evidence: Before global warming, was the Earth cooling down or heating up? [...] The challenge up to now has been that our two significant lines of evidence point in opposite directions. Paleo-environmental "proxy" data, which includes evidence from oceans, lakes, and other natural archives, point to a peak global average temperature about 6,500 years ago and then a global cooling trend until humans started burning fossil fuels. Climate models generally show global average temperatures increasing in the last 6,500 years. [...] This study highlighted uncertainties in the climate models. If the authors’ preferred interpretation—that recent global warming was preceded by 6,500 years of global cooling—is correct, then scientists’ understanding of natural climate forcings and feedbacks, and how they are represented in models, needs improvement. If they’re incorrect, then scientists need to improve their understanding of the temperature signal in proxy records and further develop analytical tools to capture these trends on a global scale.