Earth in Danger of Tipping into ‘Hothouse’ State, Scientists Warn

Global temperatures, already more than 1 degree Celsius above pre-industrial indicators, are projected to rise by at least 0.17 degree Celsius per decade. The heatwaves scorching Europe and rapid glacier melting in Greenland offer further evidence that we should not be complacent about the 2-degrees Celsius cap set by the Paris Agreement. But recently, a team of international researchers led by Will Steffen, a climate scientist from Australian National University, published a major report, “Trajectories of the Earth System in the Anthropocene,” in PNAS that warns that the climate accords may not be enough to stop Earth from tipping into an irreversible “hothouse” state. 

An alpine glacier in Albert, Canada (Source: Jim Nix/Flickr)

The paper, which attracted broad coverage from dozens of major media worldwide including CNN, BBC, Paris Le Monde, and Sina in China, sparked renewed concerns across the scientific community about thresholds of the Earth system that could lead to a runaway “Hothouse Earth” warming scenario. Among the thresholds discussed, alpine glaciers, such as those in Glacier National Park in Montana, are particularly vulnerable to global warming. What’s more, their melting is likely to trigger uncontrollable chain effects that could lead to “Hothouse Earth.”

Unlike other published papers on global average temperature rise, this paper extended its scope to the broader Earth system and components that could reinforce Earth’s decline once certain thresholds are passed.

Previously, researchers have viewed humans as an external component of this system, which consists of land, oceans, and atmosphere, and includes the planet’s natural cycles such as carbon, water, nitrogen, and phosphorus. In the PNAS article, the authors consider humans an integral component of the Earth system, with the capacity to both affect and respond to the changing climate. As we witness changes in climate, human decisions and actions are evolving as well. Knowing how anthropogenic activities have affected climate may formulate more effective solutions.

The paper presents a theory of how human activities, coupled with a natural “Tipping Cascade,” may lead to a human-driven “Hothouse Earth.” The authors argue that there is a threshold at which Earth’s natural systems can no longer support and withstand human activities. Once this limit is exceeded, abrupt changes will be evident and lead to a chain reaction of impacts. For example, a rise in temperature by 2 degree Celsius will immediately lead to Greenlandic glacier melting, followed by sea level rise. Thereafter, other effects such as changing ocean currents and coral bleaching will also become evident, as these are regulated by an intrinsic, self-reinforcing biogeophysical feedback mechanism within the Earth system.

Global map of potential tipping cascades. Individual tipping elements are color-coded according to estimated temperature thresholds. Arrows show potential interactions. (Source: Steffen et. al).

The predicted domino-like chain reaction will increase the difficulty of reversing these cascading impacts, the authors caution. The melting event of the Greenlandic glaciers is just one event that may push Earth toward a “Hothouse” pathway, moving the Earth system off its trajectory of the past 1.2 million years and toward hotter, irreparable conditions. Eventually, Earth is estimated to become 4-5 degrees Celsius hotter, with 200 foot higher sea levels, making areas of our planet inhabitable to many.

As a default mechanism of the Earth system, the biogeophysical feedback process works to activate significant interactions among different subsystems, such as glaciers, ice-sheets, ocean, forest, wind, rainfall, and others. The subsystems involved are called “tipping elements.” Some negative feedbacks can maintain a given state, while other positive feedbacks are set to drive a transition to a different state. Usually, the processes can balance each other and achieve a relatively stabilized situation. But if the climate thresholds are crossed, the authors argue certain feedbacks will be activated and become harder to predict, pushing Earth further away from its original state.

Glaciers have always been central to the Earth system, and the cascading effects of glacier melting have been consistently on the science community’s radar. The feedback processes involving glaciers and ice sheets work in at least two ways. As Will Steffen elaborated to GlacierHub, one is rather obvious. “If glaciers or sea ice retreat, they uncover darker land or sea, which absorbs more sunlight, warming the regional climate, and causing further retreat of the ice.” he said. “This a positive feedback.”

The other process is more nuanced. “Loss of significant amounts of ice from land-based glaciers and ice sheets can actually influence ocean circulation, which can then have impacts elsewhere on the planet. This is a more complex feedback process and could be positive or negative depending on the situation,” he continued.

The authors present a doomsday scenario but also provide an alternative pathway called “Stabilized Earth.” This requires radical and scalable changes in the relationship between society and the planet. For example, the paper described the need to maintain glacier volume within the Late Quaternary limits to prevent the progression toward a hothouse. At this current juncture, doing nothing is no longer an option to stop the glaciers from melting and achieve stability. Rather, humanity must commit to managing its current activities, stopping the staggering loss of ice, and perhaps even engaging in counteracting measures to neutralize previous impacts on Earth.

The authors also offered a wide range of human activities that are urgently required to hold the ultimate temperature rise to between 1.5 and 2 degree Celsius. Steffen believes this is particularly pressing for glaciers. “At these temperatures, most continental glaciers will probably disappear, as perhaps much of the West Antarctic ice sheet, as well as some erosion of marine-grounded ice sheets in East Antarctica,” he said. “A big question is whether the tipping point for the Greenland Ice Sheet would be crossed at a 1.5-2 degrees Celsius temperature rise. It is possible that the tipping point lies in this range, but there is no consensus in the scientific community yet on this. It is a critical issue for further research.”

Two towering icebergs in the freezing waters of Scoresby Sund in Eastern Greenland (Source: Marie and Alistair Knock/Flickr)

Interestingly, the discussion on tipping points is centered on predicting a certain temperature threshold without stating when that temperature threshold might occur. Most of the analysis was also based on a qualitative assessment of the current literature instead of modeling and data analysis, which has sparked some different opinions.

Steffen told GlacierHub, “Experts on each of the individual tipping elements were asked to estimate the vulnerability of the tipping element to a range of temperature increases. The experts, of course, were aware of the relevant literature in their fields, so ultimately based their judgments on their assessments of the peer-reviewed literature.”

Richard Betts, another climate scientist who previously published a paper about the model-based analysis of temperature increases and their association consequences, was consulted by Steffen. After the paper came out in PNAS, Betts offered an overview of the findings and expressed his concerns online about the researchers’ methodology. Still, Betts believes the paper, with its dramatic term “Hothouse Earth,” should serve as a good starting point for further research with modeling and data analysis. “This will help us see better whether ‘Hothouse Earth’ is our destiny, or mere speculation,” Bretts wrote in his article

There is no doubt this “initial analysis,” as the authors put it, will continue to ignite debate and further explorations to narrow the uncertainties and provide actionable suggestions to policymakers.

“We hope the glacial community gets even more support in the future,” Steffen said. “Glaciers and ice are critical parts of the Earth system, and we urgently need to know more about how vulnerable it is to human forcing.”

We are, in short, at a fork in the road. Whether humanity progresses toward a Hothouse or a Stabilized Earth depends on our social and technological trends and decisions over the next decade. Regardless of which path we choose, we will have to bear the consequences of our choices for thousands of years.