During the 2015-2016 El Niño, NASA's Orbiting Carbon Observatory-2 revealed unprecedented insights into Earth's carbon cycle, showing a two-phase response of ocean and terrestrial systems.
When the strongest El Niño in decades emerged in 2015, climate scientists around the world paid close attention. They knew this climate phenomenon would trigger significant changes across the globe, but a critical question remained: how exactly would it affect the planet's carbon cycle? As it turned out, NASA had a powerful new tool in orbit that was perfectly positioned to find answers—the Orbiting Carbon Observatory-2 (OCO-2) satellite. Launched just a year earlier in 2014, OCO-2 provided an unprecedented view of how carbon dioxide moves through our atmosphere, offering new insights into one of Earth's most vital systems 1 9 . What it revealed during this natural climate experiment challenged previous assumptions and provided scientists with a clearer picture of how our planet responds to climatic disruptions.
Launched in July 2014, perfectly timed to observe the 2015-2016 El Niño event.
The 2015-2016 event was one of the strongest El Niños ever recorded.
To appreciate OCO-2's discoveries, we must first understand Earth's carbon cycle—the continuous movement of carbon between the atmosphere, land, oceans, and living organisms. This system maintains a delicate balance through sources that release carbon dioxide and sinks that absorb it. Under normal circumstances, natural sinks like forests and oceans absorb approximately half of the CO₂ emitted by human activities, acting as crucial buffers against climate change 7 . However, this balance is fragile, and scientists have long observed that the rate of CO₂ accumulation in the atmosphere varies significantly from year to year, with a strong correlation to the El Niño Southern Oscillation (ENSO) 9 .
| Component | Role in Carbon Cycle | Impact During El Niño |
|---|---|---|
| Oceans | Absorb about 25% of human-emitted CO₂ | Reduced outgassing in tropical Pacific due to suppressed upwelling |
| Forests & Vegetation | Absorb CO₂ through photosynthesis | Reduced uptake due to drought and heat stress |
| Soils | Store vast carbon reserves | Increased respiration releases more CO₂ |
| Fossil Fuel Emissions | Human-caused source of CO₂ | Unrelated to El Niño but contributes to background levels |
| Biomass Burning | Natural and human-caused fires | Enhanced emissions, particularly in Southeast Asia |
NASA's Orbiting Carbon Observatory-2 was specifically designed to tackle one of the most pressing questions in climate science: where exactly are all the carbon sources and sinks, and how do they change over time? Before OCO-2, scientists relied on a sparse network of ground-based monitoring stations that left significant gaps in our understanding, particularly over tropical forests and oceans 4 . OCO-2 changed this paradigm by providing global, high-resolution measurements of atmospheric carbon dioxide.
The satellite employs three high-resolution spectrometers that function like sophisticated cameras, detecting the unique molecular "fingerprint" of CO₂ and oxygen in sunlight reflected from Earth's surface. By measuring how much light is absorbed at specific wavelengths, OCO-2 can determine the column-averaged carbon dioxide dry air mole fraction (XCO₂)—essentially the number of CO₂ molecules in a column of air from the satellite to Earth's surface 6 . What makes OCO-2 particularly remarkable is its precision; each measurement covers an area of just 3 km² when looking straight down at Earth, allowing it to gather up to 72,000 individual soundings on the sunlit side of each orbit 6 .
The El Niño that developed in 2015 presented a perfect opportunity to test OCO-2's capabilities. El Niño events originate in the tropical Pacific Ocean and involve a complex interplay between ocean temperatures and atmospheric pressure. The 2015-2016 event was particularly strong, with warm sea surface temperature anomalies peaking in the Niño 3.4 region (5°S–5°N, 170°W–120°W) between November 2015 and January 2016 9 . Scientists had long known that El Niño events correlate with accelerated increases in atmospheric CO₂, but the exact mechanisms and timing remained poorly understood due to limited observations 9 .
Since OCO-2 had only been operational for about a year when the El Niño began, researchers combined its measurements with data from Japan's GOSAT satellite, which had been monitoring CO₂ since 2009. This provided a longer-term context to distinguish normal seasonal variations from El Niño-related anomalies 9 .
The team calculated XCO₂ anomalies—deviations from expected values—specifically over the Niño 3.4 region in the tropical Pacific. This helped isolate the El Niño's impact from background carbon cycle processes 9 .
