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I don't understand this part: "In total, Antarctica has shed some 2.7 trillion tonnes of ice since 1992, corresponding to an increase in global sea level of mor

by mkempe 8y ago

I don't understand this part: "In total, Antarctica has shed some 2.7 trillion tonnes of ice since 1992, corresponding to an increase in global sea level of more than 7.5mm." If the melting ice of Antarctica contributes only 0.3mm per year to the annual 6mm rise of sea levels, what exactly is causing a 20x larger change? [added] On the other hand a NASA study published in 2015 [1] indicated that Antarctica has been steadily accumulating ice, thereby "taking 0.23 millimeters per year away... If the 0.27 millimeters per year of sea level rise attributed to Antarctica in the IPCC report is not really coming from Antarctica, there must be some other contribution to sea level rise that is not accounted for." [1] https://www.nasa.gov/feature/goddard/nasa-study-mass-gains-of-antarctic-ice-sheet-greater-than-losses https://www.nasa.gov/feature/goddard/nasa-study-mass-gains-o...

kaybe 8y ago
Water expands when it gets warmer.
- mkempe 8y ago
  On the other hand, "In the most recent estimate, for 1993–2008, the contribution from land ice increased to 68 percent, the contribution from thermal expansion decreased to 35 percent". [1] Something is not right. Either ice melting has been contributing 4mm/year to recent decades of sea rise, or the contribution is slightly negative (in the largest ice sheet, over Antarctica). Thermal expansion of ocean water on its own does not adequately explain it. [1] Contributions to Global Sea-Level Rise https://www.nap.edu/read/13389/chapter/5 https://www.nap.edu/read/13389/chapter/5 --- One could dogmatically down-vote, or one could provide references to scientific discussions that cover the contradictory stories... You know who you are.
  - kaybe 8y ago
    https://ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter03_FINAL.pdf https://ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter... Check page 291 for an overview of components and time lines. This is still the current scientific consensus AFAIK (no major updates).
    - mkempe 8y ago
      Thank you. The estimated contribution to sea-level rise by thermal expansion of the oceans is 1 mm/year. It's still not clear how this amount plus the measured contributions of Greenland (under 0.5 mm/year) and Antarctica (0.3 mm, possibly -0.2 mm) somehow add up to a predicted 6 mm/year (or even to a measured 3 mm/year trend).
      - southern_cross 8y ago
        Here are some things I can think of that may contribute to sea level rise but which I rarely, if ever, see mentioned; the first two are potential anthropogenic causes. 1. Every time you burn a hydrocarbon, in addition to CO2 you also produce H2O. The ratio varies, but in general all of the hydrogen atoms will burn off before all of the carbon atoms do, since carbon-carbon bonds are fairly strong. (This can lead to things like soot.) Much of the water produced ultimately ends up in the oceans. 2. For decades now we've been pumping down aquifers at a fast clip, and that water ultimately ends up in the oceans, too. Natural aquifer replenishment rates may be fairly slow, if they ever replenish at all (the oldest, deepest aquifers may not) and the land-use changes we've made (paving and construction and such) slow that process down ever more. 3. Water from deep subterranean sources maybe be leaking into the oceans via undersea vents and such; this can also lead to bottom-up warming. Ongoing isostatic rebound from the loss of ice mass over the continents may be increasing these factors. As to ocean warming itself, in general top-down warming only occurs within the uppermost layer of the ocean. (Oceans are dynamic things, of course, so some deeper flow and mixing does occur.) But that top-layer warming is also heavily mitigated by evaporation, where a great deal of the potential heat energy rises up with the water vapor and is eventually lost to space; meanwhile the water itself eventually returns. The clouds produced as the water vapor condenses and loses its heat also mitigate further warming.
        mkempe 8y ago
        Hmm. Let me know when you have some scientific evidence and specific numbers related to these notions. If we're cooking up stories, how about this arbitrary "explanation": Phytoplankton are plentiful. They're reproducing and dying and sinking in such vast quantities to the ocean floor that this floor is steadily growing a new layer, at a rate of a few mm per year. Hence the apparent sea-level rise, relative to the continents. The accumulation of biogenic sediment is accelerating with the rise in atmospheric CO2 because CO2 directly feeds the plankton.
llccbb 8y ago
0.3 mm/yr * 26 yr = 7.8 mm Antarctica's contribution to sea level rise per year is fairly small. The largest portion comes from loss of land-ice like mountain glaciers and snow pack. Mass loss from the Greenland Ice Sheet is also substantial, contributing about 2.7 mm/yr to sea level change. Groundwater withdrawal is a measurable component of modern sea level rise. Aside from these mass-transfer mechanisms, there is also ocean warming (water generally expands as it get warmer) and salinity changes that don't affect the amount of water in the oceans, but do affect its volume. Loss of sea ice (which floats on top of water and is not supported from below by solid earth) does not contribute to sea level change, as the sea ice is already displacing an amount of water equal to the amount of ice doing the displacing. That 2015 Jay Zwally paper should not be taken as truth, as there are substantial reasons to doubt the impact of the findings. In that paper Zwally uses a set of satellite laser altimeters operating over different epochs and neglects to co-register the different platforms in an intelligent way. There was actually major hubbub around that paper and most glaciologists recognize the claim that "Antarctica is gaining mass" is probably incorrect.[0] Zwally has a pretty big ego and was happy to get the publicity anyways. [0] https://www.scientificamerican.com/article/what-to-believe-in-antarctica-rsquo-s-great-ice-debate/ https://www.scientificamerican.com/article/what-to-believe-i...
- mkempe 8y ago
  Greenland does not contribute that much to global sea-level rise: "The cumulative ice loss from Greenland from 1992 to 2015 was 3,600 Gt and contributed to global sea level rise by approximately 10 mm" -- that's less than 0.5 mm/year. [1] Where did you get that Greenland contribution rate? [1] https://www.eea.europa.eu/data-and-maps/indicators/greenland-ice-sheet-3/assessment https://www.eea.europa.eu/data-and-maps/indicators/greenland...

