You will hear a lecture about using soil to reduce carbon dioxide in the atmosphere.
First, you have some time to look at questions 31 to 40.
Now, listen carefully and answer questions 31 to 40.
As we learned in the previous lecture, the rapid increase in atmospheric carbon dioxide over the past century is a major driver of climate change.
Reducing the quantity of carbon dioxide could therefore slow the rate of change.
One possible approach is to improve the ability of the soil supporting plants to absorb carbon dioxide.
Ratan Lal, a soil scientist at Ohio State University in the United States, argues that agricultural soils worldwide could potentially take up 13% of the carbon dioxide currently in the atmosphere.
That is equivalent to the amount emitted during the past 30 years.
Researchers are now examining how to achieve this.
Lal's original interest in soil did not arise from climate research.
He was concerned about the land and the communities that relied upon it.
Soil rich in carbon is dark, loose and productive, and it retains water.
By contrast, erosion becomes more likely when soil is dry, often because it contains too little carbon.
Such erosion makes both crop cultivation and livestock farming more difficult.
During the 1970s and 1980s, Lal studied African soils with so little organic matter that the ground had become extremely hard, almost like concrete.
There, he met an early researcher in global warming who proposed that the missing soil carbon had entered the atmosphere.
Evidence increasingly supports that explanation.
For millions of years, atmospheric carbon dioxide has been partly controlled through a natural partnership between plants and tiny organisms in the soil.
Plants take carbon dioxide from the air and convert it into sugars and other carbon-containing materials.
Some remain within the plant, while another portion pass up from the roots to fungi and soil microbes, which retain the carbon underground.
The development of agriculture roughly 10,000 years ago disrupted these ancient processes and caused soil carbon loss.
By draining and ploughing natural topsoil, people exposed buried carbon to oxygen.
This produced carbon dioxide, which escaped into the atmosphere.
In some places, grazing animals removed all the vegetation and released still more carbon.
Huge quantities have been taken from the soil, where they are valuable and transferred to the air.
Research now indicates that even fairly small changes in farming practices may substantially reduce atmospheric carbon.
Some farmers already follow regenerative agriculture.
This uses established methods to increase fertility and keep soil moist.
Examples include maintaining crops throughout the year and growing a greater variety of plants.
These strategies can raise the amount of carbon stored underground, and agricultural researchers are building evidence for their use against climate change.
One American project investigating carbon storage on agricultural land is underway in California.
Wendy Silver, a soil scientist from the University of California, Berkeley, is leading a pioneering study on a large cattle farm.
Her team is testing how carbon storage changes when compost is applied.
The compost combines agricultural waste, including manure and maize stalks, with garden waste such as leaves, branches and grass cuttings.
In Australia, soil ecologist Christine Jones is evaluating another promising method.
Working with 12 farmers, she aims to raise the carbon content of soil by cultivating grasses that stay green throughout the year.
Like compost application, the technique has already succeeded in experimental conditions.
Jones now wants to prove it works on commercial farms, and that the captured carbon can be measured accurately.
It is hoped that such projects will reveal how farmers and other land managers can help limit the harmful effects of greenhouse gases.
In nations such as the United States, where farms commonly apply large quantities of fertilizer, changing established practice will require a new system.
Lal believes farmers should receive payment not only for the crops or meat they sell, but also for the carbon retained in their soil.
Another study currently being conducted.
That is the end of Part 4.
You now have one minute to check your answers to Part 4.
