Sea surface temperatures in the Southwest Pacific have risen three times faster than the global average since 1980, according to the World Meteorological Organization’s State of the Climate.
And sea levels in the region have risen at almost twice the global average over the past 30 years, it found.
OK, I get that the temperature of the ocean can vary from place to place, and that it may change at differing rates in different places. But if I’m understanding them correctly, they’re saying that the average sea level in SW Pacific ocean is increasing faster than in other places around the world. This claim is at odds with my understanding of how water and gravity work. I mean if you bump up the local sea level quickly and by a large amount, it will take time for that bulge of water to spread out and achieve an equilibrium level with the rest of the world’s oceans. That’s basically how a tsunami works: a thrust fault slips and creates a mound of water in the ocean over the course of several seconds, and within hours that mound of water has spread out and achieved equilibrium level again. But climate change is taking place over decades, and the sea level rise is very small compared to what happens in a tsunami, so I’m at a loss to understand how sea level increase in the SW Pacific could be higher, over the long term, than the sea level increase elsewhere in the world. It’s like starting with a reference level of water in a bathtub, pouring some extra water in the tub, and then having that extra water stay in a pile right where you poured it.
The earth bulges a bit at the equator; the pacific ocean occupies a greater span of the equator than other oceans; this might have something to do with it.
That’s all fine and dandy, but gravity doesn’t like it when water is piled higher in one place than in some other.
The SW Pacific warmed up faster than other parts of the ocean, and now it’s bulging upward more than those other (less-warmed-up) parts of the ocean. So what’s keeping this pile of water in the SW Pacific from spreading out across the rest of the oceans and getting all of them to a common level?
The earth’s gravity is not uniform. Sea level is not flat and never has been. As you suggest, sea level follows local gravity. But local gravity here is different from local gravity there.
I gotta leave or I’d explain a lot more with cites. I suggest you start here:
“Sea level” is far more complicated than “flat smooth sphere”
There’s a massive gravity anomaly (Wiki) in the Indian Ocean that causes the water level there to be more than a hundred metres below the global mean. So yes, it’s not as if the sea level is uniform everywhere.
The fact that the ocean temerature is different in different places means that the local seal level will be different. This is in addition to the gravitational effects referred to above.
Also, there are ocean currents. That means more water is piled up in some places than others.
Yep, I get that gravity is not uniform, and sea “level” isn’t actually flat. The moon also causes tides, which change local sea level dramatically a couple of times a day. Plenty of other factors, too.
But I’m not aware of any of those other factors changing. AFAIK, gravity and all its local anomalies around the world are much as they have been since we started measuring sea level, and the moon still orbits as always.
Suppose I fill my bathtub with cold water. I put a divider in the middle, and heat up the water on one side of the divider. The hot water has expanded, so now its level is taller than that of the cold water. I remove the divider. Hot water will flow out over the cold water because it’s sitting higher, and also because buoyancy-driven convection will cause cold water to push in under the warmer water and also allow the hot water to spread out even more. A new equilibrium water level will be established in the tub; the new level will everywhere be higher than the old level because the average temperature of the water in the tub is higher than it used to be. Even assuming local gravitational anomalies that mean the surface of the water isn’t (and wasn’t) a perfectly flat surface, we should still be able to define the original shape of the surface, whatever it was, and observe that the new shape is the same, except a little higher everywhere (as opposed to being much higher in some places and only a little higher in other places).
Why would this same thing not be happening in the SW Pacific?
How is the ocean different from the bathtub scenario I described? In both cases, there are differences in water height and/or density that drive the movement of water to achieve a new equilibrium water level. The movements of water in the bathtub that I described in my previous post are analogous to ocean currents.
Just as there have always been tides and gravitational anomalies, there have always been ocean currents. What’s special about the SW Pacific?
The Earth’s atmosphere and hydrosphere are not equilibrium systems and never come to a homogenous stable condition like your bathtub analogy.
