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Ocean acidification and warming are expected to disproportionately affect high‐latitude calcifying species, such as crustose coralline algae. Clathromorphum nereostratum and Clathromorphum compactum are the primary builders of carbonate‐hardgrounds in the Aleutians Islands of Alaska and North Atlantic shelf, respectively, providing habitat and settlement substrates for a large number of species. We exposed wild‐collected specimens to 12 pCO2/T treatments (344–3322 μatm; 6.38–12.40°C) for 4 months in a factorially crossed, replicated laboratory experiment. Impacts of pCO2/T on algal calcification were quantified from linear extension and buoyant weight. Here we show that, despite belonging to the same genus, C. nereostratum exhibited greater sensitivity to thermal stress, while C. compactum exhibited greater sensitivity to pH stress. Furthermore, multivariate models of algal calcification derived from the experiment indicate that both C. nereostratum and C. compactum will commence net dissolution as early as 2120 and 2200 AD, respectively. Our results therefore indicate that near‐term climate change may lead to substantial degradation of these species and loss of the critical hardground habitats that they form. Key Points: High latitude crustose coralline algae (CCA) C. compactum and C. nereostratum will struggle to maintain their skeletons under predicted future pCO2 levelsThis reduction in CCA viability will impact a wide range of benthic organismsDifferent species of high latitude CCA react differently to the same combinations of pCO2 and temperature conditions [ABSTRACT FROM AUTHOR]