Antarctic meteorites threatened by climate warming

14 Apr 2024

Nature

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Antarctic meteorites threatened by climate warming

Most of the meteorites on the Earth’s surface are found in Antarctica. Here the authors show that ~5,000 meteorites become inaccessible per year as they melt into the ice due to climate change.

Meteorites are unique samples of extraterrestrial bodies and provide crucial information on the origin and evolution of our Solar System1,2. Antarctica is the world’s most prolific site for collecting meteorites, with more than 60% of all ~80,000 meteorites ever found on Earth being collected at the surface of the ice sheet. Antarctic meteorites are found in blue ice areas, which are atypical zones (~1% of the Antarctic surface area) where layers of snow and ice are removed from the surface through a combination of ice flow processes and local meteorological conditions, exposing meteorites that were once embedded in the ice1,3. Not all blue ice areas contain meteorites: only where processes interact favourably, a concentration of meteorites is built up over tens to hundreds of thousands of years, resulting in so-called meteorite stranding zones (Fig. 1a)4,5,6. Meteorites found in Antarctica are a few centimetres in diameter on average, but are easily detectable given their visual contrast with the underlying ice7,8. Over past decades, an average of ~1,000 meteorites per year have been collected through numerous field campaigns (Fig. 2a) and the potential of Antarctic meteorites remains far from exhausted: a data-driven approach9 recently identified over 600 meteorite-rich areas in Antarctica. Many of the identified meteorite stranding zones are not yet (fully) explored, and an estimated 300,000 to 850,000 meteorites remain to be collected from the surface of the ice sheet (Fig. 2b)9.

Once exposed at the surface, meteorites can stay there for thousands of years due to stagnant ice flow and the lack of weathering in the cold, dry conditions5,6. While most of the indicators for the presence of meteorites—for example, ice flow velocity, elevation, mountains—are thought to be stable on multidecadal to centennial timescales, the concentration of meteorites is also directly influenced by temperature4,9,10. Even when temperatures are well below zero, meteorites, with their characteristic dark crust, warm when exposed to solar radiation11 and can melt the underlying ice. The warmed meteorite generates a small water melt pocket below the stone, resulting in a surface depression that deepens over time into a hole, which (in conjunction with refreezing meltwater) results in the disappearance (‘sinking’) of the meteorite from the surface (Fig. 1)10,12,13. The sensitivity of meteorite presence to temperature is apparent from various independent lines of evidence:

(1)
Field observations: entrapped meteorites have been found covered by superimposed (refrozen) ice after the meteorite sank into the ice (Fig. 1)14,15,16.
(2)
Data on meteorite retrieval locations9,17 indicate that almost no meteorites (18.
(3)
Empirical and experimental studies of meteorite heating show that the sinking of meteorites occurs when the downward meteorite motion caused by melting into the underlying ice exceeds the local ablation rate10,12.
(4)
Thermodynamical modelling suggests that meteorites can sink into the ice with air temperatures above −10 °C (ref. 4).
(5)
Data-driven meteorite-site classifications9 indicate that near-maximum surface temperatures are an important predictor for the presence of meteorites.

Hence, the temperature susceptibility of meteorites at the surface of the ice could cause meteorite stranding zones to disappear under changing climatic conditions4,9.

To quantify the loss of Antarctic meteorites, we used a machine learning algorithm that predicts the presence of meteorites9 forced with dedicated regional climate model simulations (Methods). The machine learning algorithm captures interactions between different predictors of meteorite presence by estimating a multidimensional density distribution of observations of these different predictors (for example, ice flow velocity, surface temperature; Methods). Forcing the algorithm with future surface temperatures not only eliminates places that will become too warm for meteorites to be found in future climate conditions, but also considers interactions with other processes (for example, the ice flow velocity). While examining these interactions, the algorithm identifies locations for which future conditions become substantially different from current conditions at places where meteorites were recovered.

