Extended Data Fig. 3: Comparison of warming and OA impacts on silica dissolution.

One-dimensional representation of warming and OA effects on silica dissolution and export to depth in our model, using globally averaged vertical profiles of temperature (left) and pH (middle) under preindustrial conditions and for the year 2200 (values from the RCP8.5 simulation). The right panel depicts the model parameterization for opal dissolution, applied to the corresponding temperature and pH profiles. Averaged over the upper 1,000 m, the specific opal dissolution rate (Diss_opal) under preindustrial conditions amounts to 0.0008 (in units of m⁻¹), that is, 0.08% of sinking opal is dissolved per metre, resulting in a characteristic ‘Martin curve’ of opal flux attenuation with depth (black line). Diss_opal in the model is temperature-dependent and roughly doubles for an increase in temperature of 10 °C. Warming of ~3 °C in the upper ocean until the year 2200 (averaged over the upper 1,000 m) increases Diss_opal by about 1.3-fold (from 0.08% to 0.11%), thereby leading to a stronger flux attenuation with depth (red dashed line) compared to preindustrial conditions. At the same time, pH decreases by around 0.6 units until year 2200 (averaged over the upper 1,000 m). According to our findings, opal dissolution rates are 57% lower per unit decrease in pH, (that is, a decrease by a factor of ~2.5). This effect of pH works in the opposite direction as the temperature effect, that is, slowing down opal dissolution and thus weakening the decline of opal with depth. When considering this pH effect in isolation in our example (that is, not accounting for the effects of warming; dashed blue line), this drop in pH would decrease Diss_opal from 0.08% to 0.045% (that is, reducing it by a factor of 1.8). When considering both warming and acidification at the same time, the pH effect is still strong enough to over-compensate the temperature effect. Diss_opal amounts to 0.059%, which is notably lower than under preindustrial (0.08%) and warming-only (0.11%) conditions. Accordingly, opal dissolution in a warmer and lower-pH ocean is slower than under preindustrial conditions, thereby leading to an enhanced efficiency of silica export to the deep ocean (green line).