Here is a summary of the article:

## Summary of CLM4 Development

The article discusses the development of the Community Land Model version 4 (CLM4), which was motivated by the need to incorporate recent scientific advances in land surface processes and expand the model's capabilities.

Key points:

### Improvements in CLM4
- More sophisticated representations of soil hydrology, snow processes, vegetation-snow interactions, and surface energy balance
- Inclusion of carbon-nitrogen cycle (CLM4CN), land cover change, organic/deep soil, urban canyon model, and biogenic volatile organic compounds

### Origins in Biome-BGC
- CLM4CN builds on the earlier Biome-BGC ecosystem model, which simulated water, carbon, and nitrogen fluxes for different biomes
- Biome-BGC expanded on Forest-BGC by introducing more mechanistic calculations of photosynthesis and extending to multiple vegetation types

### Model Releases
- CLM4 was released in 2010 alongside the Community Climate System Model version 4 (CCSM4)
- CLM4 is used in CCSM4, CESM1, and CESM1.1, and remains the default land component for CESM1.2 coupled simulations.