Sea levels and sea-level rise
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Absolute (or eustatic) sea level represents the sea height relative to the centre of the Earth, whereas relative (or local) sea-level rise includes both the absolute sea-height change plus changes (up or down) in land height for the relevant coastal area.
Tidal gauges measure relative sea-level rise. Although such measurements do not distinguish between the ocean level that is rising and the land that is slowly sinking or rising—nevertheless it is the relative sea-level rise that local communities need to adapt to. However, co-locating a continuous GPS instrument near a tide gauge does enable the local land movement to be separated from the rising sea around New Zealand.
By knowing both the absolute mean sea level and the rate of land movement, then the combination of these two components will inform projected changes to the sea level for that region.
The shoreline at any time is defined by the spatial position of the land-sea interface upon the land surface, using the mean high water spring mark (which is the average high tide approximately every fortnight soon after a new or full moon). This level is determined by tide-gauge measurements (e.g., Standard Ports by Land Information NZ).
Tide gauges measure relative SLR:
The left-hand diagram shows the situation where the landmass, on which the tide gauge sits, has risen over a lengthy period of years between times t1 and t2 along with a rise in absolute sea level of the adjacent ocean. For this ongoing uplift situation (e.g., parts of Scandinavia), the measured relative SLR will be smaller than the absolute change in sea level, and conversely, if the land is subsiding long-term (e.g., lower North Island), the relative SLR will be higher.
When calculating the average global SLR to date, using tide gauges from around the world, adjustments to each dataset are needed to account for the local vertical land movement.
Satellite-based altimeters measure absolute SLR:
The right-hand diagram shows sea level being measured by altimeters installed on board orbiting satellites such as Jason 2, and the recently-launched Jason 3 in January 2016. Essentially altimeters are a pulsed radar system that detects the average sea-surface height continuously along repeat satellite-orbit tracks, approximately every 10 days, that cover the Earth from 66° latitude either side of the Equator. The satellite orbit has to be accurately tracked and its position and height is determined to a stable reference frame such as the International Terrestrial Reference Frame with its origin at the centre of mass for the Earth.
Absolute sea-level rise calculated from satellites also includes adjustments to account for the large-scale vertical movement of the Earth’s crust and ocean floor (e.g., from past glacial loading during the Ice Age) and changes in gravity by the GRACE satellite (e.g. redistribution of ice and water masses on the Earth from melting glaciers and ice sheets).
The main contributors to the global rise in sea level since the mid-1800s are:
Warming of ocean waters mostly in the upper 0–2000 metre depth layer. As the ocean warms the seawater expands in volume. Most of the increased seawater volume occurs as an increase in the level of the ocean, as the world’s shorelines constrain the ocean boundaries. The thermal contribution varies substantially in different ocean regions and is strongly influenced by El Niño/La Niña and longer climate cycles.
Water mass is added to the oceans from melting or break-up of land-based ice stores such as glaciers and polar ice sheets (particularly Greenland and West Antarctica).
Changes in water properties or flowpaths of the main ocean currents.
Changes in the net storage of terrestrial freshwater e.g., groundwater/river extraction, reservoirs, changes in rainfall and evaporation from climate variability e.g. El Niño/La Niña.
In the last two decades, the relative contributions to the global sea-level rise budget has altered, with land-ice contributions now larger than the thermal-expansion contribution as shown in the Figure (IPCC AR5). For now, Greenland is the largest contributor to the polar ice-sheet component, however Antarctica is making an increasing contribution. Net changes in water storage and oceanic currents are much smaller contributors to global sea-level rise.
Local SLR (also called relative SLR) is also affected by the vertical land movement. This is influenced by various natural or human factors, such as:
• subsidence of large river-delta systems or from pumping of underground aquifers or oil/gas reservoirs.
• tectonic effects of uplift or subsidence arising from earthquakes or stresses between major earthquake events. More information for New Zealand can be found here.
• ongoing rebound or relaxation of the Earth’s crust following retreat of the ice sheets since the last Ice Age (or Glacial Maximum) – which scientists call glacial isostatic adjustment (GIA). NZ’s landmass overall is rising slowly with an average GIA of around 0.3 mm/year of relative sea level.
The Diagram shows the effect of land subsidence relative to sea-level rise. It is this relative sea-level rise that has to be adapted to locally or regionally. Conversely, uplift of the land mass reduces the coastal impact of rising ocean levels.
Global average projections
Heat is being trapped in the atmosphere by increasing concentrations of carbon dioxide (CO2) and other greenhouse gases, and the climate–ocean and ice systems are responding. One of the major and most certain (and so foreseeable) consequences is the rising sea level.
