The ability of the Paris Agreement to achieve its ambitious long-term temperature goal depends on mechanisms that can promote a ramp-up of ambitions. Pledge-and-review may facilitate deeper cooperation over time by enhancing trust between parties. It has been proposed that starting out with easier cooperation that entails larger shares of domestic benefits, such as cooperation on short-lived climate pollutants, could also help promote the credibility and trust necessary for the ramping-up of contributions over time. Using a lab experiment we explore how these two mechanisms perform across two stages in a controlled and stylized setting where individual participants face incentives that mirror those of countries deciding on national contributions. We find that starting with easier cooperation does not produce higher levels of trust, and it leads to lower levels of cooperation when cooperation becomes more difficult in stage 2. The pledge-and-review approach does not, in itself, lead to significantly higher levels of cooperation, but it does offset the negative effect of starting with the easy cooperation. It can be seen as a safeguard against failed strategies.