Thread by @mikamckinnon

Ahhhhhhhhhhhhhhhhh

No. Just no.

Can we take a Time Out on the frankly bizarre attempts to shoehorn sciencey-sounding nonsense into climate change denial until we don’t need to constantly debunk volcano myths?

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Ready for a quick’n’dirty dive into sea level?

1. Sea level is not level.
2. Sea level is changing both globally & locally.
3. Global & local (relative) sea level change are independent.
4. Sea floor sediment depth has zero impact on surface water volume.

Our planet isn’t really a sphere: It’s a lumpy oblate spheroid.

This is the geoid, an imaginary surface of mean sea level for the entire planet as if its shape was determined exclusively by gravity & rotation.

x1000 vertical exaggeration:

Just like Earth’s solid surface has valleys and mountains, so does the ocean. Bathymetry & ocean temperature create stable highs & lows, while currents, tides, & waves create more dynamic topography.

More:
science.nasa.gov/earth-science/…
gizmodo.com/meet-nasa-s-ne…

Instead of dealing with constantly-shifting dynamic sea levels, we talk about changes in mean sea level relative to the geoid.

Globally, sea levels are rising as ice sheets melt & oceans warm (thus expand in volume): climate.nasa.gov/vital-signs/se…

Locally, it’s more complicated.

Local sea level is all about relative motion between ocean & land.

Land sinks? Local sea level rises (on top of global sea level rise).

Land rises? Local sea level sinks (reducing impact of global sea level rise).

Major drivers of local sea level change:

Subsidence: ground compacting
Tectonics: tectonic plates bending
Isostatics: gravitational rebalancing

When rivers slow, they dump sediment. On coasts, this creates deltas.

Deltas are squishy unconsolidated sediment that slowly compacts. If no new sediment piles on top (because humans are controlling floods or stabilizing ports), delta sinks (subsides) & local sea level rises

In subduction zones, one tectonic plate dives under another.

When the plates lock, the top plate is uplifted (dropping local sea level).

When they slip (earthquake!), the top plate drops (rising local sea levels). Part of how we ID ancient earthquakes is by drown ghost forests

Isistatic rebound realizing like a cork in water, continental plates float on a ductile mantle*.

Add weight (ice sheets)? Land sinks.
Remove weight (ice melts)? Land rises.

More (by me): io9.gizmodo.com/why-are-sea-le…

* Please don’t conceptualize the mantle as a liquid ocean of magma.

S-waves can’t propagate in liquids (outer core), but travel through the mantle. Geophysicists think about flow on way different scales than normal conditions.

Local factors can get complicated.

I live in the PNW, a region of tectonic uplift AND isostatic rebound: local sea level drop.

I’m on bedrock, but the nearby Fraser River delta (home of Richmond, Dekta, & YVR Airport) is subsiding: local sea level rise.

Most importantly:
Our planet has had liquid surface water for ~4.4 billion years.

Oceans are a few kilometers deep.

Sea floor sedimentation rates are (very very VERY roughly) 1mm/year.

If “Fill the oceans with sand” were a thing, it would’ve happened long ago.

Earth is the only planet that definitively has plate tectonics (although we’ve got strong suspicions).

This means we get quakes, a wider variety of volcanoes, fresh mountains, & (relevant today) seafloor recycling (including carbon sequestration).

Yes, crap falls in the ocean.

No, not enough crap falls in the ocean fast enough to impact sea level.

Yes, sea level is change is complicated, but it’s not THAT complicated. It all tl;dr’s down to “Global sea level is rising, and it’s a problem.”

Bonus: Any sediment washing off land & into the ocean is right near the coast.

That means it’s also the sediment that gets washed back onto land as beaches, or subducted & recycled into new rock.