Climate change is heating the oceans, altering currents and circulation patterns responsible for regulating climate on a global scale. If temperatures dropped, some of that damage could theoretically be undone. But employing 'emergency' atmospheric geoengineering later this century in the face of continuous high carbon emissions would not be able to reverse changes to ocean currents, a new study finds. This would critically curtail the intervention's potential effectiveness on human-relevant timescales.