake, our one-month image-pair shows a five-fold increase in land subsidence and an apparent shift of subsidence center. In this study, we suggest mechanisms that contribute to land subsidence in pre-seismic, co-seismic and post-seismic. We tend to believe that the circular/elongated pattern shown in our interferograms are caused by a point-source deformation. Besides, strain also plays a very important role in accelerating land subsidence shown in the post-seismic differential interferogram. It causes a very sudden, step-like surge in groundwater. The shaking of the earthquake as well as the increase of groundwater trigger the occurrence of soil liquefaction, in return, accelerating land subsidence. We propose there are two center of land subsidence right after the Chi-Chi earthquake though only one subsidence center can be observed in our differential interferogram.

Here’s what the Taipei Times shows happening as a result of land subsidence:

Here’s an interesting passage from the Geography Department at NTU titled The Hazards of Taiwan:

The fish-farming industry in western and northeastern Taiwan requires several times more ground water than is needed for irrigation. This kind of over-pumping of ground water results in serious land subsidence or sinking in the coastal areas. According to a recent survey, an area of up to 1,097 square kilometers suffers from subsidence: this is 3% of the island’s total land area and 9% of its flat area. This problem obviously needs an immediate and effective solution.

So even though there is plentiful evidence that local land use abuse resulting in subsidence is the primary cause of seawater incursions, the reporter, Benjamin Yeh, chooses instead to make “global warming” the primary culprit.

His paragraph says it all:

The temple of Matsu, ironically often described as the Goddess of the Sea, is only one example of how global warming is slowly, almost imperceptibly piling pressure on Taiwan.

Religion and global warming, a match made in heaven.

From this Taiwan Government Report on Water Resources we find this paragraph, red emphasis mine:

Land Subsidence

Lured by profits, many farmers in the coastal areas of Yunlin, Changhua, Pingtung, Chiayi, and Ilan have expanded into aquaculture. Aquaculturalists have dug 170,000 illegal wells and pumped excessive amounts of groundwater, because it is cheap and stable in temperature. In addition to being used in aquaculture, groundwater is also pumped for industrial, residential, and standard agricultural uses. Recent data shows that while 5.94 billion cubic meters of groundwater is being pumped annually, only four billion cubic meters is being replaced. This deficit has caused land in many areas to subside, especially along the southwestern coast and on the Ilan Plain. Overall, almost 865 square kilometers of Taiwan’s plains, or a full 8 percent, tend to subside. The most serious subsidence has occurred around Chiatung in Pingtung County, where sites have sunk by as much as 3.06 meters. The average rate of subsidence in the coastal areas is between five and 15 centimeters each year.

The Temple of Matsu is in Yunlin County which is located on this map:

Another study on groundwater and subsidence from the Department of Geomatics, National Cheng Kung University says:

For example, the overall amount of subsidence in Yunlin area in the past 30 years reaches about 2 meters, and the total affected area of subsidence is about 516 km2. Land subsidence has increased the vulnerability in this area, and a large portion of which lies below the mean sea level.

When badly flawed articles like this one from AFP’s Benjamin Yeh appear, blaming global warming for flooding clearly caused by land subsidence as a result of poor land use practice, we need to complain loudly to editors.

http://www.afp.com/afpcom/en/contact