Global ocean temperatures are rising, yet the impacts of such changes on harmful algal blooms (HABs) are not fully understood. Here we used high-resolution sea-surface temperature records (1982 to 2016) and temperature-dependent growth rates of two algae that produce potent biotoxins, Alexandrium fundyense and Dinophysis acuminata, to evaluate recent changes in these HABs. For both species, potential mean annual growth rates and duration of bloom seasons significantly increased within many coastal Atlantic regions between 40°N and 60°N, where incidents of these HABs have emerged and expanded in recent decades. Widespread trends were less evident across the North Pacific, although regions were identified across the Salish Sea and along the Alaskan coastline where blooms have recently emerged, and there have been significant increases in the potential growth rates and duration of these HAB events. We conclude that increasing ocean temperature is an important factor facilitating the intensification of these, and likely other, HABs and thus contributes to an expanding human health threat.Alexandrium | Dinophysis | climate change | sea-surface temperature | bloom duration H armful algal blooms (HABs) negatively affect aquatic ecosystems, fisheries, tourism, and human health. HABs such as Alexandrium fundyense and Dinophysis acuminata are particularly concerning, as they produce saxitoxin and okadaic acid, respectively, toxins that can cause the human health syndromes paralytic and diarrhetic shellfish poisoning (PSP and DSP, respectively). The global range, regional intensity, and frequency of HABs have increased in recent decades (1, 2). This phenomenon is, in part, related to increasing awareness and improved monitoring of HABs (2) and, in some cases, the intensification of anthropogenic nutrient loading in coastal zones (3). Although there have been multiple predictions regarding the response of HABs to future climate change (2, 4), the ability to conclusively relate changes in HAB phenology and distribution to rising ocean temperatures has been a challenge.Globally, the geographic ranges of phytoplankton are frequently controlled by sea-surface temperatures [SSTs (2, 5)], and the realized niches of HABs are often defined by a narrow range of temperatures (2, 5-8). As global oceans warm (9, 10) and the distribution of ocean temperatures changes (11,12), it is expected that the distribution and range of phytoplankton and HABs will also shift (2, 4). Observations and modeling studies have shown that climate change-driven warming of ocean water is unevenly distributed (12), particularly along coastlines (11, 13). Consequently, temperature-driven changes in HAB distributions are likely to vary along coastlines and among ocean basins. Presently, the extent to which changes in HAB occurrence and intensity are related to changing ocean temperatures is unresolved.To assess the relationship between HABs and global temperature change, some recent studies have used physical and biogeochemical output from global circulation ...