<p>A massive new review of ancient atmospheric carbon-dioxide levels and corresponding temperatures lays out a daunting picture of where the Earth’s climate may be headed. The study covers geologic records spanning the past 66 million years, putting present-day concentrations into context with deep time. Among other things, it indicates that the last time atmospheric carbon dioxide consistently reached today’s human-driven levels was 14 million years ago—much longer ago than <a href="https://climate.nasa.gov/climate_resources/7/graphic-carbon-dioxide-hits-new-high/#:~:text=Welcome%20to%20the%20Pliocene.,high%20as%20they%20are%20today.">some existing assessments </a>indicate. It asserts that long-term climate is highly sensitive to greenhouse gas, with cascading effects that may evolve over many millennia.</p>