The Intergovernmental Panel on Climate Change (IPCC) released its Fifth Assessment Report (AR5) in 2013/14, with Chapter 13 of the Working Group I report providing a detailed synthesis of past, present and future change in sea level.
Due to the non-linear and delayed responses of ocean and ice environments to ongoing climate change, natural variability and uncertainty on rate of global emissions, it is not appropriate to extrapolate historic or even recent trends in sea-level rise observations.
Instead, future projections of sea-level rise are relied on for planning and design purposes. These projections use a range of modelling and statistical approaches to quantify changes to climate-ocean-ice systems. Projections can also include surveying a wider group of experts for uncertain components, for example, polar ice sheet response.
IPCC and researchers world-wide base their projections for sea-level rise on four Representative Pathway Concentrations (RCPs). These 4 scenarios are representative of four different groupings of future radiative forcing (warming) by greenhouse gas emissions and associated social, economic, population and land-use projections. See Appendix C of Coastal Hazards and Climate Change Guidance for further details.
IPCC AR5 bases projections of global temperature rise and sea-level rise on simulations of various global climate–ocean modelling groups using these four RCPs as the input radiative forcing transients representing different pathways of human development. Further information on how projections for sea-level rise are determined can be found in Appendix D of Coastal Hazards and Climate Change Guidance.
IPCC AR5 produced projections for the global average increases in sea level for each RCP out to 2100 in their Summary for Policymakers, as illustrated in the adjacent figure. This covers the likely ranges for the lowest and highest RCP2.6 and RCP8.5 scenarios up to 2100, and shown as bars on the right for all four RCPs averaged over the two decades 2081–2100. In this context, likely range means there is a 33% chance the rise it could lie outside this range.
Across all 4 RCPs, there is near certainty that by mid-century (2050) sea-level rise will lie within a narrow band of around 0.15-0.3 m, but the uncertainty band widens toward the end of this century and beyond, depending on how global emission reductions transpire.
Projections relevant for New Zealand
Research has shown that sea level in the wider New Zealand area will rise by 5-10% more than the global average (e.g. Ackerley et al., 2013, Kopp et al., 2014). Regions within New Zealand will also be affected by vertical land movement and local changes in tides (e.g. harbours and estuaries), which need to be factored in, as each region needs to adapt to the local relative sea-level rise.
Four sea-level rise scenarios out to 2150 for the wider seas around New Zealand are provided in Chapter 5 of the Coastal Hazards and Climate Change Guidance. They are purposefully constructed to meet the requirements in the NZ Coastal Policy Statement to plan at least 100 years ahead and for use in developing and stress-testing dynamic adaptive pathways to plan ahead for adaptation in coast areas.
No, not in the foreseeable future, as annual average sea levels will continue to show year-to-year and decadal variability arising from natural climate cycles like El Niño–Southern Oscillation and the longer 20-30 year Inter-decadal Pacific Oscillation (IPO).
For instance, a change around 1999 in the phase of the IPO led to a step-jump in sea levels around New Zealand and the wider SW Pacific, followed by a few years of relatively stable mean sea levels, before rising again more recently (as shown in the Figure showing the Moturiki Island sea-level series). Peaks also occur in annual sea level in New Zealand during La Niña events and lower annual sea levels tend to occur in El Niño events. This pattern of faster and slower short-term changes in sea level will continue to be superimposed on an underlying longer-term trend of rising sea level due to climate change.
Historically, the gradual rise in sea level has accumulated to be around 20 cm over the 20th century through to present. This can be enough, in combination with a severe storm-tide event, to cause seawater inundation of low-lying coastal margins, as occurred in parts of Auckland in 2011 and 2014. Further rises in sea level will further exacerbate such situations with more frequent coastal inundation and erosion of vulnerable coastal areas (Parliamentary Commissioner for the Environment 2015 report).
The oceans store the majority (90%) of excess heat within the Earth’s climate system, with considerable lag times in the ocean response. Consequently, sea-level rise is also a very consistent and reliable indicator of climate change, albeit one of the last indicators to respond, as the oceans integrate increasing heat input and additional water mass from ice sheets, glaciers and net water loss from the land. So the slower historic (1.8 mm/year since 1900) or even recent rates of global sea-level rise (e.g., 3.4 mm/year from satellite observations since 1992) are not a reliable indicator of future sea-level change by the end of this century, and considerably underestimate the rate at which sea level could rise.