Redistribution of Light Absorption by Chlorophyll Decline Delays Warming by Up to a Decade

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Abstract
Chlorophyll regulates solar radiation absorption in the upper ocean, influencing the ocean's mean state and climate. However, its response to climate change in Earth System Models (ESMs) remains unclear. Here, we show that in addition to well‐known effects on the carbon cycle, projected chlorophyll decline reduces the absorption of solar radiation within the ocean's mixed layer, effectively redistributing 0.1–0.87 W m −2 of globally‐averaged radiative flux to the upper thermocline underneath it. Based on an idealized assessment of the heat‐equivalent effect, this redistribution may delay surface warming by 2–12 years under the SSP5‐8.5 scenario. The uncertainty arises from inter‐model differences in the simulation of chlorophyll and its solar absorption in ESMs that simulate these processes. Such differences also contribute to biases in the mean pre‐industrial state of ESMs. Our findings highlight the need to reduce uncertainty in physical, not just biogeochemical, feedbacks associated with chlorophyll in ESMs.

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