The article discusses the target atmospheric CO₂ level for humanity to avoid dangerous climate change. Paleoclimate data show that climate sensitivity is about 3°C for doubled CO₂, including fast feedback processes, and 6°C when including slower surface albedo feedbacks. Current CO₂ levels are already in the dangerous zone, and without prompt policy changes, CO₂ levels will exceed 450 ppm, leading to irreversible effects. To preserve a planet similar to that on which civilization developed, CO₂ must be reduced to at most 350 ppm, though likely less. The largest uncertainty in the target arises from possible changes in non-CO₂ forcings. An initial 350 ppm target may be achievable by phasing out coal use and adopting carbon sequestering agricultural and forestry practices. However, if current CO₂ overshoot is not brief, there is a possibility of irreversible catastrophic effects. The article also discusses climate sensitivity, paleoclimate data, and the Pleistocene epoch. It shows that the current global mean CO₂ level of 385 ppm is already in the dangerous zone. Paleoclimate data indicate that the Earth's climate has high sensitivity to climate forcings, and that the current global mean CO₂ level is already in the dangerous zone. Despite rapid current CO₂ growth, it is conceivable to reduce CO₂ this century to less than the current amount, but only via prompt policy changes. The article also discusses the Cenozoic era, the role of CO₂ in long-term climate change, and the implications of different CO₂ levels. It concludes that the equilibrium climate sensitivity for doubled CO₂ is about 6°C, and that this sensitivity is relevant to GHGs that remain airborne for centuries-to-millennia. The article also discusses the time scales of climate change, the concept of "warming in the pipeline," and the implications of different CO₂ levels. It concludes that the target atmospheric CO₂ level should be 350 ppm to avoid dangerous climate change.The article discusses the target atmospheric CO₂ level for humanity to avoid dangerous climate change. Paleoclimate data show that climate sensitivity is about 3°C for doubled CO₂, including fast feedback processes, and 6°C when including slower surface albedo feedbacks. Current CO₂ levels are already in the dangerous zone, and without prompt policy changes, CO₂ levels will exceed 450 ppm, leading to irreversible effects. To preserve a planet similar to that on which civilization developed, CO₂ must be reduced to at most 350 ppm, though likely less. The largest uncertainty in the target arises from possible changes in non-CO₂ forcings. An initial 350 ppm target may be achievable by phasing out coal use and adopting carbon sequestering agricultural and forestry practices. However, if current CO₂ overshoot is not brief, there is a possibility of irreversible catastrophic effects. The article also discusses climate sensitivity, paleoclimate data, and the Pleistocene epoch. It shows that the current global mean CO₂ level of 385 ppm is already in the dangerous zone. Paleoclimate data indicate that the Earth's climate has high sensitivity to climate forcings, and that the current global mean CO₂ level is already in the dangerous zone. Despite rapid current CO₂ growth, it is conceivable to reduce CO₂ this century to less than the current amount, but only via prompt policy changes. The article also discusses the Cenozoic era, the role of CO₂ in long-term climate change, and the implications of different CO₂ levels. It concludes that the equilibrium climate sensitivity for doubled CO₂ is about 6°C, and that this sensitivity is relevant to GHGs that remain airborne for centuries-to-millennia. The article also discusses the time scales of climate change, the concept of "warming in the pipeline," and the implications of different CO₂ levels. It concludes that the target atmospheric CO₂ level should be 350 ppm to avoid dangerous climate change.