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Q&A: The Finer Points of Rising Sea Levels

Rousbeh Legatis* interviews MARY-ELENA CARR, associate director of the Columbia Climate Centre at the Earth Institute of Columbia University in New York

NEW YORK, Nov 17 2011 (IPS) - Long before the Pacific will rise to a level that will leave its estimated 30,000 islands submerged, most of them might be severely affected by frequent flooding and storms.

Thousands of people living on islands scattered across the world’s largest ocean are already fleeing their homes and lands because of altered climate conditions.

Still, “an extraordinarily cold or warm winter in a region or even globally is not proof of climate change,” said Mary-Elena Carr, biological oceanographer at the Earth Institute in New York. Real climate change can only be concluded from shifting weather conditions observed over 20 to 30 years.

Carr¸ associate director of the Columbia Climate Centre, spoke with IPS U.N. correspondent Rousbeh Legatis about the human impact on rising sea levels, how islanders will be affected and what can be done to mitigate adverse consequences for people in the Pacific.

Excerpts from the interview follow.

Q: Is it still arguable that the increased natural disasters we are seeing are due to climate change? A: At this point, we cannot attribute any weather event to climate change, anthropogenic or natural. The climate system is extremely complex and there are many factors that determine what we experience from day to day.

While we can assert that climate change due to greenhouse gas emissions will lead to changes in the patterns of rainfall or temperature, we cannot assign a single cause to any specific event like a flood or a hurricane.

Q: From a scientific perspective, who or what is responsible for the rising sea levels and how do human actions contribute to them? A: Globally rising sea level is a consequence of a warmer planet, which is due to increased amounts of atmospheric greenhouse gases (GHGs). Historically, developed nations are responsible for the current levels of GHGs in the atmosphere. However, rapidly developing nations are increasingly contributing to GHG emissions.

At a local level, land use choices can directly impact the relative height of ocean and land: groundwater extraction, destruction of coral reefs, construction choices, can all lead to local sea level rise.

Q: How do sea levels change and why does this harm human life? A: Sea level changes when there is a change in either the mass or the volume of water in the ocean. If we imagine the ocean basin like a very large bathtub, you can change the total mass by adding or removing water; in the ocean, that would be through evaporation or precipitation, or when water flows from land to sea, either as rivers or ice.

The mass of seawater in the bathtub can change its location due to currents or winds. The same mass of seawater changes in volume, expanding when it warms or freshens.

Sea level also changes with vertical land motion (if the sides or bottom of the bathtub were to sink or rise). Such motion can occur over very long time scales. Land also undergoes vertical motion over short time scales, due to groundwater extraction or tectonic activity.

While all of these processes have occurred throughout the history of the earth, humans impact sea level rise directly, by manipulating the flow of ground and surface water, and indirectly, through GHG emissions which raise the average global temperature.

This warming affects both the mass and volume of seawater primarily due to increased melting of land ice and higher ocean temperatures, both of which translate into a global rise in sea level.

Global average sea level measured by tide gauges and altimeters was relatively constant between 1900 and 1930. Since that time, sea level has not only risen, but the rate of sea level rise has also increased: tide gauges estimate sea level rising about 1.8 millimetres per year between 1930 and 2000, while the altimeters measured approximately 3.1 millimetres per year between 1993 and 2009.

Q: Do you see a certain time when islands could be below the sea level? A: The answer to that depends on the elevation of the island and on the tidal range in addition to storm activity and sea level rise. Both storm activity and sea level rise are affected by climate change. Even the orientation of the island relative to prevailing winds affects the likelihood of flooding.

While it may be more than 150 years before sea level is three or four metres higher than in the late 20th century, islands with average elevations of four metres will undergo flooding because tides and storms raise sea level on top of the global average rise.

Predictions vary depending on both the island characteristics and projections for sea level rise, but it is likely that in the early 21st century there will be frequent flooding in most small island states.

Q: What must be done to mitigate the impact of climate change for island inhabitants around the world? A: To mitigate climate change we should reduce emissions. To adapt to the impacts of sea level rise, we need careful land use choices and adaptable infrastructure. Coastal vegetation such as mangroves can help reduce the impacts of flooding. Conservation of coral reefs also plays a huge role in protecting atolls.

Q: Is climate change an unstoppable phenomenon of contemporary times? A: We are committed to warming, and sea level rise, even if all emissions stop today, because carbon dioxide remains in the atmosphere for hundreds of years. If we continue emitting GHGs without any reduction, the climate change impacts will be greater and last much longer.

*This is the third in a three-part series on the impacts of climate change in the Pacific region.

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