New research suggests that climate- driven changes are amplifying seasonal CO₂ exchange patterns in parts of the Indian Ocean, which could influence its role as a climate buffer, though this does not automatically mean greater overall carbon absorption.

Published on Phys.org on October 18, 2026, and written by Hannah Bird, the article covers a study in Geophysical Research Letters (Joshi et al., 2026; DOI: 10.1029/2026GL123472) led by Dr. Apurva Padamanbh Joshi of the Indian National Center for Ocean Information Services. It analyzes data from 1980- 2019 on how seasonal air- sea CO₂ exchange has evolved across Indian Ocean regions.

Key findings

• Seasonal variation in CO₂ exchange increased by 48% in the equatorial Indian Ocean and 23% in the southern Indian Ocean (greater contrast between seasonal highs and lows).
• It decreased by 14% in the Arabian Sea and rose modestly (~7%) in the Bay of Bengal.
• Average CO₂ release to the atmosphere decreased in the Arabian Sea, Bay of Bengal, and equatorial Indian Ocean, while absorption increased in the southern Indian Ocean. Larger seasonal swings do not necessarily equal higher net carbon uptake.

Drivers

• In the equatorial and southern Indian Ocean, changes in the CO₂ partial- pressure difference between surface water and the atmosphere were key.
• In the Arabian Sea and Bay of Bengal, shifts in wind speed (which affect gas transfer across the air-sea interface) played a larger role.
• Similar seasonal amplification has been noted in parts of the Pacific and Atlantic, indicating broader ocean responses.

Context and implications

Oceans absorbed roughly 26% of human- caused CO₂ emissions from 2014- 2023 (per the Global Carbon Budget 2024), but capacity growth has slowed since around 2016 amid rising climate variability.

Temperature, winds, circulation, and existing dissolved CO₂ levels all influence the exchange: cooler water tends to absorb more, while warming can favor release, modulated by currents and other factors.⁠

The study focuses on historical changes rather than future projections. It highlights that seasonal processes are being reshaped regionally, which matters for accurate carbon- budget estimates (annual averages can miss timing shifts) and for marine ecosystems.

Greater CO₂ absorption increases ocean acidification, reducing carbonate ions needed by corals and shell-forming organisms; repeated seasonal extremes could stress recovery and affect biodiversity and productivity.

Overall, the findings provide a finer picture of the Indian Ocean’s evolving contribution to the global carbon cycle and its buffering of atmospheric CO₂. Further work is needed to determine long- term effects on storage capacity as warming continues.

The Indian Ocean plays a significant but regionally complex role in the global carbon cycle.

It acts as a modest net sink for atmospheric CO₂ overall, absorbing a portion of human- caused emissions and helping buffer climate change, though its behavior differs sharply between northern and southern regions and is strongly modulated by the monsoon system, ocean circulation, temperature, biology, and climate variability.

Overview of the Ocean Carbon Cycle (Relevant to the Indian Ocean)

Oceans exchange CO₂ continuously with the atmosphere. When the partial pressure of CO₂ in surface seawater (pCO₂) is lower than in the air, the ocean absorbs CO₂ (a sink); when higher, it releases CO₂ (a source).

Key controls include:

• Solubility pump: Cooler water holds more CO₂. Warming reduces solubility and can promote outgassing.
• Biological pump: Phytoplankton take up CO₂ via photosynthesis; some organic carbon sinks to deeper waters, sequestering it.
• Physical factors: Winds enhance gas transfer across the air-sea interface; upwelling brings CO₂-rich deep water to the surface; currents and stratification redistribute carbon; freshwater inputs (e.g., rivers) affect alkalinity and pCO₂.
• Anthropogenic influence: Rising atmospheric CO₂ drives net ocean uptake, but also causes ocean acidification (lower pH) as CO₂ forms carbonic acid.

Globally, oceans absorb roughly 25- 28% of anthropogenic CO₂ emissions. The Indian Ocean contributes a share of this, estimated at a net uptake of about −0.07 to −0.19 PgC/yr (petagrams of carbon per year; negative = sink) north of ~37.5°S for recent decades.

