carbon dioxide has been relatively low

These wobbles, combined with the planet elliptical orbit,cause summer temperatures to vary depending on whether the summer solstice happens when Earth is closer or farther from the sun.Approximately every 100,000 years,these factors align to create dramatically colder conditions that last for millennia.Cool summers that aren't warm enough to melt the preceding winter snow allow ice to accumulate year after year.These ice sheets produce additional cooling by reflecting more solar energy back into space.