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The United States federal government is lagging behind other industrialized countries in developing policies to address climate change. At the same time, however, many U.S. states are seeking to implement aggressive, mandatory emissions controls.

To understand the state-federal relationship and its implications for climate change policy in the United States, we reviewed eleven successful and two unsuccessful instances of policy diffusion from the state to the federal level, known as vertical diffusion. In addition, we looked specifically at two state/regional policy initiatives that will likely impact federal policy: the Regional Greenhouse Gas Initiative (RGGI), and the California vehicle emissions standards.

Based on these cases, we identified seven factors related to the successful vertical diffusion of policy, and analyzed the cases from the perspective of those factors:

1. Policy learning (states as laboratories)
2. Available cost data
3. Spillover effects on surrounding states
4. Horizontal policy diffusion to other states
5. Federal assistance
6. Business support for federal action
7. Push for diffusion by state champions

The most important factors appear to be "Push for diffusion by state champions," closely linked to the "spillover effect." The existence of a strong spillover effect coupled with state advocacy appears to be sufficient to catalyze vertical diffusion even without the widespread horizontal diffusion. "Policy learning" is also important. However, "Cost data" does not seem to be a significant factor, perhaps because cost was not an issue, data were unavailable, or federal policymakers considered the data irrelevant.