Regional Patterns in the Indian Ocean

The basin is not uniform:

• Northern Indian Ocean (north of ~15–18°S): Generally a net source of CO₂ to the atmosphere.
  • Arabian Sea (northwestern): Strong perennial source, especially due to intense upwelling during the Southwest Monsoon that brings carbon-rich deep waters to the surface.
  • Bay of Bengal (northeastern): Often a moderate net sink, influenced by large freshwater inputs from rivers that lower salinity and can suppress pCO₂.
• Southern Indian Ocean (south of ~15- 18°S, especially 15- 50°S): A significant sink. Cooler temperatures, subduction of surface waters, and biological activity favor CO₂ absorption. This region helps make the overall Indian Ocean a weak net sink.⁠

The equatorial Indian Ocean is closer to neutral (neither strong source nor sink) due to relatively deep thermocline and weaker upwelling compared to the equatorial Pacific.

Seasonal and Interannual Variability

The Indian Ocean’s carbon cycle is strongly seasonal because of the reversing monsoon winds and currents:

• Large seasonal swings in pCO₂ and air- sea CO₂ flux are linked to monsoon- driven changes in temperature, winds, upwelling, and biological production.
• Recent research (1980- 2019 data) shows these seasonal variations have amplified: +48% in the equatorial Indian Ocean and +23% in the southern Indian Ocean, with a decrease (−14%) in the Arabian Sea and a modest increase (~7%) in the Bay of Bengal. Drivers include changes in the air-sea pCO₂ difference (especially equatorial and southern regions) and wind speeds (Arabian Sea and Bay of Bengal). Average outgassing has decreased in northern and equatorial areas while southern uptake has increased.⁠

Interannual variability is strongly influenced by the Indian Ocean Dipole (IOD) and its interactions with El Niño- Southern Oscillation (ENSO). Positive IOD events can enhance outgassing in parts of the southeastern tropical Indian Ocean via upwelling and changes in dissolved inorganic carbon (DIC) and alkalinity. Circulation changes (e.g., Indonesian Throughflow, Southern Ocean inflows) can also cause large interior carbon accumulation anomalies.

Biological and Other Processes

• Primary production (photosynthesis) is substantial, with satellite- based estimates around 8 Gt C/yr for the basin.
• Respiration in deeper (aphotic) layers recycles carbon, influencing DIC distributions.
• Dust and iron deposition from the atmosphere can stimulate biological uptake in some areas, though effects vary north- south.
• The Indian Ocean is relatively undersampled compared to other basins, leading to greater uncertainty in flux estimates

Climate Change Impacts and Buffering Role

Warming, changing monsoons, and rising atmospheric CO₂ are altering the cycle. Amplifying seasonal CO₂ exchange does not automatically increase the net annual sink, but it reshapes when and how strongly exchange occurs.

This matters for accurate carbon budgets (annual averages can miss timing shifts) and for marine ecosystems: greater CO₂ uptake increases acidification, which reduces carbonate availability for corals, shellfish, and other calcifiers. Extreme seasonal conditions may hinder ecosystem recovery.⁠

Overall, the Indian Ocean continues to act as a climate buffer by taking up anthropogenic CO₂, with evidence of a strengthening sink in some analyses (~0.02 PgC/yr per decade in certain periods).

However, its capacity and reliability depend on ongoing physical and biological changes. Continued observations and modeling are essential, especially given data sparsity in parts of the basin.

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The Indian Ocean and Atlantic Ocean both act as net sinks for atmospheric CO₂ and contribute meaningfully to the global ocean carbon sink, but they differ substantially in magnitude, regional patterns, drivers, seasonal behavior, and efficiency of carbon storage.

Net CO₂ Flux (Air-Sea Exchange)

• Indian Ocean: A relatively modest net sink, typically estimated at about −0.07 to −0.19 PgC/yr (north of ~37.5°S) for recent multi- decadal periods (e.g., 1985- 2018). Northern areas (especially the Arabian Sea) are sources, while the southern subtropical region is a stronger sink. One reconstruction puts the full Indian Ocean sink near −0.30 PgC/yr.⁠
• Atlantic Ocean: A stronger net sink overall, with basin- wide estimates around −0.36 to −0.47 PgC/yr (1985- 2018, including the Mediterranean in some assessments). The North Atlantic (particularly subpolar regions) is an intense sink; the tropical Atlantic is a moderate source due to upwelling. The North Atlantic alone (a small fraction of global ocean area) can account for a disproportionately large share of global uptake.

