Robust projections of combined humidity and temperature extremes

• Published in: Nature climate change 2013-02, Vol.3 (2), p.126-130
• Main Authors: Fischer, E. M., Knutti, R.
• Format: Article
• Language: English
• Subjects: 704/106/694; Climate Change; Climate Change/Climate Change Impacts; Climatology. Bioclimatology. Climate change; Earth and Environmental Science; Earth, ocean, space; Environment; Environmental impact; Environmental Law/Policy/Ecojustice; Exact sciences and technology; External geophysics; letter; Meteorology; Relative humidity
• ISSN: 1758-678X; 1758-6798
• DOI: 10.1038/nclimate1682

This study investigates uncertainties in impact assessments when using climate projections. The uncertainties in health-related metrics combining temperature and humidity are much smaller than if the uncertainties in the two variables were independent. The finding reveals the potential for joint assessment of projection uncertainties in other variables used in impact studies. Impacts of climate change such as the effects on human discomfort, morbidity and mortality often depend on multiple climate variables. Thus, a comprehensive impact assessment is challenging and uncertainties in all contributing variables need to be taken into account. Here we show that uncertainties in some impact-relevant metrics such as extremes of health indicators are substantially smaller than generally anticipated. Models that project greater warming also show a stronger reduction in relative humidity. This joint behaviour of uncertainties is particularly pronounced in mid-continental land regions of the subtropics and mid-latitudes where the greatest changes in heat extremes are expected. The uncertainties in health-related metrics combining temperature and humidity are much smaller than if uncertainties in the two variables were independent. Such relationships also exist under present-day conditions where the effect of model biases in temperature and relative humidity largely cancel for combined quantities. Our results are consistent with thermodynamic first principles. More generally, the findings reveal a large potential for joint assessment of projection uncertainties in different variables used in impact studies.