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LONDON -- The impact of human activity on the Earth is running out of control, and the amount of time in which action can be taken to prevent potentially catastrophic climate change is rapidly dwindling, a leading scientist from the Australian National University told a global scientific climate conference in London yesterday.

On February 14, some media outlets received internal documents of the Heartland Institute , a think-tank funded in part by oil and coal companies that downplays the role of human activity in climate change. The documents contained putative evidence that Heartland was funding efforts to influence what elementary schools teach about climate science

DURBAN, South Africa--By 2020, human activity could produce some 55 billion metric tons of greenhouse gases per year, up from roughly 36 billion metric tons per year currently. All the accumulating gas is enough to raise the global average temperatures by more than 3 degrees Celsius by century's end--more than triple the amount of warming that has already occurred. Emission reductions pledged under the Cancun Agreements , which cover some 85 percent of all national greenhouse gas emissions in the world, are meant to slow that warming.

Natural climate variability is extremely unlikely to have contributed more than about one-quarter of the temperature rise observed in the past 60 years, reports a pair of Swiss climate modelers in a paper published online December 4. Most of the observed warming--at least 74 percent--is almost certainly due to human activity, they write in Nature Geoscience .

To feed the world's growing and more affluent population, global agriculture will have to double its food production by 2050. More farming, however, usually means more environmental harm as a result of clearing land, burning fossil fuels, consuming water for irrigation and spreading fertilizer.