Observed early-warning signals for a Greenland-ice-sheet tipping point

2020 ◽  
Author(s):  
Martin Rypdal ◽  
Niklas Boers
Keyword(s):  
Early Warning ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Temperature Threshold ◽  
Warning Signals ◽  
Early Warning Signals ◽  
Current State ◽  
Meridional Overturning ◽  
Global Sea Level

<p>Nonlinear feedbacks, such as the melt-elevation feedback, may produce a critical temperature threshold beyond which the current state of the Greenland Ice Sheet loses stability. Hence, the ice sheet may exhibit an abrupt transition under ongoing global warming, with substantial impacts on global sea level and the Atlantic Meridional Overturning Circulation. Melting rates across Greenland and solid ice discharge at the ice sheet's margins have recently accelerated. In this work, we analyze ice sheet runoff reconstructions and process-based simulations using new methods. We compare the acceleration in the runoff with the statistical properties of fluctuations around the system's equilibrium. The analysis uncovers significant early-warning signals for an ongoing destabilization and substantial further mass loss in the near future. </p><p> </p>

scholarly journals Critical slowing down suggests that the western Greenland Ice Sheet is close to a tipping point

2021 ◽  
Vol 118 (21) ◽  
pp. e2024192118
Author(s):  
Niklas Boers ◽  
Martin Rypdal
Keyword(s):  
Dominant Role ◽  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Temperature Threshold ◽  
Critical Transition ◽  
Slowing Down ◽  
Meltwater Runoff ◽  
Global Sea Level ◽  
The Stability

The Greenland Ice Sheet (GrIS) is a potentially unstable component of the Earth system and may exhibit a critical transition under ongoing global warming. Mass reductions of the GrIS have substantial impacts on global sea level and the speed of the Atlantic Meridional Overturning Circulation, due to the additional freshwater caused by increased meltwater runoff into the northern Atlantic. The stability of the GrIS depends crucially on the positive melt-elevation feedback (MEF), by which melt rates increase as the overall ice sheet height decreases under rising temperatures. Melting rates across Greenland have accelerated nonlinearly in recent decades, and models predict a critical temperature threshold beyond which the current ice sheet state is not maintainable. Here, we investigate long-term melt rate and ice sheet height reconstructions from the central-western GrIS in combination with model simulations to quantify the stability of this part of the GrIS. We reveal significant early-warning signals (EWS) indicating that the central-western GrIS is close to a critical transition. By relating the statistical EWS to underlying physical processes, our results suggest that the MEF plays a dominant role in the observed, ongoing destabilization of the central-western GrIS. Our results suggest substantial further GrIS mass loss in the near future and call for urgent, observation-constrained stability assessments of other parts of the GrIS.

Cascading tipping behavior of the interacting Greenland Ice Sheet and Atlantic Meridional Overturning Circulation in a model of low complexity 

2021 ◽  
Author(s):  
Ann Kristin Klose ◽  
Jonathan F. Donges ◽  
Ulrike Feudel ◽  
Ricarda Winkelmann
Keyword(s):  
Ice Sheet ◽  
Greenland Ice Sheet ◽  
Coupled System ◽  
Atlantic Meridional Overturning Circulation ◽  
Meridional Overturning Circulation ◽  
Climate System ◽  
Critical Threshold ◽  
Overturning Circulation ◽  
Meridional Overturning ◽  
Tipping Behavior

<p>The Greenland Ice Sheet (GIS) and the Atlantic Meridional Overturning Circulation (AMOC) have been identified as possible tipping elements of the climate system, transitioning into a qualitatively different state with the crossing of a critical driver threshold. They interact via freshwater fluxes into the North Atlantic originating from a melting GIS on the one hand, and via a relative cooling around Greenland with a slowdown of the AMOC on the other. This positive-negative feedback loop raises the question how these effects will influence the overall stability of the coupled system. Here, we qualitatively explore the dynamics and in particular the emergence of cascading tipping behavior of the interacting GIS and AMOC by using process-based but still conceptual models of the individual tipping elements with a simple coupling under idealized forcing scenarios.</p><p>We identify patterns of multiple tipping such as (i) <strong>overshoot cascades</strong>, developing with a temporary threshold overshoot, and (ii) <strong>rate-induced cascades</strong>, arising under very rapid changes of tipping element drivers. Their occurrence within distinct corridors of dangerous tipping pathways is affected by the melting patterns of the GIS and thus eventually by the imposed external forcing and its time scales.</p><p>The conceptual nature of the proposed model does not allow for quantitative statements or projections on the emergence of tipping cascades in the climate system. Rather, our results stress that it is not only necessary to stay below a certain critical threshold to hinder tipping cascades but also to respect safe rates of environmental change to mitigate domino effects and in turn to maintain the resilience of the Earth system.</p>

Response of the North Atlantic Meridional Overturning Circulation to the Greenland Ice Sheet Freshwater input in the CESM 2.1 model

2020 ◽  
Author(s):  
Carolina Ernani da Silva ◽  
Miren Vizcaino ◽  
Caroline Katsman
Keyword(s):  
North Atlantic ◽  
Ice Sheet ◽  
Stable State ◽  
Greenland Ice Sheet ◽  
Meridional Overturning Circulation ◽  
Freshwater Input ◽  
The North ◽  
The Future ◽  
Meridional Overturning ◽  
The Impact

<p>Coupled climate models predict a weakening of the Atlantic Meridional Overturning (AMOC) circulation in the future. However, it is not clear what is the cause of the AMOC weakening. Studies have suggested that the freshwater (FW) is an important factor in the AMOC reduction. There are different sources of FW that may play a role, such as, river discharge, sea ice melt, and precipitation. Currently, due to global warming, the Greenland Ice Sheet (GrIS) melt rate is rising, which increases the amount of freshwater (ice discharge) into the ocean. Thus, it is possible that this input of freshwater would affect the ocean circulation on a regional and global scale. Hence, the GrIS freshwater cannot be neglected. The goal of this study is to understand the impact of the freshwater from GrIS on the North AMOC (NAMOC) strength in the future. We used the Community Earth System Model (CESM) version 2.1, which contains a fully coupled and an active ice sheet, to simulate an idealized greenhouse gas scenario (1% CO<sub>2</sub>). The CO<sub>2</sub> concentration is 1140 ppm at the end of the simulation. The results show that GrIS delivers, on average, about 0.062 Sv/yr of FW to the Subpolar North Atlantic Ocean. The bulk of the total freshwater input comes from the southeastern and southwestern parts of the ice sheet:  the regions where some fast-flowing marine-terminating glaciers are located (e.g. Helheim and Kangerlussuaq). The NAMOC index (maximum barotropic stream function from above 28°N and from 500 m to 5500 m depth) was calculated. It displays a fast weakening, approximately 16.7 Sv (0.11 Sv/yr), during the first 150 yrs. After that, the NAMOC reaches a stable state where the index is around 5.7 Sv (year 350). When the NAMOC index was compared to the FW from GrIS time series, we observed that change in AMOC occurs before the FW starts to increase (from year 200). Our results thus suggest that the FW input from GrIS does not cause significant changes in the AMOC strength. It is necessary to further investigate other possible causes for the strong NAMOC decline in this model.</p>

Sign in / Sign up

Export Citation Format

Share Document