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Future Floods and Melting Ice Sheets – Predicting with GRACE

A picture showing the artist's vision of the buildings of Parliament and Big Ben clock tower being overtaken by the waves, or more precisely by a muddy version of Hokusai's Big Wave. Image: NaturPhilosophie
Forecasting Sea-Level Rise

On the whole, Earth scientists agree that melting of land ice greatly contributes to sea-level rise.  And one thing’s for sure.  Future global warming will exacerbate the risks posed to human civilisation.  But…  What if you could forecast major floods?  You can. 

NASA developed a forecasting tool that reveals which cities will be affected as different parts of the ice sheet melt over the next decades.

The simulation looks at how water on Earth gets redistributed over time.

 

Gravitational effects and the Earth’s spin are taken into account to predict how water will potentially be redistributed.

 

A New Visualisation Tool by NASA

A slide from NASA JET reproducing a modelled sea level rise simulation with GRACE.
Global Relative Sea-Level Rise simulation. This model computes local sea level rise (or fall) using GRACE observations of melt in Greenland and Antarctica, for present-day. Source: NASA

The online simulation reveals which cities will be affected as different portions of the ice sheet melt.

Nasa’s Jet Propulsion Laboratory in California published their findings in “Should coastal planners planners have concern over where the land ice is melting?” in Science Advances magazine.

For each city, the model provides a picture of which glaciers, ice sheets, and polar ice caps are specifically important.

 

London

For London, the model suggests that sea-level rise could be significantly affected by changes in the north-western part of the Greenland ice sheet.

New York

Whereas for New York, the area of concern is the ice sheet’s entire northern and eastern portions.

Sydney

In Sydney, forecasts of sea-level changes are “very strongly influenced” by ice changes that occur along the north-northeast and north-northwest coasts of Antarctica.

Contribution of Greenland Catchment Basins to Local Sea-Level

Gradient Fingerprint Mapping (GFM)-derived contribution of five catchment basins (Petermann Glacier, Helheim Glacier, NorthEast Greenland Ice Stream, and Jakobshavn Glacier) to Local Sea-Level (LSL) in London, New York and Sydney, based on an average of two SeaRISE* model runs over a 200-year time period.

Map and close up representation of the affected coastlines.
Sensitivity of SLR to worldwide variations in ice thickness. The gradient units reflect a measure of SLR change (in mm) per unit of ice thickness change (in m) per area unit of ice (in km2), which is equivalent to SLR change (in mm) per unit change in ice mass. Source: Larour et al. (2017)

For each basin, the projected GMSL value is provided for reference.

For each city, a value of LSL is provided in centimetres, along with a ratio to Global Mean Sea Level (GMSL) in %.

Contributions to Local Sea-Level from Glaciers + IcesheetsPetermann Glacier: 8.22 cmHelheim Glacier: 1.4 cmNorth-East Greenland Ice Stream: 10.6 cmJakobshavn Glacier: 4.41 cm
London3.38 cm (41%)−0.174 cm (−12%)1.7 cm (16%)0.257 cm (6%)
New York5.1 cm (62%)0.497 cm (36%)7.2 cm (68%)1.54 cm (35%)
Sydney8.96 cm (109%)1.39 cm (99%)11.3 cm (106%)4.43 cm (100%)

*The SeaRISE experiments provide quantitative projections of Greenland and Antarctica over the next 500 years.  The purpose in using these experiments was to demonstrate the impact of amplitude and spatial structure of ice mass changes on SLR at select coastal cities, using an ensemble representative of the spread in current model projections of the Greenland Ice Sheet.

Source: Larour et al. (2017)

A new diagnosis tool.

 

The Virtual Earth System Laboratory (VESL) software tool can be accessed online for research.

 

Gradient Fingerprint Mapping (GFM)

The interactive tool provide values for the gradient of local sea-level with respect to ice thickness changes over all glaciated areas of the World.

This is called the Gradient Fingerprint (dS/dH)

where

S is local relative sea level rise

H is ice thickness changes.

Unlike other simulations in VESL, this not an interactive computation.

All results are pre-calculated.  They can be downloaded by clicking on the Download button prior to displaying.

The simulation relies on the ISSM-SESAW (Adhikari et al., GMD 2016) and ISSM-AD (Larour et al., GMD 2016) capabilities to compute the gradient dS/dH (where S is local relative sea level rise) and H is ice thickness changes across the World.

The forward model itself (upon which the gradient computation is based) captures sea level rise over the entire planet, taking into account:

  • eustatic sea level – sea level rise from perturbations due to the gravity and rotational potential of the Earth, and
  • sea level rise from local elastic rebound of the Earth’s crust.

