LETTER Causes and projections of abrupt climate-driven ecosystem shifts in the North Atlantic Gre ´ gory Beaugrand, 1 * Martin Edwards, 2 Keith Brander, 3 Christophe Luczak 1,4 and Frederic Ibanez 5 Abstract Warming of the global climate is now unequivocal and its impact on EarthÕ functional units has become more apparent. Here, we show that marine ecosystems are not equally sensitive to climate change and reveal a critical thermal boundary where a small increase in temperature triggers abrupt ecosystem shifts seen across multiple trophic levels. This large-scale boundary is located in regions where abrupt ecosystem shifts have been reported in the North Atlantic sector and thereby allows us to link these shifts by a global common phenomenon. We show that these changes alter the biodiversity and carrying capacity of ecosystems and may, combined with fishing, precipitate the reduction of some stocks of Atlantic cod already severely impacted by exploitation. These findings offer a way to anticipate major ecosystem changes and to propose adaptive strategies for marine exploited resources such as cod in order to minimize social and economic consequences. Keywords Abrupt ecosystem shift, critical thermal boundary, North Atlantic Ocean, plankton, the Atlantic cod, variance. Ecology Letters (2008) 11: 1157–1168 INTRODUCTION Warming of the global climate is now unambiguous and its impact on EarthÕs functional units has become more apparent (Intergovernmental Panel on Climate Change 2007). In recent years, evidence has grown that climate variation can impact the biodiversity, structure and func- tioning of marine ecosystems (Beaugrand et al. 2002; Drinkwater et al. 2003; Edwards & Richardson 2004; Ottersen et al. 2004). Many significant covariations between changes in climate and in the abundance of marine species, ranging from plankton to fish to seabirds, have been reported (Aebischer et al. 1990; Beaugrand & Reid 2003). Latitudinal or biogeographical shifts have been identified and interpreted as reflecting the response of the ecosystems to rising temperature (Beaugrand et al. 2002; Perry et al. 2005). Some works have suggested that climate may also modify the timing of important developmental and behavio- ural events of organisms (Edwards & Richardson 2004). Such phenological shifts have been detected for some planktonic groups in the North Sea (Edwards & Richardson 2004). Generally, biological changes are species-dependent (Beaugrand et al. 2002; Edwards & Richardson 2004), which can involve community reassembly in time and space (Parmesan & Matthews 2006). Community reassembly is thought to be among the most worrisome consequences of climate change on ecosystems (Parmesan & Matthews 2006) because this process may unbalance the trophodynamics of ecosystems, having the potential to involve trophic mis- match or to perturb prey–predator relationships (Beaugrand et al. 2003; Edwards & Richardson 2004). In some regions, climate variation has been at the origin of large-scale abrupt 1 Centre National de la Recherche Scientifique, Laboratoire dÕOce ´ anologie et de Ge ´ osciencesÕ, UMR LOG CNRS 8187, Station Marine, Universite ´ des Sciences et Technologies de Lille – Lille 1 BP 80, 62930 Wimereux, France 2 Sir Alister Hardy Foundation for Ocean Science, Citadel Hill The Hoe, Plymouth PL1 2PB, UK 3 DTU Aqua, Charlottenlund Slot, 2920 Charlottenlund, Denmark 4 Universite ´ d’Artois, IUFM Nord-Pas-de-Calais, Centre de Grav- elines, 40, rue Victor Hugo, BP 129, 59820 Gravelines, France 5 Laboratoire dÕOceanographie de Villefranche (LOV), BP 28, 06234 Villefranche-sur-Mer Cedex, France *Correspondence: E-mail: gregory.beaugrand@univ-lille1.fr Ecology Letters, (2008) 11: 1157–1168 doi: 10.1111/j.1461-0248.2008.01218.x Ó 2008 Blackwell Publishing Ltd/CNRS