Given that temperatures in the Arctic are 36 degrees higher than they would normally be this time of year, this news may not surprise you: In the worst-case scenario envisioned by Intergovernmental Panel on Climate Change researchers, global temperatures could rise as much as 4.8°C by the end of the century. A new study suggests that apocalyptic scenario might be too optimistic: if emissions continue unchecked, the world could warm as much as 7.36°C.
The study projects that warming could be between 4.78 and 7.36 degrees Celsius, with 5.9°C of warming most likely. For context, when the world was about 5.9°C cooler, Manhattan was covered in 1,000 feet of ice.
Even at the low end—with 4.8 degrees of warming—we would risk the security of the global food system, severe impacts to ecosystems would lead to mass extinctions, coastal cities would go underwater, and high temperatures and humidity would make it difficult to spend time outside.
The new predictions are higher than those in the past because of the way they calculate "climate sensitivity"—how much more the planet is likely to warm up when it's already warm. Most past predictions of climate change have been calculated using complex computer models of the climate system. The new study looked at 784,000 years of data from the Earth's history in addition to computer simulations.
"Looking at natural climate swings during glacial cycles, we were wondering what we can learn from Earth's past about future global warming," says lead author Tobias Friedrich. "For our calculations, we wanted to take the most robust approach possible. This is why exploited data from different kinds of sources. We combined data from marine sediments, from ice cores and from computer simulations."
When the planet is in a warm period between ice ages, as it is now, it warms more quickly. There are many reasons why that can happen. As massive ice sheets melt, for example, and stop reflecting sunlight, planet absorbs more heat. As permafrost melts and releases methane, a potent greenhouse gas, that also speeds up warming. While researchers can try to model each of these effects, looking back at historical data can give a fuller picture.
Most researchers say that 2°C of warming is an upper limit for avoiding catastrophic climate change—and 1.5°C, the "stretch goal" in the Paris climate agreement, would be safer. Right now, we're not on track to stay below either.
Some headlines about the new study suggested that this might be "game over," but this is the wrong time to lose hope and give up.
"From a scientific perspective I don't think that a 'game over' attitude is justified," says Friedrich. "Yes, future projections are very alarming. In particular for the business-as-usual scenario. But we can change course. We do have a good chance to alleviate some consequences of climate change and avoid some of its most devastating effects."
"From a personal view, I deeply believe that we simply don't have the right to give up," he says. "Our children and grandchildren will have to live with the consequences of the decisions that we make and the actions that we take."