Stabilization of global temperature at 1.5°C and 2.0°C: implications for coastal areas

Abstract

The effectiveness of stringent climate stabilization scenarios for coastal areas in terms of reduction of impacts/adaptation needs and wider policy implications has received little attention. Here we use the Warming Acidification and Sea Level Projector Earth systems model to calculate large ensembles of global sea-level rise (SLR) and ocean pH projections to 2300 for 1.5°C and 2.0°C stabilization scenarios, and a reference unmitigated RCP8.5 scenario. The potential consequences of these projections are then considered for global coastal flooding, small islands, deltas, coastal cities and coastal ecology. Under both stabilization scenarios, global mean ocean pH (and temperature) stabilize within a century. This implies significant ecosystem impacts are avoided, but detailed quantification is lacking, reflecting scientific uncertainty. By contrast, SLR is only slowed and continues to 2300 (and beyond). Hence, while coastal impacts due to SLR are reduced significantly by climate stabilization, especially after 2100, potential impacts continue to grow for centuries. SLR in 2300 under both stabilization scenarios exceeds unmitigated SLR in 2100. Therefore, adaptation remains essential in densely populated and economically important coastal areas under climate stabilization. Given the multiple adaptation steps that this will require, an adaptation pathways approach has merits for coastal areas.This article is part of the theme issue 'The Paris Agreement: understanding the physical and social challenges for a warming world of 1.5°C above pre-industrial levels'.

Keywords: climate adaptation; climate mitigation; coastal impacts; ocean pH; sea-level rise.

Figure 1.

Future temperature rise, sea level and surface ocean pH projections to 2300 for a large (9 × 10⁴) ensemble using the WASP Earth system model. (a) Global temperature anomaly relative to pre-industrial, ΔT (°C), (b) global mean sea-level (GMSL) rise relative to 1986–2005 (m) and (c) surface ocean pH. The median ensemble projections over time (lines) and the 90% ranges within the ensemble simulations (shaded areas, from the 5th to 95th percentiles) are shown for RCP8.5 (red) and 2.0°C (blue) and 1.5°C (grey) stabilization scenarios.

Figure 2.

Climate stabilization and global coastal flooding. (a) Expected number of people flooded (millions yr⁻¹) versus time from 2000 to 2300 for stabilization and unmitigated SLR scenarios across all socio-economic scenarios for each emissions pathway. (b) Relative comparison of impacts under stabilization, showing the percentage of impact, normalized by unmitigated (RCP8.5) impacts for the same socio-economic scenario. In both cases, this assumes no adaptation (i.e. dike upgrade) and the numbers are indicators, not projections. The mean ensemble projections over time (lines) and the range are shown for RCP8.5 (red) and 2.0°C (blue) and 1.5°C (grey) stabilization scenarios.

Figure 3.

Estimates of the dike heights required to protect the major coastal cities under the three median climate scenarios from 2005 to 2300 (red: RCP8.5; blue: 2°C; grey: 1.5°C). The bars show the fraction of the floodplain population protected by the respective dike height while the lines show cumulative population. The numbers in the panels are cumulative number of people protected (in millions).