How reversible is sea ice loss?

Abstract. It is well accepted that increasing atmospheric CO₂ results in global warming, leading to a decline in polar sea ice area. Here, the specific question of whether there is a tipping point in the sea ice cover is investigated. The global climate model HadCM3 is used to map the trajectory of sea ice area under idealised scenarios. The atmospheric CO₂ is first ramped up to four times pre-industrial levels (4 × CO₂), then ramped down to pre-industrial levels. We also examine the impact of stabilising climate at 4 × CO₂ prior to ramping CO₂ down to pre-industrial levels. Against global mean temperature, Arctic sea ice area is reversible, while the Antarctic sea ice shows some asymmetric behaviour – its rate of change slower, with falling temperatures, than its rate of change with rising temperatures. However, we show that the asymmetric behaviour is driven by hemispherical differences in temperature change between transient and stabilisation periods. We find no irreversible behaviour in the sea ice cover.