The California’s Ocean Protection Council’s 2018 Sea Level Rise Guidance projects up to 2 meters (6.9 feet) of sea level rise by 2100 . In 2019, researchers at the US Geological Survey published a study of the risk from inundation from up to 2 m of sea level rise plus 100 year storm surge (Barnard et. al.) . They estimated that up to 612,000 residents on the California coast (uncertainty = up to 1,017,000) and 534,000 employees (uncertainty = up to 798,000) would be at risk. The rate of sea level rise will determine how long major population centers in coastal regions have to adapt.
A Nov 8 2021 press release by mountain and polar groups raises alarm bells about consideration of sea level rise at COP26, stating:
Mountain and polar groups at the twenty-sixth United Nations Climate Change Conference of the Parties (COP26) are calling for more consideration of the dire global impacts that will result should governments not take greater action as the climate talks in Glasgow begin their second and final week.
These research, environmental and indigenous organizations span the entire global “cryosphere” (snow and ice regions), from the poles to the high mountain Asia “Third Pole.” They believe that the real implications of this science for climate goals remain so poorly understood by governments, that an entire day should be devoted to their impacts, especially as regards thresholds and tipping points.
The groups note that despite the dire consequences noted in the IPCC’s latest report, released in August of this year, there has never been a full discussion of projected changes, especially their irreversibility at the climate talks. The UNFCCC has in the past held Dialogues on oceans and terrestrial systems or lands; but never on the Earth’s cryosphere.
Robert DeConto, a leading researcher with the Scientific Committee for Antarctic Research (SCAR) co-authored a paper with David Pollard and others which warned that “rapid and unstoppable sea-level rise from Antarctica will be triggered if Paris Agreement targets are exceeded.”  In the press release, DeConto states, “Negotiators may think they know about melting ice caps, but what they don’t realize is that the impacts are essentially permanent on human timescales, and catastrophic for humanity.” Jonathan Bamber, Director of the Bristol Glaciology Centre, states, “Once the collapse of the West Antarctic Ice Sheet begins, it will be effectively impossible to halt.”
The Greenland Ice Sheet is also losing mass faster than current state-of-the art ISMIP6 models project. A 2021 preprint by Andy Aschwanden et. al.  states that “analysis of a recent effort to project Greenland’s contribution to future sea-level suggests that few models reproduce historical mass loss accurately, and that they appear much too confident in the spread of predicted outcomes.”
Figure from Aschwaden et. al. 2021. The caption states, “Observed and simulated historical mass changes from the Greenland Ice Sheet 2000–2020 in gigatons (Gt) and centimeters of sea level equivalent (cm SLE). GRACE and GRACE-FO JPL RL06Mv2 Mascon Solution (Wiese et al., 2020) (blue), reconstruction from Mouginot et al. (2019) (red) and a consensus estimate (The IMBIE Team, 2019) (green), and their respective uncertainties (shaded). ISMIP6 Goelzer et al. (2020) historical simulations and projection (gray lines) and the 90% credibility interval (light gray shading). The 90% credibility interval of ISMIP6 barely overlaps with observations.” (emphasis added)
The Nov 8, 2021 press release by the mountain and polar groups concludes,
“This science is so clear, yet complex; and the changes so drastic and permanent that more dialogue is essential,” said Pam Pearson, Director of the International Cryosphere Climate Initiative, which is hosting a Cryosphere Pavilion at the conference. “The questions by governments during discussion of the latest IPCC report last week underscore that this issue needs more than a 15-minute presentation.”
We agree. Decisionmakers need to understand current research on nonlinear feedbacks in mass loss of polar ice sheets and tipping points.
This post was updated on November 9, 2021.
 California Ocean Protection Council and California Natural Resources Agency, State of California Sea Level Rise Guidance,: 2018 Update.
 Barnard PL, Erikson LH, Foxgrover AC, Hart JAF, Limber P, O’Neill AC, van Ormondt M, Vitousek S, Wood N, Hayden MK, Jones JM. Dynamic flood modeling essential to assess the coastal impacts of climate change. Sci Rep. 2019 Mar 13;9(1):4309. doi: 10.1038/s41598-019-40742-z. PMID: 30867474; PMCID: PMC6416275.
 DeConto, R.M., Pollard, D., Alley, R.B. et al. The Paris Climate Agreement and future sea-level rise from Antarctica. Nature 593, 83–89 (2021). https://doi.org/10.1038/s41586-021-03427-0
 Aschwanden, A., Bartholomaus, T. C., Brinkerhoff, D. J., and Truffer, M.: Brief communication: A roadmap towards credible projections of ice sheet contribution to sea-level, The Cryosphere Discuss. [preprint], https://doi.org/10.5194/tc-2021-175, in review, 2021. https://tc.copernicus.org/preprints/tc-2021-175/