Critical thresholds or “tipping points” are familiar to most of us. Think of a boat out at sea, which can stay afloat wave after wave after wave, without much help from a crew. This ability to keep bouncing back after disturbance after each wave is what we call resilience. Yet, if we could slowly increase the size or frequency of waves over time, we would eventually reach a tipping point, where the boat tips over and capsizes. And even if these powerful waves subside, the boat does not return to its upright position without considerable outside help. These types of tipping points have been identified in the human body, financial systems and in our infrastructure.
There is a growing concern that human-caused climate change could reach a tipping point in many grasslands, resulting in dramatic environmental transformations that prove difficult or impossible to reverse in our lifetime. Such transformations could have far-reaching consequences because grasslands improve water filtration and house many large mammals that feed many communities (cattle) or bring in tourists (bison, elephants, and so on).
In our chapter of Grasslands and Climate Change, we tried to get a handle on whether there is evidence that grasslands have climate change tipping points, and if so, how close are we to reaching them?
We should start by saying that many grasslands are resilient to some variation in weather, climate and disturbances. In general, many grassland plants have adaptations that help them bounce back after disturbances, whether it’s fires, droughts, heatwaves, flooding, or repeated grazing. This is partly why grasslands are so common in the hotter, drier, more disturbance-prone parts of the world.
But that doesn’t mean grassland resilience is unlimited. Grassland resilience could erode and eventually reach a tipping point, if the abundance of key species falls or important self-sustaining feedbacks become too weak.
Tipping points often occur when self-reinforcing feedbacks break down. Drier grasslands – like those in the Chihuahuan desert or the Sahel – are prime examples. In these areas grassland plants can build a network of roots and dead plant material, which increases the amount of water soil can hold and decreases the types of soil erosion that might otherwise tip the system into becoming a desert-like ecosystem. In these grasslands, a warming climate could increase mortality rates of plants, potentially setting off a destabilizing feedback cycle—less grasses means more erosion, more erosion means less grasses, which means yet more erosion, and so on. In this hypothetical situation, the system has tipped. Feedbacks are prevalent in many other grasslands too, ranging from tundra to prairie and savanna.
To search for example where grasslands have crossed tipping points, we looked at grassland responses to climate extremes. Climate extremes are periods where weather is far outside-the-ordinary, by historical standards. These include weeks to years of exceptionally low rainfall, heatwaves, or periods of especially high rainfall. Many climate change projections call for more frequent and more intense climate extremes in grasslands. Given that these climate extremes can push ecosystems to their limit, climate extremes offer a potential first-look at whether grasslands have climate-change tipping points.
We were able to track down twenty-three studies that documented grassland community responses to climate extremes and had at least a couple years of data to see if grasslands bounced back afterward. There were some clear examples of climate extremes having significant and lasting effects, including an instance where heatwaves or droughts led to the expansion of highly flammable invasive species in California, a switch from tall grasses to short grasses in Israel, decreases in grass abundance in the European Alps, and switches from tallgrasses to forbs in the central US. But we found counter examples too, with grasslands showing resilience to floods, heatwaves, and a slew of multi-year droughts.
One clear trend did stick out—many of the grasslands where climate extremes had a lasting effect were facing additional pressures. For example, two of the grasslands that appeared to cross a tipping point were also subject to accidental wildfires. Other examples of likely tipping points include grasslands with chronic heavy grazing or fertilization. These anecdotes suggest that local and regional actions might play an important role in determining if and when grasslands exhibit tipping points as climate changes.
So, will climate change push grasslands past tipping points, or will grasslands be resilient? Clearly, some grasslands have tipping points that climate change could cross, and in general, we should probably keep a look-out for when grassland resilience declines. On the other hand, we probably shouldn’t just assume that climate change will cross tipping points in all or most grasslands. We also see that local actions can play an important role in whether grasslands are resilient to climate change. Finally, identifying tipping points before we cross them will require a multi-pronged approach that blends observation, experimentation, simulations and theory.
Zak Ratajczak, University of Wisconsin–Madison, USA
Grasslands and Climate Change is part of the Ecological Reviews series. BES members get 25% off all titles in the series when buying directly from Cambridge University Press. See also, David Gibson’s blog post: Grasslands and Climate Change.