Policy thresholds in mitigation

Journal name:
Nature Geoscience
Year published:
Published online

Some climate change impacts rise fast with little warming, and then taper off. To avoid diminishing incentives to reduce emissions and inadvertently slipping into a lower-welfare world, mitigation policy needs to be ambitious early on.

At a glance


  1. Climate change impacts for various sectors.
    Figure 1: Climate change impacts for various sectors.

    For climate impacts such as coral reef degradation2, river floods1, changes in terrestrial vegetation3, yield losses in staple cropland5, UNESCO world heritage sites in danger from sea-level rise4, population threatened by sea-level rise4 and population with scarce access to freshwater6, impacts rise steeply relative to the total expected impact at low levels of warming and then begin to saturate, usually as a result of either an approach towards the total potential loss or adaptation. The impacts displayed represent the median of an ensemble of simulations; lines are based on a sigmoidal fit. Impacts are normalized by the total sector size (left) and the maximum potential impacts (right) as determined by the sigmoidal fit, ranging from 10 to 100% of the sector for these examples.

  2. Policy implications of different scalings of climate change impacts.
    Figure 2: Policy implications of different scalings of climate change impacts.

    a,b, In the canonical model, climate change impacts and mitigation costs are convex (a): impacts rise more steeply than linearly with warming, whereas costs of mitigation decline more steeply than linearly. Under these conditions, a single equilibrium point indicates the optimum policy where marginal impacts and marginal costs are equal (b). c,d, If, however, climate impacts are sigmoidal with convex mitigation costs (c), multiple policy equilibria can exist, corresponding to different levels of mitigation and therefore, probably, overall welfare (d). A policy tipping point marks the level of warming where the low-mitigation temperature target becomes the rational choice.


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Author information


  1. Department of Global Ecology, Carnegie Institution for Science, Stanford, California 94305, USA

    • Katharine L. Ricke &
    • Ken Caldeira
  2. School of Economics, Georgia Institute of Technology, Atlanta, Georgia 30332, USA

    • Juan B. Moreno-Cruz
  3. Potsdam Institute for Climate Impact Research, 14473 Potsdam, Germany

    • Jacob Schewe &
    • Anders Levermann

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