Carbon storage and sequestration are key ecosystem services critical to human well-being and biodiversity conservation. In a warming context, the quantification and valuation of carbon storage and sequestration is important in ensuring that effective incentives are put in place to tackle climate change. The quantification and valuation of ES such as carbon storage and sequestration requires the calculus of actual values and prediction, however, it usually does not include key processes that can indirectly influence carbon dynamics (i.e., risk, conservation or management). Here, we define a multifactorial approach to value ecosystem services based on two stages: (1) a biophysical approximation that integrates yearly supporting ecosystem services (i.e., quantification of carbon storage and sequestration) and (2) a weighing approach including factors that indirectly influence carbon storage and sequestration or that deserve specific attention (i.e., risk, conservation or management factors). The quantification of carbon storage and sequestration indicated that Spanish forests store on average 43 Mg C ha-1 and sequestrate on average 1.02 Mg C ha-1 year-1. Forest structure was a strong determinant of carbon storage and sequestration in Iberian forests, hence there was a strong spatial variation in the carbon sink. We adapted the weighting values to a financial cap and the monetary value of carbon increased more than four times when the weighting factors were taken into account. Finally, we argue that a multifactorial approach to value supporting ecosystem services incorporating aspects related to conservation and risk prevention can facilitate ecosystem service valuation and assist policy makers and stakeholders to establish payment service policies.