Scientists know trapping carbon dioxide in plants keeps it out of the atmosphere, thus slowing the effects of climate change. What they don’t know is what drives the natural cycle that determines whether carbon dioxide stays in plants instead of returning to the atmosphere and warming the Earth’s surface.

A number of environmental scientists throughout the world, including Christopher Schwalm, assistant research professor in the School of Earth Sciences and Environmental Sustainability and affiliated professor with the Center for Ecosystem Science and Society (Ecoss) at Northern Arizona University, are clearing up that question. In studying the land carbon cycle, which is the movement of carbon dioxide from vegetation to the atmosphere and back again, some scientists have discovered temperature is the primary factor in determining how much atmosphere is retained in plants, while other scientists have found water availability to the be the main factor.

This study, led by researchers at the Max Planck Institute in Jena, Germany, and published in Nature, found, both water and temperature contribute to the land carbon cycle, with the relative importance of each changing as a function of the spatial scale.

For his role in the study, Schwalm mapped out existing data from a network of sites in both space and time, allowing them to better observe and analyze patterns. Using this data, the researchers found at a local level, water availability is the primary driver of the variability of carbon dioxide in plants, while globally, the variability is largely driven by temperature fluctuations.

Thus, the seeming contradiction is less a paradox than a relationship between temperature and water. Put simply, it’s not simple at all, Schwalm said.

“The apparent relationship between temperature and the global land carbon cycle should be treated with caution and neither be used to understand biological process nor as the basis for predictions,” he said.

Besides clearing up findings that previously contradicted each other, the outcome of this study also demonstrates the need for researchers to focus on how climate variables change at different scales and under global warming conditions.