In relative terms, the Atlantic often contributes a larger share of the global ocean sink relative to its size than the Indian Ocean. Both basins show evidence of a strengthening sink over recent decades in response to rising atmospheric CO₂, though interannual variability (e.g., linked to ENSO and IOD in the Indian Ocean or AMOC changes in the Atlantic) can modulate this.

Regional Contrasts

• Indian Ocean: Strong north- south contrast driven by the monsoon. The northwestern Arabian Sea is a perennial source (strong summer upwelling brings CO₂- rich deep water to the surface). The Bay of Bengal is closer to neutral or a mild sink (influenced by large river freshwater inputs that lower salinity and pCO₂). The equatorial region is near neutral. The southern Indian Ocean (south of ~15- 18°S) is the main sink region.⁠
• Atlantic Ocean: Stronger high- latitude sink in the North Atlantic due to cooling of poleward- flowing waters (increasing solubility) and biological drawdown, combined with deep convection and the Atlantic Meridional Overturning Circulation (AMOC). The tropical Atlantic is a source (upwelling), while the South Atlantic is a sink. Coastal and open-ocean differences exist, but the North Atlantic’s efficiency is notable.

Unlike the equatorial Pacific (a major source) or equatorial Atlantic (a source), the equatorial Indian Ocean is closer to neutral because of its unique wind patterns and deeper thermocline.

Drivers and Physical Processes

• Indian Ocean: Dominated by seasonal monsoon wind reversal, upwelling (especially Arabian Sea), riverine freshwater inputs (Bay of Bengal), and connections via the Indonesian Throughflow and Southern Ocean inflows. Both solubility and biological pumps matter, with solubility more influential in the north and both important in the south. Climate modes like the Indian Ocean Dipole strongly affect interannual flux variability.
• Atlantic Ocean: Strongly influenced by the AMOC, which ventilates deep waters and efficiently transfers anthropogenic carbon (Cₐₙₜ) into the interior. The North Atlantic is a key entry point for Cₐₙₜ into the deep ocean. Heat loss, biological export, and western boundary currents (e.g., Gulf Stream cooling) enhance uptake. A significant fraction of Atlantic Cₐₙₜ accumulation comes from lateral transport from the Southern Ocean.

The Atlantic stores a large share of global oceanic anthropogenic carbon relative to its area (historically ~23% despite covering ~15% of the ocean), thanks to deep ventilation. The Indian Ocean stores less efficiently in its northern (landlocked) portion but receives carbon via throughflow and Southern Ocean connections.

Seasonal Variability

Both oceans show pronounced seasonal cycles in CO₂ exchange, with cooler seasons favoring stronger sinks.

• In the Indian Ocean, monsoon-driven changes produce large seasonal swings in pCO₂ and flux; recent studies show these seasonal amplitudes have increased substantially in equatorial and southern regions (e.g., +48% equatorial, +23% southern from 1980- 2019).
• In the Atlantic, seasonal amplitude is also significant (especially subtropics and high latitudes), with winter cooling and biological activity enhancing uptake. Amplification of seasonal pCO₂ cycles due to anthropogenic carbon accumulation is a broader ocean phenomenon observed in both basins.

Climate Change and Buffering Role

Both basins help buffer atmospheric CO₂ rise and are experiencing acidification.

The Atlantic’s deep ventilation makes it particularly important for long- term sequestration of anthropogenic carbon.

The Indian Ocean’s role is more surface- and intermediate- depth oriented in many areas, with greater sensitivity to monsoon and IOD changes.

Warming reduces solubility in both, but circulation changes (e.g., potential AMOC weakening vs. monsoon shifts) will affect future uptake differently. The Indian Ocean remains more undersampled than the Atlantic in many datasets (e.g., SOCAT), adding uncertainty to its estimates.

In summary: The Atlantic is generally a more efficient and stronger net CO₂ sink per unit area (especially in the North Atlantic), with superior deep- ocean carbon storage via overturning circulation.

