Satellites and climate

Climate change raises many questions. Satellites could have a decisive role in answering them.

Artist’s rendering of the ESA’s GOCE satellite in orbit. This spacecraft is part of a growing fleet of satellites expected to provide data to understand and monitor climate change. Credit: ESA - AOES Medialab

NB: On account of the United Nations Climate Change Conference (official website) taking place in Copenhagen from 7 to 18 December, we are once again recommending our feature on "Satellites and climate"; initially published in June 2009, it is more than ever a topical issue.

The climate change observed today tends towards warming according to a majority of scientists and is caused mainly by greenhouse gas emissions due to human activity. Increased awareness has emerged and many countries are pooling their efforts to group data from all over the planet. Thus, the UN defined 45 ECVs (Essential Climate Variables): these include such measurements as temperatures, CO2 levels and the salinity of oceans that should contribute to quantifying climatic evolution.

Measurements in the long term
For policymakers, it is important to have reliable data to be able to make decisions on the basis, first, of the direction of climate change and, second, of its cause or causes. This is where satellites become essential thanks to their capacity to provide highly accurate measurements over the entire planet in addition to those made on the ground, but they are necessarily more limited. The institutions in charge of managing weather satellites have fortunately stored the data obtained from the beginning. Thus, EUMETSAT, the organization in charge of European weather satellites, emphasizes that it has made all its archives available to scientists since 1981. On a global level, the GEOSS (Global Earth Observation System of Systems) initiative conducted by GEO (Group on Earth Observations) with 77 countries and the European Commission, aims to harmonize the gathering and sharing of all Earth observation data whether from the ground, sea, air or space. This future “common fund” of environmental data is not limited to climate — since it also concerns the management of natural resources — but the approach tends towards rigorous sustainable assessment of the planet’s health. There is indeed a danger, including from satellites: if the current satellites are not renewed in time, we can fear there may be gaps in the data and then, who will be able to determine whether the missing data are decisive for the future or not? This imperative need for uninterrupted reliable measurements in the long term has led space agencies to reinforce their various Earth observation programmes.

From their orbit, satellites can make accurate measurements over broad areas. Here, the EarthCARE project aims to study the role of the cloud cover in climatic equilibria with support from ESA and the Japanese JAXA Agency for a launch scheduled in 2013. Credit: ESA

In Europe, GEOSS has become GMES (Global Monitoring for Environment and Security - recently named Kopernikus) with the European Union and the European Space Agency (ESA). There again, this programme concerns more than climate and also aims to gather useful data for civil security, resource management and territorial development. The resulting infrastructure, however, based in part on satellites, should help provide measurements made from space durably and continuously.

The need for a reference
Environmental surveillance from space is already a reality, which should be reinforced by the above-mentioned international initiatives. For several years now, Earth observation satellites have made increasing use of instruments specifically designed to study climate. In October 2005, ESA’s CryoSat satellite should measure the thickness of pack ice in Arctic and Antarctic regions with unequalled accuracy. The Agency even officially declared that “its prime objective is to test whether the melting of the ice pack results from global warming.” Alas! failure of the Russian-European Rocket (an old Soviet SS-19 missile modified to become a launcher), in charge of putting the satellite into orbit, brought the mission to an early end.

ESA’s CryoSat satellite was to measure the thickness of pack ice, but its launch resulted in failure. The Agency rapidly set up CryoSat-2 to replace it. Credit: ESA-P. Carril

It can be noted, however, that its importance was such in the ESA’s eyes that the Agency confirmed its replacement with CryoSat-2 as early as February 2006, just 4 months after the failed launch: intended to measure an indicator deemed crucial for global warming, CryoSat-2 should take off in November 2009. Obviously, this is not ESA’s only recent climate-related mission. Last 17 March, the same rocket that failed to launch CryoSat was successful with GOCE (Gravity field and steady-state Ocean Circulation Explorer). Placed at an altitude of only 260 km, this satellite will be affected by variations in the Earth’s gravitational field that disrupt its orbit, because our planet’s weight is not uniformly distributed, which considerably complicates determination of a reference level (the zero point from which the altitude of mountains is calculated, for instance).

The Earth is not a perfect sphere and GOCE will help draw up its precise geoid as a reference to determine ocean levels (among other things). The irregularities are greatly exaggerated in this diagram. Credit: ESA

Obviously such a reference must be accurate to quantify precisely the rise (or drop) in ocean levels, another major indicator of climate change. A precise geoid (the name given to the reference sphere GOCE will measure more accurately) will also facilitate analysis of the dynamics of the main ocean currents that are primordial in our present climate. It can also be noted that GOCE is the first satellite in orbit for ESA’s Living Planet programme. According to the European Agency itself, this programme has a very clear ambition: “to help us improve our forecasts of the possible effects of climate change.”

Ocean currents have a key role in climatic equilibria; it is the purpose of various satellites, including GOCE, to study them further. Credit: ESA-AOES Medialab

On the American side, monitoring climate by satellite is also developing. Unfortunately, NASA’s promising OCO (Orbiting Carbon Observatory), which was to study the carbon cycle on Earth, failed when launched, since the fairing protecting it did not separate from the Taurus rocket as planned. It is obviously not the American Agency’s only Earth observation satellite nor is it in any way the only one dedicated to climate change. NASA has several missions focusing on our planet including the “A-Train” that was to work with OCO. At present, this space “train” is made up of 5 satellites that follow each other in the same orbit at an altitude of 690 km. The French space agency CNES is very actively associated with this programme essentially concentrated on studying our atmosphere. The latest news has it that NASA is interested in studying the possibility of launching a new OCO with the hope of doing so by 2011. In the longer term, for many people, Barack Obama’s election should give rise to more American space programmes dedicated to climate change, after the previous administration’s clearly wait-and-see attitude.

The A-train, a succession of 5 Franco-American satellites probing the Earth’s atmosphere. Credit: NASA

With this intensification of space programmes dedicated to Earth observation, it is now clear that satellites will contribute to the debate on climate change. They may even serve as arbiters!

Published on 2 June 2009