A seminal book by Milankovitch in 1941 proposed that the sequence of ice ages that has characterized long term changes in the Earth’s climate over the past hundreds of thousands of years was due to changes in the amount of solar radiation reaching the Earth as a result of small variations in the Earth’s orientation and orbit with respect to the sun. Since then research has shown that Milankovitch cycles by themselves do not determine the timing of glacial and interglacial cycles and that we still lack a unified mechanism that links changes in Earth’s orbit to ice ages.
Global surface temperatures reconstructed over the last deglaciation show that temperature is correlated with but generally lags CO2 concentration. However, at the beginning of the deglaciation a global warming of about 0.3 °C preceded the initial increase in CO2 concentration suggesting that rising CO2 concentration amplified but did not initiate deglacial warming.
During the last deglaciation atmosphere CO2 concentration rose by about 80 ppm amplifying climate warming. This study of radiocarbon in deep sea corals found that Southern ocean deep water was radiocarbon-depleted throughout the last ice age, but this depletion disappeared between 16,600 and 14,600 years ago consistent with Southern Ocean CO2 outgassing that corresponded to the first pulse of increased atmospheric CO2 early in the deglaciation.
Improved data coverage and analysis has made it possible to reconstruct temperature profiles across most ocean basins and at all depths to 6000 meters (m) from 1960 to 2015. The reconstructions reveal accelerating heating in the upper layers above 2000 m. Ocean warming is stronger since the late 1980s compared to the 1960s to the 1980s.