Action-oriented summits like the 2018 Global Climate Action Summit and 2019 UN Climate Action Summit, have become a major feature of global climate governance. Their emphasis on cooperative initiatives by a host of non-state and local actors creates high expectations, especially when, according to the IPCC, governments’ policies still set the world on course for a disastrous 2.7 °C warming. While earlier studies have cautioned against undue optimism, empirical evidence on summits and their ability to leverage transnational capacities has been scarce. Here using a dataset of 276 climate initiatives we show important differences in output performance, with no improvement among initiatives associated with more recent summits. A summit’s focus on certain themes and an emphasis on minimal requirements for institutional robustness, however, can positively influence the effectiveness of transnational engagement. These results make an empirical contribution towards understanding the increasingly transnational nature of climate governance.