Thus, we take 4.5 °C as our best estimate for LGM cooling, implying an amplification of surface temperature change by a factor of two relative to
deep ocean temperature change for this climate interval.
«
deep ocean temperature change does not provide a good indication of surface temperature change when the deep ocean approaches the freezing point, as quantified by Waelbroeck et al. (2002).
It seems we are still at a very early stage in acquiring knowledge on
the deep oceans temperature changes.
As Earth became colder and continental ice sheets grew, further increase of δ18O was due in equal parts to
deep ocean temperature change and ice mass change.
HS12 assume that
deep ocean temperature change was similar to global mean surface temperature change for Cenozoic climates warmer than today, but this relationship does not hold true for colder climates.
Due to time lags in moving energy / heat around, there is no reason to think that
deeper ocean temperature changes will be in lock - step with surface changes.