Dr. Jeff Masters ·

During the Jurassic Period, 208 to 146 million years ago, CO2 levels were approximately 2000 ppm (five times the current level), and a “Hothouse Earth” climate with global temperatures 3°C or more above current levels existed. Lush tropical vegetation was widespread world-wide, and dinosaurs ruled the Earth. Above: Painting of a late Jurassic Scene in the Lower Saxony basin in northern Germany. It shows an adult and a juvenile specimen of the sauropod Europasaurus holgeri and iguanodons passing by. There are two Compsognathus in the foreground and an Archaeopteryx at the right. Image credit: Gerhard Boeggemann.

Global energy-related emissions of carbon dioxide jumped by 1.7% in 2018, reaching the highest levels ever recorded, 33.1 metric gigatons, announced the International Energy Agency (IEA) last week. The United States’ CO2 emissions grew by 3.1% in 2018, reversing a decline a year earlier, while China’s emissions rose by 2.5% and India’s by 4%. The global CO2 growth rate was the highest since 2013. Global energy consumption rose 2.3% in 2018, nearly twice the average rate of growth since 2010, and was driven by a robust global economy as well as higher heating and cooling needs in some parts of the world.

Figure 1. Global energy-related CO2 emissions hit a new record high of 33.1 gigatons in 2018, compared to levels just over 20 metric gigatons in 1990. Image credit: International Energy Agency.

A "Hothouse Earth" in our future?

The discouraging news on record-high CO2 emissions in 2018 should be a reminder to go back and look at the most talked-about climate science paper of the past year—“Trajectories of the Earth System in the Anthropocene“, which was the subject of 460 news stories in 326 news outlets. Using existing results from climate models but no new modeling of their own, the researchers’ analysis found that a warming threshold likely exists beyond which we would set in motion a series of vicious cycles (feedbacks) in the climate system that would catapult us into a “Hothouse Earth” climate extremely dangerous to the existence of modern civilization--defined as having a much higher global average temperature than any period of the past 1.2 million years. This threshold might be crossed even if we manage to limit global warming to the Paris Accord target of 2.0°C above pre-industrial levels, they said.

The past 1.2 million years of Earth's history have alternated between long intervals of glaciation and warmer interglacial periods, such as the one we're in now, dubbed the Holocene. The hottest period of the past 1.2 million years was the last interglacial, the Eemian, which occurred between 115,000 and 130,000 years ago. The Eemian was up to 2°C (3.6°F) warmer than the pre-industrial climate of the 1800s, and sea levels were 20 – 30 feet (6 – 9 meters) higher than they are now. A “Hothouse Earth” climate could easily end up 4 - 5°C (7 - 9°F) warmer in a few centuries, with sea levels stabilizing at up to 200 feet (60 meters) higher than today. According to the 2014 IPCC report (our review here), a 4°C warming can be expected to result in "substantial species extinction, global and regional food insecurity, consequential constraints on common human activities, and limited potential for adaptation in some cases (high confidence)."

Figure 2. Stability landscape showing the pathway of the Earth System from the era of ice ages, through the Holocene (the geological epoch that ended when humans began significantly modifying the climate, often cited as 1945), and into the hotter Anthropocene, our proposed new geologic epoch. We are at a fork in the road where the climate can follow one of two paths (broken arrows). Currently, the Earth System is on a Hothouse Earth pathway driven by human emissions of greenhouse gases and biosphere degradation toward a planetary threshold at about 2°C, beyond which the system follows an essentially irreversible pathway to a climate hotter than any in the past 1.2 million years. The other pathway leads to Stabilized Earth, a pathway of wise Earth System stewardship into a human-maintained basin of attraction. “Stability” (vertical axis) is defined here as the inverse of the potential energy of the system. Systems in a highly stable state (deep valley) have low potential energy, and considerable energy is required to move them out of this stable state. Systems in an unstable state (top of a hill) have high potential energy, and they require only a little additional energy to push them off the hill and down toward a valley of lower potential energy. Image credit: Steffan et al., 2018, “Trajectories of the Earth System in the Anthropocene”, Proceedings of the National Academy of Sciences

Even at the ambitious target temperature rise of no more than 2 °C by the end of the century--endorsed by 195 nations in Paris in 2015--humans might trigger a cascade of feedbacks that would set the planet sliding into a Hothouse Earth climate, the authors wrote. One of the more dangerous likely feedbacks is the release of heat-trapping carbon dioxide and methane from the melting Arctic permafrost. One 2017 study warned that every 1°C (1.8°F) of additional warming would thaw 25% of permafrost, which contains twice as much carbon as the atmosphere does today. Other dangerous amplifying feedbacks they listed included diebacks of the Amazon rainforest and boreal forests, reduction of northern hemisphere snow cover, loss of Arctic summer sea ice, and reduction of Antarctic sea ice and polar ice sheets. Triggering just a few of these feedbacks would likely “activate other tipping elements in a domino-like cascade” leading to an irreversible transition to a Hothouse Earth (Figure 1).

We're on course for 3.3°C (6°F) of warming by 2100

Unfortunately, we’re on a trajectory to exceed the dangerous 2°C warming threshold. The planet has already warmed by 1°C since pre-industrial times, and temperatures are climbing at a rate of 0.17 °C per decade. The decadal climate outlook issued by the UK Met Office in January predicted that global temperatures in the period 2019-2023 will likely average between 1.03°C and 1.57°C above the preindustrial levels of 1850-1900. They predicted a roughly 10% chance that the global average will spike above 1.5°C during that period.

As I wrote in my March post, Stopping Human-Caused Air Pollution Would Prevent 5.6 Million Air Pollution Deaths Per Year, air pollution is masking an additional 0.5°C of warming that will be realized once we stop emitting so much sunlight-reflecting small particles into the atmosphere.

According to the Climate Action Tracker (CAT), a consortium of three independent European research groups, the pledges and targets agreed to by the 195 nations that signed the 2016 Paris Climate Accord would result in global warming of 3°C (5.4°F) warming by 2100. That agreement counted on strong additional actions and leadership by the biggest emitting nations to force additional cuts in greenhouse gas emissions and keep warming below 2°C. However, the current policies we’ve adopted would result in 3.3°C (6°F) of warming, said CAT.

The authors of the Hothouse Earth paper have given us a convincing argument that even strong action to control greenhouse gas emissions and limit global warming to 2°C may not be enough to prevent the destruction of a livable climate for humans. They applaud the significant progress that has been made in driving the renewable energy revolution and in slowing down population growth, but emphasize that “widespread, rapid, and fundamental transformations will likely be required to reduce the risk of crossing the threshold and locking in the Hothouse Earth pathway.”