SEA LEVEL RESPONDS TO WARMING WITH RAPID PULSES OF RISE

Most of the sea level rise models project a gradual acceleration of sea level rise through this century and beyond as Greenland and Antarctic ice melt gradually accelerates. Our building knowledge of how sea level rose out of the past ice age paints a very different picture of how sea level responds to warming.

Following the last glacial maximum (LGM) about 20,500 years ago, sea level rose from about -134 meters to its present level mainly as a series of rapid pulses of rise followed by pauses. The pauses are recorded and preserved across our continental shelves as sandy barrier islands, flood tidal inlet complexes, bay-head deltas, reefs, coastal mud flats and wetlands, and inter-tidal notches biologically carved into rocky coasts. Rapid sea level rise pulses of 1-10m, from the rapid disintegration of one ice sheet sector after another, then rapidly over-rode, drowned out, and stranded these coastal deposits across the continental shelf.

Documented sea level pause buildups include: Webster et al., 2018 (13 reef growth levels at -134 to -50m; 5 reef levels between -120 and -80 m); Locker et al., 1996, and Jarrett et al., 2005 (cemented carbonate beach ridges at -80, -71, -65m); Khanna et al., 2018 (6 reef terraces between -94 and -70.5m); Pretorius et al., 2016 (7 sandy barrier islands between -70 and -30m); Anderson et al., 2016 (4 tidal inlet and bay-head deltas between -24 and -10 meters); Milliken et al., 2008 (bay-head deltas between -17 and -5.5m); and Dominguez and Wanless (1991), and Wanless et al. (1995), (Coastal mud-flat ridges and sandy barrier islands at -2.7, -1.8, and -1m).

We are still recognizing and cataloging these features. There is still the job of refining exactly when, how fast and how long each pulse was; adjusting for differential subsidence/uplift from area to area; and weeding out included older relic features. The latter is generally not a problem since most of these shelves have significantly aggraded since the LGM and have buried earlier features.

It is the integration of valid age dating with the preserved geomorphic form and distribution and sediment body history of former coastal sedimentary environments (and coastal archaeological sites) that permits an understanding of this refined detail of sea level history. It is also this understanding that must be applied to forecasting future sea level rise.