12/3/2015 Is the earth now a carbon sink? | The Indian Express

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Published:Nov 22, 2015, 0:40

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Trees and plants act as a carbon sink and slow down the rate of global warming.

Home Technology Science Is the earth now a carbon sink?

Is the earth now acarbon sink?A study on how elevated atmospheric carbon dioxide levels andnitrogen depositions have enhanced the productivity of plants incomparison to the baseline year of 1850

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12/3/2015 Is the earth now a carbon sink? | The Indian Express

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We know that plants and trees absorb carbon dioxide for photosynthesis, a process

by which sunlight is stored as chemical energy for the survival of plants and

animals.

This carbon dioxide would have otherwise been in the atmosphere, increasing the

concentrations of greenhouse gases. Trees and plants, therefore, act as a carbon

sink and slow down the rate of global warming.

Oceans also serve as carbon sinks. Of all the carbon dioxide emitted in the

atmosphere, about 25 per cent is absorbed by oceans and 25 per cent by land

plants.

The other 50 per cent remains in the atmosphere and is the cause of global

warming.

A higher concentration of atmospheric carbon dioxide, however, is also beneficial

for land plants, as it increases their rate of photosynthesis.

Something similar is observed for the process of nitrogen deposition. Nitrogen is an

essential nutrient for plants.

Bacteria present in the roots of leguminous plants help fix the atmospheric nitrogen

in the soil in a form that can be easily assimilated by plants.

In the 20th century, increased application of nitrogen fertilisers and emission of

nitrogen compounds by industrial processes have also resulted in greater

deposition of nitrogen compounds that plants can readily assimilate.

Industrial activities have caused a six-fold rise in the deposition of these reactive

nitrogen compounds.

While the benefits of both phenomena are well known, we have not been able to

quantify the impact of increased concentrations of carbon dioxide and nitrogen

deposition on the ‘productivity’ of trees.

This is what we set out to find— how elevated carbon dioxide levels in the

atmosphere and the nitrogen deposition have enhanced the carbon sink in

terrestrial ecosystem in comparison to the baseline year of 1850, which is a

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12/3/2015 Is the earth now a carbon sink? | The Indian Express

http://indianexpress.com/article/technology/science/istheearthnowacarbonsink/ 3/12

standard comparison year in climate change science as it is generally taken to be

the start of the industrial period resulting in large-scale emissions of greenhouse

gases.

The question was addressed using a global climate model which was run on a

supercomputer at the Indian Institute of Science.

We ran several computer simulations to understand the role of many key processes

that affect carbon storage on land. Huge amounts of data were generated and

analysed.

Our study arrived at three key conclusions. First, the global net productivity of the

land plants has gone up by eight per cent as compared to the year 1850 in spite of

the massive deforestation and global warming during this period.

Net productivity is a measure of the rate at which plants store carbon in

themselves. Our simulations indicate that the increase in plant productivity is

mainly due to the increase in atmospheric carbon dioxide and nitrogen deposition.

Secondly, when we consider the entire industrial era, the land ecosystem has lost 45

billion tonnes of carbon (about 180 billion tonnes of carbon dioxide) mainly

because of the large-scale deforestation in the early 20th century.

Around the world, forests were cleared for agriculture and building cities, and

nearly one third of humanity’s carbon dioxide emissions is from deforestation.

Thirdly, the deforestation rates have come down in recent decades and the

beneficial effects of elevated carbon dioxide levels and nitrogen deposition on

plants are turning land into a carbon sink.

Our model simulations indicate that the carbon stock in the land ecosystems has

gone up by about 10 billion tonnes of carbon (about 40 billion tonnes of carbon

dioxide) in the last three decades.

Our model results are consistent with research based on recent satellite data which

shows that there is a general increase in plant productivity around the world in the

last three decades.

12/3/2015 Is the earth now a carbon sink? | The Indian Express

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Our results are important for making policy decisions regarding reducing

deforestation and degradation of forests and encouraging afforestation

programmes.

It shows that the slowing down of deforestation in the last three decades is already

having positive impacts on land carbon sink.

The writer is associated with Divecha Centre for Climate Change, Indian Institute of

Science, Bangalore

For your research to be considered for this column, please write to Senior Editor

Amitabh Sinha at amitabh.sinha@expressindia.com

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