As sea levels rise, rocky coasts will erode at a dramatic rate

Climate change is accelerating the rise of sea levels worldwide. The mean sea-level rise is predicted to exceed 1 m by 2100, unless greenhouse gas emissions are reduced. Attempts to model and predict the hazardous consequences of this rise, on coastlines, have often focused on sandy beaches, marshes and river estuaries. Rocky coasts have been neglected, in general, even though more than 50 percent of the world’s coastlines are rocky. 

Rising seas are expected to generate waves that travel further inshore and are likely to have significant impacts on coastal cliffs. Erosion of the rocky coast in these areas will lead to cliff retreat, where cliff top edges move landward, threatening settlements, farmlands and infrastructure. Although several models predicting cliff retreat rates have been developed, none of them has been tested in the field. In addition, these models often focus on soft rock types and are not necessarily applicable to coasts composed of hard rock. Consequently, there is a great deal of uncertainty about how rocky coasts will erode as sea levels change. 

According to new research, conducted by scientists from Imperial College London, the University of Glasgow, and the Australian Nuclear Science and Technology Organization, the erosion of rock cliffs is likely to proceed more rapidly in the future, due to rising sea levels. The study, published in the journal Nature Communications, is the first to validate models of the expected erosion of hard rock coasts from sea level rise, using observational data over prehistoric timescales.   

The researchers collected samples from hard rock coastlines near Scalby in Yorkshire, and Bideford in Devon, U.K. They analyzed these for the presence of rare isotopes called cosmogenic radionuclides (CRNs) that build up in rocks exposed to cosmic rays. Concentrations of CRNs in rock reveal how quickly, and for how long, the rock has been exposed to the atmosphere, reflecting the rate of erosion and retreat. 

They combined these data with information on observed coastal topography, to calibrate a model that tracks the evolution of these rock coasts over time, before comparing them with rates of past sea level change dating back 8000 years. They found that the rate of coastal erosion at these two sites has closely matched the rate of sea level rise in the past, indicating a causal relationship between cliff retreat and sea level. They then used this relationship to model likely future changes as the sea level rises due to global warming. 

The results indicate that, by 2100, the coastlines near Scalby will retreat by 13–22 m inland, and those near Bideford will retreat by 10–14 m. This represents a rate of erosion that is between three and seven times today’s rate, and is potentially up to ten times faster. The researchers found that even rock coasts that traditionally have been thought of as stable (compared to sandy coasts and soft cliffs) are likely to retreat at a rate not seen for 3,000–5,000 years. The researchers say that their forecasts are consistently higher than those predicted in previous studies.

This will put coastal regions at risk all over the world. Millions of people live near rocky coasts, and billions of dollars are invested in these areas, in the form of infrastructure like homes, businesses, nuclear power stations, transport links, and agriculture. The increasing wave energy from sea level rise could erode all of this into the sea if greenhouse emissions are not reduced significantly.

“Coastal erosion is one of the greatest financial risks to society of any natural hazard,” said study senior author Dr. Dylan Rood. “Some rock cliffs are already crumbling, and within the next century, rock coast erosion rates could increase tenfold. Even rock coasts that have been stable in the last hundred years will likely respond to sea level rise by 2030.” 

Study lead author Dr. Jennifer Shadrick conducted the work in Imperial’s Department of Earth Science and Engineering as a member of the NERC Science & Solutions for a Changing Planet Doctoral Training Partnership, and now works in the marine and coastal risk management team at JBA Consulting.

“Sea level rise is accelerating, and our results confirm that rock coast retreat will accelerate in line with this. It isn’t a matter of if, but when,” said Dr. Shadrick. “The more positive news is that, now that we have a better idea of magnitudes and timescales, we can adapt accordingly. The more data we have on the effects of climate change on sea level rise and coastal erosion, the more we can prepare by championing urgent policies that protect coasts and their communities.”

Although the rates of projected erosion and cliff retreat are higher in this study than previously thought, they may even be conservative estimates. Although the researchers considered the impact of seal level rise, they did not factor in the possible impacts of increased storminess. Storms are predicted to become more frequent and extreme in future, and this is likely to accelerate erosion, particularly at wave-dominated coastal sites. The researchers say this will increase the speed of cliff retreats beyond the estimates they have made in the current study. 

“Our study did not account for the effect of increased storms, which may become stronger and more frequent in the future as the climate changes, on wave-driven cliff erosion. However, increased storms would only speed up the cliff retreat even more than our forecasts,” said Dr. Rood. “This is another angle to the climate crisis we will account for in future studies to give a more complete picture of likely rates of rock coast erosion. We are also looking to improve our models for softer rock coasts where erosion other than by waves is more important.” 

“The findings are a stark warning that we must better adapt to coastal retreat or face the loss of the people, homes, and infrastructure that call coastal areas home,” said Dr. Shadrick.

“The implication is that rock coasts are more sensitive to sea level rise than previously thought. We need to pay more attention to how our rock coasts continue to erode as sea levels rise,” said study co-author Dr. Martin Hurst. “Heightened erosion risks at our coasts will continue throughout this century. Even if we achieve Net Zero tomorrow, a substantive amount of sea level rise is already baked in as our climate, glaciers and oceans continue to respond to the emissions that have already taken place.”

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By Alison Bosman, Earth.com Staff Writer