Smaller but not more frequent: assessing decadal trends in jökulhlaup cycle

Subaerial lakes dammed by glacier ice have a long history of scientific and public interest, as they can drain catastrophically in outburst floods (jökulhlaups) and refill repeatedly. The hazard associated with this jökulhlaup cycle may persist for decades to centuries through a cascade of processes that includes ice-dam flotation, enlargement of a subglacial drainage tunnel, and its partial or complete resealing after the flood. Theory suggests that both the height and climate-driven decay of the ice dam fundamentally control the frequency and magnitude of jökulhlaups during this cycle: thinning and retreat of the dam may lower pre-outburst lake levels and flood volumes, while reduced lake accommodation space could allow drainage events to occur more frequently, potentially several times within a single year.

Given that glacier dams have been losing elevation at accelerating rates over recent decades, I hypothesize that the jökulhlaup cycle may intensify, leading to a faster-than-linear decrease in outburst flood magnitude accompanied by an increase in jökulhlaup frequency. Testing this hypothesis has so far been constrained by sampling biases and gaps in historical flood records. Here, I present results from manual mapping of pre-outburst lake areas for at least 71 ice-dammed lakes that drained at least ten times between 1984 and 2025 in northwestern North America, Patagonia, Iceland, Scandinavia, the European Alps, and High Mountain Asia. Using optical satellite imagery from Landsat, Sentinel-2, and Planet, I systematically reconstruct jökulhlaup time series to assess decadal trends in pre-outburst lake area and drainage frequency. I fit linear and quadratic models with potential change points to test for gradual or abrupt acceleration in the jökulhlaup cycle.

Preliminary results indicate that >75% of lakes in the sample currently exhibit declining pre-jökulhlaup areas, with 15% showing evidence for an accelerated decrease in outburst magnitude. In approximately one quarter of all cases, the models favour breakpoint behaviour, with increasing pre-jökulhlaup lake areas prior to a subsequent decline. Notably, 22% of lakes have not yet reached this peak and instead show ongoing increases in pre-outburst surface area. Most of these lakes formed during the study period and actively erode their ice dams, shifting the lake toward the dam and enlarging their own basin area. This process enables lakes to buffer even high local rates of dam thinning and helps identify locations where deglaciation may favour increasing hazard, contrary to the global trend. Furthermore, only about one third of lakes show a trend toward more frequent outbursts. For most lakes, drainage frequency remains unchanged—most commonly at one event per year—consistent with the idea that subglacial tunnel systems close slowly after drainage, inhibiting multiple outbursts within a single year. Overall, the prevalence of declining outburst magnitudes occurring at largely unchanged frequencies suggests that, at the global scale, the hazard posed by ice-dammed lakes has been declining over the past four decades.