Evidence for global warming includes a recent string of very warm years. Scientists report that 1998 was the warmest year in measured history, with 2005 coming in second. Meanwhile, readings taken from ice cores show that the greenhouse gases, carbon dioxide and methane have hit their highest levels in the past 420,000 years. Arctic sea ice is also . According to NASA studies, the extent of Arctic sea ice has declined about 10% in the last 30 years.