Large contribution of natural aerosols to uncertainty in indirect forcing

摘要

明确气溶胶对云反照率的影响（实质上是它们对气候的作用力效应）是现代气候科学的最大挑战之一。人们经常默认：持续的高不确定性主要与由人类活动造成的排放有关。换句话说，如果人类的影响能够被更好地了解，总体效应也就会被更好地了解。现在Ken Carslaw 及同事发表了代表对可能会影响云亮度的气溶胶和前体气体排放以及其他因素的28个参数所做的一项分析。他们发现，自工业化前期（公元1750年左右）以来气溶胶作用力所发生变化的只有34%与人类活动造成的排放有关，而45%的变化与火山二氣化硫的自然排放、海洋二甲基硫化物和其他自然来源有关。这项工作向纯粹通过在了解人类活动产生的气溶胶方面的发展所能获得的进展程度提出了质疑，同时也表明我们需要对工业化之前的环境机理有更多了解，当时的自然气溶胶居主导地位。%The effect of anthropogenic aerosols on cloud droplet concentrations and radiative properties is the source of one of the largest uncertainties in the radiative forcing of climate over the industrial period. This uncertainty affects our ability to estimate how sensitive the climate is to greenhouse gas emissions. Here we perform a sensitivity analysis on a global model to quantify the uncertainty in cloud radiative forcing over the industrial period caused by uncertainties in aerosol emissions and processes. Our results show that 45 per cent of the variance of aerosol forcing since about 1750 arises from uncertainties in natural emissions of volcanic sulphur dioxide, marine dimethylsulphide, biogenic volatile organic carbon, biomass burning and sea spray. Only 34 per cent of the variance is associated with anthropogenic emissions. The results point to the importance of understanding pristine pre- industrial -like environments, with natural aerosols only, and suggest that improved measurements and evaluation of simulated aerosols in polluted present-day conditions will not necessarily result in commensurate reductions in the uncertainty of forcing estimates.