The UK recently invested £23 million (~US$40 million) in a program in Quantifying and Understanding the Earth SysTem (QUEST). The final product of this investment is the state-of-the-art QUEST Earth System Model (QUESM), which is comprised of the most comprehensive modeling and understanding of terrestrial, atmosphere, and marine processes ever built. QUESM is to be linked with the world-leading Hadley Centre’s GCM to assess and understand changes in the Earth system with respect to climate and global change. Here, I present the state of UK climate modeling, an overview of the QUEST Earth System Model, and detail the individual sub-models of the terrestrial ecosystems modeling component (QUERCC). The key questions we seek to address in QUEST are: (i) What and how large are biosphere feedbacks to 56 climate change? (ii) How is atmospheric composition naturally regulated? (iii) How much climate change is dangerous, and can be avoided through biosphere management?