Understanding the complex interdependencies of processes in our climate system has become one of the most critical challenges for society with our main current tools being climate modeling and observational data analysis, in particular observational causal discovery. Causal discovery is still in its infancy in Earth sciences and a major issue is that current methods are not well adapted to climate data challenges. We here present an overview of a NeurIPS 2019 competition on causal discovery for climate time series. The Causality 4 Climate (C4C) competition was hosted on the benchmark platform {www.causeme.net}. C4C offers an extensive number of climate model-based time series datasets with known causal ground truth that incorporate the main challenges of causal discovery in climate research. We give an overview over the benchmark platform, the challenges modeled, how datasets were generated, and implementation details. The goal of C4C is to spur more focused methodological research on causal discovery for understanding our climate system.