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WIREs Clim Change
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Quantifying the irreducible uncertainty in near‐term climate projections

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If the Paris agreement at the Conference of Parties 21 is implemented very effectively, greenhouse‐gas emissions might decrease after year 2020. Whether this would lead to identifiable near‐term responses in “iconic” climate quantities of wide scientific and public interest is unclear, because the climate response would be obscured by quasi‐random internal variability. I define the climate response as an increase or decrease in a linear climate trend over the period 2021–2035, compared to 2006–2020, and establish the probability of such a trend change being caused by an assumed policy shift toward emissions reductions after 2020. I quantify the irreducible uncertainty in projecting such a trend change through very large (100‐member) ensembles of the state‐of‐the‐art climate model MPI‐ESM‐LR. Trends in global‐mean surface temperature (GMST) are higher over the period 2021–2035 than over 2006–2020 in one‐third of all realizations in the mitigation scenario RCP2.6, interpreted as implementing the Paris agreement, compared to around one‐half in the no‐mitigation scenario RCP4.5. Mitigation is sufficient to cause a GMST trend reduction with a probability of 0.40 and necessary with a probability of 0.33. Trend increases in Arctic September sea‐ice area and the Atlantic meridional overturning circulation are caused by the emissions reductions with a probability of only around 0.1. By contrast, emissions reductions are necessary for a trend decrease in upper‐ocean heat content with a probability of over one‐half. Some iconic climate quantities might thus by year 2035 exhibit an identifiable response to a successful Paris agreement but sometimes with low probability, creating a substantial communication challenge.

This article is categorized under:

  • Climate Models and Modeling > Knowledge Generation with Models
Near‐term global‐mean surface temperature (GMST) in Max Planck Institute Grand Ensemble (MPI‐GE). I ask whether the post‐2020 greenhouse‐gas emissions reductions in scenario RCP2.6 cause a GMST trend reduction, as would be expected theoretically for the forced response (Gregory & Forster, ; Marotzke & Forster, ). (a) GMST time series for each realization, scenario RCP2.6. (b) As (a) but for scenario RCP4.5. The thick blue, red, and green lines show, respectively, the RCP2.6 and RCP4.5 ensemble means and the observations. (c–e) Frequency distribution of linear‐trend sizes of GMST across the RCP2.6 ensemble over the periods (c) 2006–2020, (d) 2021–2035, and (e) for the trend difference between these two periods. (f–h) As (c–e) but for scenario RCP4.5. Bin size is 0.025 K/decade
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Scenario RCP2.6, time series for each realization for (a) Arctic September sea‐ice area (SIA) and (b) the Atlantic meridional overturning circulation (AMOC) at 26.5°N. The thick blue, red, and green lines show, respectively, the RCP2.6 and RCP4.5 ensemble means and the observations
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Near‐term changes in ocean heat content of the upper 2,000 m (OHC2000). Frequency distribution of linear trend sizes of OHC2000 across the RCP2.6 ensemble over the periods (a) 2006–2020, (b) 2021–2035, and (c) for the trend difference between these two periods. (d–f) As (a–c) but for scenario RCP4.5. Bin size is 0.5 × 1022 J/decade. Most realizations lie below the observed trend (Levitus et al., , green vertical lines) over the 11‐year period 2005–2015, for which dense‐coverage, high‐quality observations are available
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