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oceanriseSea levels are rising, that’s an indisputable fact. But by how much and why exactly, those are grayer issues. In the former case, it’s because there are several variables that can affect the rise: warming waters, glacial and ice sheet melts, land subsidence, erosion and overdevelopment, to name a few. In the latter case, most scientists say that climate change is the prevailing factor, but the Earth’s natural cycles certainly play a part.

In terms of how much climate change is affecting sea level rise, the geologic record tells us quite a bit. A study published last month in the Proceedings of the National Academy of Sciences studied 35,000 years of ocean fluctuations. The researchers found that, right up until the nineteenth century, ocean levels remained fairly static for the last 6,000 years. Then, boom.

The sea has risen approximately 20 centimeters since the beginning of the 20th century. Doesn’t sound like a lot, but in geo-historical terms, it’s a major shift.

“What we see in the tide gauges, we don’t see in the past record, so there’s something going on today that’s wasn’t going on before,” lead author Kurt Lambeck, a professor at Australian National University, told the Australia Broadcasting Corporation. “I think that is clearly the impact of rising temperatures.”

Recently, another study published in Environmental Research Letters forecast the upper limit of sea level rise by the end of this century. They found that, at the most extreme end of the spectrum, sea levels could rise up to 190 centimeters (or above six feet) by 2100. This reading was based on the assessments of 13 ice sheet experts and using climate models. The number is much higher than the one projected by the Intergovernmental Panel on Climate Change’s Fifth Assessment Report, which already predicted a “significant increase” in sea levels by the end of the century. The IPCC projects a sea level rise between 26 and 82 cm.

However, the IPCC’s AR5 did not take into account the Greenland and Antarctic ice sheets’ current “unprecedented rate” of melt.

Granted, the new study admits that sea level rises larger than 180 cm are less than 5 percent probable, but knowing the upper limit of the encroaching ocean is extremely important to both scientists and policymakers.

As Robert McSweeney of the Carbon Brief writes, “Designers of flood protection need to be sure that they will be able to cope with more than just the ‘likely’ increase in sea levels; therefore, other scenarios have been developed to consider larger, albeit less likely changes. [. . .] While large changes in the ice sheets are unlikely, the thinking goes that there is too much at risk to plan for only an average amount of sea level change. These scenarios are not there to worry people, but to ensure our flood protection can cope with whatever the sea might throw at us.”

Such planning is necessary because, even though only two percent of the Earth’s land is situated on Low Elevation Coastal Zones (areas less than 10 meters above sea level), these zones contain 10 percent of the world’s population. They also contain 13 percent of the world’s urban population.

So however many centimeters the ocean rises, 26 or 190, it would behoove us all to be prepared.

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