Beyond the hockey stick: Climate lessons from the Common Era

More than two decades ago, my coauthors, Raymond Bradley and Malcolm Hughes, and I published the now iconic "hockey stick" curve. It was a simple graph, derived from large-scale networks of diverse climate proxy ("multiproxy") data such as tree rings, ice cores, corals, and lake sediments, that captured the unprecedented nature of the warming taking place today. It became a focal point in the debate over human-caused climate change and what to do about it. Yet, the apparent simplicity of the hockey stick curve betrays the dynamicism and complexity of the climate history of past centuries and how it can inform our understanding of humancaused climate change and its impacts. In this article, I discuss the lessons we can learn from studying paleoclimate records and climate model simulations of the "Common Era," the period of the past two millennia during which the "signal" of human-caused warming has risen dramatically from the background of natural variability.