A scientific critique of the two-degree climate change target

Journal name:
Nature Geoscience
Volume:
9,
Pages:
13–18
Year published:
DOI:
doi:10.1038/ngeo2595
Received
Accepted
Published online

Abstract

The world's governments agreed to limit global mean temperature change to below 2 °C compared with pre-industrial levels in the years following the 2009 climate conference in Copenhagen. This 2 °C warming target is perceived by the public as a universally accepted goal, identified by scientists as a safe limit that avoids dangerous climate change. This perception is incorrect: no scientific assessment has clearly justified or defended the 2 °C target as a safe level of warming, and indeed, this is not a problem that science alone can address. We argue that global temperature is the best climate target quantity, but it is unclear what level can be considered safe. The 2 °C target is useful for anchoring discussions, but has been ineffective in triggering the required emission reductions; debates on considering a lower target are strongly at odds with the current real-world level of action. These debates are moot, however, as the decisions that need to be taken now to limit warming to 1.5 or 2 °C are very similar. We need to agree how to start, not where to end mitigation.

At a glance

Figures

  1. Projected climate change for different global mean temperature targets.
    Figure 1: Projected climate change for different global mean temperature targets.

    20 year average annual mean surface temperature and precipitation change for a warming of 1.5 °C, 2 °C, or 4 °C relative to 1870–1889 in the CMIP5 models. Stippling implies significant changes that are robust across models, the hatching indicates changes within natural variability and white areas mark inconsistent model responses (see Methods).

  2. Dependence of different quantities on global mean surface temperature change.
    Figure 2: Dependence of different quantities on global mean surface temperature change.

    a, Global annual mean change in precipitation for each CMIP5 model (grey) and the mean (red). b, Relative increase in the number of hot days (mean and CMIP model uncertainty, from ref. 16). c, Near equilibrium sea-level rise (data from ref. 13). d, Change in Barents Sea March sea-ice area for two transient climate model simulations. e, Risks associated with different 'reasons for concern' at a global scale assessed by the IPCC (data from ref. 17).

  3. Changes in the water cycle.
    Figure 3: Changes in the water cycle.

    Area showing a change towards wetter or drier conditions in precipitation (P), evaporation (E), E–P, relative atmospheric humidity, soil moisture and runoff, aggregated for winter and summer (data from ref. 24). Lighter brown and green indicate robust but non-significant changes across models, darker colours indicate robust significant changes (see Methods). Grey indicates changes within natural variability, corresponding to the hatching in Fig. 1. White indicates significant changes in individual models but disagreement on sign or magnitude across models, implying unknown risks.

  4. Historical and future global CO2 emissions from fossil-fuel burning and industry-related activities.
    Figure 4: Historical and future global CO2 emissions from fossil-fuel burning and industry-related activities.

    The thick black line shows historical data until 2013 (data from ref. 48). Orange to red coloured areas represent filtered subsets of the IPCC AR5 scenario database that are in line with limiting warming to below 2 °C during the twenty-first century with at least 66% probability. 2 °C-compatible scenarios are filtered based on the time when they assume globally coordinated climate action. Lighter shading represents the minimum-maximum range. Darker shading shows the interquartile range.

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Author information

Affiliations

  1. Institute for Atmospheric and Climate Science, ETH Zürich, Universitätstrasse 16, CH-8092 Zürich, Switzerland

    • Reto Knutti,
    • Joeri Rogelj,
    • Jan Sedláček &
    • Erich M. Fischer
  2. Energy Program, International Institute for Applied Systems Analysis (IIASA), Schlossplatz 1, A-2361 Laxenburg, Austria

    • Joeri Rogelj

Contributions

All authors jointly wrote the paper. J.S. produced Figs 1–3. J.R. produced Fig. 4.

Competing financial interests

The authors declare no competing financial interests.

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