Concurrent with rising air temperatures, ocean surface temperatures are also increasing, causing surface waters to expand and sea-levels to rise. Warming surface waters also reduce the degree of vertical mixing—quelling the upward transfer of the deep, cool, nutrient-rich waters that encourage the growth of phytoplankton (Behrenfeld et al. 2006). At polar latitudes the problem is exacerbated by the melting of the ice caps and arctic permafrost, which causes freshwater to inundate the oceans. This creates a low density surface layer that further inhibits the vertical flow of nutrients (Jacobs et al. 2002). The decline in plankton biomass, the foundation of primary production in the oceans, has implications that resonate up the marine food chain from krill to fish and ultimately to marine mammals and seabirds.

The behavioural, social and life-history traits of seabirds may render them particularly sensitive to climate change (Grémillet and Boulinier 2009). Generally, seabirds have highly specialised diets, being reliant on just a few prey species, the abundance and distribution of which can alter dramatically in response to abrupt environmental changes. Seabirds’ behavioural characteristics may constrain their adaptation to such changes. A seabird colony can take decades to establish and many birds display considerable breeding site philopatry—sometimes remaining faithful to an area even after conditions have become unfavourable (Grémillet et al. 2008). Species with extremely restricted geographical ranges, such as Galápagos Penguin Spheniscus mendiculus, are probably at the most immediate risk of extinction (Vargas et al. 2007). Climate change, however, is likely to adversely affect many seabird populations, exacerbating the already considerable threats posed by overfishing, bycatch and habitat destruction.

Changes in the abundance and distribution of prey are already having a detrimental impact on many seabird populations. In Antarctica, rising sea-surface temperatures have led to a reduction in krill Euphausia superba, a key prey species for many seabirds, and a sharp increase in less favoured food items such as salps, transparent gelatinous organisms (Moline et al. 2004). This change to the food web has already been implicated in the decline of several seabird populations (e.g. Cresswell et al. 2008, Le Bohec et al. 2008). For example, Emperor Penguin Aptenodytes forsteri colonies in Terre Adélie declined by 50% during a period of abnormally warm temperatures and poor krill production (Barbraud and Weimerskirch 2001). A 39-year study of Southern Fulmar Fulmarus glacialoides found that the birds forego breeding altogether during warm water anomalies, probably because the availability of krill is so reduced (Jenouvrier et al. 2003).

In the north-eastern Atlantic, climate change and overfishing have severely depleted the lesser sandeel Ammodytes marinus population. At the same time, the warming ocean conditions have favoured the rapid proliferation of snake pipefish Entelurus aequoreus (Kirby et al.2006). Whilst sandeel are an important food-base for the region’s seabirds, especially black-legged kittiwakes Rissa tridactyla, snake pipefish have minimal calorific value and can present a choking hazard to seabird chicks (Harris et al. 2007a, 2007b).

Several large-scale oceano-climatic fluctuations exist, most notably the El Niño-Southern Oscillation (ENSO). Although the impact of climate change on these complex cycles is difficult to predict, it has been suggested that global warming may result in more frequent and intense El Niño events (Timmermann et al. 1999). Whatever the precise impact of climate change on large-scale climate oscillations, extreme weather events and abrupt shifts in oceanic regimes are more likely in a rapidly warming world (Alley 2003). What is known is that during El Niño years, nutrient rich upwellings in the eastern Pacific are reduced, meaning overall productivity declines, and fish stocks plummet. This can have devastating consequences for seabirds. For example, the population of the Endangered Galápagos Penguin, has halved since the early 1970s because the adults become malnourished during severe El Niño conditions and fail to breed (Boersma 1998).