 

Sea-Level Mapping Simulation

As land ice is lost to the oceans, both the Earth’s gravitational and rotational potentials are perturbed, resulting in strong spatial patterns in SLR, termed sea-level fingerprints.

The forecasting tool reveals which cities will be affected as different portions of the ice sheets melt.

Here, the Ice Sheet System Model (ISSM) team hosts simulations related to glaciers, ice sheets, sea level, and solid earth.

 

The Earth Science simulations fall into a number of categories:

  • Simulations of Glacier Flow and Sensitivity to Climate

Model the evolution of glaciers in Alaska, Patagonia and the Himalayan regions in response to variations in surface temperature, snow precipitation, and other factors related to climate change.

  • Simulations of Ice Sheet Flow and Sensitivity to Climate and Forcings

Model the evolution of ice sheets such as Greenland and Antarctica.  Understand what factors control their evolution.  And how they will contribute to sea level rise in the coming decades and centuries.

This includes basal friction at the ice/bed interface, snow precipitation, temperature, etc.

  • Simulations of Sea Level Rise and Contribution from the Cryosphere

Model the evolution of sea level over the entire Earth, and understand the contribution of glaciers and polar ice caps to its rise or decline in a changing climate.

Key processes influence the “sea-level fingerprint”, or the pattern of sea-level change worldwide:

  • The first process is gravity.

Ice sheets are huge masses that exert an attraction on the ocean.  When the ice shrinks, the attraction diminishes and the sea moves away from that mass.

  • Along with this “push-pull influence” of ice, under a melting ice sheet, the ground expands vertically, having previously been compressed by the sheer weight of the ice.
  • Finally, the last factor is Earth’s rotation on itself.

As the Earth spins, it wobbles and as masses on Its surface change, that wobble also changes over time, forcing the redistribution of water around the surface.

… With Grace

An artist's impression of GRACE flying through the upper layers of the Earth atmosphere.
Artist’s impression of GRACE (Gravity Recovery and Climate Experiment) Twin Satellites. Source: NASA

Launched in March of 2002, the GRACE (Gravity Recovery and Climate Experiment) mission was designed to map variations in Earth’s gravity field accurately.

These gravitational variations include:

  • changes due to surface and deep currents in the ocean
  • runoff and ground water storage on land masses
  • exchanges between ice sheets or glaciers and the ocean
  • variations of mass within Earth.

The Mission

Designed for a nominal mission lifetime of five years, GRACE consists of two identical spacecraft, flying about 220 kilometres (137 miles) apart in a polar orbit 500 kilometres (310 miles) above the Earth’s atmosphere.

GRACE mapped the Earth’s gravity field by making accurate measurements of the distance between its two satellites, using GPS and a microwave ranging system.

NASA’s GRACE provided scientists all over the world with an efficient and cost-effective way to map Earth’s gravity field with unprecedented accuracy.

Now, the results from the GRACE mission are yielding crucial information about the distribution and flow of mass within Earth and its surroundings.

GRACE Time Series

From 2003 to 2011, monthly changes in Antarctic ice mass, in Giga-tones were measured by NASA’s Gravity Recovery and Climate Experiment (GRACE) satellites.

The data illustrate the continuing loss of ice from the continent.

The plots depict results from five different IMBIE team members using different methods.  The data were adjusted to reflect new models of post-glacial rebound.

An x-y diagram of part of the information recorded by GRACE.
GRACE Time Series Graph: Monthly Changes in Antarctic Ice Mass, in Gigatones.  Source: NASA-JPL/Caltech; NASA GSFC; CU-Boulder; Technical University of Munich; Technical University of Denmark; Delft University of Technology, Aerospace Engineering, Netherlands; Durham University, UK; Leeds University, UK

The mass balance of the ice is not modelled, but forced by GRACE time series of ice thickness change in Greenland and Antarctica and other glaciers around the world from 2003 to the present day.

Additionally, the forecast model also provide the Local Sea Level contribution and the Ice Thickness change for each glaciated area.

Future Floods

A NASA slide showing a simulation of the Greenland Basal Friction.
Greenland Basal Friction simulation. This simulation of Greenland is the result of work carried out by the ISSM team for the SeaRISE experiments.  Source: NASA

Overall, scientists do agree that land ice melting greatly contributes to sea-level rise.

And the certainty is…  Future global warming will exacerbate the risks posed to human civilisation.

As cities and countries attempt to build plans to mitigate flooding, they may have to be thinking at least as far as 100 years in the future.