We found that in the coming decades, independent of the emissions scenario used (SSP1-2.6 or SSP5-8.5; Supplementary Fig. 1), ~5,000 meteorites yr−1 disappear from the surface of the Antarctic ice sheet in response to present warming conditions (Fig. 2a and Supplementary Fig. 1). This rate outpaces the rate at which Antarctic meteorites are found by about a factor of five (Fig. 2a). The estimated meteorite losses under the low- and high-emissions scenarios only start to deviate in the second half of the century (that is, after 2050). We estimated meteorite losses independent of these emissions scenarios by comparing the losses directly with the wide range of potential temperature increases captured under SSP5-8.5. The Antarctic continent-wide meteorite losses are strongly correlated with the increase in global air temperature: 5,100 to 12,200 meteorites (~1–2% of all current meteorites) are lost from the surface of the ice sheet for every tenth of a degree in temperature increase (r = −0.946 to −0.968 (Supplementary Fig. 2), the uncertainty range stems from the range in precision and sensitivity estimates of the machine learning algorithm; Methods). This fragile state can also be related to climate policy targets. If global warming is limited to 1.5 to 2.0 °C compared with global pre-industrial levels (Supplementary Fig. 3), the loss of meteorites can be constrained to between 9 and 20% compared with 2020. However, under current policies (that result in an estimated global warming of approximately 2.6 to 2.7 °C)19,20, 28–30% of the meteorites become unrecoverable. This share increases to 35% under scenarios with 3 °C of warming and 55% under 4 °C of warming. Under the high-emissions scenario (SSP5-8.5), 76% of the meteorites are lost by the end of this century and only 150 meteorite stranding zones (Methods) with an area of 3,180 km2 would remain, representing a decrease of 76% in the number of zones and of 78% in their areal extent (Fig. 2c–e).

The projected meteorite losses are not uniform across the continent. For some of the known dense meteorite collection areas17, we project that up to 50% of the total number of meteorites could be lost from the ice surface before 2050 (Supplementary Table 1). One example of these sensitive areas is the Grove Mountains in East Antarctica, a prime meteorite collection site where already more than 12,000 meteorites have been recovered21. In the promising, yet largely unexplored, Enderby Land region in East Antarctica (Fig. 2c), similar losses are projected, with 50% of meteorites disappearing before 2054. Data show that at present, the largest concentrations of meteorites are found at elevations between 1,800 and 2,000 m (refs. 17,22), where an 88% reduction is forecasted in the number of retrievable meteorites by the end of the century under the high-emissions scenario (Supplementary Fig. 4). Only at elevations above 2,500 m will the meteorite losses be lower than 50% (Supplementary Fig. 4). Hence, to preserve the unique information contained in Antarctic meteorites, ongoing meteorite losses not only call for fast action, but also a (global) coordination to secure the most vulnerable samples in areas that are particularly exposed to meteorite loss (for example, low-elevation meteorite stranding zones such as the Hutchison Icefield). At present, decisions on which areas to visit are largely made according to the availability of logistical support and national government science priorities7. In the field, meteorites are often found by human visual identification during grid searches, conducted either on foot or by snow mobile8. To increase retrieval rates of such labour-intensive operations, we suggest a major international effort to revisit known sites or access unexplored sites with larger searching teams over the next 10–15 years. Leveraging recent developments in robotics (for example, unoccupied aerial vehicle observations23 in harsh environments), as well as high-resolution modelling, could increase the efficiency and the extent of recovery operations, although the development of robust, scalable methods are very challenging in the extreme Antarctic4,7,24. Moreover, these techniques might allow the detection of some samples under ice or transient snow cover. Snow cover can be expected to be more prevalent in a warming climate25,26,27 and results in even more meteorites becoming unrecoverable, but this process was not considered here when estimating meteorite losses (Supplementary Section 2).

The ongoing loss of Antarctic meteorites is a consequence of climate change. Despite the delayed response of the interior of the Antarctic ice sheet to climate change in terms of ice melt (with temperatures remaining well below zero, even with several degrees of warming), meteorites are affected even by very minor (decimal) increases in surface temperatures during exceptionally warm events, which are expected to occur more frequently in the future28. Rapidly and purposefully collecting all meteorites is necessary to preserve the information on our Solar System that each additional sample contains: for example, information on the emergence of life on Earth through the delivery of water and organic matter, and how the Moon was formed2,29. A concerted effort would be similar in spirit to what is currently done in ice core research, where ice samples collected from vanishing, yet unique, glaciers—such as the few remaining tropical glaciers—are stored in long-term archives30. Ultimately, however, the only way to preserve the remaining unrecovered Antarctic meteorites is to rapidly reduce greenhouse gas emissions.