Abrupt and non-uniform movement of thermal habitat boundaries in a warming world

Sen Gupta, Alex (1) ,van Sebille, Erik (2) , Brown, Jaclyn .N (3), Jourdain, Nicolas .C (4), Ganachaud, Alexandre (5), Vergés, Adriana (6)

1. Climate Change Research Centre and ARC Centre of Excellence for Climate Systems Science, University of New South Wales, Sydney, Australia, University of New South Wales, Sydney, Australia, sengupta@unsw.edu.au
2. Grantham Institue, Imperial College London,UK, e.van-sebille@imperial.ac.uk
3. Centre for Australian Weather and Climate Research, CSIRO Wealth from Oceans National Research Flagship, Hobart, Tasmania, Australia, Brown@csiro.au
4. Centre National de la Recherche Scientifique , Laboratoire de Glaciologie et Géophysique de l’Environnement, France, Jourdain@lgge.obs.ujf-grenoble.fr
5. Institut de Recherche pour le Développement, Laboratoire d’Etudes en Géophysique et Océanographie Spatiales, UMR5566, UPS (OMP-PCA), 14 ave E. Belin, 31400 Toulouse, France, ganachaud@ird.fr
6. School of Biological and Earth Sciences, University of New South Wales, Sydney, Australia, University of New South Wales, Sydney, Australia, verges@unsw.edu.au

Ocean temperatures have warmed almost everywhere over the last century and will warm at an increasing rate in the future. However, rates of warming are spatially non-uniform and vary dramatically on inter-decadal timescales. Consistent with the view that marine species are thermally constrained, there is increasing evidence that many species have already undergone poleward range shifts. Using historical observations and state-of-the-art climate model projections of ocean temperature we examine the implied movement of isotherms that mark the boundaries for species’ thermal habitats in different regions and on different timescales. We demonstrate that even with monotonic globally-averaged warming, thermal habitats in many regions can move equatorward on multi-decadal timescales. Climate model projections suggest that median isotherm migration speeds would be about seven times faster in the 21^st century compared to the 20^th century under business as usual emissions growth. However these migrations are highly non-linear in time. As migration speeds are modulated by background ocean temperature gradients, thermal habitat boundaries can exhibit abrupt relocation rather than exhibiting a gradual monotonic poleward march. In particular, subject to progressive warming, isotherms tend to remain co-located with a thermal front for extended periods of time and then abruptly shift to a new position, marked by a more poleward thermal front. We also demonstrate a strong asymmetry in the seasonality of future species migration, with summer migration generally faster than winter migrations. This suggests a general contraction in the area of thermal habitats in the future.