Manmade Meltdown

New evidence suggests that humans have been raising global temperatures for 8,000 years.

Atmospheric levels of heat-trapping greenhouse gasses like carbon dioxide and methane naturally fluctuate, partly because the planet's orbit changes over time, so the amount of sunlight that reaches Earth in a given year is variable. However, recent studies leave no doubt that humans have also had a hand in raising the level of greenhouse gasses and warming up the planet. Air bubbles trapped in ice cores from the Antarctic ice sheet show anomalous increases in carbon dioxide levels beginning about 8,000 years ago - just about the same time that farmlands were replacing forests throughout Europe and Asia. About 3,000 years later, the ice cores reflect a similar anomalous rise in methane levels - this time in tandem with increased emissions from flooded rice fields and soaring numbers of domesticated livestock. Climate change models suggest that a global cooling period should have commenced about 5,000 years ago, but these prehistoric practices were apparently widespread enough to override the predicted ice buildup.

Recent industrial emissions of greenhouse gasses have caused rising global temperatures to accelerate in the past fifty years. Experts from the National Climatic Data Center and the National Center for Atmospheric Research calculate that there is a 90 percent chance that the planet's climate will climb between 3.1 and 8.9 degrees Fahrenheit by the end of the century because of these human influences. Among the predicted consequences are more frequent heat waves, wildfires, droughts, floods and extreme precipitation events. The scientists called climate change "humanity's greatest challenge" and warned that "it is very unlikely to be adequately addressed without greatly improved international cooperation and action."

Rice paddies are among the world's biggest methane producers, contributing around 10% of the global total. Photo: Dong Lin, CAS.
Industrial emissions of heat-trapping greenhouse gasses have caused global warming to accelerate in the past 50 years.