ROOTS: THE AGENT OF CHANGE IN GLOBAL WARMING

Planet Earth is losing its roots. Plant ecosystems are so large and complex that their roots are effectively global. They shape the planet and hold it in place against change. And when we grow plants for agriculture, we change root systems which means that in the unavoidable act of growing food, we’re losing a tool that helps us resist the effects of the climate crisis.

So here’s how we’re killing our roots and what we can do about it.

Deep roots stabilize the soil around them, protecting sloped areas from landslides and erosion, and they can also break up bedrock. All of these things make roots foundational to forest ecosystems, allowing other plants to grow.

In fact, trees completely changed the planet forever when they showed up around 400 million years ago in the Devonian period. Not only do they keep plants fed and watered, they can also make it easier for water and nutrients to stay in the soil. Roots store water in them for drier days, and heavily rooted soils also stay damp longer, making these systems much more resilient to drought conditions.

Since the climate crisis is only going to increase the frequency and intensity of droughts, roots are an important bulwark against their effects. Roots can also contribute to reducing climate change as a whole, by taking carbon out of the atmosphere and storing it in the soil.

The carbon that plants use to make sugar in photosynthesis comes straight from the air in the form of CO2, and much of that gets incorporated into the plant’s biomass. And in plants with fairly extensive root systems, much of that biomass is underground, where it isn’t in contact with the atmosphere. When the plant dies and microbes help it decompose, some of that carbon stays trapped in the soil.

When root systems are extensive, carbon dioxide goes in one way and doesn’t come back out. The carbon locked up in soils is called soil organic carbon, and roots are the primary contributor to soil carbon storage, meaning that strong root systems can directly help us fight climate change. So, roots store carbon, and they protect soils from the effects of drought.

Roots are great, but unfortunately, like everything else, roots are changing. Not all roots are created equal. Some plants, like trees and other woody plants, have deep and extensive root systems, while other plants, like the crops that we grow to eat, tend to have a shallower root system. And because we’ve converted so much forest land into farmland, we’ve changed the distribution of plants across the globe, basically making it so that the proportion of shallow-rooted crop plants is much higher than it otherwise would be.

On the other hand, there’s a phenomenon called woody encroachment, which is where woody plants are spreading into areas that never used to be able to support them. Mostly, this is happening at high latitudes, where climate change is warming areas that were previously too cold to support forests. And woody encroachment does balance the scales somewhat, bringing some deeper-rooted plants back into the equation.

But in a 2022 study that used models to predict global root depth, researchers showed that even with woody encroachment happening, Earth’s roots are getting shallower across the globe. That’s because only a small portion of the Earth’s surface is experiencing woody encroachment, while agricultural land expands every day.

In fact, the study showed that Earth has likely lost 11.6 trillion cubic meters of rooted volume already, which means the average root depth globally has gotten about 8 cm shorter. That’s a loss of 5% of Earth’s total root volume.

Now, because this discovery is still so new, it’s hard to say exactly what the effect of that might be. But the amount of water soils can store is a function of root depth, and soils already don’t contain a lot of water.

This trend is likely to continue in the next hundred years, regardless of how the climate crisis plays out.

Researchers who study the climate crisis use models that make different assumptions about what humans will do. Some models assume we’ll do nothing, while others assume we’ll try harder. But under all of those scenarios, roots are in trouble.

Under a scenario where we increase our dependency on fossil fuels, models predict that the Earth could lose up to 32.4 trillion cubic meters of rooted soil by 2100. This is almost three times more than we’ve lost already, and equates to 23 centimeters of root depth all over the world. This is not even the worst-case scenario.

When researchers looked at the model for what would happen to roots if we drastically reduced emissions, in one of the most optimistic scenarios, they found that root depths decrease even more, by up to 43.5 trillion cubic meters or 30 centimeters globally. And that’s because root depth is more connected to land-use change than it is to climate change. And if the climate crisis doesn’t wreck land that's available for agriculture, it’s projected in some regions that more farmers will move in.

Most of that root loss comes from shrublands in North Africa, the Middle East, Asia, and Australia, which are expected to transition to grassland or farmland. What that means is that even though we need to limit emissions, we can’t get tunnel vision. We have to look at what it will take to preserve the things that are working like root systems, otherwise a large portion of the soil won’t be available for water or carbon storage, which could lead to the soils becoming poorer or less fertile in the future.

At this point, researchers are still working on ways to address the problem. Reducing the amount of land being used for agriculture can help reverse these trends. But we can’t exactly ask people to stop eating; however, we can reduce the amount of crops that aren’t directly used to feed people. A lot of crops are used to feed livestock, rather than humans, so reducing meat consumption could definitely help. That’s because energy is lost as you move up in the food chain. An animal can eat more crops in its lifetime than the energy that you would get out of eating the animal. So energy-wise, it’s more efficient to eat the crops directly than to eat the animal itself.

This especially matters for red meat like beef, because it takes a lot more crops to feed a cow than it does, say, a chicken or a turkey. Since beef farming is also carbon-intensive, it really is a common-sense choice. And reforestation helps to restore roots as well.

Trees have deep roots, which you’d know if you’ve ever tried to pull out an old stump, and policy changes that reduce deforestation and plant more trees could help balance root loss from agriculture. As a bonus, both of these steps also help with climate change. Learning is on-going about all the ways that we affect the planet. But thankfully, many of these problems are linked to each other, so by working on one problem, we help solve others, leading to a more sustainable world.