Global warming due to loss of large ice masses and Arctic summer sea ice

The cryosphere, encompassing large-scale ice masses such as Arctic summer sea ice, mountain glaciers, and the Greenland and West Antarctic Ice Sheets, has undergone substantial changes due to anthropogenic global warming. This research investigates the consequences of their potential future disintegration on global mean temperature (GMT) and climate feedbacks. The study's importance stems from the lack of comprehensive evaluation of these impacts, highlighting the need for quantifying the additional global warming resulting from cryosphere loss. This is crucial for accurate climate projections and informing mitigation strategies. Understanding the various feedback mechanisms involved (albedo, lapse rate, water vapor, and cloud feedbacks) is also key to accurately predicting future climate scenarios. The research employs a computationally efficient Earth system model of intermediate complexity to systematically analyze the decay of these cryosphere components and their individual contributions to global warming.

Existing literature highlights the significant observed changes in the cryosphere during recent decades. The Arctic summer sea ice area has dramatically declined, with projections suggesting a potential ice-free Arctic summer within the 21st century, even under moderate emission scenarios. Mountain glaciers worldwide have retreated significantly, resulting in substantial ice mass loss. Both the West Antarctic and Greenland Ice Sheets have experienced accelerating mass loss, potentially exceeding critical thresholds leading to irreversible consequences. These changes, driven by anthropogenic climate change, have resulted in a global mean temperature rise, raising concerns about the potential for dramatic changes and irreversibility in these cryosphere elements at potentially lower temperatures than previously anticipated. However, the comprehensive quantification of the climate feedbacks associated with these changes has been lacking. This research addresses this gap by assessing the additional global warming caused by the disintegration of these key cryosphere components.

The study utilizes the Earth system model of intermediate complexity, CLIMBER-2, due to its computational efficiency and capacity for systematic analysis. CLIMBER-2 incorporates atmosphere, ocean, sea ice, vegetation, and land-ice model components. The research employs large ensembles of equilibrium model simulations constrained by fast climate feedback strengths from global circulation models (GCMs). To mimic the complex behavior of GCMs, the CLIMBER-2 model parameters influencing feedback strength, particularly in the troposphere, clouds, and snow albedo, were altered. This calibration process resulted in an equilibrium climate sensitivity of 2.0–3.75 °C for the ensemble, encompassing the uncertainty range of the fast climate feedbacks in GCMs. The states of the Greenland Ice Sheet, West Antarctic Ice Sheet, and mountain glaciers were prescribed in the model, affecting both ice cover and topography. For Arctic summer sea ice, the albedo during summer months was lowered to average open ocean values, while maintaining dynamic ice-covered area computation to ensure energy and water conservation. The study compares long-term GMT changes in idealized scenarios where the cryosphere elements are removed to scenarios where they remain intact. The additional warming was then deconvolved into contributions from albedo, lapse rate, water vapor, and clouds in terms of net radiation perturbations at the top of the atmosphere, focusing purely on radiative effects and neglecting freshwater contributions for equilibrium responses.

The study finds that the decay of Earth's cryosphere amplifies global warming. The combined disintegration of Arctic summer sea ice, mountain glaciers, and the polar ice sheets causes an additional GMT increase of 0.43 °C (0.39–0.46 °C) at a baseline warming of 1.5 °C above pre-industrial levels (a 29% increase). Regional warming is significantly amplified around the cryosphere components, reaching up to 5 °C locally, particularly around Greenland and West Antarctica, with considerable warming also in lower latitudes. The warming magnitudes and polar amplification are consistent with past warm periods. The additional warming from Arctic summer sea ice alone is 0.19 °C (0.16–0.21 °C), while Greenland Ice Sheet, mountain glaciers, and West Antarctic Ice Sheet contribute 0.13 °C, 0.08 °C, and 0.05 °C, respectively. The effects of individual elements on GMT are approximately linearly additive. Analysis of radiative perturbations at the top of the atmosphere shows albedo changes are the dominant contributor (55%), followed by lapse rate/water vapor feedback (30%), and cloud feedback (15%). The additional warming from Arctic summer sea ice shows a decreasing trend with increasing CO2 concentrations, explained by the linear decline of Arctic sea ice area with increasing GMT. The latitudinal distribution of additional radiative perturbation highlights a higher contribution from albedo and lapse rate/water vapor in polar regions, contributing to polar amplification.

The findings address the research question by quantifying the additional global warming commitment due to cryosphere disintegration. The significance lies in revealing the substantial contribution of cryosphere loss to future warming, exceeding what is typically captured in CMIP projections, which often focus on shorter timescales. The results highlight the importance of incorporating these long-term feedback mechanisms into climate change projections. The linear additivity of the warming effects of individual cryosphere components simplifies the assessment of combined effects. The dominant role of albedo changes underscores the importance of considering ice-albedo feedback in climate models, while the significant contributions of lapse rate/water vapor and cloud feedbacks highlight the complexity of climate interactions. The study's findings emphasize the considerable risk associated with unabated climate change, particularly regarding the irreversible consequences of ice sheet disintegration.

This study demonstrates the significant contribution of cryosphere disintegration to future global warming, emphasizing the need for comprehensive consideration of these long-term feedback mechanisms in climate projections and policy decisions. Future research could focus on refining the model representation of cryosphere-climate interactions, exploring transient warming responses, and investigating the potential interactions between different cryosphere elements, such as freshwater input into the ocean circulation. Further investigation into regional impacts and the potential implications for extreme weather events is also warranted.

The study utilizes an Earth system model of intermediate complexity, which inherently involves simplifications compared to fully coupled GCMs. The focus on radiative effects neglects freshwater contributions to feedbacks and warming, which could potentially alter the quantitative results. The prescribed removal of ice masses represents a simplified representation of a complex process. Furthermore, the study primarily focuses on equilibrium responses, and transient responses may differ. Finally, isostatic rebound was not considered.