Negligible direct radiative forcing of basin-scale climate by coccolithophore blooms

The water-leaving radiance, defined as radiation from the sun reflected off particles in water and exiting the ocean surface back into the atmosphere and space, is often used to derive ocean-colour information from remotely sensed data. However, it is in itself a measure of the amount of solar irradiance reflected by oceanic constituents and, therefore, not available to the Earth's heat reservoir (changes in which can affect the Earth's energy balance and climate). A strong influence on the water-leaving radiance is observed from coccolithophore blooms, owing to the highly reflective calcareous platelets or coccoliths covering these marine algae. We analysed remotely sensed water-leaving radiances (1998-1999) over the N. Atlantic, where the blooms are spatially and temporally most abundant, and found that the direct radiative forcing of climate between 402-565 nm (the major range of optical influence) by coccolithophores in this ocean is negligible (similar to0.05 W m(-2) mean annually). This is in contrast to what in situ or laboratory measurements on the immense local intensity of coccolithophore visible light scatter in the past two decades have led us to believe.