Light and temperature limitation of poleward coral reef expansion during past warm climates

The latitudinal range of shallow-water tropical corals is controlled by temperature, and presently limited to waters warmer than 16-18 °C yearround. However, even during Cenozoic climates with such temperatures in polar regions, coral reefs are not found beyond >50° latitude. Here, we test the hypothesis that daily available solar radiation limited poleward expansion of coral reefs during warm climates, using a new box model of shallow marine coral calcification. Our results show that calcification rates start to decline beyond 40° and more quickly beyond 50°, suggesting that winter light intensity and day length prohibits further poleward expansion. This implies that fossil coral reef distribution is not a robust proxy for water temperatures and that poleward expansion of reefs is not an expected carbon cycle feedback of climate warming.