The goal of the Paris Agreement is to keep global temperature rise well below 2°C. In this agreement––and its antecedents negotiated in Copenhagen and Cancun––the fear of crossing a dangerous climate threshold is supposed to serve as the catalyst for cooperation amongst countries. However, there are deep uncertainties about the location of the threshold for dangerous climate change, and recent evidence indicates this threshold uncertainty is a major impediment to collective action. Early warning signals of approaching climate thresholds are a potential remedy to this threshold uncertainty problem, and initial experimental evidence suggests such early detection systems may improve the prospects of cooperation. Here, we provide a direct experimental assessment of this early warning signal hypothesis. Using a catastrophe avoidance game, we show that large initial––and subsequently unreduced––threshold uncertainty undermines cooperation, consistent with earlier studies. An early warning signal that reduced uncertainty to within 10% (but not 30%) of the threshold value catalysed cooperation and reduced the probability of catastrophe occurring, albeit not reliably so. Our findings suggest early warning signals can trigger action to avoid a dangerous threshold, but additional mechanisms may be required to foster the cooperation needed to ensure the threshold is not breached.