Written by Yasmin Madsen - Editor: Kata Krnács
With a changing climate, rising temperatures and a rise of anthropogenic impact, various natural elements - named essential climate variables within the scientific discourse - are changing with it. Chlorophyll-a concentrations are no exception to this change. This article presents how chlorophyll-a concentrations changed due to sea surface temperatures (SST) in the past 20 years in Danish waters. Whilst there is an association between SST and chlorophyll-a concentrations, it seems as though it is the direct anthropogenic activities, such as wastewater runoff, that have more of an impact on the chlorophyll-a concentrations in this region. It is subsequently clear that the ‘business as usual’ scenario in our current landscape is unacceptable, creating adverse changes in natural variables.
Essential Climate Variables (ECVs) are “physical, chemical or biological variable[s]...that critically contribute to the characterisation of the Earth’s climate” (WMO, 2024a). The WMO identifies 55 ECVs, of which data on these variables are collected better to predict the changes in the Earth’s climate. ECVs are determined based on their relevance in the climate system, the feasibility to measure them using scientifically approved methods and their cost effectiveness and affordability (Bojinski, 2014).
With the complexity of the Earth’s system, ECVs provide a categorical clarity in thinking and modelling the labyrinth of ecosystem reservoirs and fluxes. Of the 55 ECVs listed currently, ocean colour is one of them. So, what does ocean colour entail, how is it measured, and how does it impact the climate?
Ocean colour is the seen hue of the water that is created through the interaction between sunlight and the microscopic composition of the water (NASA, n.d.-a). It is measured through ocean colour remote sensing (OCRS) (WMO, 2024b). Ocean colour is quantified by two different aspects: water-leaving radiance and chlorophyll-a concentrations (GCOS, 2021).
Water leaving radiance is the “upwelling radiance emitted from sea to air” (Kishino & Furaya, 2015) and is measured through the OCRS by the total radiance emitted with atmospheric corrections (Kishino & Furaya, 2015). Chlorophyll-a is a green pigment that is present in phytoplankton (NOAA, n.d.-b). It is measured by measurements of the hue of the water, in which green hues mean high chlorophyll-a concentrations, whilst blue hues suggest low chlorophyll-a concentrations. Chlorophyll-a concentrations are an important product to consider as they indicate phytoplankton biomass. Phytoplanktons are crucial in mitigating a changing climate due to their ability to uptake atmospheric carbon dioxide (European Environmental Agency, n.d). Alternatively, an excessive amount of chlorophyll-a concentrations can lead to harmful algal blooms (Zhao et. al, 2010), which have dire consequences on biodiversity and local ecosystems. Thus, this article will focus on looking at the data from chlorophyll-a concentrations.
So, what does impact chlorophyll-a concentrations, and thus the ocean colour? It is imperative to highlight that, because of the complexity of the Earth system, it is difficult to suggest a direct correlation of one variable affecting another on a global scale. Therefore, the trends that will be explained here are trends that have been observed, but do not necessarily mean that they explain chlorophyll-a concentration changes in all regions. In terms of temperature changes, the increase in sea surface temperature (SST) leads to a decrease in chlorophyll-a concentrations (Zhang et. al, 2023). This is because areas with cooler SST, nutrient-rich deep water are upwelled to the surface more easily, promoting phytoplankton growth and hence promoting chlorophyll-a concentrations (NASA, n.d-c). This means that chlorophyll concentrations are high in polar regions, or equatorial regions (where cold water is brought to the surface) and in coastal regions (coastal upwelling). However, high chlorophyll-a concentrations in coastal regions could also be due to human activity and wastewater runoff, which adds excess nutrients into the water, promoting eutrophication, leading to high chlorophyll-a concentrations (Zhang et. al, 2023). Areas with heavy rainfall could also lead to high concentrations of chlorophyll-a, as the rain can lead to nutrient-rich soil running into the water, thus promoting the growth of phytoplankton and increasing chlorophyll-a concentrations.
