Even with zero carbon dioxide emissions, sea level will continue to rise… How likely is it that “Japan will sink”?

Half of the rise in sea level is due to “thermal expansion of seawater

Half of the current rise in water levels is thought to be due to the melting of mountain glaciers in the Himalayas and ice sheets in Antarctica and Greenland, with the remainder due to thermal expansion of seawater.

The rest is thought to be due to thermal expansion of seawater,” says Tatsuo Suzuki of the Japan Agency for Marine-Earth Science and Technology.

As we learned in elementary school, water expands when it is heated. Ninety percent of the thermal energy accumulated on the Earth’s surface is stored in the oceans. When seawater rises 1°C due to global warming, its volume expands by 0.025%.

Right now, thermal expansion is having more of an impact. But once it reaches a certain point, it doesn’t expand as much, and then the effect of melting ice sheets becomes more significant. However, it is unlikely that all the ice sheets in Antarctica and Greenland will melt during this century.

Even if they do not melt during this century, they will continue to melt after 2200 or 2300. In addition, we cannot deny that the sea level will rise by more than 15 meters in 2300.

Sea level has risen 20 cm in the past 100 years…is Japan sinking too?

Sea level rise due to global warming has long been a hot topic. In Tuvalu, the rising sea level has even forced the islanders to move out. However, the ocean covers the entire planet. It is unnatural that only Tuvalu is sinking. Is Japan sinking too?

Compared to pre-industrial times, the global temperature has risen about 2°C and the sea level has risen about 20 cm. Of course, Japan is no exception.

I don’t know if Japan’s sea level is rising, but I don’t really feel it.

Water levels around Japan have always been subject to large seasonal fluctuations, and there are also tidal changes. So even if the water level rises 20 cm or so, I don’t think we would notice. If no measures are taken, there is a strong possibility that the situation will become quite serious by the end of this century. Not only Tuvalu, but also Japan is not safe.”

The water level is rising globally, but Japan is unaware of it. Why is Tuvalu in trouble?

I think the infrastructure is probably weak. If it were Japan, we could take measures such as raising levees, but some countries are economically unable to do so. I think there are other areas besides Tuvalu that have been affected.”

Fears of frequent flooding around the world…

In fact, Japan is not completely unaffected.

For example, if a lot of water enters a bay due to high tides, for example, the river may not be able to flow to the sea, and this can affect upstream areas, causing flooding when it cannot drain. As the sea level rises, the water level at the base rises, and areas that used to be able to drain may no longer be able to drain, resulting in flooding.”

In September 2021, a major hurricane hit New York City and sent muddy water rushing onto subway platforms, but this was also caused by the rising sea level. In Japan, too, typhoons have been causing more flooding recently, and when flooding occurs in coastal areas, it is thought to be the result of rising sea levels, he said.

When the Great East Japan Earthquake hit, the land subsided by about 30 cm, cutting off urban infrastructure such as sewage and drainage systems and causing major problems. Rising sea levels could cause the same thing to happen on a global scale.

Even with zero CO2 emissions, the sea level will rise 50 cm by 2100, and 1.1 m at this rate!

According to a study by the Intergovernmental Panel on Climate Change (IPCC), if nothing is done, temperatures will rise by 4°C by 2100 and sea levels will rise by up to 1.1 m. The possibility of a 15 m rise by 2300 cannot be ruled out. The possibility of a 15-meter rise by 2300 cannot be ruled out.

The reason for the wide range of predictions is the complexity of the melting mechanism of the ice sheets.

Glaciers are flowing. The speed of flow changes, and there is a complex mechanism that has not been fully understood. That’s why we don’t know how much it will melt if the temperature rises by 1°C. That’s why the range of predictions is so large. That’s why we have such a wide range of predictions.”

In any case, it is clear that we cannot let this continue. We must do our best to reduce carbon dioxide emissions.

Even if we do our best to limit the temperature rise to 2°C, the sea level will continue to rise. This is because the sea water will continue to warm and expand thermally due to the temperature increase, and in addition, the ice sheets will continue to melt.

Even if carbon dioxide emissions are reduced to zero, sea level will continue to rise, and by 2100, the sea level will rise 50 cm. The only way to stop the sea level rise is to lower the temperature from the current level. Will Japan sink someday?

The report warns that a 100-year storm surge will occur at more than half of all tide gauge locations, and at least once a year by 2100. The rate of strong typhoons is also likely to increase, resulting in more flood damage. Assuming the worst case scenario is pretty daunting.”

What should we do?

First, we need to reduce our carbon footprint. If we stop using fossil fuels, I think it will decrease considerably. Then, we need to improve our infrastructure, such as by building higher levees. I think we have to do what we can do one by one.

Tatsuo Suzuki belongs to the Global Environment Division of the Japan Agency for Marine-Earth Science and Technology (JAMSTEC). He specializes in ocean physics and climatology. He conducts research on ocean general circulation models, climate model development, climate change, and sea level rise.