The phrase "poleward shift" refers to a movement or shift of something, like a pattern or phenomenon, towards the Earth's poles. It means that something is moving closer to the North or South Pole. Full definition
A wide range of other observations (such as reduced Arctic sea ice extent and increased ocean heat content) and indications from the natural world (such as poleward shifts of temperature - sensitive species of fish, mammals, insects, etc.) together provide incontrovertible evidence of planetary - scale warming. (royalsociety.org)
Using mechanistic species distribution models, we find that rising temperatures this century will cause poleward shifts in species» thermal niches and a sharp decline in tropical phytoplankton diversity in the absence of an evolutionary response. (science.sciencemag.org)
Meanwhile, increasing temperature and ocean warming may lead to the reduction of diatom production as well as cell size, inducing poleward shifts in the biogeographic distribution of diatoms. (news-oceanacidification-icc.org)