The coldest period was between the 1830s and 1870s, after which a steadily increasing warming trend began. Temperatures in the 1900s have been higher than in any other period captured by the tree rings from all over the northern hemisphere. The 10 highest growth intervals are all after 1920 and the highest 25-year growth period was between 1944 and 1968. 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 temperature rise 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. The research was supported by the National Science Foundation.