evidence: Despite the many climate "skeptics" in key positions of power today, 97 percent of working climate scientists agree that the warming of Earth’s climate over the last 100 years is mainly due to human activity that has increased the amount of greenhouse gases in the atmosphere. Why are they so sure? [...] Scientists can track these earlier natural changes in climate by examining ice cores drilled from Greenland and Antarctica, which provide evidence about conditions as far back as 800,000 years ago. The ice cores have shown that rising CO2 levels and rising temperatures are closely linked. [...] "We have independent evidence that says when you put in greenhouse gases, you get the changes that we see," said Schmidt. "If you don’t put in greenhouse gases, you don’t. And if you put in all the other things people think about—the changes in the earth’s orbit, the ocean circulation changes, El Niño, land use changes, air pollution, smog, ozone depletion—all of those things, none of them actually produce the changes that we see in multiple data sets across multiple areas of the system, all of which have been independently replicated." In other words, only when the emissions from human activity are included, are the models and data sets able to accurately reproduce the warming in the ocean and the atmosphere that is occurring.