但是，有一个问题。大气中的碳随着地球磁场强度和太阳活动而波动，更不用说人类投入其中的东西了。你必须知道在有机体死亡时大气中的碳含量（放射性碳“储层”）是什么样的，以便能够计算自有机体死亡以来已经过了多少时间。你需要的是一个标尺，一个可靠的水库地图：换句话说，一组有机物跟踪年度大气碳含量，一个你可以安全地确定日期，测量其14C含量，从而建立基线水库在某一年。幸运的是，我们确实拥有一套有机物，每年都会记录大气中的碳。树木保持并记录其生长环中的碳14平衡 – 其中一些树木每年都会生存一个环;树木年代学的研究，也称为树木年代测定，是基于自然界的事实。虽然我们没有任何有5万年历史的树木，但我们确实有重叠的树木年轮（迄今为止）可以追溯到12，594年。换句话说，我们有一个非常可靠的方法来校准我们这个星球过去最近12，594年的原始放射性碳日期。但在此之前，只有零碎的数据可供使用，因此很难明确确定13，000年以前的任何数据。可靠的估计是可能的，但具有大的+/-因子。
However, there is a problem. The carbon in the atmosphere fluctuates with the strength of the Earth’s magnetic field and solar activity, not to mention the things humans put into it. You must know what the carbon content of the atmosphere (radiocarbon “reservoir”) is when the organism dies, so that you can calculate how much time has passed since the death of the organism. What you need is a ruler, a reliable reservoir map: in other words, a group of organics tracks the annual atmospheric carbon content, one you can safely determine the date, measure its 14C content, and thus establish a baseline reservoir in a given year. Fortunately, we do have a set of organic matter that records the carbon in the atmosphere every year. The trees maintain and record the balance of carbon 14 in their growth rings – some of which survive a ring every year; the study of tree chronology, also known as tree dating, is based on the facts of nature. Although we don’t have any trees with 50,000 years of history, we do have overlapping tree rings (so far) dating back to 12,594 years. In other words, we have a very reliable way to calibrate the date of our planet’s original radioactive carbon in the last 12,594 years. But before that, only piecemeal data was available, so it was difficult to clearly identify any data before 13,000 years. A reliable estimate is possible, but with a large +/- factor.