Before making inferences and analysing the data, it is imperative to question the sources of data and the advancement of current climate data tools. In GCOs’ last status report in 2021, they outlined the progress of the collection of ocean colour data.
In general, none of the ECV observations fall into the ‘very good’ category. This means that they are not adequate in fulfilling the resolution and uncertainty requirements, and/or there are regional gaps, and the data is not always available worldwide and is not always or completely free for users to access. ECVs that fit into the ocean category overall are classified as ‘medium’ (⅗) in adequacy and ‘good’ in availability and stewardship (⅘). There have been great advancements in satellite and in situ observations for ocean data in the past few decades, allowing the adequacy parameters to significantly improve. Specifically to ocean colour, it scores an adequacy mark of ‘medium’ (⅗) and an availability and stewardship mark of ‘good’ (⅘). Data on ocean colour is quite robust, with several satellite missions collecting data on ECV. There is also often a high resolution on the data of 1 km^2 covering most of the globe, and data available nearly daily. For chlorophyll observations specifically, data is measured at a 4 km^2 spatial resolution and an 8-day temporal resolution for the majority of the globe. Data is not easily available in regions and/or seasons with high cloud cover, and data is not available at all in regions and/or seasons when the sun's angle is low, predominantly polar nights.
There are several satellite and reanalysis models, with the OC-CCI product being one of the datasets with the longest amount of ocean colour observations (Sathyendranath et. al, 2019). They have observations starting from September 1997 up until December 2024. I have chosen to compare 2004 with 2024 for two reasons. Firstly, a 20-year comparison provides a rounded number of years to compare, and secondly, there is a lot of missing data from before 2004, especially in coastal regions. The data that I have chosen is global satellite data of chlorophyll-a concentration at a spatial resolution of 1km, a monthly temporal resolution, version 6.
I’m interested to see how chlorophyll-a concentrations have changed in the past two decades in the coastal areas and seas surrounding Denmark. Firstly, it’s important to show that sea surface temperatures (SST) are steadily increasing over time. Graph 1 shows that the global monthly SST has been increasing overtime, with Table 1 confirming that this increase in SST occurs throughout the whole year. Additionally, Table 2, which is collected from NOAA’s interactive map, compares the annual SST in the region and shows a subtle difference in temperature between 2004 and 2024 by approximately 1 degree Celsius (~11 degrees C to 12 degrees C). Since we know that SSTs are increasing, have there been changes in chlorophyll-a concentrations over time?
The monthly global SST averages for 2004 and 2024 are as follows:
To account for seasonal changes, I will look at the monthly data of March (spring), June (summer), September (autumn), and December (winter). Unfortunately due to the constraints of the composite brower, a mean of the the seasonal concentrations cannot be calculated (e.g. mean chlorophyll-a concentrations of winter - November, December, January), therefore the months listed previously will provide insight to the season’s SST or chlorophyll-a concentrations, but should be taken with a grain of salt.
March 2004 vs 2024
June 2004 vs 2024
September 2004 vs 2024
December 2004 vs 2024
The maps show that overall, there has been an increase in chlorophyll-a concentration, which is inconsistent with Zhang et. al’s assertion that increased SST leads to lower chlorophyll-a concentrations. However, we can see that there are fewer changes in chlorophyll-a concentrations further away from the coast. This suggests that there is another factor other than SST that hugely affects the chlorophyll-a concentrations along the coast. The coastal changes could be explained by human activity. Ærtebjerg et. al (2003) asserts that the most prominent direct sources for the increase in nutrients in Denmark are from agriculture and discharge from urban wastewater treatment plants and industries. The runoff of the additional nutrients from these industries could explain the increase in chlorophyll-a concentrations along the coast of Denmark. Additionally, the transference of nutrients from ocean circulation can also affect nutrient levels and, thus, in turn, chlorophyll-a concentrations. Higher chlorophyll-a concentrations can suggest more productivity and therefore more CO2 absorption; however, this increase in chlorophyll-a suggests a higher likelihood of eutrophication, depleting oxygen levels and destroying ocean wildlife. According to Ærtebjerg et. al (2003), the concentration of 1.25 ug1-1/m m-3 is used for the standard chlorophyll concentration in Danish waters. Visually, this is a light green/yellow green hue on the graph (Figure 1). Although there isn’t a standard concentration that leads to eutrophication, it is clear that areas that are over 10 ug1-1/m m-3 (dark red hue on map) far exceed this standard and thus can suggest a possibility for eutrophication.
