Weathering in Mountainous Regions may Act as Thermostat for Earth

Tree root growth is affected by temperature, and new research suggests how tree roots in mountain regions may function as a sort of thermostat that regulates long-term global temperatures.

Writing in the journal Geophysical Research Letters, scientists working in Peruvian mountain rainforests report that temperature affects the thickness of leaf litter and organic soil layers, as well as the rate of tree root growth. This led the team to theorize that a warmer climate will cause more tree root growth in the mineral layer of the soil, which leads to rocks breaking down into component parts that will eventually combine with carbon dioxide.

The so-called "thermostat effect" is caused by this rock weathering as carbon dioxide is drawn out of the atmosphere, leading to a cooling effect.

The researchers suggest that in mountain ecosystems this CO2-drawing process plays a role in preventing the Earth from the "risk of 'catastrophic' overheating or cooling over millions of years."

Working from the warm Amazonian lowlands to the cooler mountain ranges of the Andes, the researches obtained measurements of the growth of tree roots at a depth of 30 centimeters below the surface every three months over the course of several years.

The thickness of the organic layers of the soil was also recorded during each visit, and those data were combined with existing records of temperature, humidity, monthly rainfall and soil moisture.

A synthesis of the variables allowed the researchers to calculate the likely breakdown process of the basalt and granite rocks found in the mountain ranges of Peru. Then, by extrapolating their data to a global scale, the researchers were able to make estimates on the contribution of mountain forests worldwide to global weathering rates.

The model allowed the scientists to calculate the likely amount of carbon to be pulled out of the atmosphere through weathering when Earth became very hot, as well as weathering process and carbon feedback after Earth's cooling 45 million years ago, when great mountain ranges like the Andes and the Himalayas were first formed.

The researchers' model suggests that volcanic rock that constitutes most mountainous regions plays an important role in drawing carbon out of the atmosphere because the rock is highly reactive to weathering upon disintegration.

"This is a simple process driven by tree root growth and the decomposition of organic material," said University of Oxford's Chris Doughty, one of the lead authors of the study. "Yet it may contribute to Earth's long-term climate stability. It seems to act like a thermostat, drawing more carbon dioxide out of the atmosphere when it is warm and less when it is cooler.

"A series of climatic events over the last 65 million years ago have resulted in global temperatures rising and falling. However, the weathering process that regulates carbon dioxide in the atmosphere may be buffered by forests that grow in mountainous parts of the world. In the past, this natural process may have prevented the planet from reaching temperatures that are catastrophic for life," Doughty said.