When the sea level rises, more of the planet's area is watery and less of it is land. Does this change in surface condition result in a change in global warming, such as a decrease in the net radiation watts that we expect for the globe? How does this change affect other "forcings" like radiative and water vapor?
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4$\begingroup$ Why would you think this? Water absorbs incoming radiation much better than does land, and much, much better than ice. $\endgroup$– David HammenCommented Feb 18, 2019 at 20:30
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1$\begingroup$ I'm hoping for a generalized answer that somehow accounts for the W/m^2-difference, both for incident radiation and other forcings (like evaporation / ocean heat retention / ocean cloud & weather patterns etc). $\endgroup$– elliot svenssonCommented Feb 18, 2019 at 21:17
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1$\begingroup$ @DavidHammen then I suppose that water would also radiate into the clear sky well, especially when considering convection that maintains warmer surface temp through the night compared with earth or ice. $\endgroup$– elliot svenssonCommented Feb 18, 2019 at 21:32
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$\begingroup$ @DavidHammen, it seems, too, that the iodine emissions in tidal waters have a non-trivial impact to cloud cover, so I wonder if that tends to increase or decrease under sea level rise. $\endgroup$– elliot svenssonCommented Feb 18, 2019 at 23:13
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1$\begingroup$ @DavidHammen, also it sounds like oceans can move heat around geographically, and then send it back to space at the poles... that can't happen with terrestrial insolation. $\endgroup$– elliot svenssonCommented Feb 19, 2019 at 0:17
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As you say, land and water albedos are very different. Meaning that a square meter of water, in average, absorbs more energy from the sun than a square meter of land. Also water transfers heat vertically more efficiently than land due to convection and it also affects climate by providing a ready moisture source.
That said, changes in the surface of land vs water do have an impact on climate. However, the change in water surface area due to sea level rise is very small compared with other changes in land/water surfaces. Let's crunch some numbers to see it more clearly...
Lets consider a good approximate of the length of the global shoreline of 620,000 km (a precise number is impossible to get due to the coastline paradox). Now, at current sea level rise of 3.2 mm/year, 30 years would mean about 10 cm of sea level rise. If we assume that the typical slope is such that this moves the shoreline also 10 cm forward, the associated area increase would be:
$620000\, \text{km} \times 0.0001\, \text{km} = 62\, \text{km}^2$
And this is negligible compared with other effects like the drying of the Aral sea (~28,000 km$^2$), the construction of the Three Gorges Dam (1,084 km$^2$), or even the construction of the Palm Islands in Dubai (80 km$^2$).
Even if our very rough calculation above is multiplied by ten or even one hundred, the contribution of sea level rise would remain small compared with other changes.
At a global scale over the last 30 years there was a net increase of 58,000 km$^2$ of water surface, but mostly due to natural processes or large infrastructure projects. Sea level rise played just a very minor role in those changes. Indeed the following figure from the paper Earth's surface water change over the past 30 years shows changes from land to water in blue and changes from water to land in green:
Now, what's the effect of this increased water surface area (even if it is not due to sea level rise), that's another question that I won't get into in detail. But in general it will be a warming effect, both because of the lower albedo of water, and because of the warming effect of low clouds. However, the actual effect will depend on where the new water bodies are located; some extra water near the sea wouldn't have the same impact as some extra water in the middle of a desert.
answered Feb 19, 2019 at 22:01
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$\begingroup$ Thank you for this good order-of-magnitude comparison of sea-level rise to other causes for more water on the earth. Is the 58,000 km^2 that you mentioned a part of the "other forcings / land use" contribution that IPCC counts toward global warming? $\endgroup$ Commented Feb 19, 2019 at 22:13
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$\begingroup$ @elliotsvensson I just added a paragraph at the end of the answer. And yes, I think that falls into the land use change forcing category. $\endgroup$ Commented Feb 19, 2019 at 22:15
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$\begingroup$ I guess it's clear that that's how it ought to be categorized... do you remember whether it was included in the 2014 IPCC reports? $\endgroup$ Commented Feb 19, 2019 at 22:17
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$\begingroup$ @elliotsvensson It was but not those figures, that paper is from 2016. $\endgroup$ Commented Feb 19, 2019 at 22:21