The 2015-2016 El Niño and the Response of the Carbon Cycle
Findings from NASA's OCO-2 Mission
The Planet's Fever: A Climate Mystery Unfolds
In 2015 and 2016, a powerful El Niño event caused the Earth to run a fever. The hottest temperatures ever recorded were accompanied by a worrying climate mystery: the largest annual increase in atmospheric carbon dioxide (CO₂) levels in the entire human record—a jump of 3.4 parts per million (ppm) in 2016, keeping global concentrations permanently above the symbolic 400 ppm threshold 7 .
For climate scientists, this surge presented a pivotal question: was this dramatic flicker in the global carbon cycle merely a temporary anomaly, or was it a warning sign that the Earth's natural systems were beginning to switch from slowing climate change to accelerating it 7 ?
Solving this mystery required a new kind of detective. Fortunately, orbiting high above the planet was NASA's Orbiting Carbon Observatory-2 (OCO-2), a mission dedicated to studying atmospheric carbon dioxide. Launched in 2014, OCO-2 provided the precise, global data needed to piece together the puzzle, offering unprecedented insights into how our planet's carbon cycle responds to extreme events 1 . This is the story of how a record-breaking El Niño and a pioneering satellite mission revealed the delicate balance of Earth's carbon sinks and sources.
3.4 ppm
Record CO₂ increase in 2016
400+ ppm
CO₂ concentration threshold crossed
2015-2016
Strongest El Niño on record
The Carbon Cycle and the El Niño Trigger
To understand the significance of the 2015-2016 event, one must first understand the Earth's natural carbon cycle. This cycle is a continuous process where CO₂ is exchanged between the atmosphere, the oceans, and land-based ecosystems (the biosphere). "Sources" release CO₂ into the atmosphere, while "Sinks" absorb and store it 5 .
Processes that release CO₂ into the atmosphere:
- Fossil fuel combustion
- Respiration by living organisms
- Wildfires
- Decomposition
Processes that absorb and store CO₂:
- Photosynthesis by plants
- Ocean absorption
- Soil carbon storage
Human activities, primarily the burning of fossil fuels, add billions of tons of CO₂ to the atmosphere each year. Remarkably, natural sinks in the ocean and on land absorb about half of these human emissions, substantially blunting the rate of climate change 5 . The identity and stability of these sinks, however, are not fully known.
The El Niño Southern Oscillation (ENSO) is the planet's most significant mode of interannual climate variability, and it acts as a massive stress test on this system 7 . During an El Niño, a pool of warm water shifts across the Pacific, disrupting global weather patterns. This typically leads to hotter and drier conditions in the tropical land regions, which are home to vast carbon-rich forests like the Amazon 7 .
Scientists had long observed that El Niño events correlate with a surge in atmospheric CO₂ growth rates, with the source traced to the terrestrial tropics 7 . The 2015-2016 El Niño provided a crucial opportunity to move beyond correlation and uncover the precise mechanisms at work.
A Detective in the Sky: How OCO-2 Sees Carbon Dioxide
The OCO-2 satellite does not directly measure carbon dioxide sources and sinks. Instead, it uses a sophisticated indirect method. Its three high-resolution spectrometers measure the intensity of sunlight reflected from the Earth's surface. As this light passes through the atmosphere, CO₂ and oxygen molecules absorb specific wavelengths, leaving unique molecular fingerprints 4 .
OCO-2's Spectrometer Bands
- The oxygen (O₂) A-band to determine the total atmospheric pressure and the length of the light path.
- A weak CO₂ band most sensitive to CO₂ near the Earth's surface.
- A strong CO₂ band that provides an independent measure of CO₂ abundance and is sensitive to atmospheric aerosols 4 .
By analyzing the absorption in these bands, scientists can calculate the column-averaged CO₂ dry air mole fraction (XCO₂)—the total number of CO₂ molecules in a column of air from the satellite to the Earth's surface 4 . With a footprint of about 3 km² and the ability to gather up to 72,000 measurements on the sunlit side of the planet, OCO-2 provides a massive, high-resolution map of global XCO₂ 4 . This dense data coverage, especially over remote tropical regions with few ground-based monitors, was the key that previous studies lacked.
NASA's OCO-2 satellite provides precise global CO₂ measurements
The Scientist's Toolkit: Key Instruments for Carbon Cycle Investigation
Solar-Induced Chlorophyll Fluorescence (SIF)
A remote sensing measure from OCO-2 that detects the faint glow emitted by plants during photosynthesis, serving as a direct proxy for photosynthetic activity 9 .
