Reduction of climate sensitivity to solar forcing

An accurate assessment of the role of solar variability is a key step towards a proper quantification of natural and anthropogenic climate change. To this end, climate models have been extensively used to quantify the solar contribution to climate variability. However, owing to its large computational cost, the bulk of modeling studies to date have been performed without interactive stratospheric photochemistry: the impact of this simplification on the modeled climate system response to solar forcing remains largely unknown. Here we quantify this impact, by comparing the response of two model configurations, with and without interactive ozone chemistry. Using long integrations, we first obtain robust surface temperature and precipitation responses to an idealized irradiance increase. Then, we show that the inclusion of interactive stratospheric chemistry significantly reduces the surface warming (by about one third) and the accompanying precipitation response. This behavior is linked to photochemically-induced stratospheric ozone changes, and their modulation of the surface solar radiation. Our results suggest that neglecting stratospheric photochemistry leads to a sizable overestimate of the surface response to changes in solar irradiance. This has implications for simulations of the climate in the Last Millennium and geoengineering applications employing irradiance changes larger than those observed over the 11-year sunspot cycle, where models often use simplified treatments of stratospheric ozone that are inconsistent with the imposed solar forcing. 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33