  Recent attempts at cooperating on climate change mitigation highlight the
limited efficacy of large-scale agreements, when commitment to mitigation is
costly and initially rare. Bottom-up approaches using region-specific
mitigation agreements promise greater success, at the cost of slowing global
adoption. Here, we show that a well-timed switch from regional to global
negotiations dramatically accelerates climate mitigation compared to using only
local, only global, or both agreement types simultaneously. This highlights the
scale-specific roles of mitigation incentives: local incentives capitalize on
regional differences (e.g., where recent disasters incentivize mitigation) by
committing early-adopting regions, after which global agreements draw in
late-adopting regions. We conclude that global agreements are key to overcoming
the expenses of mitigation and economic rivalry among regions but should be
attempted once regional agreements are common. Gradually up-scaling efforts
could likewise accelerate mitigation at smaller scales, for instance when
costly ecosystem restoration initially faces limited public and legislative
support.
