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Climate breakdown: Forests and soils are reaching the limit of CO2 absorption

Every year, the concentration of carbon dioxide in the atmosphere increases. The CO2 concentration currently stands at 411.36 parts per million. In 2019, the UK Met Office predicts that the increase in CO2 in the atmosphere will be larger than in 2018. The increase in 2019 is likely to be one of the largest in 62 years of measurements.

Professor Richard Betts of the Met Office Hadley Centre explains the predicted increase as follows:

“Since 1958, monitoring at the Mauna Loa observatory in Hawaii has registered around a 30 per cent increase in the concentration of carbon-dioxide in the atmosphere. This is caused by emissions from fossil fuels, deforestation and cement production, and the increase would have been even larger if it were not for natural carbon sinks which soak up some of the excess CO2. This year we expect these carbon sinks to be relatively weak, so the impact of record high human-caused emissions will be larger than last year.”

The reason that the Met Office anticipates that carbon sinks will be “relatively weak” this year is because the tropical Pacific will be warmer than usual. The Met Office states that, “In years with a warmer tropical Pacific, many regions become warmer and drier, which limits the ability of plants to grow and absorb CO2.”

Currently, the oceans and forests absorb about half of all CO2 emissions caused by human activities. Clearly, we cannot continue our current rates of burning fossil fuels, clearing forests, and building with concrete. We also cannot rely on forests to continue absorbing our greenhouse gas emissions.

Soil moisture and carbon uptake

Last week, a study published in Nature showed that vegetation may not be able to continue absorbing these emissions.

The paper is titled, “Large influence of soil moisture on long-term terrestrial carbon uptake”, and was written by Julia K. Green (Department of Earth and Environmental Engineering, Columbia University), Sonia I. Seneviratne (Department of Environmental Systems Science, ETH Zurich), Alexis M. Berg (Department of Civil and Environmental Engineering, Princeton University), Kirsten L. Findell (Geophysical Fluid Dynamics Laboratory, Princeton), Stefan Hagemann (Institute of Coastal Research, Helmholtz-Zentrum
Geesthacht), David M. Lawrence (Climate and Global Dynamics Laboratory, Terrestrial Sciences, National Center for Atmospheric Research, Boulder) and Pierre Gentine (Department of Earth and Environmental Engineering, Columbia University, and The Earth Institute, Columbia University).

Pierre Gentine, associate professor of earth and environmental engineering at Columbia University and affiliated with the Earth Institute led the study. In a press statement, he said that,

“It is unclear … whether the land can continue to uptake anthropogenic emissions at the current rates. Should the land reach a maximum carbon uptake rate, global warming could accelerate, with important consequences for people and the environment. This means that we all really need to act now to avoid greater consequences of climate change.”

When the soil is dry, plants are stressed and growth is slower. As the global climate warms, droughts in the tropics are becoming more common.

Gentine worked with his PhD student, Julia Green, aiming to understand how droughts, floods, and long-term drying trends were affecting the capacity of terrestrial carbon sinks, including forests, to absorb CO2 emissions.

“This is a big deal!”

Gentine and Green analysed net biome productivity (NBP) that the Intergovernmental Panel on Climate Change defines as the net gain or loss of carbon from a region. They used data from four Earth System Models from the Global Land Atmosphere Coupling Experiment—Coupled Model Intercomparison Project experiments.

They found that soil moisture had a dramatic impact on the amount of carbon absorbed. Green said that,

“We saw that the value of NBP, in this instance a net gain of carbon on the land surface, would actually be almost twice as high if it weren’t for these changes (variability and trend) in soil moisture.

“This is a big deal! If soil moisture continues to reduce NBP at the current rate, and the rate of carbon uptake by the land starts to decrease by the middle of this century—as we found in the models—we could potentially see a large increase in the concentration of atmospheric CO2 and a corresponding rise in the effects of global warming and climate change.”

REDD-Monitor has written many posts about the dangers of relying on forests to offset emissions from burning fossil fuels. As the climate worsens, forests are becoming an extremely unstable store of carbon. Reducing deforestation is urgently needed. But to address climate breakdown (and to prevent forests going up in smoke) we have to leave fossil fuels underground.

The following links are to posts on REDD-Monitor based on what climate scientists have said over the past 10 years about the impact of the changing climate on tropical forests:


 

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