by mkempe 8y ago

kaybe 8y ago

Water expands when it gets warmer.

mkempe 8y ago
On the other hand, "In the most recent estimate, for 1993–2008, the contribution from land ice increased to 68 percent, the contribution from thermal expansion decreased to 35 percent". [1] Something is not right. Either ice melting has been contributing 4mm/year to recent decades of sea rise, or the contribution is slightly negative (in the largest ice sheet, over Antarctica). Thermal expansion of ocean water on its own does not adequately explain it. [1] Contributions to Global Sea-Level Rise https://www.nap.edu/read/13389/chapter/5 https://www.nap.edu/read/13389/chapter/5 --- One could dogmatically down-vote, or one could provide references to scientific discussions that cover the contradictory stories... You know who you are.
- kaybe 8y ago
  https://ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter03_FINAL.pdf https://ipcc.ch/pdf/assessment-report/ar5/wg1/WG1AR5_Chapter... Check page 291 for an overview of components and time lines. This is still the current scientific consensus AFAIK (no major updates).
  - mkempe 8y ago
    Thank you. The estimated contribution to sea-level rise by thermal expansion of the oceans is 1 mm/year. It's still not clear how this amount plus the measured contributions of Greenland (under 0.5 mm/year) and Antarctica (0.3 mm, possibly -0.2 mm) somehow add up to a predicted 6 mm/year (or even to a measured 3 mm/year trend).
    - southern_cross 8y ago
      Here are some things I can think of that may contribute to sea level rise but which I rarely, if ever, see mentioned; the first two are potential anthropogenic causes. 1. Every time you burn a hydrocarbon, in addition to CO2 you also produce H2O. The ratio varies, but in general all of the hydrogen atoms will burn off before all of the carbon atoms do, since carbon-carbon bonds are fairly strong. (This can lead to things like soot.) Much of the water produced ultimately ends up in the oceans. 2. For decades now we've been pumping down aquifers at a fast clip, and that water ultimately ends up in the oceans, too. Natural aquifer replenishment rates may be fairly slow, if they ever replenish at all (the oldest, deepest aquifers may not) and the land-use changes we've made (paving and construction and such) slow that process down ever more. 3. Water from deep subterranean sources maybe be leaking into the oceans via undersea vents and such; this can also lead to bottom-up warming. Ongoing isostatic rebound from the loss of ice mass over the continents may be increasing these factors. As to ocean warming itself, in general top-down warming only occurs within the uppermost layer of the ocean. (Oceans are dynamic things, of course, so some deeper flow and mixing does occur.) But that top-layer warming is also heavily mitigated by evaporation, where a great deal of the potential heat energy rises up with the water vapor and is eventually lost to space; meanwhile the water itself eventually returns. The clouds produced as the water vapor condenses and loses its heat also mitigate further warming.
      - mkempe 8y ago
        Hmm. Let me know when you have some scientific evidence and specific numbers related to these notions. If we're cooking up stories, how about this arbitrary "explanation": Phytoplankton are plentiful. They're reproducing and dying and sinking in such vast quantities to the ocean floor that this floor is steadily growing a new layer, at a rate of a few mm per year. Hence the apparent sea-level rise, relative to the continents. The accumulation of biogenic sediment is accelerating with the rise in atmospheric CO2 because CO2 directly feeds the plankton.

llccbb 8y ago

0.3 mm/yr * 26 yr = 7.8 mm Antarctica's contribution to sea level rise per year is fairly small. The largest portion comes from loss of land-ice like mountain glaciers and snow pack. Mass loss from the Greenland Ice Sheet is also substantial, contributing about 2.7 mm/yr to sea level change. Groundwater withdrawal is a measurable component of modern sea level rise. Aside from these mass-transfer mechanisms, there is also ocean warming (water generally expands as it get warmer) and salinity changes that don't affect the amount of water in the oceans, but do affect its volume. Loss of sea ice (which floats on top of water and is not supported from below by solid earth) does not contribute to sea level change, as the sea ice is already displacing an amount of water equal to the amount of ice doing the displacing. That 2015 Jay Zwally paper should not be taken as truth, as there are substantial reasons to doubt the impact of the findings. In that paper Zwally uses a set of satellite laser altimeters operating over different epochs and neglects to co-register the different platforms in an intelligent way. There was actually major hubbub around that paper and most glaciologists recognize the claim that "Antarctica is gaining mass" is probably incorrect.[0] Zwally has a pretty big ego and was happy to get the publicity anyways. [0] https://www.scientificamerican.com/article/what-to-believe-in-antarctica-rsquo-s-great-ice-debate/ https://www.scientificamerican.com/article/what-to-believe-i...

mkempe 8y ago
Greenland does not contribute that much to global sea-level rise: "The cumulative ice loss from Greenland from 1992 to 2015 was 3,600 Gt and contributed to global sea level rise by approximately 10 mm" -- that's less than 0.5 mm/year. [1] Where did you get that Greenland contribution rate? [1] https://www.eea.europa.eu/data-and-maps/indicators/greenland-ice-sheet-3/assessment https://www.eea.europa.eu/data-and-maps/indicators/greenland...