The Indian Ocean is a weaker overall sink with sharper regional contrasts (northern sources vs. southern sink) tightly coupled to monsoon dynamics.

Both are critical components of the global ocean carbon cycle, and ongoing changes in their seasonal and interannual behavior will influence how reliably they continue to act as climate buffers.

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The Pacific Ocean is the largest ocean basin and plays a dual role in the global carbon cycle: it is the world’s biggest oceanic source of CO₂ in its equatorial region, yet overall, it functions as a net sink of comparable magnitude to the Atlantic (and larger than the Indian Ocean in absolute terms), thanks to strong uptake in its subtropical and higher- latitude zones.

Net CO₂ Flux (Air- Sea Exchange)

• Pacific: Basin- wide net uptake is typically estimated at roughly −0.4 to −0.5 PgC/yr (e.g., −0.41 ± 0.12 PgC/yr from observation- based pCO₂ products and −0.51 ± 0.16 PgC/yr from models for (−0.7 PgC/yr), offsetting substantial natural outgassing. The net sink has been strengthening at rates around −0.08 to −0.09 PgC/yr per decade.
• Comparison: This places the Pacific’s net sink in a similar range to the Atlantic (−0.36 to −0.47 PgC/yr) and larger in absolute terms than the Indian Ocean (−0.07 to −0.30 PgC/yr depending on the exact domain). Despite covering nearly half the global ocean area, the Pacific’s net contribution is moderated by its powerful equatorial source. In some climatologies, the three basins contribute roughly similarly to global open- ocean uptake when coastal zones are included (Pacific and Atlantic each ~25%, Indian ~20%).

Regional Patterns

• Equatorial Pacific: The dominant global oceanic CO₂ source. Strong upwelling of carbon-rich deep water (especially in the eastern “cold tongue”) releases ~0.4- 0.7 PgC/yr or more on average, often accounting for a large fraction of total oceanic outgassing. This far exceeds the tropical Atlantic source and the more localized northern Indian Ocean sources (e.g., Arabian Sea). The western equatorial Pacific is closer to neutral or occasionally a weak sink.⁠
• Subtropical Pacific: Strong sinks, particularly in the North Pacific subtropical gyre, driven by cooling of waters and biological activity. These help offset equatorial outgassing.
• Higher latitudes: Uptake occurs in subpolar regions, though less intense per unit area than the North Atlantic’s deep-convection zones. The North Pacific lacks the Atlantic’s vigorous deep-water formation, so anthropogenic carbon (Cₐₙₜ) penetration is shallower (mainly intermediate depths).

In contrast:

• The Atlantic has intense high- latitude (especially North Atlantic) sinks and efficient deep ventilation via the AMOC.
• The Indian Ocean shows a clear north– south divide (northern sources vs. southern sink) tightly tied to the monsoon, without a single dominant equatorial source like the Pacific’s.

Drivers and Variability

• Pacific: Dominated by the El Niño- Southern Oscillation (ENSO). During El Niño, equatorial upwelling weakens and outgassing decreases sharply (sometimes nearly shutting off the source); during La Niña, outgassing intensifies. This makes the tropical Pacific the largest contributor to interannual variability in the global ocean carbon sink. Subtropical and higher -latitude uptake is influenced by gyre circulation, seasonal temperature changes, and biology.
• Seasonal cycles: Present but often secondary to interannual ENSO- driven swings in the tropics. Seasonal amplification of pCO₂ cycles (linked to anthropogenic carbon accumulation) occurs, similar to other basins.
• Comparison: The Indian Ocean’s variability is more monsoon- and IOD- driven with strong seasonal signals that have recently amplified. The Atlantic has notable seasonal cycles (especially high latitudes) and is influenced by AMOC strength and North Atlantic Oscillation variability but lacks the Pacific’s extreme interannual equatorial swings.

Anthropogenic Carbon Storage and Efficiency

The Pacific stores a large absolute amount of Cₐₙₜ due to its size, but less efficiently per unit area than the Atlantic. The North Atlantic’s deep convection allows deeper and more rapid sequestration. Pacific Cₐₙₜ is mostly confined to intermediate depths via mode and intermediate water subduction. The Southern Ocean (shared influence) is a major entry point for all basins, with subsequent redistribution.

