Can Global Warming Stay Below 1.5 Degrees? Views Differ Among Climate Scientists

The Paris Climate Agreement seeks to keep global warming well below 2 degrees Celsius relative to pre-industrial temperatures. In the best case scenario, warming would go no further than 1.5 degrees.

Many scientists see this as an impossible goal. A recent study by Peter Cox et al. postulates that, given a twofold increase in atmospheric carbon dioxide, there is only a 3% chance of keeping warming below 1.5 degrees.

But a study by Richard Miller et al. provides more reason for hope. The Miller report concludes that the 1.5 degree limit is still physically feasible, if only narrowly. It also provides an updated “carbon budget”—a projection of how much more carbon dioxide we can emit without breaking the 1.5 degree limit.

Dr. Joeri Rogelj, a climate scientist and research scholar with the Energy Program of the International Institute for Applied Systems Analysis, co-authored the Miller report. For Rogelj, the updated carbon budget is not the paper’s most important point. “Our paper shows to decision makers the importance of anticipating new and updated scientific knowledge,” he says.

Projected “carbon budgets” are rough estimates based on limited observations. These projections need to be continually updated as more data becomes available. Fortunately, the Paris Agreement calls for countries to periodically update their emission reduction pledges based on new estimates. Rogelj is hopeful “that this paper has put the necessity for a strong [updating] process on the radar of delegates.”

For scientists who have dismissed the 1.5 degree limit as impossible, the updating process might seem pointless. But Rogelj stresses that his team looked only at geophysical limitations, not political ones. Their report assumes that countries will agree to a zero emissions commitment—a much more ambitious scenario than other researchers have considered.

There is a misconception, Rogelj says, that the report claims to have found an inaccuracy in the Earth system models (ESMs) that are used to estimate human-driven warming. “We are using precisely those models to estimate the carbon budget from today onward,” Rogelj explains.

The problem is not the models, but rather the data fed into them. These simulations are often run using inexact projections of CO2 emissions. Over time, small discrepancies accumulate and are reflected in the warming predictions that the models make.

Given information about current CO2 emissions, however, ESMs make temperature predictions that are “quite accurate.” And when they are provided with an ambitious future scenario for emissions reduction, the models indicate that it is possible for global temperature increases to remain below 1.5 degrees.

So what would such a scenario look like? First off, emissions have to fall to zero. At the same time, the carbon budget needs to be continually reevaluated, and strategy changes must be based on the updated budget. For example, if emissions fall to zero but we’ve surpassed our carbon budget, then we’ll need to focus on making our emissions negative—in other words, on carbon dioxide removal.

Rogelj names two major processes for carbon dioxide removal: reforestation and bio-energy with carbon capture and storage. Some negative emissions processes, such as reforestation, provide benefits beyond carbon capture, while others may have undesired side effects.

But Rogelj is quick to add that these negative emissions technologies are not “silver bullets.” It’s too soon to know if carbon dioxide removal at a global scale will actually be necessary—we’ll have to get to zero emissions before we can tell. But such technologies could also help us reach zero in the first place.

What else will get us to zero emissions? According to Rogelj, we need “a strong emphasis on energy efficiency, combined with an electrification of end-use sectors like transport and building and a shift away from fossil fuels.” This will require a major shift in investment patterns. We want to avoid “locking into carbon dioxide-intensive infrastructure” that would saddle future generations with a dependency on non-renewable energy, he explains.

Rogelj stresses that his team’s findings are based only on geophysical data. Societal factors are a different matter: It is up to individual countries to decide where reducing emissions falls on their list of priorities.

However, the stipulation in the Paris Climate Agreement that countries periodically update their pledges is a source of optimism. Rogelj, for his part, is cautiously hopeful: “Looking at real world dynamics in terms of costs of renewables and energy storage, I personally think there is room for pledges to be strengthened over the coming five to ten years as countries better understand what is possible and how these pledges can align with other priorities.”

But not everyone in the scientific community shares the hopeful tone struck by Rogelj and his team. An article by the MIT Technology Review outlines “the five most worrisome climate developments” from 2017.

To start, global emissions are on the rise, up 2% from 2016. While the prior few years had seen a relative flattening in emissions, this more recent data shattered hopes that the trend would continue. On top of that, scientists are finding that observable climate trends line up best with “worst-case scenario” models of global warming—that is, global temperatures could rise five degrees in the next century.

And the arctic is melting much faster than scientists predicted. A recent report by the U.S. National Oceanic and Atmospheric Administration (NOAA) declared “that the North Pole had reached a ‘new normal,’ with no sign of returning to a ‘reliably frozen region.’”

Melting glaciers and sea ice trigger a whole new set of problems. The disappearing ice will cause sea levels to rise, and the “reflective white snow and ice [will] turn into heat-absorbing dark-blue water…[meaning] the Arctic will send less heat back into space, which leads to more warming, more melting, and more sea-level rise still.”

And finally, natural disasters are becoming increasingly ferocious as weather patterns mutate. The United States saw this first-hand, with massive wildfires on the west coast—including the largest ever in California’s history—and a string of hurricanes that ravaged the Virgin Islands, Puerto Rico, and many southern states.

These consequences of global warming are beginning to affect areas of social interest beyond the environment. The 2017 Atlantic hurricane season, for example, has been a massive economic burden, wracking up more than $200 billion in damages.

In Rogelj’s words, “Right now we really need to find ways to achieve multiple societal objectives, to find policies and measures and options that allow us to achieve those together.” As governments come to see how climate protection “can align with other priorities like reducing air pollution, and providing clean water and reliable energy,” we have reason to hope that it may become a higher and higher priority.