The Importance of Monitoring Sea Ice and Sea Surface Salinity

Oceans cover over 70% of Earth's surface, and their vastness means they play a crucial role in regulating the planet's climate. They store significant heat and redistribute it from the equator to the poles. Measurements of the oceans serve as both significant markers of a changing climate, and key variables in modelling future climate scenarios. In this article we look at two ocean variables which can be measured from space, what measurement capabilities satellites offer, and how to discover and utilise the data.

Sea ice measurements allow for the monitoring of coverage. When sea ice thickness measurements are included, sea ice volume can be calculated.

Sea surface salinity or how salty is the sea? Salinity rises in areas where sea ice forms as salt is largely left behind when the water freezes. This cold, salty, and therefore dense water sinks into the oceans’ depths as part of a process known as thermohaline circulation, which is key to understanding how the oceans transport heat around the world.

These two measurements support our understanding of the climate in a number of ways.

Climate Feedback Mechanisms: Sea ice plays a pivotal role in climate feedback mechanisms, acting as an insulator and moderating the exchange of heat between the ocean and the atmosphere. Changes in the distribution and thickness of sea ice can trigger feedback loops that amplify climate change effects.

Temperature Regulation: The reflective properties of sea ice, referred to as albedo, help regulate Earth's temperature by reflecting solar radiation. As sea ice cover decreases, less sunlight is reflected, leading to greater absorption of heat in the ocean.

Climate Change Indicators: Trends in sea ice distribution and thickness provide metrics that are sensitive indicators of atmospheric and oceanic changes, offering early markers of shifts in climate patterns.

Ocean Circulation: Both sea ice and sea surface salinity influence global ocean circulation patterns, crucial for redistributing heat and maintaining balance in the global climate.

Freshwater Flux: The freshwater melt from ice sheets lowers salinity in some areas, meaning that in these areas sea water can freeze more easily. This can in turn affect water column stratification and the distribution of related physical and biochemical properties.

Marine Ecosystems: During winter, the Arctic Ocean ice pack grows to an area between 14 and 16 million square kilometres, dropping to four to five million square kilometres by the end of summer. This fluctuation in sea ice habitat is roughly equivalent to the size of Europe. This habitat provides food, shelter, and transport for the many species who live on, near, or under it, and monitoring changes is essential for understanding broader impacts on biodiversity.

What Capabilities Can Satellites Bring?

Sea Ice - CryoSat Mission ESA’s Earth Explorer mission, CryoSat, is dedicated to measuring the thickness of polar sea ice and monitoring changes in the ice sheets that blanket Greenland and Antarctica. Its primary payload is an advanced radar altimeter specifically designed to monitor the most dynamic sections of Earth's cryosphere.

CryoSat’s radar altimeter instrument is named SIRAL (Synthetic Aperture Radar, or SAR Interferometer Radar Altimeter), and is designed for measuring changes at the margins of vast ice sheets and floating ice in polar oceans.

Using both Synthetic Aperture Radar and interferometry techniques, CryoSat measures the distribution of sea ice, the difference in height between sea ice and adjacent water (known as 'freeboard'), and ice sheet altitude. These measurements enable the tracking of changes in ice thickness, and therefore the estimation of the volume of sea ice.

Lean more about CryoSat

Salinity - Soil Moisture and Ocean Salinity Mission

ESA’s Earth Explorer mission SMOS, or the Soil Moisture and Ocean Salinity mission, is dedicated to making global observations of soil moisture over land and surface salinity over oceans. It carries an L-band passive microwave instrument called MIRAS, the Microwave Imaging Radiometer using Aperture Synthesis.

Electromagnetically, MIRAS measures natural emissivity at L-band frequencies (1-2 GHz) with sensitivity centred around a frequency of 1.4 GHz, corresponding to a 21 cm wavelength. Seawater emits natural microwave radiation at these frequencies as a function of its dielectric properties. Increased salinity decreases the emissivity of seawater, thereby reducing the microwave radiation emitted. Sea Ice also emits natural radiation around the same frequency, and so by measuring these changes in emissivity, MIRAS is able to measure changes in sea surface salinity and sea ice thickness, in particular for relatively thin sea ice.

Learn more about SMOS

What kind of data do satellites contribute?

CryoSat - Sea Ice Data

CryoSat data are available freely and openly to all users and are processed to generate a suite of higher-level products. These include standard products addressing sea ice concentration and distribution, as well as advanced products providing measurements of sea ice thickness. The Cryo-TEMPO family of products currently provide a 14-year record of global sea ice and sea levels provided by the mission.

The CRYO2ICE campaign has sought to address a major source of error in ice thickness measurements: snow on sea ice. In 2020, ESA and NASA collaborated on a change to the CryoSat orbit to periodically align with NASA's ICESat-2. This provided radar and lidar measurements of the same ice, at nearly the same time. These coincident observations allow for the estimation of the snow depth from space on both sea and land. This improved the accuracy of sea ice thickness measurements, and helped map snow over the poles. The orbit of the CryoSat and ICESat-2 satellites is naturally growing closer together over time, resulting in shorter times between observations of the same area.