Through analysing chlorophyll-a data from the OC-CCI, a long-term and good resolution satellite product, and comparing this data analysis to existing literature, it is concluded that the increase in chlorophyll-a concentrations in coastal areas suggests that it is not as influenced by SST. Rather, it is predominantly an excess of nutrients that has seeped into the coastal waters - an effect of pure human negligence.
References
Ærtebjerg, G., Andersen, J. H., & Hansen, O. S. (2003). Nutrients and eutrophication in Danish marine waters. A challenge for science and management. National Environmental Research Institute, 126.
Bojinski, S., Verstraete, M., Peterson, T. C., Richter, C., Simmons, A., & Zemp, M. (2014). The concept of essential climate variables in support of climate research, applications, and policy. Bulletin of the American Meteorological Society, 95(9), 1431-1443.
European Environmental Agency. (n.d.). Chlorophyll in Europe’s transitional, coastal and Marine Waters. European Environment Agency’s home page. https://www.eea.europa.eu/en/analysis/indicators/chlorophyll-in-transitional-coastal-and
GCOS (2021). The Status of the Global Climate Observing System 2021: The GCOS Status Report (GCOS-240), pub WMO, Geneva
KISHINO, M., & FURUYA, K. (2015). A new simplified method for the measurement of water-leaving radiance. La mer, 53(1-2), 29-38.
NASA. (n.d-a). NASA Ocean Color. https://oceancolor.gsfc.nasa.gov/
NASA. (n.d.-b). Chlorophyll. National Marine Ecosystem Status. https://ecowatch.noaa.gov/thematic/chlorophyll-a#:~:text=Chlorophyll%20a%20is%20a%20green,as%20%E2%80%9Cocean%20color%E2%80%9D
NASA. (n.d.-c). Sea surface temperature & chlorophyll. https://earthobservatory.nasa.gov/global-maps/MYD28M/MY1DMM_CHLORA#:~:text=In%20places%20where%20ocean%20currents%20cause%20upwelling%2C%20sea%20surface%20temperatures,is%20evident%20in%20multiple%20places
Sathyendranath, S., Brewin, R. J., Brockmann, C., Brotas, V., Calton, B., Chuprin, A., ... & Platt, T. (2019). An ocean-colour time series for use in climate studies: the experience of the ocean-colour climate change initiative (OC-CCI). Sensors, 19(19), 4285.
WMO. (2024, December 20, a). About essential climate variables. World Meteorological Organization. https://gcos.wmo.int/site/global-climate-observing-system-gcos/essential-climate-variables/about-essential-climate-variables
WMO. (2024, November 14, b). Ocean colour. World Meteorological Organization. https://gcos.wmo.int/site/global-climate-observing-system-gcos/essential-climate-variables/ocean-colour
Zhang, K., Zhao, X., Xue, J., Mo, D., Zhang, D., Xiao, Z., ... & Chen, Y. (2023). The temporal and spatial variation of chlorophyll a concentration in the China Seas and its impact on marine fisheries. Frontiers in Marine Science, 10, 1212992.
Zhao, D., Xing, X., Liu, Y., Yang, J., & Wang, L. (2010). The relation of chlorophyll-a concentration with the reflectance peak near 700 nm in algae-dominated waters and sensitivity of fluorescence algorithms for detecting algal bloom. International Journal of Remote Sensing, 31(1), 39-48.
Graph and Data from:
https://www.oceancolour.org/portal/
https://climatereanalyzer.org/clim/sst_monthly/?dm_id=world_60s-60n&var_id=sst
https://www.nnvl.noaa.gov/view/globaldata.html#SURF