Economic development and population growth have unleashed unprecedented change in the Earth’s atmosphere. Earth observation (EO) satellites have propelled not only our ability to investigate this change and to monitor the consequences in real-time but also to make issues around weather and climate more visible and understandable for the general public.

Every year, the Conference of the Parties (COP) brings together the nations that are signatories to the UN Framework Convention on Climate Change (UNFCCC). This year will be the 21st such meeting, the two most emblematic having been Kyoto in 1997 and Copenhagen in 2009. Copenhagen set a target of capping long-term global warming at 2°C above pre-industrial temperatures. That marked a decisive step, but we now need to translate this commitment into actions. To reach this target, the conference in Paris aims to secure a specific agreement on reducing greenhouse gas emissions after 2020.

To meet the objective of 2°C, we need to start by breaking the emissions spiral by 2020 and then reduce our greenhouse gas emissions by at least a factor of two, ideally three, between 2020 and 2050 to achieve carbon neutrality – i.e., zero net carbon emissions – by the end of the century. The best way to view the problem is to look at our energy requirements, which currently are met largely by burning fossil fuels. To reach the long-term 2°C target, we need to keep nearly 80% of fossil fuel reserves obtainable with current technologies and costs in the ground. We must not use more than 20% of these easily accessible reserves. Seen in that light, it is obvious that we need to completely rethink our current development model.

The role of the Intergovernmental Panel on Climate Change (IPCC) in this process is of the utmost importance. Indeed the IPCC’s mission is to provide a diagnosis and scientific elements on how the climate system works, the impacts of disruptions to this system and possible solutions to inform policymakers’ decisions.

Satellite data have truly revolutionized how we see the climate system. The most striking example is sea level, which is currently rising at a rate of 3mm a year. Satellite measurements at the polar ice caps and on temperate glaciers have shown that roughly half of this rise is due to ice melt. Slightly less than the remaining half results from expansion of the warming ocean, which stores 93% of excess heat. The remaining proportion comes from extracted groundwater, which obviously ends up in the sea; it is not huge, but it accounts for 13% of sea level rise all the same. Satellite data enable us to precisely calculate these different contributions. Without such data, we would not have that kind of precision, especially when determining regional characteristics. It’s the kind of thing we didn’t even dare dream about 30 years ago!

CEOS ensures international coordination of the civil Earth-observing programmes of more than 30 of the world’s leading space agencies.

These agencies are collectively investing billions of dollars in space infrastructure with the capability to provide sophisticated, continuous and sustained observations of the entire planet. The global nature of climate change issues presents special challenges in terms of the need for global information and data on key planetary indicators that can provide the information required for governments and policy makers to make well-informed decisions. Recognising that no single country can satisfy all of the observational requirements necessary for monitoring of the Earth system, governments are taking steps through CEOS to harmonise and integrate their observing network.

This report explores how satellite EO can contribute to the challenges of providing the information that society needs in order to accurately characterise the nature of changes to our climate and to help define the most cost-effective strategies for mitigation and adaptation. Satellite EO data have already proven to be fundamental to the development of the evidence that has persuaded governments to act. Satellite EO data will continue to be an essential source of evidence necessary for informed decision-making, supporting the science that underpins climate policies.