The Indian Ocean is undergoing significant changes in its seasonal exchange of carbon dioxide (CO₂) with the atmosphere potentially influencing its role in regulating global warming according to a new study published in Geophysical Research Letters.
The research analysed four decades of data, from 1980 to 2019, to investigate how seasonal carbon exchange has changed across different regions of the Indian Ocean. The findings show that these changes vary considerably with some regions experiencing stronger seasonal fluctuations while others show a decline.
Oceans play a crucial role in slowing climate change by absorbing carbon dioxide released through human activities. When seawater takes up more CO₂ than it releases, it acts as a carbon sink, removing some greenhouse gases from the atmosphere. However, oceans can also release dissolved carbon dioxide back into the air.
According to the Global Carbon Budget 2024, the ocean absorbed approximately 26 per cent of human caused CO₂ emissions between 2014 and 2023. Scientists have raised concerns that the ocean’s capacity to absorb additional carbon has shown little growth since 2016, partly due to increasing climate variability.
Four decades of changing carbon exchange
The study led by Apurva Padamanbh Joshi of the Indian National Centre for Ocean Information Services and colleagues, examined seasonal changes in carbon exchange across four regions: the Arabian Sea, Bay of Bengal, equatorial Indian Ocean and southern Indian Ocean.
Between 1980 and 2019, seasonal variation in carbon exchange increased by 48 per cent in the equatorial Indian Ocean and 23 per cent in the southern Indian Ocean. This means the differences between seasonal highs and lows became more pronounced in these regions.
In contrast, seasonal variation declined by 14 per cent in the Arabian Sea. The Bay of Bengal recorded a more modest increase of around 7 per cent.
The researchers also found that average CO₂ release into the atmosphere decreased in the Arabian Sea, Bay of Bengal and equatorial Indian Ocean. Meanwhile, carbon absorption increased in the southern Indian Ocean.
These findings indicate that the Indian Ocean is not responding uniformly to changing climate conditions. They also highlight an important distinction greater seasonal variation does not necessarily mean that a region absorbs more carbon overall.
Temperature, winds and ocean circulation
The exchange of carbon dioxide between the ocean and atmosphere is influenced by water temperature, wind speed ocean circulation and the concentration of dissolved CO₂ in seawater.
Cooler water generally holds more dissolved carbon dioxide than warmer water, making it easier for the ocean to absorb the gas from the atmosphere. Warming can encourage CO₂ to escape from seawater although the overall outcome also depends on ocean currents and the amount of carbon already present in the water.
The study identified differences in the factors driving seasonal changes across the Indian Ocean.
In the equatorial and southern Indian Ocean, changes in the difference between surface water CO₂ concentrations and atmospheric levels played a major role in shaping carbon exchange.
In the Arabian Sea and Bay of Bengal, wind speed was an important influence. Winds affect how quickly gases move across the ocean atmosphere boundary meaning changes in wind conditions can alter the rate and seasonal pattern of carbon exchange.
Similar increases in seasonal carbon exchange have previously been observed in parts of the Pacific and Atlantic oceans, suggesting that shifting seasonal patterns may be a broader feature of changes in the global ocean carbon cycle.
Implications for marine ecosystems
The changing patterns of carbon exchange could also have consequences for marine ecosystems. When seawater absorbs carbon dioxide it becomes more acidic reducing the availability of carbonate ions needed by corals and other marine organisms to build their skeletons and shells.
Repeated climate driven extremes in seasonal conditions could place additional pressure on sensitive marine environments, potentially affecting biodiversity, species distribution and ecosystem productivity. Such pressures may also make it harder for some ecosystems to recover from environmental disturbances.
The findings underline why scientists need to look beyond annual averages when assessing the ocean’s role in regulating the climate. Annual figures can conceal changes in when and where the ocean absorbs or releases carbon dioxide making it more difficult to understand the processes shaping its overall carbon balance.
Although the research reveals substantial changes in seasonal carbon exchange, it does not establish that continued global warming will necessarily make the entire Indian Ocean a stronger carbon sink.
The study examines historical patterns rather than directly predicting the ocean’s future carbon storage capacity. Further research is needed to determine how these seasonal changes will influence the Indian Ocean’s long term ability to absorb and retain carbon dioxide as temperatures continue to rise.
Understanding these regional and seasonal differences will be important for improving climate projections and assessing how reliably the world’s oceans can continue to buffer the effects of human caused greenhouse gas emissions.