According to a recent report published by the Intergovernmental Panel on Climate Change (IPCC), climate change has already caused irreversible effects on both natural and human systems. Ocean color sensors are one of many sophisticated technologies that have been developed in an effort to monitor the impact of global warming on the ocean’s composition.
Phytoplankton, and the Color of the Ocean
Sunlight can penetrate more than 600 feet below the surface of the ocean. Beyond this depth, the entire ocean floor is enshrined in darkness. However, between the surface and this range, most water molecules can absorb all colors except for blue, thus causing the often blue hue to be reflected from most oceans.
Phytoplankton, which is microscopic marine algae, form the base of the marine food web, often contributing to the color of the ocean’s surface. The ability of phytoplankton to alter the color of the ocean is largely due to their high concentration of chlorophyll, a green pigment that absorbs sunlight to generate food. Therefore, in addition to the colors absorbed by the ocean’s water, the concentration of phytoplankton present within a given region of the ocean will also contribute to its appearance as seen from space.
Image Credit: Choksawatdikorn/Shutterstock.com
The Impact of Climate Change on the Color of the Ocean
One of the primary causes of climate change is the increased release of carbon dioxide (CO2) and other greenhouse gasses (GHG) into the environment. Unfortunately, much of the excess heat that is released as a result of GHG emissions is absorbed by the ocean, thus causing ocean temperatures to rise across the planet.
As these temperatures rise, currents in the ocean become more irregular, which has been shown to prevent warmer parts of the ocean from mixing well with colder areas. Several thousand phytoplankton species are adapted to thrive in either warm or cold regions; the continuous warming of these waters may cause some species to die, others to thrive, and many to move to migrate elsewhere, which may disrupt existing ecosystems.
In addition to monitoring any changes in phytoplankton movement and survival, several other applications benefit from ocean color monitoring. For health purposes, ocean color monitors can assist in identifying pollution, eutrophication, and the presence of harmful algal blooms.
There are also climate benefits from ocean color monitoring. Allowing researchers to assess the impact of carbon emissions on oceans, marine ecosystems, fisheries, and aquaculture, provides insight into how changes to the composition of the sea have impacted different environments and industries.
As indicated in the IPCC report, there is an urgent need to limit global warming to 1.5 °C as compared to the projected 2 °C rise. The utility of ocean color monitoring technologies can assist researchers in tracking changes in ocean acidity and oxygen levels, both of which the IPCC report finds have been associated with rising global temperatures.
What are Ocean Color Sensors?
Over the past two decades, satellite ocean color measurements have been obtained to allow marine biologists and other interested scientists to study the activity of phytoplankton in the oceans of the world. Typically, measurements are obtained via sensors that monitor the reflectance, which is otherwise known as water-leaving radiance, at the top of the atmosphere over a range of wavelengths.
Remotely sensed reflectance (RRS) is defined as the ratio of the upwelling radiance to the downwelling irradiance at the ocean surface. Typically, most space agencies will utilize this product to discuss ocean color. In addition to RRS, another method to measure ocean color is by monitoring chlorophyll-a (Chl-a) values, the primary pigment utilized in photosynthesis.
To infer either RRS or Chl-a concentration values, various algorithms have been constructed. Notably, any optically significant constituents present within the atmosphere that could interfere with the accuracy of these readings are excluded from the final measurement. However, due to these unwanted signals, the ocean color signal comprises only about 10% or less of the total satellite-detected top-of-atmosphere (TOA) radiance.
A History of Ocean Color Sensors
Within the ocean-color field, the International Ocean Color Coordinating Group (IOCCG) functions as the main forum for space agencies and any professionals involved in monitoring the color of the oceans.
In 1978, the world’s first ocean color sensor was developed by Coastal Zone Color Scanner as a proof-of-concept and remained in operation until 1986. It was not until 1996 that the next ocean color sensor was reported in Japan, known as the Japanese Ocean Color Temperature Scanner. Shortly after, the Sea-viewing Wide field-of-view Sensor (SeaWiFS) was developed in 1997 in the United States as the first contiguous ocean color sensor.
In 2002, the United States National Aeronautics and Space Administration (NASA) launched the Aqua MODIS instrument, which has been in service for the past 16 years. This system, which operates daily and continuously, is equipped with nine bands in the visible/near-infrared (NIR) and wide swath.
