Nasa and climate satellites share a common history. Even as the fledgling organisation was struggling with the early frustrations of its initial manned space flight programme, a Juno II rocket successfully launched Explorer 7 into low Earth orbit and ushered in the era of satellite climate monitoring. The date was 13 October 1959 and Nasa was little more than a year old.

Fast-forward to the organisation’s 50th anniversary on 29 July 2008 and the Earth-observing constellation had grown to 21-strong. Unsurprisingly, it is not only the numbers that have changed over the intervening half-century. The climate-monitoring element in that first payload simply consisted of a flat-plate radiometer to quantify the heat reaching and leaving the Earth. Today, satellites employ multisensor arrays to monitor everything from glacial ice to the planetary water cycle.

However, while the technologies may have been transformed almost beyond recognition in their sophistication, the logic behind satellite-based research remains exactly the same – though many would argue that the need for it is now greater than ever.

It is as simple as it is compelling. Before the satellite era, climatologists could only look upward, limiting any attempt to gather data on the middle or upper atmosphere, and largely restricting world-scale understanding to a compendium
of discrete, localised observations. With satellites, the whole thing went global.

“Before the satellite era, climatologists could only look upward, limiting any attempt to gather data on the middle or upper atmosphere.”

A global picture

Achieving the kind of worldwide overview required to meet the needs of increasingly complex climate models inevitably requires cutting-edge technologies across a wide spectrum of sectors, aside from the obvious aerospace demands implicit in the spacecraft themselves.

The instruments on Nasa’s Aqua satellite – part of a huge mission instigated to elucidate the intricacies of the Earth’s water cycle – reads like a checklist of state-of-the-art monitoring devices. The technology includes atmospheric infrared sounding, advanced microwave sounding and scanning radiometry units, and moderate-resolution imaging spectro-radiometry. Aqua and its fellow Earth observation satellites have come a long way since the pioneering days of Explorer 7.

The technology has grown, but then so has the task asked of it. The net result has been to provide today’s climate scientists with access to an unprecedented global picture, replete with levels of detail that would have been almost unimaginable to the likes of professor Verner Suomi – the meteorologist behind that first flat-plate radiometer. While Aqua has been examining the inter-relationships of oceanic evaporation, atmospheric water vapour, clouds, precipitation, soil moisture, ice and snow, other missions have been inexorably adding to our understanding of Earth’s changing climate.

From CloudSat and Calipso, for instance, come near-simultaneous 3D measurements of cloud structure, from Aura, details of the composition, chemistry and dynamics of the atmosphere and from the aptly-named IceSat, data on the size and thickness of ice sheets.

Still more investigate rainfall, wind, ocean flows and solar radiation. If all goes according to plan, 2010 will add another two satellite and 2013, a third – and all three are set to push the climate monitoring envelope even further.

Aquarius and Glory

Currently scheduled for launch on 22 November 2010, Glory will explore the Earth’s energy balance and the effect it has on the climate. Established in a low orbit, the satellite has two main goals. Firstly, to collect information on “black carbon”, and other atmospheric aerosols, in order to complement existing knowledge of the seasonal variability in their properties. Secondly, it will amass data on solar irradiance to contribute to studies of long-term climate shift.

These tasks are potentially enormously significant, since they could go a long way to answering question as to whether temperature increase and climate changes are largely anthropogenic, or merely the consequences of natural events.

Aquarius, also due to launch in 2010, will pioneer the observation of sea surface salinity from space, closing a notable gap in climatologists’ current understanding by gathering more data in two months than has been collected by conventional means over the last 100 years.

Capable of a detection accuracy of 0.2psu – equivalent to a pinch of salt in a gallon of water, according to Nasa – the satellite will extend the boundaries of our knowledge of oceanic circulation and the global water cycle, enabling more comprehensive climate models to be developed.

The next generation

Most of the world’s water is contained in the oceans; only 3% is freshwater and two-thirds of that is in the form of permanent ice. That 1% which is available, however, forms a vitally important component of the Earth’s hydrological cycle – socio-economically as much as bio-climatically – and precipitation represents one of its most critical elements.

“The only practical way to quantify rain, snow and ice fall is to do it from space.”

It is easy to see why. The world’s population has doubled since 1950 – and water use has tripled as a result.

With an estimated one billion people already denied access to clean potable supplies, and against a backdrop of changing climate and burgeoning demand, the future availability of freshwater is clearly of massive social importance. It also has ramifications for virtually every other environmental issue too.

Without an accurate measurement of the global distribution and intensity of precipitation, climate study lacks one of its most crucial factors, yet quantifying rain, snow and ice fall arguably remains the biggest challenge facing Earth science. The only practical way to do it is from space.

Nasa’s global precipitation measurement (GPM) mission, scheduled for launch sometime in 2013, arose in response. The satellite will carry a conically-scanning radiometer and dual-frequency cross-track scanning radar and provide the calibration standard for other members of the GPM constellation. The overarching scope of the mission should lead to a better understanding of the role of precipitation within the global system and help examine the wider context of natural and human-induced climate change.

Soundness of observations

For all the successes, the climate monitoring programme has not been without its set-backs. In February 2009, for instance, the launch of the $278m Orbiting Carbon Observatory ended in failure when it fell into the South Pacific.

In 1665, scientist and philosopher Robert Hooke wrote “the science of nature has been already too long made only a work of the brain and the fancy; it is now high time that it should return to the plainness and soundness of observations.” With satellite-based missions, today’s climate science seems to have firmly embraced that ideology.