So far the signal of a discernible human contribution to global climate change has not emerged from this natural variability or background noise.
The Late Ordovician Period [approximately 450 million years ago] was also an Ice Age while at the same time CO2 concentrations then were nearly 12 times higher than today-- 4400 ppm. According to greenhouse theory, Earth should have been exceedingly hot. Instead, global temperatures were no warmer than today. Clearly, other factors besides atmospheric carbon influence earth temperatures and global warming.