Fifty-five paleolimnological records from lakes in the circumpolar Arctic reveal widespread species changes and ecological reorganizations in algae and invertebrate communities since approximately anno Domini 1850. The remoteness of these sites, coupled with the ecological characteristics of taxa involved, indicate that changes are primarily driven by climate warming through lengthening of the summer growing season and related limnological changes. The widespread distribution and similar character of these changes indicate that the opportunity to study arctic ecosystems unaffected by human influences may have disappeared.climate change ͉ paleolimnology ͉ Anthropocene ͉ warming ͉ indicators P olar amplification of anthropogenic warming is consistently predicted by general circulation models, largely because of positive feedback mechanisms involving cryospheric processes (1). This heightened climatic sensitivity is supported by recent accelerations of glacier retreat (2), sea-ice thinning (3), and permafrost degradation (4). Although the instrumental record of temperature across the Arctic is incomplete and generally of short duration, warming appears to be concentrated in the decades between approximately anno Domini 1915-1940 and approximately anno Domini 1965. However, proxy data indicate that much of the Arctic began to warm considerably earlier, in the mid-19th century (6). Such generalized trends, however, are neither spatially nor temporally uniform (7), because of regional differences in continentality, ocean heat transport, glacier and sea ice distribution, topography, and vegetation. For example, whereas much of Beringia and central Siberia have warmed Ͼ0.5°C in the last 50 years, there has been little change or even cooling (7) in parts of the North Atlantic sector. Nonetheless, all subregions of the Arctic are considered highly ecologically sensitive, implying that anthropogenic warming will test ecosystem resilience and potentially induce dramatic shifts in community composition.In the absence of long-term climatic and environmental monitoring data, proxy data from the sediments of lakes and ponds, which are ubiquitous features of most arctic landscapes, can be used to provide a long-term perspective of environmental change (8, 9). Siliceous algal remains, specifically the valves of diatoms (Bacillariophyceae) and the stomatocysts and scales of chrysophytes (Chrysophyceae and Synurophyceae), as well as chitinous invertebrate remains (Chironomidae, Diptera and Cladocera, Crustacea), are the primary paleoindicators in lake sediments that provide reliable records of changes in water quality, habitat, and catchment processes (10). Here, we synthesize a large number of paleolimnological records from arctic lakes and ponds, providing a circumpolar assessment of recent ecological changes. These data show that striking and often unprecedented ecological changes have occurred within the last Ϸ150 years, following several millennia of relatively stable communities.High-latitude lakes are extremely responsive t...

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