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Episodic and non-uniform migration of thermal habitats in a warming ocean

Alex Sen Gupta, Jaci Brown, Nicolas Jourdain, Erik van Sebille, Alexandre Ganachaud, and Adriana Vergés
In Deep Sea Research II, 2014, in press doi:10.1016/j.dsr2.2013.12.002

Abstract

Ocean temperatures have warmed in most regions over the last century and are expected to warm at a faster rate in the future. Consistent with the view that marine species are thermally constrained, there is growing evidence that many species have already undergone poleward range shifts inline with warming trends. This study uses historical observations of ocean temperature and climate model projections to examine the implied migration of isotherms that mark the boundaries for species thermal habitats. In particular, we compare the rates of isotherm migration between different ocean regions and at different time scales and examine to what extent the implied migration is uniform or sporadic. Widespread long-term warming implies poleward migration of isotherms in almost all regions. However, as the speed of isotherm migration is inversely related to local meridional SST gradients and the pattern of ocean warming is heterogeneous, migration speeds vary considerably between regions, season and over time. On decadal timescales, changes due to low frequency natural SST variability dominate over human induced changes. As such, there are multi-decadal periods in certain regions when we would expect to see range shifts that are much faster or in the opposite direction to that implied by anthropogenic warming. Based on climate model projections, median isotherm migration speeds will be about seven times faster in the 21st century compared to the 20th century under a business as usual emissions scenario. Moreover, SST warming is projected to be greater in summer than in winter in most regions, contrary to what happens over land. As such net isotherm migration speeds, particularly in the northern hemisphere summer, are projected to be much faster than winter migrations. Finally we show that isotherms may exhibit erratic migration rates over time, even under uniform warming. Isotherms tend to remain co-located with a thermal front for extended periods of time and then rapidly shift to a new position, marked by a more poleward thermal front. This implies that species ranges would also be expected to undergo sudden rapid shifts rather than exhibiting a gradual monotonic poleward march.