The oceans are warming only half as fast as ecosystems on land, but marine species have evolved in an environment whose temperature varies relatively little over the course of days, weeks, or millennia. Do these characteristics make ocean animals more or less vulnerable to ongoing and future warming?
Using a unique global database, we show that marine species have narrower thermal tolerance breadths than animals on land, and will hit physiologically harmful thermal limits sooner than populations on land across all latitudes. In addition, marine animals lack the access to microclimate refuges that allow some terrestrial species to escape warming. In essence, ocean animals are more extreme versions of notoriously climate-sensitive species living in tropical lowlands. This finding implies a wave of local extinctions in the ocean, and existing records suggest local extinctions are occurring in the ocean at twice the rate as on land. Local losses are amplified by species interactions and are likely to ripple through ecosystems to negatively affect conservation efforts, food supplies, and international politics. Ecological history suggests that climate change can cause catastrophic losses of marine biodiversity, but history need not repeat itself if we aggressively mitigate and adapt to climate impacts in the ocean.