The Ocean

The ocean is the largest ecosystem on the planet and covers 71 per cent of its surface. Ocean is a vital component of the climate system. It influences climate by absorbing solar radiation and releasing heat needed to drive the atmospheric circulation. It also absorbs carbon dioxide (CO2) in two manners. First from the dissolution of CO2 into water (process called the solubility pump) and secondly from the absorption of CO2 by phytoplankton (process called the biological pump).  

UN Climate Change stated that up to 2021, the ocean has absorbed around 90 per cent of the heat generated by rising greenhouse gas emissions trapped in the Earth’s system and has taken in 30 per cent of carbon emissions. This has resulted in several fallouts, including the rise in the temperature of seas and oceans and marine heatwaves, sea-level rise, or ocean acidification.  

Such consequences have severe impacts on oceans and coastal life and activities, and lead to marine biodiversity loss, endanger population of coastal villages, towns, and megacities that in total amount to almost half of the global population, and impacts people working in fishery and aquaculture sector. 

Another consequence of the temperature rise of the ocean is the increase in toxic algae blooms, setting into motion a series of cascading effects. Those algae reduce the amount of oxygen (O2) available for phytoplankton to develop and stock the carbon dioxide (CO2) disrupting global climate further. 

Space technology and applications are instrumental in dealing with major consequences of the impacts of climate change on the ocean. They are used to monitor the ocean temperature, the sea-level or the colour of the ocean, from which is inferred the chlorophyll-a concentration, used to better understand the role of phytoplankton in, amongst other, the global carbon cycle, ocean acidification or the response of marine ecosystems to climate variability. 

From the 54 Essential Climate Variables < https://gcos.wmo.int/en/essential-climate-variables >  identified by the Global Climate Observing System (GCOS) 19 are solely dedicated to oceans. 

Sea Surface Temperature (STT)  

Infrared radiometers (sensors measuring electromagnetic radiation) onboard polar-orbiting satellites, in combination with in-situ measurements for validation, are used to retrieve information about the sea-surface temperature.  

The ocean emits radiations that vary with the temperature. Those radiations of the sea surface occur in the infrared (for a depth of the surface of 10 micrometres) and the microwave (for a depth of the surface of 1 millimetre) wavelengths. 

The temperature of the surface is then retrieved from the spaceborne measurements of the radiations using Planck formula and taking into account the effects of the atmosphere on the measured radiations (also called atmospheric correction). 

Daily produced maps of SST produced are essential for understanding, monitoring and predicting ocean-atmosphere interactions that drives the weather patterns and influences the slow and subtle changes in our climate.  

They also help monitoring marine heatwaves and marine cold spells (the cold version of heat waves).  

Indeed, extreme temperatures of the ocean lead to devastating consequences for marine life and dependent communities, causing for instance widespread coral bleaching and reef degradation. Also, CO2 is less soluble in warmer water, therefore an increase in ocean temperature slows down the solubility pump mechanism of the ocean.  

In recent decades, marine heatwaves have doubled in frequency, and have become longer-lasting, more intense and extensive. According to IPCC, in 2021, nearly 60 percent of the world’s ocean surface experienced at least one spell of marine heatwaves.  

More information on the sea surface temperature ECV: https://gcos.wmo.int/en/essential-climate-variables/sst  

Sea-level 

Radar altimetry satellites can measure the ocean surface topography, meaning the sea-surface height and depression. Sea-surface height can be derived from the measure of the time it takes for radar pulses to hit the ocean surface and bounce back to the spaceborne altimeter. Knowing the speed of propagation of the radar pulse, and combined with precise satellite location data, precise sea-surface height in relation to the ellipsoid can be retrieved. 

Latest data from the World Meteorological Organization shows that sea-level has accelerated in recent decades. Global mean sea level has been measured since the early 1990s by high-precision altimeter satellites. The sea-level rose by 2.1 millimetre per year between 1993 and 2002. However, in the recent decade (2013 -2021), the sea-level rise per year was 4.5 millimetre, more than double that of the previous period. In addition, the data from altimetry satellites was used to observe that sea level has occurred unevenly across the globe as some regions are affected more by the rise. 

Rising sea level is a critical issue for millions of people living in coastal communities around the world and is a direct consequence of climate change. Therefore, monitoring sea-surface height is essential to grasping the changes inferred by climate disruption to take action to protect vulnerable communities. 

More information on the sea level ECV: https://gcos.wmo.int/en/essential-climate-variables/sea-level/  

Ocean colour 

Ocean colour data plays an essential role in understanding climate change. The colour of ocean measured by satellite is used for monitoring the state of phytoplankton, which are the basis of marine life, produce about half of the oxygen in the Earth’s atmosphere, and absorb carbon dioxide (CO2) during photosynthesis, participating in the global carbon cycle. Ocean temperature, currents, acidification, surface winds and nutrients can affect phytoplankton populations, and further impact marine ecosystems. 

The presence of chlorophyll (the primary pigment of phytoplankton) in the water changes the reflection and absorption of sunlight properties of the water, and detecting concentration of chlorophyll enables to map the amount and location of phytoplankton. From the satellite detected marine reflectance (the water-leaving radiance, i.e the part of the sunlight backscattered by the water after interaction with it) the concentration of chlorophyll can be retrieved, correcting the effects of surface and atmosphere and using algorithms proven to be effective. 

More information on the ocean colour ECV: https://gcos.wmo.int/en/essential-climate-variables/ocean-colour/