Did you know that mankind caused Global Warming at least once before?

A news article today started out: "LOS ANGELES – On a remote island in the Canadian Arctic where no trees now grow, a newly unearthed mummified forest is giving researchers a peek into how plants reacted to ancient climate change.That knowledge will be key as scientists begin to tease out the impacts of global warming in the Arctic..."

I do not understand how "ancient man" could have caused Global Warming, but its the only explanation for what scientists found -- isn't it?