The paper by Solomon et al. highlights the irreversibility of climate change due to carbon dioxide emissions, emphasizing that the impacts of increased atmospheric CO2 concentrations are largely irreversible for at least 1,000 years after emissions cease. Key findings include: 1. **Atmospheric CO2 Perturbation**: The removal of CO2 from the atmosphere is a multi-step process involving exchange with the land biosphere and the ocean. Even if emissions stop, a significant amount of CO2 (about 40% of the peak concentration) will remain in the atmosphere for a millennium, leading to persistent warming and sea level rise. 2. **Global Average Temperature**: Global temperatures will remain approximately constant (±0.5°C) until the end of the millennium, despite zero further emissions. This is due to a balance between the decrease in radiative forcing from CO2 decay and the reduced cooling through heat loss to the oceans. 3. **Precipitation Changes**: The paper discusses the expected drying of subtropical regions, particularly in southern Europe, northern Africa, southern Africa, and southwestern North America. These regions are projected to experience irreversible decreases in dry-season rainfall, similar to those seen during major droughts like the "dust bowl" era. 4. **Sea Level Rise**: The thermal expansion of the ocean and the loss of land ice are the primary contributors to sea level rise. Even with zero emissions after reaching peak CO2 concentrations, irreversible global average sea level rise of at least 0.4–1.0 m is expected if 21st-century CO2 concentrations exceed 600 ppmv. For peak CO2 concentrations exceeding ~1,000 ppmv, the rise could be as high as 1.9 m. 5. **Policy Implications**: The authors argue that the irreversibility of climate change due to CO2 emissions poses significant challenges for policy-making. Discount rates used in economic trade-offs often neglect the long-term effects of CO2 emissions, and the irreversibility of climate change highlights the need for more comprehensive and long-term planning. The paper underscores the urgency and magnitude of the climate change challenge, emphasizing that the decisions made today will have long-lasting impacts on the planet.The paper by Solomon et al. highlights the irreversibility of climate change due to carbon dioxide emissions, emphasizing that the impacts of increased atmospheric CO2 concentrations are largely irreversible for at least 1,000 years after emissions cease. Key findings include: 1. **Atmospheric CO2 Perturbation**: The removal of CO2 from the atmosphere is a multi-step process involving exchange with the land biosphere and the ocean. Even if emissions stop, a significant amount of CO2 (about 40% of the peak concentration) will remain in the atmosphere for a millennium, leading to persistent warming and sea level rise. 2. **Global Average Temperature**: Global temperatures will remain approximately constant (±0.5°C) until the end of the millennium, despite zero further emissions. This is due to a balance between the decrease in radiative forcing from CO2 decay and the reduced cooling through heat loss to the oceans. 3. **Precipitation Changes**: The paper discusses the expected drying of subtropical regions, particularly in southern Europe, northern Africa, southern Africa, and southwestern North America. These regions are projected to experience irreversible decreases in dry-season rainfall, similar to those seen during major droughts like the "dust bowl" era. 4. **Sea Level Rise**: The thermal expansion of the ocean and the loss of land ice are the primary contributors to sea level rise. Even with zero emissions after reaching peak CO2 concentrations, irreversible global average sea level rise of at least 0.4–1.0 m is expected if 21st-century CO2 concentrations exceed 600 ppmv. For peak CO2 concentrations exceeding ~1,000 ppmv, the rise could be as high as 1.9 m. 5. **Policy Implications**: The authors argue that the irreversibility of climate change due to CO2 emissions poses significant challenges for policy-making. Discount rates used in economic trade-offs often neglect the long-term effects of CO2 emissions, and the irreversibility of climate change highlights the need for more comprehensive and long-term planning. The paper underscores the urgency and magnitude of the climate change challenge, emphasizing that the decisions made today will have long-lasting impacts on the planet.