Keeping PACE with NASA's Plankton, Aerosol, Cloud, ocean Ecosystem mission
Video Credit: NASA Scientific Visualization Studio/YouTube.com
Future Impacts of Ocean Color Sensors
By 2020, the NASA Plankton, Aerosol, Cloud ocean Ecosystem (PACE) mission will launch. It will carry the Ocean Color Instrument (OCI), which offers a significantly improved resolution of 5 nanometers (nm) while also offering the ability to record ocean color data for climate studies.
The OCI, which is currently being built at Goddard Space Flight Center (GSFC), is a highly advanced optical spectrometer that consists of a cross-track rotating telescope, thermal radiators, half-angle mirror, and solar calibration mechanisms. Notably, the tilt of the OCI will prevent sun glint from interfering with measurements, while this instrument’s single science detector will prevent image striping.
In addition to the OCI, PACE will also carry two polarimeters that will function to measure polarized light at multiple view angles. These polarimeters will include the Hyperangular Rainbow Polarimeter 2 (HARP2) and the Spectro-polarimeter for Planetary Exploration (SPEXone).
Overall, the objective of missions like PACE and several others that will be initiated in the near future is to gain a better understanding of the carbon cycle and how the ocean ecosystem responds to a changing climate. In addition to determining how a changing environment impacts the ocean and its composition, PACE will also aim to understand how differences in the biological and photochemical processes of the world’s waters can affect the atmosphere as a whole.
As emphasized throughout the IPCC report, human activities must be adjusted to ensure that global warming does not cause temperatures to rise beyond 1.5 °C. In addition to changing these behaviors worldwide, several technologies can also be utilized to detect and address environmental changes due to global warming preemptively. To this end, ocean color sensors offer climate scientists the opportunity to closely monitor the composition of the oceans in response to climate change.
Industrial Response to Climate Change
This article is a part of the IPCC Editorial Series: Industrial Response to Climate Change, a collection of content exploring how different sectors are responding to issues highlighted within the IPCC 2018 and 2021 reports. Here, Sensors showcases the research institutions, industrial organizations, and innovative technologies driving adaptive solutions to mitigate climate change.
References and Further Reading
IPCC, (2018) Summary for Policymakers. In: Global Warming of 1.5°C. An IPCC Special Report on the impacts of global warming of 1.5°C above pre-industrial levels and related global greenhouse gas emission pathways, in the context of strengthening the global response to the threat of climate change, sustainable development, and efforts to eradicate poverty [Masson-Delmotte, V., P. Zhai, H.-O. Pörtner, D. Roberts, J. Skea, P.R. Shukla, A. Pirani, W. Moufouma-Okia, C. Péan, R. Pidcock, S. Connors, J.B.R. Matthews, Y. Chen, X. Zhou, M.I. Gomis, E. Lonnoy, T. Maycock, M. Tignor, and T. Waterfield (eds.)]. World Meteorological Organization, Geneva, Switzerland. In Press
IPCC, (2021) Summary for Policymakers. In: Climate Change 2021: The Physical Science Basis. Contribution of Working Group I to the Sixth Assessment Report of the Intergovernmental Panel on Climate Change [Masson-Delmotte, V., P. Zhai, A. Pirani, S. L. Connors, C. Péan, S. Berger, N. Caud, Y. Chen, L. Goldfarb, M. I. Gomis, M. Huang, K. Leitzell, E. Lonnoy, J.B.R. Matthews, T. K. Maycock, T. Waterfield, O. Yelekçi, R. Yu and B. Zhou (eds.)]. Cambridge University Press. In Press.
Johnson, B., (2021) Monitoring the Oceans’ Color for Clues to Climate Change. [online] NIST. Available at: https://www.nist.gov/blogs/taking-measure/monitoring-oceans-color-clues-climate-change
Dutkiewicz, S., Hickman, A. E., Jahn, O., et al. (2019) Ocean colour signature of climate change. Nature Communications 578(10). Available at: https://doi.org/10.1038/s41467-019-08457-x
Gibbens, S., (2019) Climate change will shift the oceans’ colors, study predicts. [online] National Geographic. Available at: https://www.nationalgeographic.com/environment/article/climate-change-alters-oceans-blues-greens
Groom, S., Sathyendranath, S., Ban, Y., et al. (2019) Satellite Ocean Colour: Current Status and Future Perspective. Frontiers in Marine Science 29. Available at https://doi.org/10.3389/fmars.2019.00485
Pace.oceansciences.org. (2021) Ocean Color Instrument. [online] Available at: https://pace.oceansciences.org/oci.htm