We explore the combined effects of a climate threshold (a potential ocean thermohaline circulation collapse), parameter uncertainty, and learning in an optimal economic growth model. Our analysis shows that significantly reducing carbon dioxide ðCO 2 Þ emissions may be justified to avoid or delay even small (and arguably realistic) damages from an uncertain and irreversible climate change-even when future learning about the system is considered. Parameter uncertainty about the threshold specific damages and the CO 2 level triggering a threshold can act to decrease near-term CO 2 abatements that maximize expected utility. r