Warmer water will be less dense, and will therefore occupy more volume for the same mass, and thus will contribute to greater rise in mean sea level. There are other contributors but this is the driving cause of increase in mean sea level in the South Pacific. The tropical and subtropical South Pacific islands of Melanesia, Micronesia, and Polynesia are also uniquely vulnerable to sea level change as they are low laying, with small water tables that are often only segregated from seawater by osmotic pressure, so they are doubly threatened by sea level rise both consuming living and agricultural spaces as well as compromising freshwater reservoirs.
Your bathtub doesn’t have trillions of gallons of water in it, and isn’t constantly in an uneven state of heating, and the Americas are still dividing the Pacific and Indian Ocean.
I vaguely recall a trivia bit that the Pacific sea level is 14 feet higher than the Atlantic/Caribbean at the Panama canal.
Also, cause for alarm, there are a lot more inhabited small low-lying islands in the Pacific, so more affected - also meaning that things like storm surges are more impactful. And as mentioned, these islands close to the equator in warm water are more likely to encounter tropical storms.
Also, some southern or not-so-southern pacific coastal cities have some of the highest subsidence rates on earth. They’re high enough that sea level rise will compound subsidence rather than the other way around.
I couldn’t find any math on how the South Pacific per se could retain a lot of its sea level rise from warming water. But one of the links I found did mention that high waves that distribute sea level change toward the east coast of the US could take years to hit it even though they start in the Atlantic.
This is new information to me, since I had assumed that water would redistribute itself, in the absence of active factors like gravity anomalies or currents, at the rate of the waves that I had imagined in my mind, namely, around as fast as a walking pace at the very minimum, and would thus only take a year or so at the very most to travel around the world, with no one spot in the ocean being permanently higher via thermal expansion.
But if the waves that redistribute this water do move very slowly, then it makes logical sense that the south pacific could have a permanent sea level rise compared to the rest of the globe.
There have always been difference in sea level around the world. The volume of liquid water in the oceans is increasing and it’s not going to be distributed equally.
Another thing I couldn’t find exact details on is how much the outsized increase is due to being on the opposite side of the retreating Greenland ice sheet, which releases water around it due to lowered regional gravity. Not knowing any better, my simplistic math tells me it’s plausible that this could account for some of the millimeters but I can’t find anything firm from my searches. Plus it’s probably a measurable factor since articles are reporting it as a factor.
Sea level can vary ocean to ocean- but not by a lot. But a centimeter change is quite a large change, even tho that sort of small sea level difference would not be remarked on.
No. Sea level rise is uneven, the two main reasons being ocean dynamics and Earth’s uneven gravity field.
First, ocean dynamics is the redistribution of mass due to currents driven by wind, heating, evaporation and precipitation. For example, during La Niña events, sea level goes down because some rain that usually occurs over the ocean shifts to land, and the same phenomenon produces low latitude currents that redistribute seawater. Regional climate cycles, like El Niño and La Niña, and longer-term effects, like the Pacific Decadal Oscillation, change ocean circulation, which changes sea level.
A visualization of Earth’s gravity field using Gravity Recovery and Climate Experiment (GRACE) data. Gravity is determined by mass; Earth’s mass is not distributed equally, and it also changes over time. Credit: NASA/JPL/University of Texas Center for Space Research
Second, because the distribution of Earth’s mass is uneven, Earth’s gravity is also uneven. Therefore, the ocean’s surface isn’t actually a perfect sphere or ellipsoid; it is a bumpy surface. As the land-based ice sheets of Greenland and Antarctica continue to unload their mass (lose ice) from far above sea level and far from the tropics, that mass reaches the sea in the form of meltwater that is then redistributed along Earth’s gravity field.
At Panama the difference is striking due to unusual currents and tides-
I was under the same impression as the OP and found this thread very frustrating to read as no one seemed to quite understand the question, even the NASA site is confusing, until I finally got to this graphic, which blows my mind:
That’s the change in sea level between 1992 and 2019 - blue is falling, red is rising!
I’m starting to see that sea level is something like a very complex standing wave that persists over very long time spans. Smack my gob.
Great cites both of you; thank you. I’ll just point out since it’s hard to read that the total height range on that color map is ~30cm or very very roughly 1 foot.
They’re measuring something very very big to a very very fine tolerance considering its overall size.