Robinson Meyer writes:

The year 2018 was not an easy one for planet Earth.

Sure, wind and solar energy kept getting cheaper, and an electric car became America’s best-selling luxury vehicle. But the most important metric of climatic health—the amount of heat-trapping gas entering the atmosphere—got suddenly and shockingly worse.

In the United States, carbon emissions leapt back up, making their largest year-over-year increase since the end of the Great Recession. This matched the trend across the globe. According to two major studies, greenhouse-gas emissions worldwide shot up in 2018—accelerating like a “speeding freight train,” as one scientist put it.

U.S. emissions do remain 11 percent below their 2007 peak, but that is one of the few bright spots in the data. Global emissions are now higher than ever. And the 2018 statistics are all the more dismal because greenhouse-gas emissions had previously seemed to be slowing or even declining, both in the United States and around the world.

Many economists expect carbon emissions to drop somewhat throughout the next few decades. But maybe they won’t. If 2018 is any indication, meekly positive energy trends will not handily reduce emissions, even in developed economies like the United States. It raises a bleak question: Are we currently on the worst-case scenario for climate change?

“We’re actually a lot closer than we should be; I can say that with confidence,” says Rob Jackson, an Earth scientist at Stanford and the chair of the Global Carbon Project, which leads the research tracking worldwide emissions levels.

When climate scientists want to tell a story about the future of the planet, they use a set of four standard scenarios called “representative concentration pathways,” or RCPs. RCPs are ubiquitous in climate science, appearing in virtually any study that uses climate models to investigate the 21st century. They’ve popped up in research about subjects as disparate as southwestern mega-droughts, future immigration flows to Europe, and poor nighttime sleep quality.

Each RCP is assigned a number that describes how the climate will fare in the year 2100. Generally, a higher RCP number describes a scarier fate: It means that humanity emitted more carbon dioxide into the atmosphere during the 21st century, further warming the planet and acidifying the ocean. The best-case scenario is called RCP 2.6. The worst case is RCP 8.5.

“God help us if 8.5 turns out to be the right scenario,” Jackson told me. Under RCP 8.5, the world’s average temperature would rise by 4.9 degrees Celsius, or nearly 9 degrees Fahrenheit. “That’s an inconceivable increase for global temperatures—especially when we think about them being global average temperatures,” he said. [Continue reading…]