This article proposes a new formula for a post-Kyoto Protocol on climate change. The article critiques the Kyoto Protocol insofar as it allocates emissions reduction entitlements to some countries but not to others. All countries, and not just developed countries, must be allocated emissions entitlements. The article further critiques the Kyoto Protocol for imposing emission reduction obligations on only the developed countries, leaving developing countries without emission reduction and/or avoidance obligations. The article refutes the assumption underlying the Kyoto Protocol that the principle of common but differentiated responsibilities necessarily exempts developing countries on account of their historically low GHG emissions from emissions reductions and/or avoidance obligations. Our proposal assigns real, common, but equitably differentiated responsibilities to all nations of the world, and requires that all countries maximize the efficiency of their use of energy. The post-Kyoto Protocol climate change mitigation regime must 1) stipulate an absolute cap on allowable GHG emissions, which the global atmospheric system can tolerate without dangerous climate change; 2) allocate GHG emissions under that cap to all countries on the based on a combination of four factors: the Human Development Index; the efficiency of energy use; historical and present per capita emissions; and projected future per capita emissions; 3) allow all countries to use flexible mechanisms in meeting reduction commitments; and 4) provide for technology transfer and capacity building to enable poor countries to embark on a sustainable development path based on low carbon, energy efficient, and renewable energy technologies.