'Hothouse Earth': Climate Domino Effect Could Lead to Runaway Global Warming

The combined effects of several climatic processes, such as the thaw of permafrost, deforestation and melting sea ice, could push the Earth into a state of runaway warming, according to a study published in the journal Proceedings of the National Academy of Sciences.

This "hothouse Earth" would experience global average temperatures of 4 to 5 degrees Celsius (7.2-9 degrees Fahrenheit) above pre-industrial levels—far higher than the 2 degrees Celsius (3.6 degrees Fahrenheit) limit agreed by the nations of the world at the Paris climate talks.

Although a relatively arbitrary figure, most experts take the view that limiting warming to below this level is essential for mitigating some of the worst impacts of climate change (although a host of significant impacts may still be seen before then). But the new paper suggests keeping below this limit may be harder than previously thought.

In light of the findings, the authors say we must urgently accelerate the transition toward a greenhouse gas emission-free economy. At present, global average temperatures are hovering at just over 1 degree above pre-industrial levels and rising by roughly 0.17 degrees Celsius (around 0.3 degrees Fahrenheit) per decade.

According to lead author of the study, Will Steffen, from the Australian National University and Stockholm Resilience Center, human-induced global warming of 2 degrees Celsius may trigger other Earth system processes, often referred to as "feedbacks", which can drive further warming—even if we stop emitting greenhouse gases.

"There is a risk that feedback processes intrinsic to the Earth System could form a "tipping cascade", in which the feedbacks act like a row of dominoes," Steffen told Newsweek. "Such a cascade would very likely take the trajectory of the Earth System out of human control and towards much hotter conditions that we call "Hothouse Earth"".

The study focuses on the interplay between 10 natural feedback processes, some of which are "tipping elements" that could lead to rapid change if critical thresholds are crossed, resulting in uncontrollable warming. The timescales involved in these feedbacks generating significant effects are large, likely a matter of centuries, however, several of them could be triggered within the next hundred years.

These feedback processes are: loss of Arctic summer sea ice, reduction of Antarctic sea ice and polar ice sheets, weakening land and ocean carbon sinks, increasing bacterial respiration in the oceans, destruction of plant life in the Amazon rainforest, reduction of the world's northern coniferous forests, thaw of permafrost, weakening land and ocean carbon sinks, increasing bacterial respiration in the oceans, reduction of northern hemisphere snow cover and loss of methane hydrates from the ocean floor.

"It may be very difficult or impossible to stop the whole row of dominoes from tumbling over," Johan Rockström, executive director of the Stockholm Resilience Center, said in a statement. "Places on Earth will become uninhabitable if "Hothouse Earth" becomes the reality."

As a whole, the climate system is quite complex and intricately intertwined, Rhawn Denniston, a professor of geology at Cornell College, who was not involved in the study, told Newsweek. For example, melting glacial ice can influence ocean circulation, which can in turn influence Arctic air temperature, and subsequently, rainfall patterns, and so on.

"Greenhouse gases are constantly being added to and subtracted from the atmosphere by natural processes," he said. "Human activity has profoundly interfered with these systems. When we burn fossil fuels, we are taking carbon that was slowly pulled out of the air over millions of years and stored in the ground as coal and oil, and pumping it back to the atmosphere in a geologic blink of an eye. This causes the atmosphere to warm, which causes all sorts of side effects."

For example, enormous amounts of the powerful greenhouse gas methane are stored in the Arctic permafrost. "Warming causes the permafrost to melt, releasing the methane, causing more warming, and thus releasing more methane," Denniston said. "It's a vicious cycle and one that, once ramped up, could cause temperatures to sky rocket, even if we sharply reduce our reliance on fossil fuels."

Furthermore, we're also reducing the ability of natural processes to soak up greenhouse gases, such as the way plants suck carbon dioxide out of the air through photosynthesis.

"In some areas, rising temperatures make it harder for plants to photosynthesize, thereby reducing the plants' ability to act as a sink or reservoir of carbon dioxide," he said.

The authors of the latest study note that it remains unclear whether we can limit global temperature rises to near or above 2 degrees Celsius, or if the "Hothouse Earth" envisaged in the study will become a reality.

"We don't know for sure how likely this scenario is," Steffen said. "However, we know enough about the dynamics of the Earth System and about the nature of the tipping elements to say that this scenario is credible scientifically." Moreover, "the risk of crossing a planetary threshold rises sharply with temperature rises above 2 degrees Celsius, and cannot be completely ruled out for a temperature rise of 2 degrees itself (the upper Paris target)."

Reducing the chances of this situation materializing will require urgent reductions in human greenhouse gas emissions, the researchers argue. In addition, we will need to enhance or create new biological carbon stores, through improved forest, agricultural and soil management, for example; ensure the conservation of Earth's biodiversity; and develop effective and scalable technologies that can remove carbon dioxide from the atmosphere and store it underground.

"The implications of these thresholds and positive feedbacks scare the hell out me, and should be frightening to anyone who bothers to think about them," Denniston said.

"Particularly troubling is that the temperature at which we start to cross such thresholds may be quite near," he said. "Exactly where these tipping points are located and exactly how large the climatic response will be once the thresholds are crossed is a question that is both extremely important and hard to precisely define. It's like we are walking in the dark toward a cliff, with only a walking stick to tell us when we are getting close to the edge."

If we don't take immediate steps to mitigate humanity's impact on the Earth, we may found ourselves in "a very different world", according to Denniston, with a climate that poses huge challenges to many aspects of society, including agriculture, fire management, infectious disease transmission, and water resources.

Sarah Green, a researcher in the Department of Chemistry at Michigan Technological University, who was also not involved in the study, told Newsweek that the extent of human influence on the climate is clear.

"Because we now influence the Earth system to the same degree as some natural geological forces, we have to rethink our place in the world," Green said. "We are an integral part of the system, not bystanders or victims of natural forces. Humans now must take the next step on the difficult road from influencing to managing the planet."

"Even though we do not know the exact conditions that will trigger one of these tipping points, we can be very sure that we don't want to go there," she said. "We don't test the hull of a ship by ramming progressively larger icebergs until we find one that will break it. Instead, we scout ahead to be alert for danger and try to steer clear. We are already driving the planet's climate; now we need to learn how to steer it."

This article has been updated to include additional comments from Sarah Green and Rhawn Denniston.