A recent surge (see my last post) of published papers on sea-level rise generated the following paper: “Ice sheet collapse following a prolonged period of stable sea level during the last interglacial” (subs. req’d) by O’Leary et al. in Nature Geoscience. The authors embarked on this work to provide information on potential events in our future as sea levels rise in response to increasing greenhouse gas concentrations. From their abstract: “During the last interglacial period, 127–116 kyr ago, global mean sea level reached a peak of 5–9 m above present-day sea level.” An “interglacial” is a time period in between periods with extensive polar ice sheets. The last interglacial is also known as the Eemian. During the Eemian, sea levels were 16-30 feet higher than today’s level. Scientists estimate that 4 feet of additional sea level rise is already locked-in due to past GHG emissions. Our current emissions pathway through 2100 locks in 23 feet of future sea level rise (over the course of the next few hundred years).
Most analysis of future sea-level rise suggests that the rise is likely to occur over the course of decades to centuries. The results are dramatic – more than 1,400 coastal US municipalities would be mostly underwater with an additional 23 feet of sea level – regardless of the timescale in which the seas rise. Society can respond more easily if the time is century-scale than if it is decadal-scale. The costs are high regardless, but societal strife might be less if the timescale is longer. Imagine telling one million people they have to abandon their homes in the next decade due to rising seas while their governments are forced to move infrastructure such as roads and airports (billions of dollars’ worth) and businesses have to move inland as well. I don’t think the result would be pretty.
O’Leary et al. focused on an even more striking event within the Eemian (which was as warm as projections of likely late 21st century temperatures): a rapid, not gradual, increase in sea levels. “We show that between 127 and 119 kyr ago, eustatic sea level remained relatively stable at about 3–4 m above present sea level. [...] We suggest that in the last few thousand years of the interglacial, a critical ice sheet stability threshold was crossed, resulting in the catastrophic collapse of polar ice sheets and substantial sea-level rise.”
Their results (along with others’) show that sea levels were stable a few meters above the current level. Then, something caused polar ice sheets to melt “rapidly” and sea levels to rise quickly from 3-4 m above today’s to 9 m above today’s. So sea levels rose from 10-12 feet above today’s to nearly 30 feet above today’s in less than one thousand years. If such an event happens again in our future, one thousand years is a relatively long time to respond to the primary change. Secondary changes might challenge our responses, but that isn’t my focus today.
If O’Leary’s estimate of one thousand years is too long by one order of magnitude, serious challenges would exist. If sea levels rise by 17 feet in less than one hundred years, how would we respond? How much infrastructure would we leave to the rising seas because moving it is just too costly? In addition to continued sampling of former underwater sites in Australia and elsewhere to gauge total sea level change, scientists need to refine methods to pin down the timing of past changes. How frequently did past rapid changes occur? What were their magnitude? Then, decision makers need to establish two-tiered response plans. The first should address the most common type of change: gradual rise/fall over hundreds to thousands of years. The second therefore should address the more serious challenge of sub-centennial rapid sea level change. We can’t implement plans that aren’t formulated beforehand.
It is worth noting that Mercer first wrote about polar ice sheet collapse 35 years ago. This phenomenon is not new but is being better informed by subsequent research. Hat tip New York Times.