The seemingly inconsequential jump from 1.5 degrees to 2.0 degrees Celsius of global warming is anything but — that half of a degree would mean 5 million people across the world will need to move, or find a way to live in flooded areas.

The 2015 Paris climate agreement seeks to stabilize global temperatures to less than 2 degrees Celsius above pre-industrial levels, with efforts made to ensure we never go more than 1.5 degrees Celsius above pre-industrial levels. And that sounds like a reasonable, safe goal. An international team from the Princeton, Rutgers, and Tufts Universities, alongside researchers from Climate Central and ICF International, however, wanted to find out what such a scenario would entail for people living in the most at-risk areas — coastal areas.

Still underwater

They drew on a global network of tide gauges and a framework of local sea-level projections to estimate how the frequency of storm surges and other extreme sea-level events would fare under three scenarios: global temperature increases of 1.5°, 2.0°, and 2.5°C.

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Their results suggest that by 2150, the minute difference between an increase of 1.5° and 2.0°C would equate to the permanent inundation of areas that currently house 5 million people, including 60,000 who live in small island nations.

“People think the Paris Agreement is going to save us from harm from climate change, but we show that even under the best-case climate policy being considered today, many places will still have to deal with rising seas and more frequent coastal floods,” said first author DJ Rasmussen.

Unsurprisingly, the team reports that higher temperatures will make extreme sea level events much more common than they are today. Based on long-term hourly tide gauge records, they estimated current and future return periods of such events throughout the 22nd century. Under the 1.5°C scenario, this value is still expected to increase. For example, New York City is expected to see one Hurricane Sandy-like flood event every five years by the end of the 21st century under these conditions.

Extreme sea levels can be borne of high tides, storm surges, or a combination of these two — sometimes referred to as a storm tide. When whipped by hurricanes or other large storms, such events can flood into coastal areas, threatening life and property alike. A background of rising average sea levels will only compound the destructiveness and frequency of such events.

How much sea levels rise, on average, depends on how global mean surface temperatures evolve in the future. However, caution to the wise, the team notes that even if temperatures stabilize, sea levels are expected to continue to rise for centuries — because carbon dioxide lingers in the atmosphere for a long time and ice sheets will have inertia in responding to climate change.

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Still, the researchers predict that by the end of the century, coastal flooding may be among the costliest impacts of climate change in some regions. Overall, a 1.5°C temperature increase by the end of the century (the best-case scenario under the Paris agreement) would equate to a roughly 1.6 feet (48 cm) mean sea level rise, and a 2.0°C increase to about 1.8 feet (56 cm). A 2.5°C increase would result in an estimated 1.9 feet (58 cm) increase, the team adds.

The paper “Extreme sea level implications of 1.5 °C, 2.0 °C, and 2.5 °C temperature stabilization targets in the 21st and 22nd centuries” has been published in the journal Environmental Research Letters.