The California Current System is one of four ocean currents that make up the North Pacific Gyre—a swirling vortex rotating clockwise around the northern basin of the Pacific Ocean. The 200 mile wide California Current moves cold northern waters southwards along the western coast of North America from British Columbia to Baja California. The waters are further cooled by extensive coastal upwelling driven by the prevailing northwesterly winds. The nutrient-rich waters that are brought to the well-lit surface promote the growth of phytoplankton. Consequently, the area is highly productive and capable of supporting large populations of seabirds and other marine life.

Long-term records show an overall trend of increasing ocean surface temperature along the West Coast of North America (McGowan et al. 1998, Breaker 2006, Field et al. 2006, Hsieh et al. 2009). This warming may lead to changes in plankton biomass and primary productivity (Roemmich & McGowan 1995, Kahru et al. 2009, Chavez et al. 2011), as well as changes in the timing of peak plankton availability (Kim et al. 2009), with knock-on effects further up the food chain. Studies of population trends of seabird prey species are rare, however population declines have been reported in several fish species (Holbrook et al. 1997, Hsieh et al. 2009, Field et al. 2010). There is also evidence that seabird competitors may be increasing. For example, Humboldt squid underwent an extensive range expansion in the Californian Current System in the late 1990s and early 2000s (Zeidberg & Robison 2007).

Records of seabird abundance in the California Current System are not available over a long enough time period to attribute any declines with certainty to anthropogenic global warming (Young et al. 2012). However, population declines and shifts in distribution or phenology have been recorded for many seabird species. For example, there was a 40% decline in pelagic bird abundance from 1987-1994 during a period of ocean warming, including a 90% decline in Sooty Shearwater Puffinus griseus (Veit et al. 1996, Veit et al. 1997). Sydeman et al. (2009) found an overall decline in seabird species richness and density in the Southern California Current System over time and with increasing temperature, as well as earlier average egg laying dates for one species—the Common Murre Uria aalge.

Different seabird assemblages are adapted to exploit different oceanic conditions. In high productivity regions, such as the California Current System, diving species prevail. These species require high prey densities to meet the substantial energetic requirements associated with this foraging method, and are therefore particularly vulnerable to declines in prey species (Ainley & Hyrenbach 2010). In contrast, areas of low productivity tend to have a depauperate seabird community dominated by plunging and surface-foraging birds. Research off southern California has found that, concurrent with rising ocean temperatures, there has been a significant decline in seabird abundance—with particular losses of cold-water taxa, such as Cassin’s Auklet Ptychoramphus aleuticus and Rhinoceros Auklet Cerorhinca monocerata (Hyrenbach & Veit 2003, Sydeman et al. 2009). This suggests that the seabird communities in the region could shift from a ‘high-productivity’ assemblage, typical of subpolar oceans, to a ‘low-productivity’ community, typical of subtropical gyres. If the California Current System were to permanently switch to a less productive state, this would have devastating consequences for marine life in this vital ecosystem.

It is now apparent that even modest temperature rises can have a profound impact on marine ecosystems—disrupting complex oceanic cycles and radically altering the abundance and distribution of marine organisms. Climate change is likely to adversely affect numerous seabird populations, exacerbating the already considerable threats posed by overfishing, bycatch and associated habitat destruction. Compiling and analyzing datasets on seabird response to climate change should be a high priority, as this will help prioritise conservation actions.