A recent study reveals that New Zealand's major coastal cities are facing significant risks from both rising sea levels and land subsidence.

Research indicates that shorelines near all major New Zealand cities are subsiding by a few millimeters annually, with Christchurch experiencing some of the highest rates due to post-earthquake land adjustments.

Human activities, including groundwater extraction, dredging, and land reclamation, are exacerbating the risk of subsidence in these coastal areas.

The study identified urban hotspots of rapid land sinking, particularly in areas with reclaimed land, such as Porirua Harbour, where certain sections are sinking at rates of 3 to 5 millimeters per year.

In Christchurch, localized subsidence rates in suburbs like Southshore and New Brighton reached up to 8 millimeters per year, significantly higher than previously reported data.

Globally, sea levels have risen by an average of 18 centimeters since 1900, with the current rate of increase at approximately 4.4 millimeters per year, exacerbating storm and tidal surges.

The phenomenon of relative sea-level rise occurs when land subsidence, which can double or triple the effects of rising seas, is factored into projections for coastal communities.

Estimates suggest that relative sea-level rise in urban areas may reach or exceed 7 millimeters per year, potentially leading to a one-meter rise by the end of the century, threatening coastal defenses.

While the study indicates no recent acceleration in subsidence rates, it emphasizes that vertical land motion must be considered in future urban planning and assessments of sea level rise.

The research utilized satellite-based InSAR technology to measure vertical land movement in urban coastal strips, including Auckland, Tauranga, Wellington, Christchurch, and Dunedin, from 2018 to 2021.

New Zealand's geological activity complicates the understanding of subsidence, as the country is situated on the boundary of two tectonic plates, leading to both earthquake risks and complex land motion patterns.

Critical infrastructure, including heavy industries and wastewater treatment plants, is located in subsiding areas, raising concerns about their long-term viability.