Climate Change Lends New Color to the Ocean

The Ocean’s Changing Color: A Sign of Ecosystem Shifts

The deep-blue sea is turning a touch greener. While that may not seem as consequential as, say, record warm sea surface temperatures, the color of the ocean surface is indicative of the ecosystem that lies beneath. Communities of phytoplankton, microscopic photosynthesizing organisms, abound in near-surface waters and are foundational to the aquatic food web and carbon cycle. This shift in the water’s hue confirms a trend expected under climate change and signals changes to ecosystems within the global ocean, which covers 70 percent of Earth’s surface.

Global Sea Surface Color Changes

Researchers led by B. B. Cael, a principal scientist at the U.K.’s National Oceanography Centre, revealed that 56 percent of the global sea surface has undergone a significant change in color in the past 20 years. After analyzing ocean color data from the MODIS (Moderate Resolution Imaging Spectroradiometer) instrument on NASA’s Aqua satellite, they found that much of the change stems from the ocean turning more green.

Mapping Ocean Surface Color Changes

The map above highlights the areas where ocean surface color changed between 2002 and 2022, with darker shades of green representing more-significant differences (higher signal-to-noise ratio). By extension, these are places we can detect a change in the ocean ecosystem in the last 20 years. The study focused on tropical and subtropical regions, excluding higher latitudes, which are dark for part of the year, and coastal waters, where the data are naturally very noisy.

The black dots on the map indicate the area, covering 12 percent of the ocean’s surface, where chlorophyll levels also changed over the study period. Chlorophyll has been the go-to measurement for remote sensing scientists to gauge phytoplankton abundance and productivity. However, those estimates use only a few colors in the visible light spectrum. The values shown in green are based on the whole gamut of colors and therefore capture more information about the ecosystem as a whole.

Long-Term Trends in Ocean Color

A long time series from a single sensor is relatively rare in the remote sensing world. As the Aqua satellite was celebrating its 20th year in orbit in 2022—far exceeding its design life of 6 years—Cael wondered what long term trends could be discovered in the data. In particular, he was curious what might have been missed in all the ocean color information it had collected. There’s more encoded in the data than we actually make use of, he said.

By going big with the data, the team discerned an ocean color trend that had been predicted by climate modeling, but one that was expected to take 30-40 years of data to detect using satellite-based chlorophyll estimates. That’s because the natural variability in chlorophyll is high relative to the climate change trend. The new method, incorporating all visible light, was robust enough to confirm the trend in 20 years.

Ecological Changes and Ocean Stratification

At this stage, it is difficult to say what exact ecological changes are responsible for the new hues. However, the authors posit, they could result from different assemblages of plankton, more detrital particles, or other organisms such as zooplankton. It is unlikely the color changes come from materials such as plastics or other pollutants, said Cael, since they are not widespread enough to register at large scales.

What we do know is that in the last 20 years, the ocean has become more stratified. Surface waters have absorbed excess heat from the warming climate, and as a result, they are less prone to mixing with deeper, more nutrient-rich layers. This scenario would favor plankton adapted to a nutrient-poor environment. The areas of ocean color change align well with where the sea has become more stratified, said Cael, but there is no such overlap with sea surface temperature changes.

Future Insights with PACE Satellite

More insights into Earth’s aquatic ecosystems may soon be on the way. NASA’s PACE (Plankton, Aerosol, Cloud, ocean Ecosystem) satellite, set to launch in 2024, will return observations in finer color resolution. The new data will enable researchers to infer more information about ocean ecology, such as the diversity of phytoplankton species and the rates of phytoplankton growth.

NASA Earth Observatory image by Wanmei Liang, using data from Cael, B. B., et al. (2023). Story by Lindsey Doermann.

SDGs, Targets, and Indicators

1. Which SDGs are addressed or connected to the issues highlighted in the article?

• SDG 14: Life Below Water
• SDG 13: Climate Action

The article discusses the changes in the color of the ocean surface, which indicates shifts in the ecosystem beneath. These changes are connected to climate change and have implications for life below water. Therefore, SDG 14, which focuses on conserving and sustainably using the oceans, seas, and marine resources, is relevant. Additionally, SDG 13, which addresses climate action, is also connected as the changes in ocean color are a result of climate change.

2. What specific targets under those SDGs can be identified based on the article’s content?

• SDG 14.1: By 2025, prevent and significantly reduce marine pollution of all kinds, particularly from land-based activities, including marine debris and nutrient pollution.
• SDG 14.2: By 2020, sustainably manage and protect marine and coastal ecosystems to avoid significant adverse impacts, including by strengthening their resilience, and take action for their restoration to achieve healthy and productive oceans.
• SDG 13.3: Improve education, awareness-raising, and human and institutional capacity on climate change mitigation, adaptation, impact reduction, and early warning.

Based on the article’s content, the targets related to these SDGs can be identified. The changes in ocean color can be an indicator of the health of marine ecosystems and can help monitor marine pollution and the impact of climate change on the oceans. Therefore, targets related to preventing marine pollution (SDG 14.1) and sustainably managing marine and coastal ecosystems (SDG 14.2) are relevant. Additionally, improving education and awareness about climate change (SDG 13.3) is important to understand the implications of these changes.

3. Are there any indicators mentioned or implied in the article that can be used to measure progress towards the identified targets?

• Indicator for SDG 14.1: Marine pollution levels, including nutrient pollution and marine debris.
• Indicator for SDG 14.2: Health and resilience of marine and coastal ecosystems.
• Indicator for SDG 13.3: Education and awareness levels on climate change mitigation and adaptation.

The article mentions the changes in ocean color as an indicator of the health of marine ecosystems. This can be used to measure progress towards the targets related to marine pollution (SDG 14.1) and the health of marine and coastal ecosystems (SDG 14.2). Additionally, improving education and awareness about climate change (SDG 13.3) can be measured through surveys or assessments of knowledge and understanding among the general population.

Table: SDGs, Targets, and Indicators

Behold! This splendid article springs forth from the wellspring of knowledge, shaped by a wondrous proprietary AI technology that delved into a vast ocean of data, illuminating the path towards the Sustainable Development Goals. Remember that all rights are reserved by SDG Investors LLC, empowering us to champion progress together.

Source: earthobservatory.nasa.gov

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