Earth is warming and sea levels are rising as land-based ice is lost to melt, and oceans expand due to accumulation of heat. The pace of ice loss and steric expansion is linked to the intensity of warming. How much faster sea level will rise as climate warms is, however, highly uncertain and difficult to model. Here, we quantify the transient sea level sensitivity (TSLS) of the sea level budget in both models and observations. Models show little change in sensitivity between the first and second half of the 21st century for most contributors. The exception is glaciers and ice caps (GIC) that have a greater sensitivity pre-2050 (2.8±0.4 mm/yr/K) compared to later (0.7±0.1 mm/yr/K). We attribute this change to the short response time of glaciers and their changing area over time. Model sensitivities of steric expansion (1.5±0.2 mm/yr/K), and Greenland Ice Sheet mass loss (0.8±0.2 mm/yr/K) are greater than, but still compatible with, corresponding estimates from historical data (1.4±0.5 mm/yr/K and 0.5±0.1 mm/yr/K). Antarctic Ice Sheet (AIS) models tends to show lower rates of sea level rise with warming (‑0.0±0.3 mm/yr/K) in contrast to historical estimates (0.4±0.2 mm/yr/K). This apparent low bias in AIS sensitivity is only partly able to account for a similar low bias identified in the sensitivity of GMSL excluding GIC (3.2±0.5 mm/yr/K vs 2.2±0.4 mm/yr/K). The balance temperature, where sea level rise is zero, lies close to the pre-industrial value, implying that sea level rise can only be mitigated by substantial global cooling.

Numerical, process-based simulations of tidewater glacier evolution are necessary to project future sea-level change under various climate scenarios. Previous work has shown that nonlinearities in tidewater glacier and ice stream dynamics can lead to biases in simulated ice mass change in the presence of noisy forcings. Ice sheet modeling projections that will be used in the upcoming IPCC Assessment Report 6 (AR6) utilize atmospheric and oceanic forcings at annual temporal resolution, omitting any higher frequency forcings. Here, we quantify the effect of seasonal (<1 year) tidewater glacier terminus oscillations on decadal-scale (30 year) mass change. We use an idealized geometry to mimic realistic tidewater glacier geometries, and investigate the impact of the magnitude of seasonal oscillations, bed slope at the glacier terminus, and basal friction law. We find that omitting seasonal terminus motion results in biased mass change projections, with up to an 18% overestimate of mass loss when seasonality is neglected. The bias is most sensitive to the magnitude of the seasonal terminus oscillations and exhibits very little sensitivity to choice of friction law. Our results show that including seasonality is required to eliminate a potential bias in ice sheet mass change projections. In order to achieve this, seasonality in atmospheric and oceanic forcings must be adequately represented and observations of seasonal terminus positions and tidewater glacier thickness changes must be acquired to evaluate numerical models.