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| 09/03/15

Nasa: sea level rise accelerating


Scientists from the National Aeronautics and Space Administration have said that the world is likely to experience an average sea level increase of one metre by the end of the century. The new forecast puts on track to hit the upper end of the United Nations’ worst-case scenarios.

The US space agency last week released fresh data and visualisations based on satellite-based altimetry (assessing elevation) from the last two decades illustrating their latest understanding of ice melt in Greenland and Antarctica.

It concludes that as a result of such losses, global sea levels have risen by 7.4 centimetres since 1992, a faster rate than previous estimates of annual increases of 2.6 milimetres per year since that time.

To put this in perspective, the seas have risen by roughly 19 cm since 1900. In other words, more than a third of the increase has happened in the last two decades.

Particularly worrying is that the last UN Intergovernmental Panel on Climate Change report issued in 2013 said that the world would see a rise of between 52 and 98 cm by 2100, if greenhouse gas emissions were to stay on the uppermost business-as-usual path (RCP 8.5).

The researchers also say that the amount of ice lost is accelerating. Since 2004, Greenlandic ice has been melting at an average of 303 billion tonnes each year, a rate that is accelerating by some 30 billion tonnes per year, every year. Antarctica for its part is losing ice at a slower pace, some 118 billion tonnes per year since 2004, accelerating by 28 billion tonnes per year.

Nasa’s Josh Willis did little to calm reporters nerves, saying bluntly: “You ain’t seen nothing yet.”

This year’s new estimates come after last year’s revelation that a section of the West Antarctic ice sheet is now irreversibly melting into the ocean, a phenomenon that will yield sea level rises of between 1.5 and 3 metres. Glaciologist Eric Rignot said at the time during another Nasa debriefing: “It has passed the point of no return.”

The reassuring caveat to that sobering news was that this huge increase would at least happen over the course the coming centuries, not by century’s end. However, in July, James Hansen, former chief climate scientist with Nasa, made headlines around the world with the distribution of a discussion paper that sped up the timeline considerably.

In the lengthy document, published in Atmospheric Chemistry and Physics Discussion, an open-access journal published by the European Geosciences Union, Hansen and 16 colleagues from various climate-related disciplines said that feedback loops could produce sea-level rises as high as 5 metres over the next half century. The paper, which has not yet been subject to rigorous peer review, has been very much contested within the climate science community, with some researchers suggesting Hansen’s extrapolation is unwarranted.

But trying to grapple with these different heights can be confusing. What do they mean for British Columbia? To put them in context, note that the provincial government’s sea-level adaptation plans already assume a rise at the upper end of projections: 50 cm by 2050, one metre by 2100 and two metres by 2200.

At the same time, the global average does not mean the same rise will happen everywhere, like water filling a bathtub. Other global and local geographical effects play a role as well, such as upward or downward movements at the edges of tectonic plates. These influence local rates of sea level rise in BC.  Existing estimates place the rise by century’s end on the southern BC coast, for example, as  being about 80 cm in Nanaimo and 120 cm on the Fraser Delta.

None of this means that this week’s news from Nasa should do nothing to change the province’s plans. The worst case scenario projected two years ago by the UN now seems to be very likely. The next cycle of UN IPCC assessment report drafting begins this October and can be expected to re-evaluate future worst-case sea-level rise projections.

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