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Target atmospheric CO2: Where should humanity aim? (0804.1126v3)

Published 7 Apr 2008 in physics.ao-ph

Abstract: Paleoclimate data show that climate sensitivity is ~3 deg-C for doubled CO2, including only fast feedback processes. Equilibrium sensitivity, including slower surface albedo feedbacks, is ~6 deg-C for doubled CO2 for the range of climate states between glacial conditions and ice-free Antarctica. Decreasing CO2 was the main cause of a cooling trend that began 50 million years ago, large scale glaciation occurring when CO2 fell to 450 +/- 100 ppm, a level that will be exceeded within decades, barring prompt policy changes. If humanity wishes to preserve a planet similar to that on which civilization developed and to which life on Earth is adapted, paleoclimate evidence and ongoing climate change suggest that CO2 will need to be reduced from its current 385 ppm to at most 350 ppm. The largest uncertainty in the target arises from possible changes of non-CO2 forcings. An initial 350 ppm CO2 target may be achievable by phasing out coal use except where CO2 is captured and adopting agricultural and forestry practices that sequester carbon. If the present overshoot of this target CO2 is not brief, there is a possibility of seeding irreversible catastrophic effects.

Citations (1,275)

Summary

  • The paper reveals that incorporating slow feedbacks shows climate sensitivity nearly 6°C for CO₂ doubling, justifying the 350 ppm target.
  • Utilizing paleoclimate data and modern observations, the authors challenge conventional models by integrating slow feedback mechanisms like ice sheet melt.
  • The study advocates urgent policy reforms, including an aggressive coal phase-out and investment in carbon capture, to prevent irreversible climate impacts.

Evaluation of Proposed Targets for Atmospheric Carbon Dioxide Reduction

The paper, "Target Atmospheric CO₂: Where Should Humanity Aim?" authored by James Hansen et al., presents a comprehensive exploration of climate sensitivity based on paleoclimate data and contemporary climate observations to propose a target atmospheric carbon dioxide level necessary to maintain the Earth's climate stability. The authors set forth an analysis that suggests a much lower atmospheric CO₂ concentration than is currently present, aiming to prevent potential irreversible consequences of climate change.

Key Findings

The core thesis advocates for reducing atmospheric CO₂ levels to a maximum of 350 parts per million (ppm), a significant decrease from the present (at the time of writing) level of 385 ppm. This conclusion is drawn from extensive evaluation of paleoclimate records, revealing that climate sensitivity, when including slow feedback processes such as ice sheet disintegration and vegetation changes, is approximately 6°C for doubled CO₂, doubling the standard Charney sensitivity estimate of 3°C. This finding underscores the inadequacy of current climate models in accounting for long-term feedback mechanisms.

Paleoclimate and Climate Sensitivity Analysis

The authors utilize data spanning the Pleistocene and Cenozoic eras to elucidate the Earth's climate sensitivity to CO₂ concentration changes. Empirical evidence indicates that slow feedback mechanisms significantly amplify the climate response, a crucial insight for modeling future climate scenarios. The analysis highlights that during the early Cenozoic, atmospheric CO₂ was as high as 1500 ppm, indicating that current levels might predispose the Earth towards a climate state that does not support current biodiversity and human civilization.

Implications

The implications of this research are profound, suggesting that current levels of CO₂ are within the dangerous zone capable of triggering irreversible climate phenomena such as large-scale ice sheet collapse and significant biodiversity losses. The research advocates for immediate policy intervention to reduce CO₂ emissions, particularly focusing on a phased approach to eliminate coal use unless carbon capture technology is employed.

Scenario Modeling and Climate Policy

The paper presents different scenarios under which CO₂ levels could be managed to stay below the critical threshold. A notable scenario entails an aggressive reduction in coal usage, advocating for its near-complete phase-out by 2030. The authors suggest that with concerted efforts involving both emission reductions and carbon sequestration through changes in agricultural and forestry practices, it is feasible to lower the atmospheric CO₂ levels back to 350 ppm. The argument for a carbon pricing mechanism and investment in carbon capture technology emphasizes the need for coordinated global policy approaches.

Future Outlook and Recommendations

Given the urgency highlighted in the paper, further exploration into carbon cycle feedbacks and the development of technologies for carbon capture and sequestration are imperative. The authors call for enhanced climate models that incorporate both fast and slow feedback mechanisms to reliably predict outcomes of current anthropogenic activities. Moreover, reevaluation of GHG reduction targets might be required as new data and models become available, ensuring that strategies are aligned with the latest scientific insights.

In conclusion, Hansen et al.'s work provides a rigorous, evidence-based target for atmospheric CO₂. Its insights compel a rethinking of current climate policies, offering a pathway to mitigate anthropogenic impacts on climate—a critical consideration for safeguarding environmental balance and ensuring a habitable planet for future generations.

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