Carbon capture by expanding temperate mangrove forests

Mangrove forests provide a wide range of ecosystem services that benefit both people and nature. While many countries have lost nearly 50 % of their mangroves in the past 50 years, Aotearoa New Zealand's mangroves are rapidly expanding in many locations due to increased sediment inputs from land-use changes and urbanisation. In the Auckland region, where mangrove extent has increased 4.5-fold since 1940, we quantified the blue carbon sequestration benefits from seven mangrove expansion sites. Our results indicate that New Zealand's mangroves sequester 86,000 t CO₂ annually, with an additional 3,400 t CO₂ sequestered in 2020 due to mangrove expansion. Specifically, mangrove carbon sequestration in the Auckland region has risen from 7,700 t CO₂ yr⁻¹ in 1940 to 34,600 t CO₂ yr⁻¹ in 2020 - equivalent to 50 % of the region's current emissions from forestry, fishing, and mining. Although the expansion of New Zealand's mangroves may impact perceived coastal amenity values, their role as significant carbon sinks is critical for mitigating global carbon emissions and supporting the country's commitments to the Paris Agreement. However, like many other countries, New Zealand has yet to include mangrove carbon sequestration in its national policies and management strategies. To move forward, we recommend: 1) integrating blue carbon into the national greenhouse gas inventory; 2) monitoring mangrove expansion and its likely drivers; and 3) working with local communities to better understand conflicting socio-ecological values of expanding mangrove forests.