Intensive farming in the northern hemisphere has changed the way the Earth is breathing, research has shown.

Seasonal variations in carbon dioxide (CO2) levels have became more extreme as the ‘green revolution’ developed between 1961 and 2010.

During that period, the seasonal amplitude − the difference between the peaks and troughs − of CO2 was boosted by 15 per cent.

Today, carbon dioxide levels are about six parts per million higher in winter than in the summer in the northern hemisphere.

Lead researcher Ning Zeng, from the University of Maryland, US, said: “What we are seeing is the effect of the green revolution on Earth’s metabolism. Changes in the way we manage the land can literally alter the breathing of the biosphere.”

Changes in the way we manage the land can literally alter the breathing of the biosphere

Since the 1950s, scientists have known that atmospheric CO2 levels hit an annual low during late summer in the northern hemisphere, which has more plant life than the southern half of the planet.

Levels fall as plants reach their maximum growth, taking in carbon dioxide and releasing oxygen. In the autumn, when plants are decomposing and releasing stored carbon, CO2 levels rapidly increase.

Writing in the journal Nature, the scientists point out that between 1961 and 2010, the amount of land planted with major crops grew by 20 per cent but crop production tripled.

The factors making up the green revolution − such as improved irrigation, increased use of fertiliser, and higher-yield strains of corn, wheat, rice and other crops − led both to higher productivity and greater seasonal growth and decay.

This was reflected in greater swings in the amount of carbon dioxide in the atmosphere.

US co-author Mark Friedl, from Boston University, said: “Over the last 50 years, the area of croplands in the northern hemisphere has been relatively stable, but production has intensified enormously.”