On melting ice and rising seas

Only weeks after legislators in North Carolina passed a law that prohibited the state’s planners from taking account of sea level rise when making deciding on coastal development, new scientific work appears to highlight the stupidity of the decision.

In June, North Carolina’s Senate approved a law that required the state’s Coastal Resources Commission (CRC), which sets rules and policies for coastal development and grants permits, to base predictions of future sea level rise on a steady, linear rate of increase, banning them from taking into consideration any new factors, such as the melting of the polar ice sheets.

Now, scientists in Bristol have discovered that the two most recent episodes of rapidly rising seas, the so-called Meltwater pulse1a, which began around 14.600 years ago and the 8,200 year event, were both caused by ice melting in North America.

The research, published in the journal Nature, was led by Dr Lauren Gregoire of Bristol’s School of Geographical Sciences. Working from climate models, they explained both events in terms of so called “saddle collapse”, the rapid melting of the relatively low altitude ice that linked the massive ice sheets that covered North America, leading in turn to a collapse of the ice sheets and consequent rise in sea levels.

Both events were known, but the causes have remained contentious. The Bristol team believe that they have uncovered a pattern of ice melting that explains the rise in sea levels.

The ice that covered North America was contained in domes up to 3 kilometres thick in the mountains to the west and lining corridors at lower altitude. As the planet warmed, the higher altitude ice melted, eventually affecting the saddle areas between the domes. As the saddle ice disappeared, the meltwater it produced was sufficient, according to the Bristol team’s model, to produce the observed sea level rise of some nine metres over 500 years. The second event produced a rise of 2.5 meters.

The North Carolina law remains contentious. Ironically, around the time that the Senate was making it illegal to take climate science into account in planning, the US Geological Survey issued a report warning that, along a stretch of the US coast from North Carolina to Long Island, sea levels were rising four times faster than the global average.