Grant R. Bigg , Mattias Green

It is well established that the Earth’s long-term orbital variation affects global climate through insolation changes, particularly with respect to glacial cycles. However, there is little research into the impact of this slowly varying insolation forcing on timescales well below the smallest main periodicity, the precession, of ~ 21,000 years. Here, intermediate complexity climate modelling demonstrates that allowing a smooth annual variation in orbital parameters and atmospheric CO2 levels through 4,000 years of a glacial climate simulation leads to significantly more variability on a range of timescales in key measures of oceanic climate, compared to simulations with combinations of fixed or variable forcing. This includes a more vigorous ocean overturning circulation on a global basis. These results raise the possibility that past climate change signals, often ascribed to internal natural variability, may instead be initiated by slowly-evolving external orbital and atmospheric CO2 variation.