Climate Constraint Represents Forced Signal
Scientists at Lawrence Livermore National Laboratory in collaboration with colleagues from the University of Washington assess a recently proposed emergent constraint on the climate’s sensitivity to a doubling of atmospheric carbon dioxide. The research shows that the constraint, which is based on climate variability, is sensitive to climate forcing, the time period considered, and the models used in the analysis.
The proposed emergent constraint was highlighted as substantially reducing uncertainty in future climate change, allowing us to rule out really large changes in global mean surface temperature. This research shows that such conclusions are premature.
The Earth’s global surface temperature sensitivity to a doubling of atmospheric carbon dioxide is remarkably uncertain with a likely range of 1.5 – 4.5 K. A recent paper purported to drastically reduce the uncertainty in Earth’s climate sensitivity to carbon dioxide change (to 2.2 – 3.4 K) using observations of global mean surface temperature variability. We revisit this finding and determine that the estimated climate sensitivity value is sensitive to the model forcing used and the climate models and time period considered. We find that although the magnitude of internal climate variability in models does scale with climate sensitivity, the relatively short historical record and uncertainty in the 20th century forcing limit our ability to constrain climate sensitivity using surface temperature observations.