About Essential Climate Variables

Infographic illustrating various components of Earth's systems such as atmosphere, cryosphere, biosphere, hydrosphere, and different ocean and surface systems, under the GCOS framework.
Infographic showcasing the Essential Climate Variables.

An Essential Climate Variable (ECV) is a physical, chemical or biological variable or a group of linked variables that critically contributes to the characterization of Earth’s climate. GCOS currently specifies 55 ECVs.

ECV datasets provide the empirical evidence needed to understand and predict the evolution of climate, to guide mitigation and adaptation measures, to assess risks and enable attribution of climate events to underlying causes, and to underpin climate services. They are required to support the work of the UNFCCC and the IPCC.

ECV are identified based on the following criteria:

  • Relevance: The variable is critical for characterizing the climate system and its changes.
  • Feasibility: Observing or deriving the variable on a global scale is technically feasible using proven, scientifically understood methods.
  • Cost effectiveness: Generating and archiving data on the variable is affordable, mainly relying on coordinated observing systems using proven technology, taking advantage where possible of historical datasets.

ECV Observation Requirements

Current ECV requirements according to the 2022 GCOS ECV Requirements (GCOS-245):

  • Atmosphere
  • Ocean
  • Land

GCOS Climate monitoring principles

The GCOS Climate Monitoring Principles are intended to provide guidance to those involved in the design, development, deployment, and management of observing systems for climate-related needs across all domains (surface, above surface, and subsurface), all observing platforms, and all essential climate variables (ECVs). They address the needs for generating datasets for monitoring the climate system and its changes, for supporting climate applications, for underpinning assessments such as the IPCC, and for informing action by Parties to the UNFCCC. Therefore, these GCOS Climate Monitoring Principles should be integrated into national, regional, and global observing plans, in response to GCOS Implementation Plans and GCOS Status Reports and aiming at a consistent global observing system for climate through an internationally coordinated effort.

The GCOS Climate monitoring principles update

The previous version of the climate monitoring principles was adopted by UNFCCC COP-9 in 2003, with Decision 11/CP.9. In 2022, the GCOS Implementation Plan, Action C1 “Develop monitoring standards, guidance and best practices for each ECV”, Activity C1.4 “Review the GCOS climate monitoring principles”, raised the need to update these principles, considering the significant advancements in best practices and monitoring capabilities occurred in the last two decades. Consequently, a review was undertaken in 2023 for relevancy and applicability of the GCOS climate monitoring principles, resulting in a revised set of principles applicable across all observing domains and technologies.

Several of these principles are also related to the observing network design principles specified in the WMO Integrated Global Observing System (WIGOS) Manual, therefore the updated principles were included in the new WIGOS Manual (WMO-1160) that was adopted at the 78th session of the WMO Executive Council, in June 2024. Consistent with the WIGOS Manual, principles are expressed using “should” rather than “shall”. However, failure to abide by these principles is likely to significantly limit the utility of the collected observations for climate-relevant purposes.

In November 2024, at COP29 in Baku, GCOS requested the Parties to UNFCCC, under the negotiation on Research and Systematic Observations by SBSTA (the Subsidiary Body for Scientific and Technological Advice), to consider the updated set of the GCOS Climate Monitoring Principles in their reporting on systematic observation. The SBSTA, in its draft conclusions (FCCC/SBSTA/2024/L.17), “noted the updated GCOS global climate monitoring principles” and “encouraged Parties to consider the updated principles.”

The 2024 list of GCOS Climate Monitoring Principles:

  1. Spatial and Temporal Sampling: It is critical for observations to sample the Earth system in such a way that climate-relevant diurnal, seasonal, interannual and long-term changes can be resolved. When the opportunity exists to fill gaps in the existing observing system high priority should be given to data-poor regions, poorly observed parameters, regions sensitive to change, and key measurements with inadequate temporal resolution.
  2. System Design: Observing systems should encompass both in-situ and remote sensing platforms as appropriate based on their respective strengths and limitations. Climate monitoring requirements regarding appropriate spatial and temporal sampling, instrument precision and accuracy, and stability should be specified to observing system designers, operators, and instrument engineers at the outset of system design and implementation. Observing systems should include reference observations to ensure well characterized measurement time series traceable to SI and/or community standards with robustly quantified uncertainties that can be used with confidence. Periodic reviews should be conducted to assess the feasibility and benefits of incorporating new technologies into the observing systems. 
  3. System Sustainability: For in-situ observations, operation of historically uninterrupted stations and observing systems that meet the specified calibration, stability, and siting requirements should be maintained for as long as possible.  For satellite measurements, continuity should be ensured through appropriate launch and orbital strategies. Relevant climate research  observing systems and networks should be sustained and transitioned to operational status. 
  4. System Change Management: The impact of new systems or changes to existing systems should be assessed prior to implementation. A suitable period of overlap between new and old instruments and observing systems should be ensured for a period adequate to determine inter-instrument biases and maintain the homogeneity and consistency of time-series observations.
  5. Metadata: In order to ensure the utility of the observations, the details and history of local conditions, site location, instruments, operating procedures, data processing algorithms, data errors and biases, and other factors pertinent to interpreting data (i.e., metadata) and their changes over time should be documented and treated with the same care as the data themselves.  
  6. Calibration: Prior to the deployment of a new instrument or observing platform, its technical characteristics, such as accuracy, precision, and stability, should be rigorously documented and calibrated in order to ensure consistency with climate-relevant requirements. Calibration should be traceable to SI units or to reference observations. Following deployment, all system components should be regularly recalibrated or otherwise evaluated to ensure the highest data quality.
  7. Data Quality and Homogeneity: The quality and homogeneity of data should be regularly assessed as a part of routine operations. Random errors and biases in the observations should be identified and documented.
  8. Data and Metadata Preservation: Data and metadata should be preserved for secure, long-term storage and retrieval in an appropriate repository according to relevant international standards.
  9. Data Access: Data management systems that facilitate access, use, and interpretation of data and products should be included as essential elements of climate monitoring systems. These systems should facilitate open user access to climate products, metadata, and raw data, including key data for delayed-mode analysis, in line with the WMO Unified Data Policy (Resolution 1).
  10. Data Exploitation: The collected observations should be used to generate datasets of ECVs. In order to keep pace with evolving technologies, climate-relevant requirements, and methods, these datasets should be sustained, regularly assessed, and reprocessed as needed.