For the first time, scientists could see the invisible connections between climate and carbon.
The year 2015 saw the birth of an environmental giant. A powerful El Niño event—a periodic warming of the central and eastern Pacific Ocean—had begun, altering weather patterns across the globe. While its impacts on temperature and rainfall were widely reported, a more subtle but critical drama was unfolding in the atmosphere: the largest annual increase in atmospheric carbon dioxide levels in millennia.
For scientists, this event presented a unique natural experiment. For the first time, they had a powerful new tool to observe it—NASA's Orbiting Carbon Observatory-2 (OCO-2) satellite. This mission provided the detailed, global data needed to finally crack the code on how El Niño disrupts the planet's natural carbon cycle, revealing the specific regions on Earth that turned from carbon absorbers into massive carbon emitters 9 .
The 2015-2016 El Niño provided the first opportunity to observe with precision how climate phenomena affect the global carbon cycle, thanks to NASA's OCO-2 satellite.
To appreciate the significance of the OCO-2 findings, one must first understand the Earth's natural carbon cycle.
Think of the planet as a living entity that breathes. On each "inhale," vast forests and oceans absorb carbon dioxide (CO₂) from the atmosphere. Trees use it for photosynthesis, and oceans dissolve it into their waters.
On each "exhale," natural processes like plant respiration, ocean gas exchange, and decomposition release CO₂ back. This cycle typically maintains a delicate balance.
However, human activities, primarily the burning of fossil fuels, have added an extra, uncontrolled "exhale," flooding the atmosphere with more CO₂ than the system can handle. El Niño events act as a massive stressor to this already strained system.
For decades, scientists have observed a strong correlation between El Niño and a sharp acceleration in the growth of atmospheric CO₂. They knew the tropical regions were the key players, but without precise, global observations, they couldn't pinpoint the exact mechanisms or quantify the contributions of South America's droughts versus Indonesia's fires. The OCO-2 satellite was built to solve exactly this kind of puzzle 3 7 9 .
OCO-2 Satellite Visualization
Launched in July 2014, just months before the onset of the massive 2015-2016 El Niño, OCO-2's timing was serendipitous. It was NASA's first satellite dedicated to measuring atmospheric carbon dioxide with the precision needed to identify regional sources and sinks 4 8 .
Unlike ground-based stations that take measurements at specific points, OCO-2 provides a sweeping, global view. It doesn't measure CO₂ concentration directly. Instead, it uses three high-resolution spectrometers to analyze sunlight reflected off the Earth's surface.
As this light passes through the air, CO₂ and oxygen molecules absorb specific colors. OCO-2 measures the intensity of this absorption to determine the number of CO₂ molecules in a vertical column of air, a quantity known as the column-averaged dry-air mole fraction (XCO₂). By comparing it with the well-known amount of oxygen, scientists can calculate the XCO₂ with an accuracy better than 0.3% 1 8 .
The satellite orbits the Earth in a sun-synchronous pattern, part of the "A-train" constellation, allowing it to observe the same location at the same time of day and gather over 100,000 measurements on the sunlit side of the planet daily 1 4 . This immense data volume was the key to creating a high-resolution map of carbon dioxide that was previously impossible.
When the El Niño struck, the OCO-2 science team embarked on a grand experiment to understand how this climate phenomenon altered the carbon cycle.
OCO-2 began continuously collecting XCO₂ data across the globe, with a particular focus on the tropical regions most affected by El Niño.
Scientists compared the OCO-2 data from the El Niño period (2015-2016) with data from a reference year, 2011, which had neutral climate conditions 9 .
The team looked for significant changes—or anomalies—in atmospheric CO₂ over three key tropical continents: South America, Africa, and Indonesia.
The satellite data was combined with other datasets, including measurements from the Total Carbon Column Observing Network (TCCON) for validation, and climate data on temperature, rainfall, and fire activity 8 .
The findings revealed that three tropical regions released a staggering 2.5 gigatons more carbon into the atmosphere in 2015 than in 2011 9 .
| Region | Primary Mechanism | Impact on the Carbon Cycle |
|---|---|---|
| South America | Drought & Reduced Photosynthesis | Hot, dry conditions stressed tropical forests, reducing their ability to absorb CO₂ 9 |
| Africa | Increased Respiration | Higher temperatures accelerated decomposition, releasing more CO₂ 9 |
| Indonesia | Intense Fires | Widespread fires released vast amounts of stored carbon 9 |
Drought conditions in the Amazon reduced the forest's capacity to absorb carbon, turning a critical carbon sink into a diminished absorber.
Increased temperatures accelerated the respiration of soil and vegetation, releasing stored carbon into the atmosphere.
Intense fires, exacerbated by drought conditions, released massive amounts of carbon stored in peatlands and forests.
The OCO-2 mission's findings during the 2015-2016 El Niño were a breakthrough in climate science. They moved our understanding from theory to observed fact, confirming the long-hypothesized mechanisms behind the carbon cycle's response to climate variability.
As the OCO-2 deputy project scientist noted, these results "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 .
In a world where climate change may increase the frequency and intensity of El Niño-like conditions, this knowledge is more critical than ever. The satellite revealed that if future climates bring more prolonged droughts and hotter temperatures, the Earth's natural carbon sinks could weaken, and natural emissions could increase, creating a feedback loop that further accelerates warming.
By revealing the inner workings of our planet's carbon cycle, OCO-2 has provided us with a clearer crystal ball, offering essential insights for shaping a more informed global response to the climate challenge.