We have been changing our landscapes for thousands of years, burning grasslands and clearing forests to make way for farms and towns. Today, nearly 40 percent of Earth's land surface has been cleared for human use.
Because trees return carbon dioxide to the atmosphere when they are burned or they decay after being cut, land clearing--deforestation--contributes to global warming. But the processes involved are complex, and estimates of the impact on climate are uncertain.
Methane, a potent greenhouse gas, is produced naturally as vegetation decays--in wetlands, for instance. But paddy-rice farming produces it, too, and this kind of agriculture is becoming more widespread.
The greenhouse gas methane comes from tracts of cud-chewing animals like cows. And since more people are eating meat, more cattle are making more methane. As much as 30 percent of methane due to human activities comes from livestock. And after CO2, atmospheric methane is the gas with the highest impact on climate.