Here is a concise summary of the provided article:

## Summary

The article discusses how radioactive decay of carbon-14 (14C) in carbon pools allows for long-term dating since photosynthetic uptake. It also describes how atmospheric 14C and 13C concentrations have varied over the 20th century, first being diluted by fossil fuel emissions and then enriched by above-ground nuclear testing.

The key points are:

### 14C Radioactive Decay
- 14C pools are reduced by radioactive decay at a rate of -log/τ, where τ is the half-life of 5568 years.
- During model spinup, the radioactive decay of accelerated 14C pools is also accelerated to quickly equilibrate them.

### Historical Atmospheric 14C and 13C Concentrations
- Δ14C and Δ13C values can be set to fixed or time-varying concentrations read from a provided default file.
- The Δ14C values are provided for three latitude bands: 30°N–90°N, 30°S–30°N, and 30°S–90°S.

The article explains how the CLM model incorporates these processes to allow comparisons of carbon dynamics across multiple timescales with observed values.