w ww ww w. .f fr ro on nt ti ie er rs si in ne ec co ol lo og gy y. .o or rg g © © The Ecological Society of America E cologists and environmental scientists are constantly faced with the challenge of working at the most appropriate temporal and spatial scales to reach defendable conclusions. Few recent questions have elicited more discussion and debate than those concerning the nature, magnitude, and effects of global climate change. Although there is no longer any serious scientific debate about the influence of greenhouse-gas emissions on climate, many questions remain concerning the relative importance of natural versus anthropogenic causes of climate change, as well as the spatial and temporal scales of climate-related effects on ecosystems. Each of these questions has a temporal component, yet long-term monitoring data, which will help us to find the answers, are often scarce or non-existent. Fortunately, a variety of natural archives preserve proxy data of past environmental and ecological changes, some examples of which are highlighted in this issue of Frontiers. Here, we focus on paleolimnology -the multidisciplinary science that uses the physical, chemical, and biological information preserved in lake and river sediments to reconstruct past environmental conditions (Cohen 2003; Smol in press). We also summarize the development of ideas (and associated challenges) following an earlier paper (Douglas et al. 1994), which proposed that a worrisome record of recent climate warming is preserved in Arctic sediments.The lack of long-term instrumental data in polar regions is especially disconcerting, because Arctic ecosystems, due in part to a variety of positive-feedback mechanisms, are warming at a faster rate than are other parts of the planet (ACIA 2004). In fact, based on the brief temporal windows of available instrumental meteorological data, average temperatures in the Arctic have risen at twice the rate of the rest of the world over the past few decades (ACIA 2004). Not surprisingly, polar regions are therefore often referred to as the "miners' canary" of the planet. Using a variety We live in a constantly changing environment, yet tracking ecological change is often very difficult. Long-term monitoring data are frequently lacking and are especially sparse from Arctic ecosystems, where logistical difficulties limit most monitoring programs. Fortunately, lake and pond sediments contain important archives of past limnological communities that can be used to reconstruct environmental change. Here, we summarize some of the paleolimnological studies that have documented recent climate warming in Arctic lakes and ponds. Several hypotheses have been evaluated to determine if warming, resulting in changes in ice cover and related variables (eg increased habitat availability), was the factor most strongly influencing recent diatom and other biotic changes. Striking and often unprecedented community changes were evident in post-1850 sediments, and could be linked to ecological shifts consistent with warming. Be...