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Factors affecting carbon dioxide uptake by the terrestrial ecosystem

The Earth’s climate history has been ever-changing. After the industrial revolution, however, climate change brought about by human activities is accelerating. . Burning of fossil fuels and deforestation has resulted in massive amounts of carbon dioxide being released into the atmosphere. It is now known that a fraction of this “anthropogenic” carbon dioxide is taken back by the land, making it act like a ‘Carbon sink’. A new study conducted by Prof. Govindasamy Bala and his team at the Indian Institute of Science, has quantified the main factors influencing this terrestrial carbon uptake.

Climate patterns have influenced human economies and cultures since time immemorial. Though climate can fluctuate naturally, the recent climate change is caused directly by human activities.. Industrialization and vehicular emissions have led to an increase in atmospheric concentrations of greenhouse gases like carbon dioxide, methane, nitrous oxide etc. which are inducing global warming. At the same time, there has been an increase in carbon uptake by the land in recent years, serving to lower the atmospheric carbon dioxide and decelerate the adverse effects of climate change. A thorough understanding of this process is critical in predicting how the carbon dioxide in the atmosphere will evolve and affect climate in future.

“Qualitatively”, Professor Bala explained, “four major factors have been deemed to play a role in terrestrial carbon fixing but their quantification had remained uncertain”. The first major factor, a process known as carbon dioxide fertilization, refers to the increased uptake of carbon dioxide by plant photosynthesis – more carbon dioxide leads to more photosynthesis. The heightened photosynthetic activity has also been linked to increases in crop yields and is seen as a rare benefit amidst the many gloomy predictions of the effects of global climate change.

Increased atmospheric presence of nitrogen compounds from fossil fuel combustion has also led to larger deposition of nitrogen in the earth. This second factor, known as nitrogen deposition, can act as a ‘fertilizer’, stimulating plant growth and thereby enhancing the carbon uptake by the land. However, excess of deposited nitrogen diminishes soil fertility and reverses the effect. Climate warming, on the other hand, increases carbon release to the atmosphere by affecting respiration rates in terrestrial ecosystem. Land use and land cover change (LULCC), is another factor that results in release of land-fixed carbon into the atmosphere; an example would be deforestation, wherein a trees are cleared by burning. Thus the uptake and release of carbon by the land is tightly controlled by these opposing factors.

Professor Bala and his team used a complex climate model having atmosphere, land biosphere, ocean and sea-ice components to investigate the contribution of these multiple factors governing land carbon uptake. They performed experiments to understand the land ecosystem response to the changing climate in the industrial era from 1850-2005.

Their simulations show that though globally land has lost carbon in the entire analysed historical period mainly because of deforestation, deforestation rates have come down in the last three decades along with an increase in vegetation productivity. Even though industrialization has progressed rapidly, the study results show that in the recent past, more carbon has been taken up by the land than emitted. Bala said that their study has quantitatively shown that this was made possible by the increased atmospheric carbon dioxide leading to larger carbon dioxide fertilization, and larger nitrogen deposition. Thus, we can conclude that CO2 fertilization and nitrogen deposition the two most prominent factors responsible for enhancing the terrestrial vegetation productivity and are driving the Earth’s carbon sink in recent decades.

These modelling simulations have laid a robust quantitative framework to previously qualitatively predicted trends. The study has used a superior and more complex climate model, resulting in better estimates of each factor’s contribution to the carbon uptake by the land. Professor Bala emphasized that many factors underlie the mechanisms of climate change and the ecosystem’s response to it, however it is with the understanding of the primary components that the more complex designs will unravel. For now, a piece in the puzzle has been identified, paving the way for further explorations and discoveries.

About the authors:

Prof. Govindasamy Bala is a professor at the Divecha Centre for Climate Change and Centre for Atmospheric and Oceanic Sciences, Indian Institute of Science, Bangalore, Karnataka. Dr. Devaraju was a post-doctoral fellow in Prof. Bala’s lab at the time of the study. He is currently in LSCE, France.

Email: gbala@caos.iisc.ernet.in; devarajun@gmail.com

About the study:

The paper appeared online in the ‘Climate Dynamics’ journal on September 5, 2015.