This open access book examines climate policy effort among subnational governments participating in the Under2 Coalition. The central research question investigates whether Under2 Coalition founding members demonstrate greater climate policy effort compared to early joiners (2015-2016) and later joiners (post-2016), and whether these performance differences persist over time. The empirical analysis focuses on subnational governments in five countries that played influential roles in founding and developing the Under2 Coalition: Canada, Germany, Spain, the United Kingdom, and the United States.

The study makes three key contributions to climate governance literature. First, it systematically demonstrates how climate policy effort and leadership dynamics evolve within the Under2 Coalition framework. Second, it provides genuinely comparative insights across two dimensions—comparing different membership categories within countries and across nations. Third, it offers a replicable methodological template for expanding this research approach to other contexts.

By examining quantifiable indicators including policies, policy instruments, and greenhouse gas emissions reduction targets, the book provides valuable insights into how multilevel climate governance can be strengthened and sustained over time, bridging theory and practice in understanding subnational climate action.

Jale Tosun is Professor of Political Science at Heidelberg University, Germany, and Adjunct Professor at the University of Oslo, Norway.

Simon Bulian is a Researcher at the Institute of Political Science, Heidelberg University, Germany.

Alfie Gaffney is a Researcher at the Tyndall Centre for Climate Change Research, University of East Anglia, United Kingdom.

Joan Enguer is a Researcher at the Department of Constitutional Law and Political Science, University of Valencia, Spain.

Emiliano Levario Saad is a Researcher at the Institute of Political Science, Heidelberg University, Germany.