Chris de Freitas: Tuvalu floods, but it's not sinking

KEY POINTS:

The Monday Reuters agency report in the Herald on changing sea levels in the Pacific island group of Tuvalu, "Sinking Culture Snapped", is misleading.

Since instrumentation was installed in 1993 on Tuvalu's main island Funafuti, sea level has shown no discernible trend. There is some inundation evident on islands in Tuvalu, but global warming is not the cause.

It is the result of erosion, sand mining and construction projects causing an inflow of sea water.

Other factors are also involved.

Excessive use of freshwater for irrigation also causes destruction of underground freshwater reservoirs. A consequence is seawater encroachment into vegetable growing pits is occurring, but is not due to sea level rise.

Part of the problem is related also to the paving of the roads and the runway on Funafuti.

According to estimates, about one quarter of the island has been paved over. The effect of this has been to reduce infiltration of rainwater into the freshwater lens. When this increased runoff is combined with a high tide, flooding along the coast looks like the sea level is rising.

Perception of trends can also be affected, as increasing population on the islands means people are now living on flood prone land that was previously avoided.

Coral is capable of growing faster than most if not all past rates of sea level rise. The atolls are not static.

The islands grow as they are replenished by coral that breaks off the reefs and is thrown ashore by storms.

In that way atolls are self-maintaining, provided humans don't intervene, such as by digging the coral for use in construction work, and building flush toilets that discharge the effluent into the sea and where it affects coral growth.

Reasonable scenarios of sea level change are based on calculations that rely on scientifically sound assumptions. Over the short term, climate warming could cause sea level to rise mainly by the thermal expansion of the oceans.

Melting of polar ice caps is not involved since this is a long-term response. As only the surface water is affected, response times can be rapid, but sea level rises of only a few millimetres are possible even for the worst cases of warming.

Supporters of worst-case scenarios of global warming point to evidence of sea level rising. But sea level measurements are subject to a number of biases, usually upwards, from removal of ground water, erection of buildings, road and airports, subsidence of the measuring equipment, and surface movement due to earthquakes. Or as a result of large depositional features such as river deltas.

Despite this it is noteworthy that historical records show no acceleration in sea level rise in the 20th and 21st centuries. It is important to keep in mind that greenhouse gas-induced climate change can also act to substantially reduce sea level.

There is now a substantive body of research reported in peer-reviewed scientific journal literature that suggests that sea levels, which have been rising since the end of the last ice age (long before industrialisation), are likely to stabilise or fall in a greenhouse-warmed world.

This is because empirical evidence indicates that a modest warming of the Earth could lower sea level by increasing evaporation from the oceans. The result is increased deposition and accumulation of snow on the polar ice caps, principally in the Antarctic, thereby transferring large amounts of water from the oceans to the ice sheets.

The reasoning is that if the Antarctic air were to warm, it would still be below freezing, but its water holding capacity would increase as it warms. With more moisture in the atmosphere over the Antarctic, snowfall would increase and ice sheets would grow, locking up water that would otherwise be in the sea.

In this context, it is significant that during the strong warming episode of 1920-40, sea level rise did not accelerate but actually stopped.

Several years ago the prime minister of Tuvalu said his government is ready to sue the United States and Australia because they have not signed the Kyoto Protocol. He claimed most of his country will have disappeared into the ocean within 50 years.

Getting this suit to court might be the best that could happen, since the scientific foundation of the global warming scares would be tested in court and independent scientist would be heard as experts.

Chris de Freitas is an associate professor in the School of Geography, Geology and Environmental Science at the University of Auckland.