Scientists tracked how these anomalies evolved throughout the different phases of the El Niño, from its development in spring 2015 through its maturity in late 2015 and early 2016, and into its decline 9 .
OCO-2 measurements were cross-checked against data from the Tropical Atmosphere Ocean (TAO) mooring array and other ground-based observations to ensure accuracy 9 .
The results revealed a fascinating two-phase response that had never been so clearly observed before:
During the developing stages of El Niño, OCO-2 detected an unexpected negative XCO₂ anomaly over the tropical Pacific Ocean—meaning there was less carbon dioxide in the atmosphere than usual. This initially counterintuitive finding actually confirmed a long-held hypothesis about the ocean's role. During El Niño events, the easterly trade winds weaken, suppressing the normal upwelling of cold, carbon-rich waters in the eastern equatorial Pacific 9 . Since these cold waters normally release substantial CO₂ to the atmosphere (about 0.4-0.6 PgC per year), reduced upwelling means less ocean outgassing, effectively leaving more carbon in the ocean 9 . OCO-2 observations showed this reduction in oceanic CO₂ emissions was substantial—approximately 40-60% lower than normal during the event 9 .
As the El Niño reached its mature phase, the pattern reversed dramatically. The negative ocean anomaly was replaced by a strong positive XCO₂ anomaly, indicating more CO₂ in the atmosphere. This shift reflected the combined impact of terrestrial ecosystems: reduced carbon uptake by drought-stressed vegetation, increased respiration from soils and plants, and significant emissions from fires, particularly in Indonesia and Southeast Asia 9 . The terrestrial response was larger than the ocean effect but also lagged behind it by several months, creating the distinctive two-phase pattern that had been difficult to detect before comprehensive satellite observations.
| El Niño Phase | Timeline | XCO₂ Anomaly Pattern | Primary Driver |
|---|---|---|---|
| Development Phase | Spring-Summer 2015 | Negative anomaly (0.5-1.0 ppm below normal) | Reduced ocean outgassing in tropical Pacific |
| Transition Period | Late Summer 2015 | Shift from negative to positive anomaly | Decreasing ocean effect, increasing terrestrial effect |
| Mature Phase | Fall 2015 onwards | Positive anomaly (0.5-1.5 ppm above normal) | Terrestrial emissions from reduced uptake and fires |
OCO-2's observations during the El Niño provided several crucial insights that have reshaped our understanding of the carbon cycle:
The tropical Pacific Ocean plays an early and important role in modulating atmospheric CO₂ during El Niño events, not just the terrestrial biosphere as previously emphasized 9 .
The timing of carbon cycle responses differs significantly between ocean and land components, with the ocean responding earlier and terrestrial systems dominating later 9 .
These findings suggest that previous estimates of land carbon sensitivity to climate may need revision, since they often attributed all atmospheric CO₂ variations during El Niño to terrestrial processes without accounting for the oceanic component 9 .
"The high-density, broad-scale observations of CO₂ from OCO-2 provide a valuable tool to partition the ocean and terrestrial carbon cycle responses to El Niño" 9 . This partitioning had been nearly impossible with the sparse ground-based monitoring network alone.
NASA's OCO-2 mission has fundamentally transformed our understanding of Earth's carbon cycle, particularly during climate extremes like the 2015-2016 El Niño. By providing the first detailed space-based observations of how both oceanic and terrestrial carbon systems respond to such events, OCO-2 has revealed a more complex narrative than previously understood—one where the timing and magnitude of both ocean and land responses create a cascading effect on atmospheric CO₂ concentrations.
These insights come at a critical time in human history. As climate change intensifies, extreme weather events and climate patterns like El Niño are expected to evolve, potentially altering how Earth's natural carbon sinks function. Understanding these dynamics is essential for predicting future climate change more accurately and for informing policy decisions aimed at managing carbon emissions. The "slow-in, fast-out" dynamics of the carbon cycle—where carbon is slowly absorbed by ecosystems but quickly released during extremes—highlights the vulnerability of our planet's natural carbon balancing systems 5 .
As we continue to face the challenges of climate change, the legacy of OCO-2's El Niño observations extends far beyond academic interest. They provide us with crucial insights into how Earth's carbon cycle may respond to a warming world, reminding us that while nature currently absorbs half of our carbon emissions, this service may be subject to change in ways we are only beginning to understand. With successors to OCO-2 already planned, the future of carbon cycle science looks bright, promising ever more detailed insights into the invisible flows of carbon that so profoundly shape our climate.