The importance of GRACE

How it works

The Gravity Recovery and Climate Experiment (GRACE) is carried out by twin satellites in near polar orbit 500km above the earth. About 220km apart, they make 16 orbits of the earth a day. Each satellite is equipped with microwave sensors which measure the distance between them with extraordinary accuracy of ±5 microns. Other sensors linked to GPS satellites record the precise position of GRACE over the Earth’s surface.

The two satellites constantly maintain a two-way microwave-ranging link between them. Fine distance measurements are made by comparing frequency shifts of the link. As a cross-check, the vehicles measure their own movements using accelerometers.

As the satellites circle the globe they sense minute variations in Earth's gravitational pull. When the first satellite passes over a region of slightly stronger gravity, an anomaly, it is pulled ahead of the trailing satellite. This causes the distance between the satellites to increase. The first spacecraft passes the anomaly, then slows down again; meanwhile the following spacecraft accelerates, then decelerates over the same point.

By measuring the constantly changing distance between the two satellites and combining that data with precise positioning measurements from GPS, scientists construct a detailed map of Earth's gravity at a particular point in time, say monthly.

All of this information is then downloaded to ground stations. To establish baseline positions and fulfill housekeeping functions, the satellites also use star cameras and magnetometers.

Why record gravity?

Changes in gravity occur when the density or mass of an object changes. In other words, changes on or under the Earth’s surface can be detected by measuring the strength of gravity and comparing that measurement with a previous or subsequent measurement.

The gravitational pull exerted by a mountain range, an open plain or an ocean will remain constant over a long period since it usually takes millennia for mass and density to change. This measurement is known as mean or long-term gravity. This does not mean that mass can not change more rapidly. It does, primarily through the accumulation or loss of water in the form of snow, ice or as a liquid which is subject to movement by melting, through evaporation or by flowing to other locations. These shorter term changes are known as time variable gravity.

Over a few centuries, snow falling on a mountain can compact into ice and the ice can accumulate into an ice sheet or flow into glaciers, increasing the mass of the mountain and therefore its gravitational pull. Similarly, an aquifer may shrink in size and that change can be detected by measuring the size of change in gravity occurring at that location. The presence of water will cause an increase in gravity while a reduction in its volume reduces its mass and therefore its gravitational pull.

By measuring gravity at the same locations at different times, GRACE measures changes in time variable gravity producing data that can be used to measure:

• changes in mass of polar ice caps;
• change in other land based ice and snow cover;
• rising sea level resulting from ocean temperature and changes in mass;
• changes in water resources on and under land;
• shallow and deep ocean current transport; and
• atmosphere-ocean mass exchange.

This enhanced knowledge is expected to result in a better understanding of the forces that drive El Niño and La Niña, more accurate seasonal forecasts of Earth’s weather patterns, ability to track the changing distribution of water resources and in improved forecasting.

What GRACE tells us

Since its launch in 2002, GRACE has produced data which have been used to calculate changes in aquifers, the rate at which polar ice is melting and the loss of mountain ice and water from aquifers. From this source it has been confirmed that:

• The Greenland Ice Cap is currently loosing ice at a rate of 250 gigatonnes per annum (1 gigatonne = 1 cubic km of water) and the rate of loss has recently been increasing at about 7 per cent per annum.

The major factor has been increased discharge from glaciers, primarily due to relatively warm water melting ice below sea level at a much faster rate than ice above sea level. The effect has been to cause ice slowing glacier flows to become stressed producing increased calving enabling glaciers to flow more rapidly.

• Arctic Sea ice cover has contracted by 23 per cent (1.68m sqkm) compared with average (1973-2000) cover and the remaining cover has lost density, which surface examination confirms is due to thinning.

Problems arising from this are that existing ice is prone to more extensive and speedier melting in future years because it is thinner, resulting in continued loss of cover and complete absence of sea ice in summer within the next 25-40 years.

• The West Antarctic Ice Sheet (WAIS) is now loosing ice at a rate of 132 gigatonnes per annum and the rate of loss has been increasing over the last five years. It is expected to continue increasing.

WAIS is a marine ice sheet anchored to the sea floor and covers an archipelago west of the Trans Antarctic Mountain range. It is vulnerable to melting in summer due to warming air temperatures, particularly over the Peninsula where an increasing number of glaciers are in retreat. WAIS is particularly vulnerable to melting from contact with a warming Southern Ocean and warmer currents from equatorial regions flowing directly on to the ice.

• The East Antarctic Ice Sheet has been loosing ice at a net rate of some 55 gigatonnes per annum since 2006, indicating it was loosing ice at a slower rate in prior years.

Until data became available from GRACE, it was assumed by many scientists that increased precipitation over parts of East Antarctica contributed to net growth of the ice sheet. At the very least, the ice sheet was not affected by global warming, was holding its own and not contributing to rising sea levels.

That has been shown to be wrong and it has been necessary to revise forecasts of the extent to which ice will be lost and its effect on sea level. GRACE data not only shows that East Antarctica is loosing ice but that it is doing so at an increasing rate.

• Aquifers in Northern India and adjacent Pakistan are being pumped at an unsustainable rate because of the need to supplement rivers and rainfall to supply increasing population and irrigation of food crops grown to feed them.

This pumping is being encouraged by the Indian government in an area with a population of about 600 million. No government program exists to limit population growth. An increasing population is threatened by the prospect of exhausted aquifers and reduced river flows due to glacier retreat in the Himalaya and Hindu-Kush mountains, yielding less water.

Within the next 40 years, it will no longer be possible to supply the population with sufficient food or water. It will therefore have to move to better supplied areas, likely to be resisted since this will overstrain their resources or deal with increasingly severe famine conditions to which many will succumb.

• Californian aquifers in the Central Valley are being pumped at unsustainable rates to provide water for crop irrigation, due to reduced river flows and, in recent years, low rainfall.

Over the last seven years, Central Valley aquifers have lost the equivalent of 30 cubic km of water. The Valley is fed by the Sacramento and San Joaquin rivers. Both are fed from glaciers in the Sierra Nevada. Those glaciers are rapidly melting, producing less water, reducing both river flows and aquifer replenishment.

This presents a serious problem for one of the most important agricultural areas in the USA. The Central Valley produces more than 8 per cent of the nation’s food crops, including valuable exports. On going melting of Sierra Nevada glaciers will result in further decline of water tables, water shortages, decreasing crop sizes and continued land subsidence.

Data provided by GRACE and other satellites enables studies to be undertaken which include calculation of the magnitude and effects of ice and water loss and identifies trends. This information warns us of the consequences of continuing activity causing or contributing to global warming.

This enables government and industry to take appropriate, timely action to reduce greenhouse gas emissions responsible for global warming. GRACE provides our leaders in science, politics and industry with data warning them of the consequences of not acting and removes from them the excuse that they didn’t know.

GRACE also provides empirical evidence which refutes arguments put forward by those who contend that global warming is not occurring and claim there is no evidence that it is.