By Larry O’Hanlon

Rising sea levels will accelerate this century and beyond, exposing hundreds of millions of people to flooding and other coastal hazards by the year 2300. But acting early to lower greenhouse gas emissions can slow that rise, say scientists in two new studies in Earth’s Future, an open access journal published by the American Geophysical Union.

In one study, researchers used a sea level projection model to look at several possible future scenarios including aggressive climate mitigation that keeps global temperatures from rising by 1.5 degrees Celsius above pre-industrial levels, moderate climate mitigation that keeps temperature rise to 2.0 degrees Celsius, and no mitigation at all. When it comes to sea level rise, they found that the biggest benefit of climate mitigation won’t be seen by 2100, but will be very clear by 2300.

“Previous studies have taken a mitigation versus non-mitigation view,” said climate researcher Sally Brown of the University of Southampton in the U.K. and lead author of one of the studies. “What we wanted to do is look at the difference between 1.5 degrees C and 2 degrees C warming.” Also, many studies stop at the year 2100, whereas she and her colleagues pushed ahead two more centuries.

The Global Mean Sea Level (GMSL) anomaly relative to 1986-2005 (meters) over time from observations and future scenarios in an Earth system model. The period from years 1980 to 2300 is shown. The observation GMSL reconstruction is from Church and White (2011), downloaded 10th May 2017. Credit: Goodwin, et al. (2018).

The researchers found that with climate mitigation efforts limiting warming to 1.5 and 2.0 degrees Celsius above pre-industrial levels, coastal flood plains would grow to 610,000 square kilometers and 640,000 square kilometers (236,000 and 247,000 square miles) respectively by 2100, up from 540,000 square kilometers (208,000 square miles) in 2000. That is an increase equivalent to the area of South Korea.

By the year 2300, however, the differences become more dramatic. Flood plains are projected to increase to 700,000 square kilometers and 960,000 square kilometers (271,000 and 370,000 square miles) by 2300 if warming it limited to 1.5 and 2.0 degrees Celsius respectively. With no climate mitigation, the study’s authors found the area of coastal flood plains will increase to about 1.63 million square kilometers (629,000 square miles) by the year 2300. That’s an increase in flooded lands about the size of Egypt.

“By 2300 we found the number of people exposed to sea level rise is between 1.5 and 5.4 percent of the global population,” Brown said. That translates into as many as 620 million people living in areas exposed to flooding in comparison to 100 million people living in areas exposed to flooding today, according to Brown.

However, adapting to sea level rise, such as raising infrastructure or building coastal defenses, can greatly reduce the number of people exposed. This is especially important in low-lying highly populated areas, such as deltas which are highly threatened by sea level rise.

In a second paper in Earth’s Future, some of the same researchers dig into the next question: How can policy makers mitigate climate change so temperature goals can be met?

Normalized pattern of sea level rise applied to each sea level rise scenario in the study. This is based on RCP4.5 in Slangen, et al. (2014). Credit: Brown, et al. (2018).

Decision makers don’t know exactly which carbon emissions pathways to follow to slow and stop warming, explains researcher Philip Goodwin, lead author on the second paper, also at University of Southampton. There is a lot of uncertainty on any given pathway, he added.

That uncertainty makes it difficult for decision makers to avoid either undershooting or overshooting greenhouse gas emissions targets, he explained. To solve this problem, Goodwin and his colleagues have developed a new mitigation strategy they call the Adjusting Mitigation Pathway (AMP). This strategy is more flexible than other approaches and allows for adjustments to the paths taken to meet climate targets each decade in response to actual warming.

They tested AMP by using it in a large number of Earth climate simulations for different climate targets ranging from a 1.5-degree Celsius increase in global temperatures to a 4.5-degree Celsius increase in global temperatures from 2030 through the year 2300. Their simulations covered not just global mean sea level, but ocean acidification, surface warming, carbon emissions, and atmospheric carbon dioxide. The AMP approach automatically adjusts emissions pathways as more observations of future warming unfold.

“We find that lower 21st century warming targets will significantly reduce ocean acidification this century and will avoid up to 4 meters of sea level rise by year 2300 relative to a high-end scenario,” concluded Goodwin.

Larry O’Hanlon is an independent science writer and editor. He manages the AGU Blogosphere and coordinates AGU social media.