The governors of the world are not waiting for a bold global climate deal here at the Paris climate summit (COP21). They are mobilizing to move their states and provinces to the low-carbon economy. Our new report on the “Under 2 MOU” shows that the commitments of just 30 of the 65 signatories could keep 20 billion tons of CO2 out of the atmosphere by 2030 if fully implemented; this could be over 77% of what is needed to avoid dangerous climate change. Most of these reductions will come from slowing tropical deforestation and re-growing new tropical forests.   The Under 2 MOU was launched by Governor Jerry Brown of California less than one year ago. In signing, governors pledge to reduce carbon pollution to below 2 tons CO₂ per person annually, or 80 to 95% below baseline levels, by 2050. Its 65 signatories (expected to grow to over 80 by the end of COP 21) plan to radically reduce their greenhouse gas emissions.   The Under 2 MOU builds upon a previous subnational collaboration launched in California in 2008—the Governors’ Climate and Forests task force (GCF). This partnership has now grown to include 29 states and provinces that contain 1/4^th of the world’s tropical forests, including most of the Amazon region and most of the forests of Indonesia. The GCF recently launched the “Rio Branco Declaration”. Its signatories commit to reduce tropical deforestation 80% by 2020.   These global partnerships to tackle climate change are a new manifestation of the “California Effect”, a term that was coined to describe the adoption of California’s progressive legislation on vehicle pollution and increased fuel efficiency beyond its borders. Through the California effect, environmental legislation in the US and beyond is more ambitious. These new global partnerships are continuing a long tradition that is critical to solving climate change.   International climate deals are hugely important. But alone, they will not drive the transition to low carbon economies fast enough. Subnational action can help plug the gap.