CryoSat Data | CryoSat Products | Cryo-TEMPO Baseline C | CRYO2ICE | CryoSat Tools

SMOS - Sea Surface Salinity Data

SMOS ocean salinity products contain the retrieved sea surface salinity, an unbiased sea surface salinity anomaly with reference to a SMOS consistent climatology, and the corrected sea surface salinity (mitigating land/sea transition contamination). The product is available with a latency of 8-12 hours. These products are complemented by a variety of tailored products generated by national entities as outlined in the SMOS Data Brochure.

SMOS - Sea Ice Thickness Data

SMOS can also provide sea ice thickness products, which contain daily estimations of sea ice thickness that are particularly useful for areas of thin sea ice where CryoSat has larger uncertainties. These measurements are available at the edge of the Arctic Ocean during the Boreal winter season (October-April), and at the edge of the Antarctic region during the Austral winter season (April-October).

SMOS - CryoSat Combined Sea Ice Thickness Data

A combined sea ice thickness product based on estimates from both the MIRAS and the SIRAL instruments provides a significant reduction in the uncertainty of ice thickness measurements. A weekly average is generated daily, and a final product is provided with a latency of about 3-4 weeks. Coverage is limited to the October-April period for the Northern Hemisphere, and is available from the year 2010 onwards.

SMOS L3 Sea Ice Thickness | SMOS - CryoSat L4 Sea Ice Thickness | SMOS Data and Tools

ESA Climate Change Initiative

ESA’s Climate Change Initiative (CCI) was established in 2008, and is a significant research and development effort focused on global satellite-derived climate data records. These data records are intended to provide a contribution to the Intergovernmental Panel on Climate Change assessment of the state of the climate, to inform effective policy, decision-making, and action. For the period 2023 to 2029, the CCI is focused on addressing requirements for Earth Observation in support of the Paris Agreement.

This work is also being done in support of the Global Climate Observation System (GCOS), which has defined Essential Climate Variables representing the information required by the United Nations Framework Convention on Climate Change, to systematically monitor the Earth system.

Satellite observations form the backbone of this monitoring, with two-thirds (38) of the 55 Variables exclusively or largely measured from Space. As of January 2024, data records from the CCI are addressing 27 Variables.

Data from the CCI are available via its Open Data Portal. As of August 2024, the CCI hosts 20 datasets related to the sea ice ECV, and 10 related to the sea surface salinity ECV.

ESA Climate Change Initiative | CCI Open Data Portal | Essential Climate Variables

CCI Sea Ice Project Datasets

ESA’s CCI Sea Ice project aims to combine and extend ongoing research to develop validated long-term time series of ice concentration and ice thickness for use in climate research. These data products are available for the Arctic and Antarctic and are developed in accordance with GCOS requirements.

Products include:

• Northern and Southern hemisphere sea ice thickness from Envisat and CryoSat
• Sea Ice Concentration from SSM/I, SSMIS, AMSR-E, and AMSR-2
• Sea Ice Concentration from Nimbus-5 ESMR

The project is currently working on extending the sea ice thickness dataset to the ERS-1/2 period (1991-2011).

CCI Sea Ice Project | Sea Ice CCI Dataset Collection

CCI Sea Surface Salinity Project Datasets

ESA’s CCI Sea Surface Salinity project aims to provide data products that are specifically adapted to climate science and applications. These comprise global level 4, multi-sensor Sea Surface Salinity maps covering the 2010-2022 period at a spatial resolution of 50km and a time resolution of 1 week. A monthly product is also available, at a spatial grid of 25 km.

CCI Sea Surface Salinity Project | Sea Surface Salinity CCI Dataset Collection

Accessing Data from ESA

Visual Browsers

The ESA Earth Online Visuals allow users to interact with 3D models of ESA satellites, which visualise their design, payload, and mission data.

The CCI’s Climate from Space portal provides stories for understanding space impact on climate, as well as a visual interface to explore CCI datasets. 

ESA CCI Toolbox

ESA’s CCI Toolbox is designed to provide access to CCI data from the ESA Open Data Portal, as well as to provide functions to analyse and utilise the data. The toolkit is available on GitHub, and includes a series of Jupyter notebooks to familiarise new users with the capabilities. To set the Toolkit up locally, follow the instructions in the repository readme file, followed by the additional steps in the notebooks folder readme file regarding setting up your environment for Jupyter notebooks.

ESA CCI Toolbox

You can find a user example notebook in ESA’s CCI Toolbox, which demonstrates the loading and plotting of CCI Sea Ice data. Explore the notebook to see how to create the output below.