[garrettdreyfus]

I’m not sure this title is completely correct

“The researchers identified the type of water loss on land, and for the first time, found that 68% came from groundwater alone — contributing more to sea level rise than glaciers and ice caps on land.”

They are saying the leading loss of water loss is from ground water. The largest contributor to sea level rise I would guess is still thermosteric sea level rise due to the ocean becoming warmer and less dense

See ipcc https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-9/

9.6.1 Global and Regional Sea Level Change in the Instrumental Era

In particular, Cross-Chapter 9.1, Figure 1 | Global Energy Inventory and Sea Level Budget. Panel b

EDIT: @dang could the submission title be changed to the article or journal article title?

“New global study shows freshwater is disappearing at alarming rates”

Or

“Unprecedented continental drying, shrinking freshwater availability, and increasing land contributions to sea level rise”

[garrettdreyfus]

My reading of Figure 6 https://www.science.org/doi/10.1126/sciadv.adx0298 suggests that this study still has thermosteric effects making up the majority of sea level rise.

I also highly recommend reading up on the GRACE satellite used in this study it is amazing https://gracefo.jpl.nasa.gov/resources/50/how-grace-fo-measu...

[mturmon]

> it is amazing

Indeed!

The GRACE measurement of mass change is one of the more revolutionary advances in Earth science remote sensing in the last few decades. It has provided a unique and completely novel view of groundwater mass change. Grace is the main reason we know so much about the massive groundwater loss in the Oglala aquifer in the US Midwest, in the Central Basin in California, and in northern India. Water well data exists but it is very sparse and idiosyncratic.

It’s also our main window into mass losses in ice sheets in high latitudes (Greenland, Antarctica). We have radar altimetry data from Antarctica, but because of glacial rebound and other effects, it’s not easy to translate height changes into mass changes. Grace measures mass change directly.

Several authors of the cited study are on the science team. It is a JPL instrument.

The original Grace pair used radio to measure separation and velocity, while the follow-up Grace-FO uses a laser. I assume the small wavelength of the laser provides a more accurate measurement. It’s possible that Grace-FO has a slightly higher spatial resolution (I’ve worked with Grace but not Grace-FO); the horizontal resolution of Grace is about 100km or about 1 degree.

From an inference perspective the measurement is very interesting. They pool about a month’s worth of observations of the distance and velocity of a pair of satellites, and do a Bayesian inversion to obtain a parameterized gravitational potential for that month. The map from gravitational potential to observation is known analytically, so it’s readily possible to get a spatial covariance for the gravitational potential, as well as the point estimate.

[yboris]

Thank you for sharing, GRACE-FO feels to me like a brilliant design!

[ornel]

Quote from the paper: "the continents are now the leading contributor (44%) to mass-driven GMSL rise". As regards to non-mass-driven rise, another article[0] states, "Ice-mass loss—predominantly from glaciers—has caused twice as much sea-level rise since 1900 as has thermal expansion". I think the findings about sea level rise are as interesting as the ones about fresh water disappearance.

[0] https://www.nature.com/articles/s41586-020-2591-3

[garrettdreyfus]

The study you cite is talking about sea level rise since 1900 which is a very different story.

The IPCC section “9.6.1.1 Global Mean Sea Level Change Budget in the Pre-satellite Era” says Since SROCC, a new ocean heat content reconstruction (Section 2.3.3.1; Zanna et al., 2019) has allowed global thermosteric sea level change to be estimated over the 20th century. As a result, the sea level budget for the 20th century can now be assessed for the first time. For the periods 1901–1990 and 1901–2018, the assessed very likely range for the sum of components is found to be consistent with the assessed very likely range of observed GMSL change (medium confidence), in agreement with Frederikse et al. (2020b; Table 9.5). This represents a major step forward in the understanding of observed GMSL change over the 20th century, which is dominated by glacier (52%) and Greenland Ice Sheet mass loss (29%) and the effect of ocean thermal expansion (32%), with a negative contribution from the LWS change (–14%). While the combined mass loss for Greenland and glaciers is consistent with SROCC, updates in the underlying datasets lead to differences in partitioning of the mass loss.”

Edit: by a different story I mean a different story from what is the leading driver of sea level rise. Sea level rise from ice melt was larger since 1900 because sea level rise in general was less fast back then and global mean temperature rise was much smaller so thermosteric sea level rise played less of a role. Thermosteric sea level rise is larger than ground water factors, both will be eclipsed by ice melt in the upcoming century.

I would note the authors pointedly do not call it the leading driver of sea level rise.

[perching_aix]

So let me get this straight:

- sea level is formally referred to as Global Mean Sea Level (GMSL)

- its change is segmented into two subcategories in literature(?), mass-driven (e.g. ice melting?, freshwater runoff?, freshwater water cycle stuff?) and non-mass-driven (e.g. thermal expansion?)

- freshwater loss from land was found to be at present the lead driver of the mass-driven change as per the paper (over what timeframe?)

- title says it's the primary driver for GMSL change overall, which this alone doesn't support (i.e. the title is a lie)

- @ornel (the person posting) points to another study that claims mass-driven change is the leading change, hence the title [0, this doesn't pass my smell test but i see the logic]

- you point out that that's glossing over that that other study is counting from 1900, but if one shrunk the evaluation window, the non-mass-driven causes would be the drivers now [1, this doesn't pass my smell test either, but i see the logic here as well]

The latter point then begs the question though, what is the time window in this case then, and how stable that result is? What would be an "appropriate" time window to choose, and how would one derive that?

Regarding my non-passing smell tests, imagine the following scenario for some event:

- category A: 51% of the total

- cause A1: 26% of the total

- cause A2: 25% of the total

- category B: 49% of the total

- cause B1: 27% of the total

- cause B2: 22% of the total

In this case, category A will be the lead contributor, but individually none of its contributing causes will be, addressing [0]. The causes will be ordered like so instead: B1 > A1 > A2 > B2. More elaborate variations are possible of course. For [1], you can imagine the same scenario just in reverse.

Did I get all this right?

[garrettdreyfus]

Hi,

I appreciate the effort in your comment. I think upon further reflection my simpler objection is calling freshwater loss the main driver of sea level rise when the journal article and news article don’t. Also I would note this is only one study.

[perching_aix]

> I appreciate the effort in your comment.

Thanks for that! I do wish it wasn't necessary though, but I guess that's just how real life problems go.

> I think upon further reflection my simpler objection is (...)

Right, that's perfectly fine; just got curious and you seemed informed.

Editorializing the titles in general is against the guidelines here anyhow to be fair, I'm expecting it will be updated by the mods eventually: https://news.ycombinator.com/newsguidelines.html

[garrettdreyfus]

I very much like your categories point here by the way!

[garrettdreyfus]

See table 9.5

https://www.ipcc.ch/report/ar6/wg1/chapter/chapter-9/

[garrettdreyfus]

Although to your point it does vary alot over different windows of time.

[srameshc]

> New findings from studying over two decades of satellite observations reveal that the Earth’s continents have experienced unprecedented freshwater loss since 2002, driven by climate change, unsustainable groundwater use and extreme droughts.

The title captures the crux of the story

[garrettdreyfus]

Sorry I am referring to HN submission title not the article title

[cwillu]

@dang doesn't do anything; but they're quite responsive to email.

[dang]

Ok, I've put the second suggestion up there. Thanks!

(Submitted title was "Freshwater loss from land is the lead driver of sea-level rise")

[ornel]

Thanks dang and everyone for the better title and for clarifying this further