In deliberations over actions to stabilize atmospheric concentrations of greenhouse gases, policymakers have yet to build in an evaluation process with data and tools to address the following questions: Is it working; is the climate getting healthier; and, at what price? The answers matter because stabilizing concentrations takes more than a fiscal year. It will take decades, necessitating periodic, science-based assessment to ascertain, in fact, whether climate is improving. And subsequent actions will need balancing against their costs.