The 10 highest growth intervals are all after 1920 and the highest 25-year growth period was between 1944 and 1968. X-Ray of growth rings. In trees growing at the timberline, tree rings generally grow wider during warm periods and narrower in colder times. In general, warmer temperatures recorded by the tree rings correspond with periods of increased solar brightness before the Industrial Revolution. But solar brightness alone cannot account for the steady rising and unusually high warming that began in the late 1800s when large amounts of greenhouse gases began to build up in the atmosphere.
Volcanoes play a role by sending up particles that block sunlight and cool the atmosphere.
The cold trend in the middle 1800s coincides with several major volcanic eruptions. To find long-lived trees undisturbed by human activity, the scientists traveled to a remote region of Mongolia. They sampled trees at timberline sites more than 8,000 feet high, where temperature is the limiting growth factor. The research was supported by the National Science Foundation.