The 2015-2016 El Niño and its unprecedented impact on Earth's carbon cycle
In 2015 and 2016, something remarkable happened in Earth's atmosphere. Despite years of awareness about climate change and efforts to curb emissions, the planet witnessed the largest spikes in atmospheric carbon dioxide levels ever recorded. The culprit wasn't solely the usual suspect—human fossil fuel consumption—but a complex interaction between a powerful climate pattern and the planet's natural carbon cycle. This unprecedented event saw global average atmospheric carbon dioxide jump to 422.8 parts per million by 2024, with the increase during 2024 alone reaching 3.75 ppm—the largest one-year increase on record 2 .
Did you know? The 2015-2016 El Niño was one of the strongest since the 1950s, and its effects provided scientists with crucial insights into how our planet responds to climate stress.
At the heart of this mystery was El Niño, a natural climate phenomenon that occurs every few years. But the 2015-2016 El Niño was different—it was one of the strongest since the 1950s, and its effects would provide scientists with crucial insights into how our planet responds to climate stress. For the first time, however, researchers had a powerful new tool to investigate this phenomenon: NASA's Orbiting Carbon Observatory-2 (OCO-2) satellite, which provided unprecedented data on where and how carbon dioxide was moving through our atmosphere 9 .
This is the story of how a temporary climate event triggered a cascade of environmental responses, leading to a record-breaking surge in atmospheric CO₂, and how a novel satellite mission revealed the intricate mechanisms behind this surge, providing us with vital clues about how Earth's carbon cycle may respond to future climate change.
To understand the significance of what happened during the 2015-2016 El Niño, we first need to understand Earth's natural carbon cycle. Under normal circumstances, our planet maintains a delicate balance—carbon dioxide is emitted through both natural processes and human activities, while approximately half of these emissions are absorbed by natural "sinks," including forests and oceans 2 . This balance has been increasingly disrupted by human fossil fuel burning, which has raised atmospheric CO₂ from pre-industrial levels of 280 ppm to over 420 ppm today 2 .
Forests and oceans absorb approximately half of human CO₂ emissions under normal conditions.
Fossil fuel burning has raised atmospheric CO₂ from 280 ppm to over 420 ppm since pre-industrial times.
Enter El Niño-Southern Oscillation (ENSO), a climate pattern characterized by unusually warm ocean temperatures in the equatorial Pacific. While El Niño is a natural occurrence, its effects ripple across the globe, altering weather patterns, causing droughts in typically rainy regions and excessive rainfall in normally dry areas. Scientists had long noticed correlations between El Niño events and increases in atmospheric CO₂ growth rates, but the precise mechanisms remained poorly understood due to insufficient data, particularly over the tropics where these changes originate 3 .
What made the 2015-2016 event so significant was its intensity coupled with a new observational capability that would allow scientists to move beyond correlation to causation.
Launched in July 2014, NASA's Orbiting Carbon Observatory-2 (OCO-2) was perfectly positioned to observe the unfolding El Niño event. This dedicated carbon observatory carries three high-resolution spectrometers that function like sophisticated cameras, tuned to detect the unique molecular "fingerprints" of carbon dioxide and oxygen in Earth's atmosphere 6 .
July 2, 2014
1.29 × 2.25 km
705 km altitude
OCO-2's measurement approach is both ingenious and precise. It observes sunlight reflected from Earth's surface, analyzing how specific wavelengths are absorbed by atmospheric gases. By measuring the absorption levels in three specific wavelength bands (oxygen A-band at 0.76 μm, weak CO₂ band at 1.61 μm, and strong CO₂ band at 2.06 μm), the satellite can determine the concentration of carbon dioxide molecules in a given column of atmosphere, a measurement known as XCO₂ (column-averaged CO₂ dry air mole fraction) .
| Parameter | Specification | Significance |
|---|---|---|
| Launch Date | July 2, 2014 | Timed perfectly before strong 2015-2016 El Niño |
| Instrument | Three grating spectrometers | Measures CO₂ and O₂ simultaneously |
| Spectral Bands | O₂ A-band (0.76 μm), Weak CO₂ (1.61 μm), Strong CO₂ (2.06 μm) | Enables precise column CO₂ measurements |
| Orbit | 705 km altitude, sun-synchronous | Provides global coverage every 16 days |
| Measurement Modes | Nadir, glint, and target | Flexibility for different surfaces and validation |
This advanced technology enabled OCO-2 to collect up to 72,000 individual measurements on the sunlit side of each orbit, providing the dense, global coverage necessary to piece together how tropical regions responded to the El Niño conditions .
