A century-long linear rise in sea level/atmospheric CO ₂ ratios

Abstract Millions of people are vulnerable to future sea level rise (SLR); however, SLR predictions are less accurate than temperature and atmospheric CO 2 concentration outcomes under different gas emission scenarios. There are also lags in the analytical determinations and uncertain future changes in climate forcing functions. A linear relationship exists between sea level elevation and atmospheric carbon dioxide concentration (mm ppm −1 ; SEACO) at 10 tide gage stations across the world of 0.005911 mm ppm −1 y −1 from 1901 to 2020. This ratio can serve as a diagnostic tool for SLR tipping points, help calibrate SLR models, and be easily calculated for regional oceans. Higher or lower future regional changes may indicate tipping points in ocean current strength or direction or foreshadow global climate changes. A regional example is from the Gulf of Mexico (aka Gulf of America), where, after 2000, SEACO y −1 was more than twice as high as in the previous six to eight decades. The predicted atmospheric CO 2 concentrations in the high-end emissions IPCC scenario (RCP8.5) for 2050 and 2100 are 550 and 1000 ppm, respectively. If these concentrations occur and the SEACO index continues to rise at the same rate as during the last 120 years, then the average annual sea level rise (SLR) from 2020 to 2050 will be 164% of the 1991 to 2020 baseline years (2.17 to 3.57 mm y −1 ), and the 2020 to 2100 interval average will be 278% of the baseline years, equivalent to an annual SLR of 3.57 and 6.73 mm y −1 , respectively. At those concentrations the average global sea level in 2050 and 2100 will be 0.11 and 0.54 m, respectively, above the 2020 average elevation and considerably below the sea level predicted under the IPCC RCP8.5 high-emissions scenario.