Why Human Activity Can Make Discerning the Impacts of Climate Change So Difficult

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Dr Carleton Ray

The Hubbard Brook White National Forest in New Hampshire, site of the oldest continuous ecological study in the U.S.

A simplified version of the scientific method goes like this: ask a question, make a hypothesis, test the hypothesis in a controlled experiment, analyze results, draw conclusions. But as a team of scientists showed in a recent study of a New England forest, that approach won’t always yield perfect results—particularly in the natural world—if we don’t consider the countless externalities that can sully what would otherwise be clear-cut cause-and-effect.

When the group first set their hiking boots in New Hampshire’s Hubbard Brook Experimental Forest (HBEF), they thought they would find a clear effect of climate change on the dramatic 90% nitrogen level drop in temperate forest watersheds over the last 46 years. As a powerful driver of ecological processes, warming was a prime suspect, particularly given the higher soil temperatures and longer growing seasons of recent years. Instead, they found that the fall in nitrogen levels stems from a cocktail of human influences, including heavy logging, air pollution, and agricultural fertilizer as well as climate change—and that these inputs are obscuring the true baselines against which we can meaningfully judge any “change.”

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“The human species have made the world very complex,” study co-author and Princeton ecology and evolutionary biology professor Lars Hedin said. “We’ve changed things so much in different ways…so identifying the climate effect by itself is going to be a challenge.”

To conduct their study, Hedin’s team got hold of the world’s longest record of biological, physical, and biogeochemical factors within the HBEF and analyzed the interaction between proxies for climate change, like faster vegetation growth, and atmospheric and dissolved nitrogen. The rich half-century archive showed that climate change had indeed contributed to lower nitrogen levels in the forest. But when they modeled other disturbances such as heavy logging in 1906 and 1917 and severe storm events, the researchers also found a distinct sensitivity of nitrogen dynamics to these legacies of man-made and weather factors: up to 60% of the long-term changes in the forest’s nitrogen composition came from historical tree harvesting and extreme weather events.

“Most startling is perhaps the lack of any evidence for direct effects of climate change on net vegetation growth and plant [nitrogen] demand,” the scientists wrote in their paper, published in a February issue of the Proceedings of the National Academy of Sciences. “In contrast, our analyses indicated that historical disturbances of vegetation could have very large and lasting influences on soil N pools and in turn, patterns of nitrate export over many decades.” They did identify one potential climate effect, however: a warming-induced shift in snowmelt hydrology that may have influenced nitrate export in the watershed.

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Hedin noted that the team’s results highlight the extent to which our long record of human activity has muddled the question of which baselines are most appropriate for identifying climate effects on ecosystems. Instead of providing a solid picture of the historic conditions of HBEF, the high nitrate period of half a century ago may simply be a “transient response” to past human and weather disturbances, according to the paper – which means comparisons of current nitrate levels to those from that period could be less meaningful than previously thought.

“We have to be very clear about what a baseline is, and we cannot just assume a simple effect due to climate change,” Hedin said. He stressed the importance of scientists providing the public with accurate representations of how social and ecological factors are tied together within an ecosystem. “The process of science is often divide and conquer, but when it comes to climate change it’s the interactions that really matter.” That’s a crucial message not just for scientists, but also for policymakers and the public: that a single focus on a single issue doesn’t really work in the real-life laboratory. The world outside the test tube is a complex one – largely because of our presence in it – and when we put together our natural experiments or concoct climate legislation it’s worth remembering the messy fingerprints we leave behind every day that are muddying every equation.

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Tara Thean is a TIME contributor. Find her on Twitter at @TaraThean. You can also continue the discussion on TIME’s Facebook page and on Twitter at @TIME.