When the record CO₂ spike occurred in 2015-2016, the OCO-2 science team, led by researchers like Junjie Liu of NASA's Jet Propulsion Laboratory, embarked on an investigation comparing 28 months of satellite data against a reference year (2011) when weather and carbon cycle processes were normal 9 . Their analysis revealed that three tropical regions released approximately 2.5 gigatons more carbon into the atmosphere during the El Niño period than they did in 2011 9 . This extra CO₂ explained nearly all the difference in atmospheric carbon dioxide growth rates between 2011 and the peak years of 2015-2016.
| Region | Climate Impact | Effect on Carbon Cycle | Contribution to CO₂ Spike |
|---|---|---|---|
| Tropical South America | Driest year in 30 years, higher temperatures | Reduced photosynthesis, stressed vegetation | Less carbon absorbed from atmosphere |
| Tropical Asia | Second-driest year in 30 years | Increased severity and extent of forest fires | Direct carbon emissions from combustion |
| Tropical Africa | Higher temperatures without significant drought | Accelerated decomposition of dead organic matter | Increased natural carbon emissions |
The most significant revelation was that each region responded differently to El Niño's influence, creating a "perfect storm" of carbon release. As Annemarie Eldering, Deputy Project Scientist for OCO-2, noted: "We knew El Niños were one factor in these variations, but until now we didn't understand, at the scale of these regions, what the most important processes were" 9 .
~280 ppm CO₂
Previously the largest CO₂ increase on record
Record CO₂ spike (3 ppm/year)
422.8 ppm CO₂, with 3.75 ppm increase in one year
The data showed that during the El Niño, the tropical Pacific Ocean played "an early and important role" in modulating atmospheric CO₂ concentrations—a phenomenon that had been inferred from limited ground-based measurements but never directly observed on such a comprehensive scale 3 .
The groundbreaking findings from the OCO-2 mission relied on more than just the satellite itself. A suite of sophisticated tools and resources enabled scientists to collect, process, and validate their data:
The open-source algorithm that converts raw spectral measurements into precise XCO₂ values, available on GitHub along with its User's Guide 1 .
Absorption coefficient tables essential for running the L2 retrieval algorithm, available upon request from the project 1 .
The curated science data products, including XCO₂ Lite and Solar-Induced Fluorescence (SIF) Lite, which are publicly available through the Goddard Earth Sciences Data and Information Services Center (GES DISC) 1 .
A worldwide network of ground-based Fourier Transform Spectrometers that provides essential validation data for OCO-2 measurements by comparing space-based and ground-based observations 8 .
The insights gained from studying the 2015-2016 El Niño with OCO-2 extend far beyond explaining a single climate event. They reveal fundamental vulnerabilities in Earth's carbon cycle system that have profound implications for our climate future.
Perhaps the most significant finding is the identification of a climate feedback mechanism: as temperatures rise due to global warming, natural carbon sinks like tropical forests may become less effective at absorbing our emissions 9 . This means more of our emitted carbon dioxide would remain in the atmosphere, potentially accelerating climate change—a self-reinforcing cycle that could make climate projections more dire.
The research also demonstrated that the apparent climate sensitivity of the global carbon cycle—how responsive it is to temperature variations—has changed over time, first increasing but then decreasing, though the reasons for this remain partially understood 4 .
| Time Period | Carbon Cycle Response | Notable Features |
|---|---|---|
| 1957-2017 | Varying sensitivity to temperature | Relationship changed over time |
| 1997-1998 El Niño | Significant CO₂ increase | Previously the largest on record |
| 2015-2016 El Niño | Record CO₂ spike (3 ppm/year) | Coincided with 25% higher human emissions than 1997-1998 |
| Future Projections | Potential for enhanced carbon release | If droughts become more frequent or severe |
As we look ahead, the lessons from this research are clear. The ability of OCO-2 to monitor carbon dioxide precisely from space has opened a new chapter in our understanding of Earth's carbon cycle. Future satellite missions, including the successor OCO-3 installed on the International Space Station, will build on these findings to improve carbon cycle models and climate predictions.
"Understanding how the carbon cycle in these regions responded to El Niño will enable scientists to improve carbon cycle models, which should lead to improved predictions of how our planet may respond to similar conditions in the future" 9 .
The 2015-2016 El Niño provided a natural experiment—a glimpse into a future where Earth's natural systems may no longer buffer our emissions as reliably as before. Thanks to NASA's OCO-2 satellite, we now have a clearer picture of that future, and with it, the knowledge to make more informed decisions about the path ahead.