What global warming can do to us – Part 3: Sea level rise serious by the year 2100.

Previous articles in this series include:
Part 1: What global warming can do to us –If greenhouse gas emissions are not reduced the global average temperature is predicted to reach higher than any time in the past 800,000 years. Earth in the twilight zone.

Part 2: What global warming can do to us – Wildfires.
Australia in late 2019 and California in September 2020 experienced over 100 raging wildfires. The heating world seems to track rising greenhouse gas emissions.

Sea level is measured by tidal gauges and by satellites.
Sea levels are rising – steadily since about 1850, and everyone agrees on this. It’s easy to blame global warming because melting ice is pretty sensitive to small temperature changes and the earth has warmed by a bit more than 1°C since 1850. Most of the melting ice is in land-based glaciers or ice sheets and this makes the sea level rise. The question is: how much further will sea levels rise and what will be the consequences?

In the twentieth century mean sea level rose 11–16 cm globally, or 4.3 – 6.3 inches. The average is close to 1.4 mm/year. The shank of a normal writing pen is about 9 mm thick. Take one sixth of that and you have about 1.4 mm. This is a tiny amount in any one year.

And this amount of 1.4 mm is miniscule compared with ordinary tides (2 feet between high and low tides in the open ocean but up to 53 feet in the Bay of Fundy, Canada). Storm surges are typically several feet and in a big hurricane can be up to 30 feet.

However, since the seashore is fairly flat in many countries where people choose to live, a sea level rise of 1 foot can mean the ocean covers up almost 10 feet of beach sand. That’s still not too worrying. With all the concrete laid out building cities in the past 100 years, a small amount could have been diverted to construct a seawall just one foot high to compensate for the sea level rise. A classic example is the 17 foot seawall built in Galveston after the terrible hurricane of 1900.

The miniscule 1.4 mm per year if extrapolated ahead by 80 years gives 112 mm or 11.2 cm or 4.4 inches. Again, not much of an increase by 2100 compared with storm surges of several feet that would cause much more damage.

But what if sea level increases are accelerating? That’s what the climate modelers say will happen, and it appears to be so in Figure 1. The figure includes data measured by satellite altimetry.

Global mean seal level graphic

Figure 1. Global sea level rise in mm since 1900. Satellite data started in 1993. The red dots are averages I selected to make the plot in Figure 2.

Sea level is accelerating according to Figure 1. But some authors have disputed the acceleration of sea level rises, based on data collected previous to 2016 (Ref 1). The book also points out that successive reports by the IPCC (Intergovernmental Panel of Climate Change) between 1990 and 2007 have lowered their predictions of 2100 sea level by substantial amounts.

Sea level trend graphic
Figure 2. The data over 120 years is well matched by an exponential curve. The scale in mm on the left has been shifted from Figure 1 to make plotting and curve-matching easier.

The curve in Figure 2 hits the year 2100 at 35 inches (almost 3 feet) from 2020. That’s a sizeable increase from now, and would cover up nearly 30 feet of beach, using the rule of thumb mentioned earlier. A storm surge that used to be 3 feet in the old days would now be 6 feet, and potentially cause much more flooding damage.

But there is no guarantee the curve will continue to be exponential up to 2100, even though it’s a good match to the data.

How does this exponential projection compare with sophisticated climate models? By 2100, the expected range of sea level is 50-70 cm (19.7-27.6 inches) for low carbon emissions, and 70-100 cm (27.6-39.3 inches) for high carbon emissions. The exponential projection of Figure 2 gave 35 inches, which falls in the high carbon range. So the unsophisticated prediction of Figure 1 is not too far out.

The following are adapted from the report in Nature:

What is the overall risk? The world population is close to 8 billion. One billion people (12.5%) live less than 10 meters above current high tide lines, while 230 million people (almost 3%) live below 1 meter and are extremely vulnerable. In general the most vulnerable live in Asian countries where coastal flooding affects more than 10% of countries such as Vietnam and Bangladesh.

Consider annual flood levels under a high carbon emissions scenario. Today, 250 million people live in areas that are at risk for annual flooding. Projections out to 2100, when sea level and tides will be higher, give 630 million people (7.9%) at risk. Note: 630 million people is twice the population of USA that could be at risk for annual flooding.

The current rate of sea level rise is less than 2 mm per year — such a miniscule number. But because it is accelerating, as in Figure 1, and because so much of the world’s population resides on or close to the coast, by the year 2100 the cumulative effects could be serious.

Reports like the one discussed here are important to assess the threat of climate change on coastal populations and to give the world a chance mitigate potential problems.

7.9% of the world’s population are at risk for annual flooding under a high carbon emissions scenario (i.e. the world continuing on its present course). The risk includes damage to cars, homes, buildings, farms, crops, roads, and drainage systems.

Momentum is shifting toward renewables, catalyzed perhaps by the President Joe Biden, in the USA. It seems like everyone is talking about the transition away from fossil fuels to renewable energies with net-zero greenhouse gas emissions by 2050. This could mean the exponential rise in Figure 2 would flatten and the sea level rise by 2100 would be less than 35 inches, and maybe much less.

Inconvenient Facts, Gregory Wrightstone, Silver Crown Productions, 2017.

BLOG TOPICS: I write content (in-depth) blogs about a mix of topics: Science and Energy, and Inspiration and Hope, and Health and Hiking.
The Gray Nomad ….. Enjoy the seashore but understand future danger.
Faith – wildflower of the soul,
Thrilling the breathless night with fragrance,
And the desolate ways where silence fears to whisper praise.
With radiant delight of wonder – worship in God’s sight.

[Dowell O’Reilly, Australian poet.]

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3 years ago

Ian, thanks again for an informative and thought provoking blog. Nancy S’s comment points out that melting ice shelves are producing more methane gas. Also I wonder about wild fires, which are of greater intensity and occurring more often than in the past. These fires also increase carbon load in the atmosphere. I would expect with global warming, these fires will burn increasingly hotter, and yearly fire seasons will expand. At the behest of some well meaning environmental groups, there has been reduction in timber harvesting, which served to thin the forest by removing undergrowth and tree density. This reduced the intensity and more destructive nature of wild forest fires. We now know that improved methods of timber harvesting makes for healthier forest, less destructive wild fires, improves the economy, and less carbon is release into the atmosphere. Just a thought, I’m sure other factors can be introduced into this discussion.

Nancy S.
Nancy S.
3 years ago

Ian, I’m glad to be reading your blogs again. Very informative. There is another variable besides the rate of our use of petro products that may impact the trajectory of sea rise. I believe I saw it in a recent Atlantic article, though I can’t be sure. The author noted that the melting of the ice shelves appear to be releasing methane as ancient waters melt, which would keep up or increase the carbon load in the atmosphere over time, regardless of us switching to green energy. We are still in an exploratory phase of climate effects of our life styles. The one thing is that is certain is that our world is changing and becoming hostile to our lives.

3 years ago

Great info, Ian!

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