Climate change is perhaps the biggest threat facing human society today. Based on the results of Earth System Models (ESMs), warming is projected to be between 1.0 and 3.7°C during the course of the 21st century. Just how reliable are these predictions, given the complexity of the climate system? I will compare the contrasting fortunes of Raymond Pearl and Guy Stewart Callendar who, working in the early 20th century, developed models to predict human population growth (Pearl) and climate warming (Callendar). Based on this comparison, I will discuss the performance of contemporary Earth System Models and make the case for confidence in their projections of climate warming during the next hundred years.