Quantifying coral reef accretion in a changing world: approaches, challenges and emerging opportunities.

The long-term development of coral reef frameworks and the net vertical accretion of reefs fundamentally underpins the provisioning of most reef-related ecosystem services. One area of particular concern at present is how rates of reef accretion are changing under ecological decline and what the consequences of this may be for the capacity of reefs to keep pace with near-future sea-level rise (SLR). This may have major implications for the capacity of reefs to maintain their coastal protective functions and to support reef island stability. Both are issues relevant to understanding future tropical coastal risk. Long-term (millennial timescale) rates of reef accretion are relatively well constrained, including through past periods of sea-level fluctuations. However, widespread and persistent ecological degradation of coral communities has caused many reefs to diverge significantly from their past accretion trajectories. This renders historical analogues increasingly unreliable for projecting future accretion potential. Addressing this necessitates a reorientation towards considering reef accretion rates across shorter (ecological to geomorphic) timescales, i.e., over years to multi-decades. This is essential if we are to better constrain contemporary reef accretion rate and SLR interactions at timescales relevant to predicting emerging coastal risks and understanding future implications for reef-derived benefits. Here, we review existing approaches for quantifying vertical reef accretion rates of modern reefs. These methods span data recovered from fossil outcrops or core-derived records, the conversion of carbonate budget data, direct in situ measurements and emerging remote sensing and image-based techniques. The review explores the advantages and limitations of these different approaches and outlines options for developing an integrated framework to link past, present and future reef accretion potential.