Climate Change Context

All three basins are experiencing rising uptake of anthropogenic CO₂, ocean acidification, and changes in seasonal and interannual patterns. The Pacific’s equatorial source region is particularly sensitive to ENSO changes and warming (which can affect upwelling and solubility). Its large area means even modest per-area changes have global impact. Data coverage in the Pacific is generally better than in the Indian Ocean but still sparse in some southern regions.

Summary comparison:

• Pacific: Largest source region (equatorial) and large subtropical sinks → moderate- to- strong net sink; high interannual variability (ENSO).
• Atlantic: Strongest high latitude sinks and efficient deep storage (AMOC) → strong net sink relative to size.
• Indian: Modest net sink with sharp monsoon- driven north– south contrast and growing seasonal amplitude.

Together, the three major ocean basins (plus the Southern Ocean) form the backbone of the oceanic carbon sink that absorbs roughly a quarter of human CO₂ emissions each year.

The Pacific’s unique combination of a massive natural source and extensive sink regions makes its behavior especially important for understanding global carbon- cycle variability.

Climate Change Induced Seasonal Amplification of the Indian Ocean CO₂ Sink

Key findings from the paper (based on available summaries)

The study analyzed air- sea CO₂ exchange across the Indian Ocean from 1980 to 2019, focusing on changes in seasonal variability:

• Seasonal amplitude increased by:
  • 48% in the equatorial Indian Ocean
  • 23% in the southern Indian Ocean
  • ~7% in the Bay of Bengal
  • It decreased by 14% in the Arabian Sea
• Mean CO₂ release to the atmosphere decreased in the Arabian Sea, Bay of Bengal, and equatorial Indian Ocean.
• CO₂ absorption increased in the southern Indian Ocean.
• Drivers differed by region:
  • Equatorial and southern Indian Ocean → mainly changes in the air-sea difference in CO₂ partial pressure (ΔpCO₂).
  • Arabian Sea and Bay of Bengal → primarily changes in wind speed (affecting gas-transfer velocity).

Important caveat emphasized in the work: Larger seasonal swings do not automatically mean a stronger net annual carbon sink. The amplification reshapes the timing and intensity of exchange within the year rather than necessarily increasing total uptake.

The paper frames these changes as a response to climate change (warming, altered circulation and monsoon patterns, and rising atmospheric CO₂) and notes implications for marine ecosystems (via acidification) and for carbon- budget estimates that rely only on annual averages.

Context

This is one of the more detailed observational analyses of seasonal CO₂ flux changes specifically in the Indian Ocean. It complements broader findings that seasonal pCO₂ amplitudes are increasing across much of the global ocean due to anthropogenic carbon accumulation (which reduces the ocean’s buffer capacity and makes pCO₂ more sensitive to temperature and biology).

Journal: Geophysical Research Letters, (2026)

DOI: 10.1029/2026gl123472

First published: 30 September 2026

Authors: A. P. Joshi, Kunal Chakraborty, V. V. S. S. Sarma, Prasanna Kanti Ghoshal, B. Sridevi, Zouhair Lachkar

Abstract

This study investigates changes in the mean and seasonal states of oceanic carbon uptake across the Indian Ocean (IO) using an improved, high-resolution, observationally constrained long-term (1980–2019) surface ocean pCO2$$ data product. The Arabian Sea shows a ∼14% decline in CO2$$ flux amplitude along with reduced mean source strength. In contrast, the Equatorial IO exhibits the largest increase in seasonal amplitude (∼48%), followed by the Southern IO (∼23%), while the Bay of Bengal shows only a modest change (∼7%). Despite these differences, mean source strength decreases in both the Bay of Bengal and Equatorial IO, whereas the Southern IO shows an enhanced carbon sink. Amplitude changes in the Arabian Sea and Bay of Bengal are driven by shifts in flux sensitivity to wind speed and evolving seasonality of the ocean-atmosphere pCO2$$ gradient, whereas in the Equatorial and Southern regions, they are primarily controlled by the evolving pCO2$$ gradient alone.