Climate trajectories for 1960-2009: north Northern Hemisphere ocean

The file contains maps of climate trajectories derived from the velocity of climate change for 1960-2009 for the northern part of the Northern Hemisphere ocean.

Methods follow:

Velocity-derived trajectories to indicate global regions susceptible to effects of geographical limits to climate-driven distribution shifts. Climate trajectories are paths that points on an isotherm will travel over specific periods, integrating spatially variable speeds and directions of climate velocity along the way to show effects that static velocity maps cannot. Geographical limits to trajectories, either barriers such as coasts and mountains or lack of connections to cooler or warmer environments, suggest limits to climatic niche shifts and, by inference, local species persistence or replacement from warmer environments. Velocity fields were derived for 1960-2009 for land and ocean surface temperatures7 on a 1º grid, allowing inference at a global scale, but sacrificing small-scale detail.

We categorized types of trajectory behaviour using trajectory length and the percentages of trajectories starting in, ending in and passing through cells. Short trajectories indicated non- or slow-moving thermal niches. Cells were classed as relative climate sinks if a high proportion of trajectories terminated there. Absolute climate sinks were also distinguished: coastal climate sinks where trajectories were blocked by coasts, and internal climate sinks where velocities in neighbouring cells converged. Cells were classed as climate sources if no trajectories ended there. Thereafter, cells with a high proportion of trajectories passing through were classed as corridors. Divergence cells were identified as those where fewer trajectories ended than started in that cell, and convergence cells if the opposite were true.

Uncertainty evaluated by bootstrap resampling of annual average temperature maps gave a likely (>66% consistency) designation of types for 59% of ocean and 72% of land cells. Consistency was <66% where spatial gradients and temperature trends are most uncertain, such as where inter- and multidecadal climate variability dominates as for the El Niño-Southern Oscillation in the tropical Pacific. Very likely classification (>90%) was achieved for 40% of land and 26% of ocean cells, mainly sources, coastal sinks and low velocity areas .