|Adélie Penguin. Photo by Grant Ballard.|
This year was a good illustration. In the Ross Sea, Adélie Penguins started breeding ten days later than usual, with many pairs apparently choosing not to breed at all and others laying only one egg (normally they lay two). This was the latest start observed during our 14 years of continuous study. A late start is the last thing Adélies need in this extremely challenging environment.
We don't yet know why this group nested late, but the best guess is that more sea ice in the Ross Sea sector forced these birds farther away from their breeding colonies in the southern Ross Sea. Since the late 1970s, sea ice has decreased in the most northerly parts of Antarctica but, somewhat paradoxically, increased in the more southerly sectors, particularly in the Ross Sea.
This is because there are more windy days than ever before recorded; atmospheric patterns are changing. As long as temperatures remain below freezing, this leads to more ice production. As ice forms on freezing water, it is blown out to sea, continuously freeing new open water to be frozen. This system falls apart when temperatures rise above the freezing point of sea water, a tipping point already reached in more northerly parts of the Antarctic.
|Adélie Penguin on ice in the Ross Sea. Photo by Viola Toniolo.|
Because Adélie Penguins are year-round "sea-ice obligates," these changes are having big effects on their populations. Where ice is diminishing, penguins are decreasing in numbers, with some colonies disappearing altogether. In the Ross Sea, although numbers now are increasing, new findings by David Ainley and others show that this trend will not last more than another 20–30 years. The Ross Sea sea ice is predicted also to start decreasing over this time, at least north of 70°S. And the farther south the sea ice retreats, the less light there is in winter. According to other PRBO findings, Adélies need ice located where there is some light for part of the day, probably so they can see well enough to forage. Ultimately, ice will occur too far south for Adélies to make use of it as a platform for foraging.