This paper challenges the idea that 1.5 is a safe amount of climate change to save the coasts. They find that the safe threshold was around 1°. Even net zero will raise sea levels. They also find that the interglacial paleorecord downplays the true risk of modern day CO2 forcing on the ice sheets.
The paleo record downplays the risk:
First, if the Anarctic ice sheet and greenland ice sheet retreated (and regrew) out of phase, as has been hypothesised, this would have buffered the total amount of sea level rise experienced at any one time. Hence, relying on the peak in last inter glacial global mean sea level as an analogue for future warming may underestimate ice sheet retreat that is currently observed simultaneously in both hemispheres due to increasing greenhouse gas concentrations, rather than orbital forcing.
Thus, in the past record as the greenland sheet was melting, the anarctic sheet was growing at the same time. The sum total sea level rise at any point was not representative of the simultaneous coordinated melting combination event. This is unlike today.
This mistake is artificially baked into the climate models we are using that predict the future:
Future projections of ice sheet change using numerical modelling are informed, often qualitatively but sometimes quantitatively, by palaeo-records of ice sheet configuration and sea level from periods when Earth’s climate conditions were similar or slightly warmer than present
Satellites have measured that the melting rate is above what was predicted:
Indeed, it has been noted that mass loss (melting) from 2007 to 2017 tracked the upper range projected in the IPCC’s Fifth Assessment Report (AR5); and that if high rates of loss were to continue (+1.2 ± 0.2 mm year−1 between 2019 and 2020), they would track above the upper range projected in AR6 (+1.0 to +1.1 mm year−1 for the current decade). Furthermore, by neglecting the retreat of marine-terminating outlet glaciers, assessments of mass balance may have underestimated mass loss from Greenland by as much as 20%
This is an important observation:
A key implication of this hysteresis behaviour is that current rates of sea level rise could increase rapidly with only small changes in temperature
Surely “net zero” will put the brakes on sea level rise?
No, that’s not what they report.
Over longer timescales, the committed median sea level rise […] has been estimated at +0.7 m to +1.2 m, even if net zero greenhouse gas emissions are sustained until 2300, but ~3 m from Antarctica could not be ruled out.
/Furthermore, each 5-year delay in near-term peaking of CO2 emissions increased median SLR at 2300 by around +20 cm, and no net zero scenario gave a median SLR below +1.2 m at 2300 once global mean temperatures exceed +1.5 °C.
Based on the CO2 already in the atmosphere, we are looking at major sea level rise. It takes a while to happen.
What this says is that every 5 years of Business as Usual before we get to net zero, we commit to another 20cm of rise.
Net zero starting today could still be as much as 3m of rise in the pipeline over the next generations.
Due to the hysteresis in the system, if we actually wanted to cancel sea level rise, to refreeze the poles takes an even bigger signal in the opposite direction. So to over power the warming trend we would have to get CO2 down even more below pre-industrial than we raised it.