Oregonian: For years, climate researchers puzzled over a long hot spell about 90 million years ago, when tropical breadfruit trees flourished in Greenland, crocodiles slithered above the Arctic Circle and at least a few dinosaurs roamed Antarctica. It’s a serious concern because the warmth came at least in part from greenhouse gases. Climate scientists say they need to understand that period to better forecast how rising carbon dioxide levels affect our climate. Computer models that …