740 J. R. Ferrer‐Paris et al.
cross-validation.We compared the predicted probability or suitability index with the known occurrences in the test par- tition and in the Cordillera deMérida to calculate cut-off va- lues of equal sensitivity and specificity (balancing errors of omission and commission) and maximum accuracy (min- imizing misclassification errors). We then used the model to predict future suitability under 15 combinations of global circulation models and socio-economic pathways, assuming the alternative cut-off values represented thresholds of eco- system collapse (sub-criterion C2a). The quantitative analysis of collapse for criterion E was
based on a global hybrid model (Rounce et al., 2023) that combined a mass balance module and a glacier dynamics module to model all glaciers in the world independently using globally available datasets of glacier outlines (Randolph Glacier Inventory Consortium, 2017), glacier- wide geodetic mass balance data and regional ice volume es- timates for calibration (Farinotti et al., 2019;Hugonnet et al., 2021). In the case of small regions with no direct measure- ments (such as the Cordillera de Mérida) the model used initial estimates of ice volume based on digital elevation models and most likely overestimated initial mass, making the projections conservative (Rounce et al., 2023). The model was run with monthly and annual time steps from 2000 to 2100 for various ensembles of global circula- tion models and socio-economic pathways.We used the ice mass projections of the model for the glacier outlines of the Cordillera de Mérida (Rounce et al., 2022) to estimate the year of collapse (first year when ice mass reaches zero) for 48 combinations of global circulation models and socio- economic pathways. We used the empirical cumulative dis- tribution function of the year of collapse to calculate the proportion of models indicating collapse for each year and estimate probability of collapse. Weperformed all statistical analysis using R 4.3.1 (R Core
Team, 2023). For the suitability model we used functions in the R package caret (Kuhn, 2008).
Results
There are no direct field measurements of decline in extent of glacier ice for the last 50 years, but cartographic estimates of glacier extent suggest a decline of at least 89%over
21 years (proportional rate of decline = 5.173 ± SE 0.130%/ year) and nearly 99% if we consider the last 67 years (pro- portional rate of decline = 10.220 ± SE 0.473%/year). These rates of decline would translate to future declines of at least 94% by the year 2048 (Table 1). Given the rapid decline and disappearance of the glacier
at Bolívar Peak during 1998–2017 and that the size of the Humboldt glacier at the time of the last measurement (0.045 km2 in 2019) was comparable to that of the Bolívar glacier in 1998, it is reasonable to expect that the Humboldt glacier will disappear completely over the next 20 years. The area of glacier ice in Humboldt Peak was last measured at 0.046 ± 0.004 km2 in July 2019 (Ramírez et al., 2020) and was estimated to be c. 0.01 km2 in August 2022 (Plate 1; N. Ramírez, pers. comm., 2023). This last field observation would confirm the recent acceleration of the rate of decline of the Humboldt glacier and increases the likelihood that it will disappear sooner than predicted, prob- ably within the next 5 years. Reconstruction of the historical evolution of glaciers in
South America suggests that the most recent maximum gla- cial extent in Venezuela occurred c. 1730 (during the Little Ice Age), and glaciers retreated continuously in the follow- ing centuries, with only minor readvances c. 1760, 1820 and 1880 (Polissar et al., 2006; Jomelli et al., 2009). Thus, we can assume that the observed decline between 1910 and the pre- sent represents a lower bound of the total decline between 1750 and the present (Table 1). The extent of occurrence for the Humboldt glacier and surrounding areas is 0.892 km2. If the proglacial waters and glacier forefields of recently collapsed glaciers are in- cluded, the extent of occurrence is at most 5.957 km2. The main occurrences of known glaciers (extant and recent- ly collapsed) occupy a single 10 × 10 km cell (Fig. 1, Plate 2). This cell probably contains all occurrences of associated habitats with connected microbiota (proglacial waters and glacier forefields). The relationship between recent temporal changes in cli-
matic conditions and the disappearance of glacier ice is best illustrated with the case of La Concha Peak. The estimated maximum elevation of ice was 4,840min the year 1952, and it disappeared before 1998. The mean freezing level height was almost 10 m below the maximum elevation in 1948
TABLE 1 Estimated magnitude of past and future tropical glacier declines in the Cordillera de Mérida, Venezuela (Fig. 1), based on previous measurements (Ramírez et al., 2020) and estimated proportional rates of decline.
Start date 1910
1952 1998 1998 1998
End date 2019
2019 2019 2048 2048
Timeframe 109 years
67 years 21 years 50 years 50 years
Method
Reconstructed map + field observations Aerial photographs + field observations Aerial photographs + field observations
Projected using proportional rate of decline = 5.71 ± 0.130% Projected using proportional rate of decline = 10.20 ± 0.473%
Decline ± SE (%) 99.10 ± 0.080
98.06 ± 0.180 89.61 ± 1.149 94.72 ± 0.611 99.54 ± 0.127
Oryx, 2024, 58(6), 735–745 © The Author(s), 2024. Published by Cambridge University Press on behalf of Fauna & Flora International doi:10.1017/S0030605323001771
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