This local concentration of sea level rise feels inexplicable when we imagine monolithic oceans rising and falling each day with the tides but otherwise evenly spaced out over the vastness. We tend to picture that water finding its natural level as surely as a swimming pool finds its level after a few minutes without occupants.
But there are some easily understood reasons why the ocean is getting deeper faster in some places than in others and is even falling in some areas.
All locations are already affected to some extent by ice loss and thermal expansion of water. While melting sea ice doesn’t add to the sea’s depth, melting land ice does. This includes montane and Arctic glaciers and the Greenland and Antarctic ice caps, all of which are melting at an accelerating rate.
The other primary cause of sea level rise has been the ocean expanding as it heats. Of course, since the tropics face the sun very directly, tropical seas accumulate heat faster than elsewhere so thermal expansion is greater.
The melting of earth’s great ice sheets has some surprising effects. Isostatic rebound is one – as an ice sheet melts, the landmass on which it sat is free to rise to its more natural altitude. This causes the nearby sea to effectively fall while causing distant seas to rise.
Plus, ice sheets have gravitational pull on all the world’s oceans such that when ice sheets fade, the sea slides away to where gravity now wants it to be – toward the tropics. And the melting of the ice caps is contributing to change in the earth’s spin axis, further shifting water toward, once again, the tropics.
Tectonics has a part to play – the 2004 Indian Ocean tsunami led to measurable, permanent changes in sea level over a vast area. Then there’s subsidence, both natural and manmade such as from ground water extraction, and differences in the width and slope of continental shelves.
If geophysical forces represent the plodding, relentless side of sea level rise, atmospheric forces are dramatic, destructive and in our face right now. Witness changes in ocean currents such as the warm, massive invisible river that runs north-south down Australia’s east coast. Most of these currents are speeding up due to faster winds. There are also currents called gyres which are like whirlpools the size of oceans and they pile up water in their centre, up to a metre more than on their outer edge.
The world is getting more rain, so more water exits river deltas, adding to sea level differentials. Winds are getting stronger, notably in the western tropical Pacific, pushing Pacific Ocean bulk broadly westwards.
Wave heights are increasing. In its 5th Assessment report the IPCC states that particularly large waves are becoming more common in the tropical South Pacific. Incrementally stronger winds, changing wind directions and higher waves all add to the sea level rise differential between regions.
The strongest cyclones are most likely getting stronger and in the South Pacific they’re occurring further south. These effects working together can result, at the very least, in increasingly destructive storms in certain regions compared to others with associated amplifying effects on sea level.
But the storminess story gets even more troubling. Several factors appear to be multiplying the destructiveness of storm surges in certain regions. Storm surge refers to the unusually high water level that causes so much destruction during wind storms.
But measurements have shown that mean sea level rise is statistically correlated to a rise in sea level extremes. In other words, sea level rise breeds more sea level rise. Compounding this, the faster the underlying rise is, the shorter the return period for extreme days.
A 2015 study in Nature Climate Change came to some astounding conclusions about the confluence of rising sea level and increasing storminess. A commentary on the study stated that “even the [authors’] reduced-emissions calculations suggest a 4- to 75-fold increase in the [coastal inundation] flood index—that is, the combined heights and durations of expected floods—across the locations. With business as usual, the flood index might go up 35 to 350 times.” This is the extraordinary degree to which the confluence of two factors can amplify effects.
In 2013 the strongest storm to ever make landfall anywhere on earth barrelled into an unfortunately warm, shallow, funnel shaped bay in the eastern Philippines. The city of Tacloban was devastated by a 5-7m surge and more than 6,000 people perished, most at the hand of the monstrous sea.
Yet an 1897 typhoon had struck the same bay with a surge only half as high at Tacloban, despite having been of similar height on the open Pacific coast. Why the height difference? It’s conceivable that in 2013 the magnifying effect of a higher mean sea level in tandem with increased storminess generated a freakily amplified wave set.
Our psyches have not evolved to deal with the concept of a rising ocean. The strange reality of big regional differences in the rate of the unfolding phenomenon is even less intuitive. So a little science can help us understand why Pacific peoples are reaching out with urgency. Let’s not assume they are exaggerating about what they see. They are not.
The rising ocean is already a killer. Here’s why we must believe this counter-intuitive, slowmotion news story by Roger Goodman
March 1st, 2020
Last August the geographically and numerically miniscule Pacific state of Tuvalu hosted the Pacific Islands Forum. Australian Prime Minister Scott Morrison on landing was greeted by a dramatization of sea level rise involving semi-submerged school children, flag waving and plenty of smiles. It was clearly intended to be educational rather than confrontational.
But Morrison appears to have gone to Tuvalu with a resolve to shift not an inch on coal or on climate change. A communique materialized which failed to mention coal at all. It was noted that except for Australia all of the countries present agreed to a call for a global ban on new coalmines and coal-fired power stations.
Later the forum’s chairperson and former Prime Minister of Tuvalu, Enele Sopoaga declared himself “stunned by the un-Pacific tenor and manner of the Australian prime minister… much against the concerns and the tears that were shed by the Pacific Island leaders.”
To be fair Australia had, ahead of Morrison’s arrival promised to redirect $500 million in aid funding over five years starting in 2020 to help Pacific nations invest in renewable energy and 'climate and disaster resilience'.
Meanwhile, New Zealand Prime Minister Jacinda Arden recently committed funds to assist Fiji in relocating communities already struggling with inundation from rising seas.
Calls from low lying states for the actual polluting nations to act on emissions have been largely ineffectual, while requests for the funds required for effective adaptation, particularly seawalls have had mixed results, such that increasingly, small Pacific nations are making the case for extraction of people as in a slow motion emergency.
The western half of the Pacific Ocean in general is the most threatened part of the world, with the Solomon Islands considered to be most at risk from rising seas along with parts of Papua New Guinea, the Philippines and even northern Australia, particularly the Torres Strait and the Gulf of Carpentaria.