OCO-2 Mission Specifications 8
| Parameter | Specification | Scientific Relevance |
|---|---|---|
| Launch Date | July 2, 2014 | Began operations just before the record El Niño. |
| Orbit | 705 km, Sun-synchronous | Provides global coverage with a 16-day repeat cycle. |
| Equatorial Crossing Time | 1:35 PM local time | Ensures high sun for optimal reflected light measurements. |
| Spatial Resolution | 1.29 km x 2.25 km | Allows observation of localized sources and sinks. |
| XCO₂ Precision | < 0.8 ppm (globally) | High enough to detect the small changes driven by ecosystems 9 . |
A Deep Dive into the OCO-2 MIP: A Multi-Model Investigation
A single measurement cannot tell the whole story. To ensure robustness, the OCO-2 science team initiated the OCO-2 Model Intercomparison Project (MIP). This was a crucial experiment designed to synthesize results from an ensemble of 10 different international research teams 2 . Each team used its own atmospheric inversion model—which couples atmospheric transport with data assimilation algorithms—to interpret the OCO-2 data. The goal was to account for uncertainties in transport models and optimization techniques, providing a reliable range for the estimated carbon fluxes 2 .
Methodology: A Step-by-Step Process
Data Collection
Throughout the 2015-2018 period, OCO-2 collected hundreds of thousands of XCO₂ measurements, with a particular focus on the intense El Niño period of 2015-2016 2 .
Data Assimilation
Each of the 10 modeling teams fed the OCO-2 v9 land data into their unique inversion systems. For comparison, a baseline was created using traditional in-situ (IS) measurements from a sparse global network 2 .
Flux Estimation
The models calculated month-by-month global surface CO₂ fluxes—the net exchange of carbon between the land/ocean and the atmosphere 2 .
Independent Validation
The resulting flux estimates were evaluated against independent data not used in the assimilation, such as aircraft measurements and TCCON data, to verify their accuracy 2 .
Results and Analysis: The Carbon Cycle Revealed
The OCO-2 MIP analysis yielded a much clearer picture of how the carbon cycle responded to the El Niño stress test. The findings confirmed that the tropical land biosphere was the epicenter of the disruption.
Regional Carbon Flux Anomalies During the 2015-2016 El Niño
Tropical Asia
Large carbon release due to drier conditions leading to increased peat and forest fires 7 .
Eastern Amazonia
Large carbon release due to reduced photosynthesis from heat and atmospheric dryness 7 .
Southern Africa & Australia
Reduced carbon uptake due to widespread drought leading to decline in photosynthesis 7 .
The core finding was that the record CO₂ growth rate was driven by a combination of two factors:
- Reduced Photosynthesis: The hotter, drier conditions in the tropics caused vegetation to slow down its photosynthetic activity. Data from OCO-2's Solar-Induced Chlorophyll Fluorescence (SIF) showed a strong decline, particularly in the dry season of late 2015 in eastern Amazonia 7 . One analysis estimated a pan-tropical reduction in photosynthesis of around 1 Pg C over the duration of the El Niño 7 .
- Increased Respiration and Fire: Ecosystems also released more carbon through plant respiration, the decomposition of soil organic matter, and a dramatic increase in wildfires, especially in Indonesia 7 .
Global Carbon Budget Changes (2015-2016)
| Component | Impact | Contribution to Atmospheric CO₂ Rise |
|---|---|---|
| Direct Human Emissions | Continued steady release | ~75% (2.5 ppm) of the 3.4 ppm rise 7 |
| Tropical Land Biosphere | Shifted from a sink to a source | ~25% (0.9 ppm) of the 3.4 ppm rise 7 |
| Global Net Land Sink | Weakened significantly | Absorbed less human-emitted CO₂ than normal 2 |
The MIP study also highlighted the value of satellite data. While inversions using the traditional in-situ network agreed with OCO-2 on the global total carbon flux, they struggled to pinpoint the regional variations, particularly in data-sparse regions like tropical Africa. OCO-2's dense coverage provided a far more detailed and confident picture of these critical carbon cycle hotspots 2 .
A Warning from the Tropics: Implications for Our Climate Future
The findings from OCO-2 during the 2015-2016 El Niño are more than just a historical record; they are a window into a warmer future. The event demonstrated that the tropical carbon sink, a crucial buffer against climate change, is highly sensitive to temperature and moisture stress. As one 2025 study using OCO-2 data confirmed, in Africa, moisture levels have a much bigger impact on plant growth and carbon emissions than was previously understood, with temperature being the prime suspect 9 .
The concern is that if the conditions seen during the 2015-2016 El Niño become the new normal due to ongoing global warming, the land biosphere could lose its efficiency as a carbon sink. Recent research based on OCO-2 data has already shown alarming signs, such as a record low global net land carbon sink in 2023, a year of record temperatures, suggesting that the negative impact of warming is continuing 9 . If this buffering capacity falters, the rate of CO₂ buildup in our atmosphere will accelerate, leading to more rapid and severe climate change.
NASA's OCO-2 mission has fundamentally transformed our ability to monitor and understand the vital signs of our planet. By turning the 2015-2016 El Niño into a global natural experiment, it has provided undeniable evidence of the fragility of Earth's carbon cycle. The data streaming down from this silent sentinel in space continues to inform our climate models and policies, offering a critical reality check on the health of our world and a stark reminder of the changes already underway.
Drought conditions during El Niño reduce carbon uptake by vegetation