By Martin Bush. Reposted with permission from ClimateZone.org.

A scientific paper recently published in the Proceedings of the National Academy of Sciences (PNAS) is getting a lot of attention. Written in the dry style of systems analysis — and throughout the text referring to the planet as the “Earth System,” it nevertheless brilliantly manages to present the looming dangers of extreme climate change in a way that has powerfully resonated with many people. People who are worried about climate change, but aren’t exactly sure what the future holds, how bad it’s going to get, and how to avoid being dragged in that direction. 

Into the Hothouse

“Hothouse Earth” is where we are all headed. And those two words, after the record heat waves and raging wildfires we have witnessed this year, conjure up images that most of us can fully comprehend. We can almost feel the heat.

Not only that, the authors have created a simple graphic which although dimensioned in the metrics of time, space, and temperature, is easily understandable. The Earth is a ball gently rolling down a slope of increasing global temperatures. Two pathways are available to us. But only one can be chosen.

The pathway of least resistance is the one we are on now. The one that leads to the hothouse.

The diagram shown below perfectly captures the concept.

Credit: Steffen et al. 2018

We know that actions taken so far to reduce global emissions of greenhouse gases are hopelessly inadequate. Atmospheric concentrations of carbon dioxide and methane — the most damaging of the greenhouse gases that are responsible for global warming — continue to increase every year. There are no signs that the warming trend is slowing.

The trajectory of the planet towards Hothouse Earth is catalyzed by a series of cascading tipping points which, like a line of dominoes, tip over into the next one in line. The tipping elements fall into three interrelated groups based on their estimated threshold temperatures. The casades are triggered when a rise in global temperature reaches the first threshold–activating tipping elements such as the loss of the Greenland ice sheet or Arctic sea ice. These tipping elements push global average temperatures higher still, inducing tipping in mid- and higher-temperature threshold elements.

The text cites the example where the freshwater runoff from the Greenland and the west Antarctic ice sheets has a significant impact on on the Atlantic Meridional Ocean Circulation, which could in turn lead to heat accumulation in the Southern Ocean which would then accelerate ice loss from the east Antarctic ice sheet. Sea levels would rapidly rise and coastal cities around the world would be devastated.

But what would trigger these domino-like tipping points?

Positive Reinforcement

Scientists know of three mechanisms that could potentially force global temperatures up to the point where the tipping elements are likely to be triggered.

They are each positive feedback mechanisms — where the warming global temperature has a physical impact on the terrestrial and marine environments that in turn leads to more warming. It’s a vicious circle of escalating global temperature which–once it reaches a certain point — is impossible to control or restrain. This is the gateway to Hothouse Earth.

Two of these positive feedback mechanisms involve the principal greenhouse gases—carbon dioxide and methane. As global temperatures rise, emissions of carbon dioxide start to climb — because of the deteriorating ability of forests to act as carbon sinks; the increasing frequency and intensity of wildfires; increased bacterial respiration in the oceans; and warming soils. This positive feedback loop has almost certainly started — witness the escalating levels of CO2 in the atmosphere even though emissions from anthropogenic sources of the gas levelled off for a few years before 2017.

The second feedback mechanism involves methane — the most dangerous of the greenhouse gases. There are huge amounts of the gas locked away in the permafrost of the Arctic tundra and frozen into the methane hydrates buried in the coastal oceans. There is overwhelming evidence that the permafrost is weakening. Worse still is the specter of methane locked away in ocean sediments gradually being released by the warming coastal waters. These frozen methane hydrates hold enormous quantities of the gas.

Just like carbon dioxide, the concentration of methane in the atmosphere continues to increase. This is clear evidence that emissions are rising. Several sources are contributing to this upward trend — the oil and gas industry, enteric emissions from cattle, and wetlands are the chief suspects. But if the release of permafrost methane accelerates, and methane start to bubble up from the ocean floor the global warming effect is likely to be catastrophic.

The third mechanism doesn’t directly involve emissions of greenhouse gases. But as the ice sheets melt, the sea ice recedes, and the tundra becomes greener, the Earth reflects less incoming solar radiation. What’s called the albedo — the fraction of solar energy reflected by the planet — declines, and the Earth’s surface, including the ocean’s, gets slightly warmer as more solar radiation is absorbed. But this potentially warming feedback cycle, at least for the moment, appears to be having little effect — the albedo shows no clear trend.

The feedback loops that accelerate the global emsissions of carbon dioxide and methane from the land and the oceans have almost certainly started — but for the moment they are masked by the greater levels of emissions from anthropogenic sources.

But a genuine fear is that we have started down the road to Hothouse Earth and there are no indications that we know how to stop.

Greenhouse Hothouse Firehouse

So what would life on Hothouse Earth be like? The authors of the PNAS article are scientists — choosing their words carefully to avoid being accused of unwarranted exaggeration. They state that “The impacts of a Hothouse Earth pathway on human societies would likely be massive, sometimes abrupt, and undoubtedly disruptive.”

This is putting it mildly. With the tropics scorching hot and almost uninhabitable, all coastal cities inundated and abandoned, small island nations under water, essential ecosystems collapsing, the sixth extinction well underway, and agriculture disupted and unable to cope, the Earth cannot possibly support even its present population. This is Hothouse Earth.

The euphoria of the Paris Agreement apparent success of forging a global consensus and a coordinated effort to vigorously tackle climate change has rapidly foundered on the rocks of national self-interest and populist policies that prioritize local concerns and disparage mechanisms to reduce emissions — like pricing carbon — as costly, useless, and irrelevant.

In the firehouse the alarm bells are ringing incessantly. But is anyone listening?

For a deeper dive, check out:

• The PNAS scientific paper 
• More on the global carbon cycle and budget 
• Information about emissions of carbon dioxide and methane  
• More about permafrost methane 
• The recent bulletin by the American Meteorological Society  
• The recent National Geographic article: Some Arctic Ground No Longer Freezing — Even in Winter 

Main image: Los Angeles skyline. Credit: Pixabay/user 12019, CC0