In light of the IPCC's newest report that conclusively lays the blame for global warming on human activities, it's worth noting that this isn't the first time that we've messed around with the climate.
Fifteen thousand years ago, mammoths roamed North America and Russia, mowing down all the vegetation they could wrap their trunks around. Just like modern-day elephants do in Africa, mammoth grazing maintained a grassland ecosystem across their range, preventing trees from taking root and spreading.
When our ancestors came along, their bottomless appetite for mammoth meat drove these pachyderms to extinction. Birch trees flourished in the absence of mammoths. Darker in color than grasses, birch forests contributed to localized global warming. Researchers at the Carnegie Institution for Science have made an extraordinarily exact calculation: that 0.2 degrees of warming in Siberia at the end of the ice age were directly caused by human activity. Perhaps our hunter-gather ancestors would be proud of the warming we've managed to accomplish today?