How much do the different factors affect sea level rise?

Submitted by Natural-Cap4008 t3_11b8429 in askscience

In my minimal research and remembering university classes on environmental issues, I believe that sea level rise is caused predominantly by the increase in temperature of the ocean, which is caused by multiple different factors.

My question (well kind of 2 questions) is/are, do we know how much h the different factors cause the seawater to expand? And/or do we know how much the different factors cause the sealevel to rise?( ie how much of recent expansion was from melting of ice caps, how much was heat expansion ect).

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CrustalTrudger t1_j9y6iau wrote

> In my minimal research and remembering university classes on environmental issues, I believe that sea level rise is caused predominantly by the increase in temperature of the ocean, which is caused by multiple different factors. My question (well kind of 2 questions) is/are, do we know how much h the different factors cause the seawater to expand?

This is largely incorrect. For current rates, this is pretty easy to find e.g., this page from NASA. The total sea level rise rate is 3.4 mm/yr. Of that, 2 mm/yr (or ~60%) is from increasing ocean mass (i.e., addition of mass to the ocean from melting land-based glaciers and ice sheet) and 1.2 mm/yr (or ~40%) is from steric changes (i.e., changes in volume related to both temperature increases - thermosteric changes - and salinity decreases - halosteric changes).

> And/or do we know how much the different factors cause the sealevel to rise?

This is described on those linked NASA pages as well. For the total sea level rise, this is something that is now measured directly from satellite altimetry, i.e., we measure the surface height of the ocean over time and find average changes in height. In terms of attributing the components, we can estimate changes in mass from satellite gravity measurements and we can estimate changes in temperature and salinity (and in turn estimate their contribution to steric changes) through measurements from "floats".

It's also worth noting that the above are effectively current rates. If we look at longer term averages over the last 100+ years (e.g., Frederikse et al., 2020), we find that the long term average is ~1.5 mm/yr (i.e., the current rate represents an acceleration). In terms of long-term contributions, changes in ocean mass again dominate with the steric (whether talking about thermosteric or halosteric) components being more variable in both time and space (i.e., at a global average level, their relative contribution varies through time, but also at a given time, their relative contribution are not consistent spatially).

In short, whether we're considering current rates or average rates over the last 100 years, changes in ocean mass dominate the signal of sea level rise. Steric changes are definitely important, but it's incorrect to say they are the largest component.

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Bwyanfwanigan t1_j9y7fn4 wrote

Serious question. I've never understood how sea levels measured in the past can be compared to now that we are using satellites. Before satellites was there even a means of measuring global sea level? The same question has bothered me about climate measurements which in the past were not digital and inaccurate. Not a denier, just always wondered about this and never thought to ask.

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CrustalTrudger t1_j9yaht5 wrote

Global networks of tide gauges for the "historical" sea level, which gets us back relatively accurately to at least the late 1800s. There are a variety of geologic records of sea level which we can use to build sea level curves going back well beyond historical periods.

> The same question has bothered me about climate measurements which in the past were not digital and inaccurate

If you want a deep dive on this, starting with something like the 'physical science basis' product of the latest IPCC report would be a good start. The short version is that we can place individual temperature records into context with a vast numbers of proxy data that allow us to reconstruct temperature (e.g., oxygen isotopes, clumped isotopes, compound specific isotopes, tree rings, etc.) and climate modeling that all tell us effectively the same thing.

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racinreaver t1_ja0yw2v wrote

There's actually a lot of work that goes into cross-correlating data between different sorts of satellite measurements and different generations of similar technologies. For correlating to historical data, you can also always keep collecting the same kind of historical data.

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Natural-Cap4008 OP t1_j9zb9nr wrote

Thank you! Basically perfectly answered my question/s. So steric changes make up only 40%. Looking at the NASA website you linked, it's a bit hard to tell if increase in each type is linear or exponential. You mentioned that the current rate is accelerating, can you expand on this a little, and whether the acceleration is the same 60-40 percentages of steric vs volume, or if say the acceleration is more due to volume changes so you might expect that ratio to increase over time? (I think I'm articulating what I'm thinking)

Again thank you for your response!

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