Will United Nations headquarters in Manhattan be flooded anytime soon? We can predict which American streets will be flooded by using this tool developed by US Office for Coastal Management. Even with a 3 metre rise, the UN building is safe, but not all parts of New York City would remain dry. How likely, though, is a 3 metre rise in the sea level? Are we on track for this or worse?

In his recent visit to Samoa enroute to Tonga for the Pacific Islands Forum, UN Secretary-General António Guterres expressed apocalyptic views of climate change – as he usually does. “Today's rate of increase [sea level] is unprecedented in at least 3,000 years and likely, since the dawn of civilization, 12,000 years ago.” “The climate crisis is the gravest threat facing this country and this region – and, quite possibly, the world”. “People are suffering”. ”Economies are being battered”. “And entire territories face annihilation”. “The injustice is appalling.” “A crisis entirely of humanity’s making.”

You get the idea.

The world is treating oceans “like a sewer”, Mr Guterres said just a few days later when addressing the August 25 opening of the Pacific Islands Forum. He added, “Plastic pollution is choking sea life. Greenhouse gases are causing ocean heating, acidification and rising seas.” While I agree with his sewer comment, CO₂ isn’t pollution. Without CO₂, life of any kind couldn’t exist on Earth.

One month later at a UN high level meeting on sea level rise in New York, representatives of islands and low-lying nations “urged immediate international action to combat sea-level rise — a global crisis threatening the lives and livelihoods of 1 billion people worldwide.”

In Tonga, Mr. Guterres urged people to “listen to the science.”

That’s a great idea, so let’s do that and try to figure out how much the sea will rise based on the latest science. Let’s look at the Pacific islands which are so often referred to by the media when discussing sea level.

Pacific Islands

There are about 10,000 islands in Oceania with about 46 million people. Oceanic islands are differentiated as high volcanic-based islands, such as Hawaii, or low coral islands and atolls, such as the Marshalls. Coral reef islands are typically less than 1 km^{2, ;} elevations rarely exceed 3m; and the mean elevation is 1-2m above sea level. An estimated 90 percent of Pacific Islanders live within 5km of the coastlines and over 50 percent of their infrastructure is within 500m of the coast. Even small increases in sea level are expected to significantly increase flood events.

The map below illustrates the location of major Pacific islands. 

There’s no question that some Pacific coastal communities have been impacted by rising sea level, changing currents, human-driven and natural mangrove tree loss, etc. However, a scientific study of 104 Pacific coral reef islands gives a fuller picture. Based on aerial photographs from the 1940s and 1970s and recent satellite images, “Results revealed accretion [deposits of sediment] has been the predominant mode of shoreline change, with 46 percent of the studied shorelines showing statistically significant accretion...” While three islands with a combined area of 3.2 hectares had disappeared, three new ones had emerged. One of the islands that disappeared, Sorol, had twelve residents in 1946.

Another study published in 2023 indirectly admonished public figures who make sweeping claims about island erosion based on a limited understanding: “The lack of association between sea-level rise and shoreline change on this large sample of islands addresses the common notion of a linear relationship between rising sea levels and magnitude of erosion in reef island systems. We emphasize the critical need to move from simplistic assessments of island change...”

Somebody should give Mr Guterres a copy of this report.

Some geological history

The last glacial maximum took place about 26,000 to 20,000 years ago during the Pleistocene epoch, which ended 11,700 years ago. Earth then entered the current Holocene epoch of warming and melting. Sea levels at the last glacial maximum were between 125 and 134 meters below present. As you can imagine, such a large increase in sea level brought about dramatic changes in the landscape. For example, given that the English Channel is less than 40 metres deep in many places, there was no sea there at that time. You could have walked from Amsterdam to London.

From 11,400 to 8,200 years ago, the global mean sea level (GMSL) rose at a rate of 15 mm per year. If we go back a little further to around 14,000 years ago to the Meltwater pulse 1A event, the sea was rising at 24-44 mm/year. All of this information comes from the IPCC AR6 report . The current and geologically recent GMSL rise rates have not exceeded these rates – not even close . The UN’s latest publication on sea level rise Surging Seas claims that current sea level rise rates are unprecedented in the past 3,000 years, but not since 12,000 years ago.

Why then did Mr Guterres say that the rise in sea levels was “unprecedented” and not likely in 12,000 years? That’s simply not true, and he should explain himself.

Sea level rise today

The World Meteorological Organization is reporting—based on satellite measurements--a GMSL rise of 3.4 mm/year over the January 1993 to May 2023 period. The range of local trends, however, is large, from – 8.87mm/year at Churchill, Manitoba, a port on Hudson’s Bay, to +16.87 mm/year for Fort Phrachula Chomklao, Thailand. (Hudson’s Bay, Canada was compressed by enormous sheets of ice during the last ice age and is still decompressing today like a sponge after it has been squeezed.) Fiji, which is near Tonga, has a sea level rise of 7.95 mm/year. The diagram below by NOAA shows relative sea level rise trends based on tide gauges for some locations.

Tide gauges and satellites provide us with data on sea levels. As with all measuring instruments, neither is perfect.

Factors affecting sea level

Copernicus (EU) attributes 30 percent of sea level rise to thermal expansion, 60 percent to melting glaciers and 10 percent to changes in land water storage, such as soil moisture and groundwater. Satellite data indicates that the rate of increase of the GMSL has increased by 105 percent, from a trend of 2.1 mm/year over 1993–2003 to a trend of 4.3 mm/year over 2013–2023. That means that sea level rise is accelerating. NOAA, NASA and the WMO are also claiming that sea level rise is accelerating (see here, here , and here). Obviously, these agencies represent a lot of expertise and clout, which should not be dismissed.

Nevertheless, not everyone agrees with that assessment. 

A May 2019 Policy Brief by The Heartland Institute titled Global Sea-level Rise: An Evaluation of the Data  states, “Contrary to the IPCC’s statement that it is “very likely” sea-level rise is accelerating, the highest quality coastal tide gauges from around the world show no evidence of acceleration since the 1920s.”

For example, here is tide gauge data for Harlingen, Netherlands. Notice the overall linear increase in sea level since 1865 despite an acceleration of the CO₂ concentration rate of increase starting around 1850 (for details see hereand here). A linear increase in time means that acceleration is zero.

I encourage you to review the Heartland report to make up your own mind.