Pteropods – or sea butterflies – are tiny sea snails which play a critical role in various ocean ecosystems. Because of this critical role there is great concern about the potential impact of global change and particularly ocean acidification on these organisms. Ocean acidification is likely to make the production of calcium carbonate structures such as pteropod shells more challenging. Global warming is also expected to affect pteropod populations by modifying the geographical distribution of optimal species-specific temperatures.

Pteropods are of particular concern for the scientific community as they have an external shell that is expected to be highly sensitive to changes in environmental conditions. Importantly, changes in pteropod abundance could have strong impacts, as they play a critical role as a food source for fish, birds, whales and other zooplanktonic organisms. This role is particularly critical in high latitude oceans, where pteropods can represent a large proportion of the diet of commercially important fish such as salmon. Their role in food chains at lower latitudes has received less attention, but a reduction in pteropod abundance in lower latitudes would also be likely to modify food webs. Beyond these ecological impacts, it is likely that decreasing pteropod populations will also affect the cycling of carbon and carbonate in the oceans.

In order to model the impact of ocean acidification and global warming on pteropods, risk indicators have been calculated for major pteropod species. These were calculated for three particularly dominant species, in high latitudes, temperate waters and warm waters respectively. To create the indicators, experimental data on pteropod shell production under ocean acidification scenarios was combined with models describing chemical conditions of the oceans at present, in 2030 and 2050, under both optimistic and pessimistic carbon dioxide emission scenario projections. For pteropod shell production, five risk levels were calculated. For temperature, risk levels were calculated as a function of average surface yearly temperature deviation from the maximum temperature at which organisms from the three species are now found.

Of particular concern is the high latitude pteropod Limacina helicina, which is found in both the Arctic Ocean and the Southern Ocean. These results confirm a threat for this species. Under the pessimistic CO₂ emissions scenario, it is likely that by 2050 the chemical conditions of the water in most of the Arctic Ocean will limit the formation of pteropod shells, meaning they will not be able to thrive in most of the Arctic Ocean. In addition, the area in which environmental conditions will remain optimal for Limacina helicina will also be reduced under global warming. With warming, it is likely that the optimal ocean areas for high latitudes pteropods will contract towards the poles, while ocean acidification is likely to push them toward lower latitudes.

The temperate water species examined here (Limacina retroversa) will also very likely be affected by global change, with only 26% of the ocean representing low risk for them by 2050. Warm water species probably will not be as strongly affected by ocean acidification by 2050, but excessive temperatures will likely reduce their area of distribution, particularly in the equatorial Pacific.

The development of these indicators confirms the threat represented by global change for pteropods. Ocean acidification in particular represents a major threat for the ecosystem critical high latitudes pteropods. As ocean acidification is directly linked to carbon dioxide emissions, only a strong reduction in emissions will limit the effects of acidification on these organisms. The impact of disappearing pteropods is hard to quantify at present, but it is likely that several ecosystems will be affected with potential impacts for fisheries. In order to better assess these effects, further studies on both the impact of warming and ocean acidification on pteropods – building on the limited number of empirical datasets used here – are critically needed.