Since 1880 sea levels have risen about 8 inches. For most of us, that has not posed a problem.

Where it has posed a problem it is combined with both natural and human-caused subsidence. In parts of the globe threatened by rising tides, subsidence has played a bigger role than the rising seas.

“Subsidence is a common cause of amplified relative sea-level rise, flooding, and erosion in coastal environments. In particular, subsidence due to sediment consolidation can play a significant role in relative sea-level rise in large deltas. We use a combination of InSAR (interferometric synthetic aperture radar), leveling, and global positioning system data to map absolute vertical land motion in the Fraser River delta, western Canada. We show that primary consolidation of shallow Holocene sediments is the main cause for the slow subsidence (−1 to −2 mm/a) affecting the delta lowlands. In addition, parts of the delta undergo increased anthropogenic subsidence. Rapid subsidence rates (−3 to −8 mm/a) are associated with recent artificial loads and exhibit a first-order exponential decrease with a time constant of ~20 years, consistent with the theory of consolidation. Assuming two sea-level rise scenarios of 30 or 100 cm by the end of the twenty-first century, natural subsidence will augment relative sea-level rise in the Fraser Holocene lowlands by ~50% or ~15%. Anthropogenic subsidence will augment relative sea-level rise by ~130% or ~40%, potentially raising it to as much as 1–2 m. In deltaic, lacustrine, and alluvial environments, anthropogenic sediment consolidation can result in significant amplification and strong spatial variations of relative sea-level rise that need to be considered in local planning.”

“Subsurface fluid-pressure declines caused by pumping of groundwater or hydrocarbons can lead to aquifer-system compaction and consequent land subsidence. This subsidence can be rapid, as much as 30 cm per year in some instances, and large, totaling more than 13 m in extreme examples. Thus anthropogenic subsidence may be the dominant contributor to relative sea-level rise in coastal environments where subsurface fluids are heavily exploited.”

“Erban et al (2014) use satellite-radar interferometry (InSAR) to document recent subsidence rates of 1–4 cm yr−1 over large parts of the Mekong Delta, and show that this widespread subsidence is likely caused by groundwater pumping and associated water-level declines. They project ∼0.9 m of human-induced subsidence by 2050, versus ∼0.1 m of expected sea-level rise.”

“In some areas of the world, the obvious and expensive damage caused by anthropogenic coastal subsidence has prompted concerted efforts to arrest and reverse groundwater-level declines, often by importing additional surface water. For instance in the greater Houston (USA) area, nearly 3 m of coastal subsidence due to aquifer-system compaction caused billions of dollars in damage and in 1975 prompted establishment of a Subsidence District with regulatory authority (Galloway et al 2003).”