Blue Marble 2.0
The Blue Marble was the first full photograph of the Earth from space. The Apollo 17 mission took it on December 7, 1972 – over forty years ago. Of course, it was not the first time that a photograph of the planet had been shown widely. By then for example, the first image of the Earth as seen from the moon had been circulated. In 1969 the astronauts of the lunar landing had taken the famous shot of “Earthrise”, capturing the solitary fragility of our planet as it rises from darkness. But the blue marble represented a photograph of a different nature. It was comprehensive in scope yet detailed in nature, giving it an unusually high density of information and a powerful evocative quality – a symbol for the beginning of the Anthropocene. It was in fact during the seventies that man began recognizing its role as a member of a planetary ecosystem, and began wrestling with the question of its own impact on a surprisingly finite and vulnerable planet.
The Blue Marble shows the planet from the Mediterranean to Antarctica, with the African continent and the Arabian Peninsula in the foreground, and the Indian subcontinent and the Southern Ocean as frames. It provides an integrated single view of the planet’s atmosphere in its spellbinding complexity: the inter-tropical convergence zone, where moist air flowing equator-ward from North and South, rises in a narrow band of convective plumes that give the characteristic thunderous rainy weather to the tropics; the Sahara and Kalahari deserts around thirty north and south of the Equator, where that same air subsides from its pole-ward flow, drying out any remaining moisture as it completes the cycle of the Hadley cell, the atmospheric overturning circulation spanning the Tropics; a tropical cyclone, fed by the warm surface waters of the Arabian Sea is visible in the top right quadrant; the mid-latitude weather systems of the roaring forties over the Southern Ocean, marked by visible fronts, altocumulus, cirrocumulus clouds; the contour of Antarctica revealed in full view of the sun. A compendium of the Earth’s climate in a single shot.
Iconic geographic images can reframe how we conceive of our place on the planet. They are a recurring cultural phenomenon and a moment of synthesis that reveal the preoccupations of those who produced them. While we cannot know how widespread its adoption was, the first known map of the world – the 6th Century BC “Imago Mundi” from Mesopotamia – shows the city of Babylon as it relates to the surrounding cities, the Euphrates, and the Persian Gulf, synthesizing in one image the primary elements of threat and survival of an entire civilization. The “Peutinger Map”, almost ten centuries later, revealed the extent of the Roman state in one image, providing a map organized around the great land routes that represented the strength of Roman logistics and that had connected the empire. A different type of picture made the headlines around the world in 2015, one that is equally representative of our modern preoccupation with our own sustainability, and evocative of the challenges ahead.
“Tom and Jerry” went up in space on March 17, 2002. Two identical satellites formed the basis for the Gravity Recovery and Climate Experiment, GRACE for short. The satellites orbit the Earth 16 times every day at over 500 km altitude, sending back a map of the distribution of mass on the planet due to the variation in distribution of rocks and water. They produce this data by measuring distortions in the gravitational field caused by slight differences in the distribution. As the two satellites pass over these differences, the first is slightly accelerated or decelerated compared to the second. By measuring their relative distance to an astonishing level of accuracy – the satellites can detect a micron difference over two hundred kilometers – they provide an integrated, point-wise measurement of the gravitational field of the planet – a planetary CT scan of sorts. It is one of the great successes of modern geodesy that the resulting measurement can be inverted, filtered and analyzed to reveal the complex three-dimensional structure of the Earth.
One of the crucial applications of this technology has been to diagnose groundwater storage in the great aquifers of the world. Water is of course of a different density to the surrounding rock, thus leading to slight effects on the gravitational field, which GRACE can detect. Global hydrological assessments have always been hampered by the complex and local nature of the resource, making syntheses of the state of the world difficult to construct and of limited utility. When they do exist, they tend to be encyclopedic in nature, often published in ponderous, static volumes, listing individual rivers and nations as chapters, and based on single numbers, painstakingly inferred, compiled or estimated from sparse measurements and models: hardly an evocative synthesis to stimulate a humanist debate on our place on the planet.
But over the course of almost a decade and half since its launch, the long datasets offered by GRACE have provided the first integrated image of the state of groundwater resource use, an image global in scope, yet local in nature: a detailed, real-time diagnostic of the planet. And GRACE has shed light on the most obscure part of the water cycle – that which is hidden underneath the earth’s surface – and this is the picture that has made the news. It shows that one in three large aquifers in the world appear to be stressed, depleted by people drawing water for human use. It shows that California’s Central Valley, the Arabian Peninsula, the Indus basin all share a common fate, uniting vastly different economies and societies in a planetary challenge: our persistent inability to manage finite resources.
While scientific practice will be integrated rather than dramatically changed by GRACE’s data – remote sensing still requires significant processing, and integration with land-based measurements, to be operationally useful – the resulting images have already started to change the narrative on sustainability. That is why this data has made news in 2015, and why it will continue to be news for the coming years. Like the maps of the past or the Blue Marble photograph before it, it provides a powerful explanatory visual framework for an existential concern of our time: that another finiteness of our planet resides in the water resources we all share. GRACE has shown us that, indeed, we live on a fragile blue marble, one that, when it comes to the resources we directly depend on, is drying at an alarming rate.
