Fig. 1.

Prescribed forcings. (Top) Volcanic forcing is indicated as global visible optical depth. (Middle) Solar forcing is obtained by scaling ¹⁰Be data of ref. 3 to a Maunder Minimum reduction in solar irradiance relative to today of 0.1% (green dash-dot, low solar simulation from 1550 to 2000 AD), 0.25% (red, solid, medium solar simulation from 850 to 2000 AD), and 0.65% (blue, dash, high solar simulation from 850 to 2000 AD). The 11-year solar cycle is not included in the forcing. Anthropogenic SO₂ emissions (dash-dot) and forcing by greenhouse gases (Bottom, solid), expressed as CO₂ equivalent, are from ref. 42. The atmospheric CO₂ concentration is shown by the black, dashed line. Three long simulations were run with prescribed solar (high, medium, and low scaling) and identical volcanic, greenhouse gas, and tropospheric sulfate forcing. In addition, three “natural-only” simulations were branched off from each long integration at year 1870 AD, with greenhouse gas concentrations and anthropogenic sulfate kept constant at 1870 AD levels (green lines).