There is no question that climate change is reshaping Planet Earth and that things are getting ugly as global warming progresses. The debate now centers on just how bad things will get. But there are still major uncertainties when it comes to modeling future climate. Chief among them: How much more carbon dioxide will humans emit, and how sensitive is the climate system to all those emissions anyway?
To account for those uncertainties, the United Nations Intergovernmental Panel on Climate Change (IPCC) created several baseline scenarios, known as Representative Concentration Pathways (RCPs), to describe just how much warming might result from a range of carbon emissions. The high-emissions scenarios are bleak.
The most extreme model, RCP 8.5—sometimes called the business-as-usual scenario—describes rampant burning of fossil fuels, a global average temperature increase of nearly 5°C, and mean sea levels roughly a meter higher than they are today. But climate and energy experts disagree over how likely it is that this high-emissions scenario will come to pass. A recent study published in Environmental Research Letters found that the emissions trajectories in climate assessments from the IPCC overshot actual energy emissions over the past 15 years.
It’s not the first study to suggest that the IPCC’s worst-case climate scenarios, which describe a world with unchecked fossil fuel use and little to no climate mitigation, were overshooting actual carbon emissions. In the new research, the authors identified economic drivers of that divergence.
An Overlooked Link Between the Economy and the Environment
When Matthew Burgess, an assistant professor in the environmental studies program at the University of Colorado Boulder, first saw findings that the IPCC’s energy emissions projections were likely too high, he remembers thinking, “Of course, they are!”
Economic growth and emissions have grown more or less in tandem since the Industrial Revolution began, and the high-emissions scenarios rely on continued economic growth. But many economists are now questioning whether the high growth rate of the past century can continue throughout the present one. “There’s a debate in mainstream macro[economics] that’s exploded in the last 10 years,” says Burgess, lead author of the new study. “What if it’s just inevitable that growth is going to slow almost to a halt in this century in developed countries for a bunch of reasons that have very little to do with the environment?” Aging populations, debt, plateauing rates of educational attainment, urbanization, and women entering the workforce have all been shown to slow economic growth and could continue to do so in the coming years.
But it seemed to Burgess that not many people on the environmental side of climate discussions were paying attention to this economic debate, despite its clear implications for global warming models. “I started looking under the hood at some of the economic assumptions [in the scenarios], and there’s some pretty odd stuff,” he said.
For one thing, RCP 8.5 predicts that coal use will increase sixfold by the end of the century, although current data suggest that global coal use peaked in 2014.
For another, the scenarios don’t account for the economic impacts of climate damages. That’s a justifiable omission, Burgess noted—you can’t use a model that assumes damages to assess potential damages—but it has led to some odd contradictions. “If you look at the [Shared Socioeconomic Pathway] 8.5 scenario, it assumes that per capita GDP [gross domestic product] in every world region is going to be higher than $120,000 by 2100. Everybody’s going to be [paid as if they were] Google employees, even in the regions that are the poorest today. Then you look at other studies that say if we have that much warming, there’s going to be huge parts of the tropics that are virtually uninhabitable to human life,” he said. “Well, you’re not going to have both of those things at the same time, right?”
The team compared scenarios from the IPCC’s Fifth and Sixth Assessment Reports to observations of real-world energy emissions, population, per capita GDP, energy intensity, and carbon intensity from 2005 to 2017. They found that most of the high-emissions scenarios—not just RCP 8.5—overprojected fossil fuel emissions, largely because per capita GDP was lower than expected.
Growth Going Forward
“This paper better explains the exact drivers in the historical differences we’ve seen between these baseline no-policy scenarios and what’s actually happening in the real world,” said Zeke Hausfather, the director of climate and energy at the Breakthrough Institute who was not involved in the study. The rapid rise of renewable energy and the fall of coal allowed the energy sector to decarbonize faster than baseline scenarios predicted, but the main driver of the divergence, the team found, was overly optimistic projections of economic growth.
“That’s an interesting aspect, but it’s also one that’s challenging to figure out how to assess going forward,” Hausfather said. “Is the past our destiny on that issue? Or would it be possible for there to be faster economic growth in the future? That’s a hard-to-answer question.”
According to Burgess, there are reasons to believe that growth will continue to underperform. “To catch up by 2040, growth would not only have to suddenly stop being slower than the IPCC projections, it would have to start being quite a bit faster, and even more so if you factor in COVID,” Burgess said. It’s more plausible that growth rates could catch up by the end of the century, but an increase in climate damages, a retreat from globalization post-COVID-19, and other unpredictable economic shocks in the coming years could all continue to drag down growth rates. Although economic shocks are unpredictable, Burgess has found that on the whole, negative shocks have larger impacts than positive ones.
“Do we really think that COVID is going to be the last big shock to the global economy?” Burgess asked. “Probably not.”
Importantly, researchers emphasized, just because energy emissions are lower than predicted does not mean the world can relax its focus on climate mitigation. Indeed, burning fossil fuels is not the only source of carbon emissions. A report published in the Proceedings of the National Academy of Sciences of the United States of America that accounted for emissions from land use changes as well as fossil fuel emissions found that RCP 8.5 actually slightly underestimated total emissions. Although fossil fuel use is the main source of carbon emissions, for climate scientists who want to understand the physical risks of climate change, the total amount of atmospheric carbon dioxide may be more important than the pathways the carbon took to get there. In other words, whether RCP 8.5 and other high-emissions scenarios are relevant depends on what researchers plan to use them for.
But everyone seems to agree that RCP 8.5 shouldn’t be known as “business as usual” because we don’t live in a no-policy world. Burgess and his colleagues argue that scientists need to do a better job of distinguishing between policy-relevant scenarios and exploratory ones. If these scenarios are going to be used to inform policy, they need to be updated more often to make sure they track with real-world conditions. Knowing where the RCPs currently fall flat can help researchers build better scenarios in the future.
A Warmer World
Even if you believe the lower-emissions scenarios are now the most plausible, they’re still far from ideal. “We still expect around 3°C or more of warming by the end of the century, which is not a world we want to live in,” Hausfather said.
There’s no guarantee that warming will stop there, even if we manage to lower emissions. There are three main sources of uncertainty in these models: how sensitive the climate is to extra carbon, how the oceans and land will absorb or release carbon as the world warms, and how much more carbon humans will pour into the atmosphere.
“The presence of these big uncertainties means we can’t rule out some of these higher warming outcomes even under more moderate emissions scenarios and makes it all the more important to control the one type of uncertainty in the climate system that we have control over,” Hausfather said, “which is our future emissions.”
—Kate Wheeling (@katewheeling), Science Writer