Researchers at UC San Diego’s Scripps Institution of Oceanography have created a more accurate model of global carbon cycling. The model better accounts for the contributions of Earth’s terrestrial ecosystems to atmospheric concentrations of CO2 .
The model can verify reported global emissions over a five-year timespan with half the uncertainty of another widely used model. That five-year timespan is especially relevant to the Paris Agreement because of a scheduled assessment called the global stocktake.
Every five years, the stocktake evaluates the world’s progress on lowering greenhouse gas emissions and other facets of addressing climate change. The first global stocktake is now in progress.
The study improved performance by better accounting for the spatially and seasonally varying influences of temperature on land-based carbon cycling. The model accomplished this by incorporating the findings of previously published research that yielded a global map establishing how short-term temperature fluctuations impact the carbon balance of land ecosystems.
The model’s strong performance also suggests an unexpected stability in land ecosystems’ response to rising temperatures as climate change progresses. That runs counter to an influential 2014 paper that used similar atmospheric data to argue that tropical ecosystems had become more sensitive to temperature fluctuations in recent decades.
Refining terrestrial ecosystems’ piece of the carbon puzzle is particularly impactful because it fluctuates from year to year and has historically entailed the greatest level of uncertainty compared to the planet’s other three main carbon repositories and sources – the oceans, changes in land use or land cover such as deforestation, and emissions from human activities.
Previously, when researchers have tried to add up the contributions of these to equal the measured changes in atmospheric carbon dioxide levels, there have been up to 760 million tons of carbon – equivalent to roughly 8% of current global fossil-fuel emissions – left unaccounted for. This is the carbon budget imbalance.
Until now, there have been two main models trying to explain year-to-year variations in exchanges of carbon between the land biosphere and the atmosphere. These are the El Niño-Southern Oscillation (ENSO) model and the Global Carbon Project (GCP) model. Both of these approaches to balancing the carbon budget still leave large unexplained residuals.
These imbalances make it more challenging to accurately verify reported carbon emissions from the world’s countries and harder to understand how Earth’s systems are responding to rising temperatures.
The new model outperformed both the simple ENSO model and GCP’s complex dynamic vegetation model. At the interannual scale it left 500 million tons of carbon unaccounted for at the year-to-year scale. At the decadal scale it did even better, with a carbon budget imbalance of plus or minus 1.6 gigatons of carbon, which is less than half that from the GCP model.
“The performance of this model says to me that it’s not just temperature or anomalous warmth that matters for carbon cycling, it’s also the timing and location of that anomalous warmth,” said Benjamin Birner, the study’s lead author.
The next step for this model would be to operationalize it to “see if the last five years of carbon dioxide growth in the atmosphere are in alignment with reported emissions,” said Ralph Keeling, study co-author.
But he said the model can also help us better understand the fundamentals of carbon cycling on Earth – a key to forecasting and combating climate change.
“If we have a year of unusual carbon dioxide growth we can also use this model to check our understanding of why that took place,” said Keeling. “If this current El Niño is like past ones, we might see extra carbon dioxide growth. The fundamental assumptions of this model provide us with a new hypothesis about what’s driving the carbon cycle that we can test against what we see each year.”
To view the complete study, click here.