Ice-free period too long for Southern and Western Hudson Bay polar bear populations if global warming exceeds 1.6 to 2.6 °C

The Hudson Bay polar bear population, reliant on sea ice for hunting seals, faces significant challenges due to climate change. Over the past 30 years, Hudson Bay has warmed by over 1°C, causing earlier spring ice melt and later fall freeze-up. This extended ice-free period (IFP) forces bears onto land for longer periods, leading to prolonged fasting and decreased body condition. This impacts their reproductive success and overall survival. The study uses updated projections from CMIP6 to assess the future IFP and its impact on polar bear survival, focusing specifically on Western Hudson Bay (WHB) and Southern Hudson Bay (SHB), areas with some of the longest ice-free periods already observed. The research's significance stems from the urgent need to understand the future prospects for polar bears in this crucial habitat, and the impact of varying degrees of global warming on their survival and the associated implications for conservation efforts.

Previous research has linked the lengthening IFP in Hudson Bay to declining polar bear populations, particularly in WHB. Studies have shown a decline in the WHB population, primarily attributed to earlier sea ice breakup, impacting the availability of ringed seals—the primary prey for polar bears. Earlier studies have highlighted that bears fast for about 4 months (120 days) historically but the rapid warming of Hudson Bay and extension of the IFP has led to a decline in the population, particularly in WHB, which is one of the most extensively studied polar bear populations and home to the town of Churchill, Manitoba. The decline accelerated in recent years, with a 27% decrease between 2016 and 2021, compared to an 11% decline between 2011 and 2016, attributed largely to fewer adult females, young bears, and cubs. The total population is estimated to be around half of its 1987 numbers, although differences in estimation methods make direct comparisons challenging. The impact on adult male bears has been studied, linking prolonged fasting (exceeding 200 days) to mortality risk.

This study utilizes data from several sources: passive microwave satellite data (Bootstrap and NASA Team algorithms) to estimate IFP based on sea ice concentration (SIC); Berkeley Earth Surface Temperatures (BEST) for historical temperature data; and CMIP6 model outputs for future IFP projections. Two definitions of IFP are used for CMIP6 models: 10% SIC and 10 cm sea ice thickness. The 10 cm thickness threshold was chosen because it is the minimum thickness needed to support successful seal hunting for polar bears. They also utilized onshore/offshore dates for polar bears from satellite-linked radio tags in order to calibrate the IFP to the polar bear fasting period. For future projections, two methods were employed: (1) a delta-shift bias adjustment applied to 20 CMIP6 models, and (2) a weighted average of 49 simulations from the same models, accounting for model performance and independence. The bias adjustment corrects for discrepancies between modeled and observed IFPs. The weighting scheme gives more weight to models with shorter IFPs and stronger sensitivity to warming. Snow depth data from SnowModel-LG (MERRA2 reanalysis) was also used to assess the impact of changes in snow cover on ringed seal pup survival and polar bear denning. The study focuses on WHB and SHB, analyzing average IFP, trends, and sensitivity to global annual mean temperature. The analysis incorporated spatial gradients in IFP, considering differences in ice-free periods across Hudson Bay.

The study's key findings highlight a concerning trend: the IFP in Hudson Bay is projected to significantly lengthen with increasing global warming. Using both bias-adjusted and weighted averages from CMIP6 models, the analysis shows that at 1.5°C of global warming, the IFP increases by approximately two weeks in both SHB and WHB, reaching roughly 150-170 days. At 2°C of warming, the IFP increases further, reaching approximately 165-180 days, with retreat happening in late June/early July. Under a high emission scenario (SSP5-8.5), exceeding 4°C of global warming by the end of the century is possible. At this level, the IFP could reach approximately 255-280 days in SHB and 220-240 days in WHB, far exceeding the thresholds for adult polar bear survival. The models also showed a spatial gradient in IFP, with the longest ice-free periods in the southeast and the shortest in the west, a gradient that becomes more pronounced with increasing warming. The analysis of snow depth using CMIP6 models showed a modest decline in snow depth for WHB (about 2 cm °C⁻¹) and a stronger decline in SHB (3 to 4 cm °C⁻¹), potentially further impacting ringed seal pup survival. The current IFP already surpasses levels that negatively impact cub recruitment, suggesting that long-term sustainability is at risk. Adult male polar bear survival impact thresholds were estimated to range from 183 to 218 ice-free days. Therefore, for adult male survival, the global warming limit is estimated to be around 2.1°C (1.6°C) for SHB and 2.6°C (2.2°C) for WHB. Without bias correction, the outlook is even more pessimistic.

The findings directly address the research question of the impact of global warming on Hudson Bay polar bear populations, revealing the severe consequences of exceeding certain warming thresholds. The extended IFP resulting from warming directly affects polar bear foraging behavior, body condition, and reproductive success, ultimately threatening the long-term viability of the populations. The spatial gradient in IFP highlights the vulnerability of specific areas within Hudson Bay and the potential for more pronounced localized impacts. The projected decrease in snow depth adds another layer of complexity, affecting ringed seal pup survival and polar bear denning success. The study's implications are significant for polar bear conservation and inform climate change mitigation strategies. The results underscore the urgency for stricter emission controls, aiming to limit global warming to well below the 2°C threshold to enhance the chances of polar bear survival in Hudson Bay. The results, however, suggest that exceeding 1.5°C may be detrimental to cub recruitment and population viability. The study's findings reinforce the importance of incorporating multiple sources of data and models to produce the most realistic assessment of the impact of climate change.

This study projects a critical decline in Hudson Bay polar bear populations if global warming exceeds 1.6-2.6°C, with potentially devastating effects on both adult survival and cub recruitment. Limiting global warming to 2°C may offer some hope for adult survival, but the negative impacts on reproduction are already apparent. Future research should focus on finer-scale modeling to improve the accuracy of projections in specific Hudson Bay areas and investigate adaptive management strategies for polar bear populations in a rapidly changing climate, such as exploring the potential for supplementary feeding during prolonged ice-free periods.

The study's limitations include reliance on CMIP6 model projections, which have inherent uncertainties. The spatial resolution of some models may not capture localized variations in sea ice conditions fully. The study acknowledges the difficulty in quantifying the relationship between average snow depth and the presence of sufficiently deep snow for seal birth lairs at a finer scale. The use of a single sea ice thickness threshold for defining the IFP might not capture the full complexity of polar bear habitat preferences. Direct comparisons of population numbers across time are limited by variations in survey methods. Future improvements to the data will allow for more accurate predictions and refined analysis.