To the Editor

The Paris Agreement1 of the United Nations Framework Convention on Climate Change in December 2015 was a landmark event. Stakeholders such as cities, companies and citizens that endure the impacts of a changing climate, as well as policymakers, now need to know what impact their future choices will have on the environment. Systematic observations of the climate system and of greenhouse gas (GHG) emissions are needed to track progress and identify where additional goals may be needed. We suggest that, although the targets agreed in Paris — to hold global warming to well below 2 °C and avert loss and damage — are formulated around temperature, monitoring and meeting these targets requires a broader range of climate indicators2. Global mean surface temperature alone has proved problematic for communicating the impacts and evolution of climate change. The warming of the oceans, sea-level rise, increasing ocean acidity, melting glaciers and decreasing snow cover, and changes in Arctic sea ice also need to be taken into account.

Such a set of indicators should be able to convey the range of climate change impacts and how they are evolving. Following the Paris Agreement, policymakers will need reliable evidence of the impacts of climate change on society to assess the increasing risks, for example to infrastructure, food security and water resources.

Through the Global Climate Observing System (GCOS), a set of 51 essential climate variables (ECVs) have been defined: they aim to describe key aspects of the behaviour and composition of the land, oceans and atmosphere (see Table 1). This in turn has led to substantial investment, and to improvements in the implementation of a space-based and in situ monitoring system for the climate research community3,4. This collective effort provides remarkable understanding of the climate.

Table 1 List of essential climate variables as proposed the in the draft GCOS Implementation Plan (ref. 8)

The concept of defining ECVs has proved very effective. It has now been adopted by a number of other scientific communities, particularly linked to multilateral environmental agreements such as the UN Convention on Biological Diversity and the UN Sustainable Development Goals. Examples include — in various stages of implementation — essential ocean variables5, essential biodiversity variables6 and essential water variables7. Observing these essential variables will yield sustained global information on changes in key Earth system variables, and hence a better scientific understanding of future risks. This knowledge can feed into the development of specific prevention, adaptation and mitigation efforts. GCOS is now expanding its framework of ECVs to include those required to close the cycles of energy, water and carbon.

We need continuous and systematic climate observations of a well-thought-out set of indicators to monitor the targets of the Paris Agreement, and the data must be made available to all interested users. The Paris Agreement serves as a strong reminder to the observing community to deliver the data that will underpin progress.