Last year was the hottest year on record, and this year’s temperatures may reach even higher.
With so many extraordinary heat waves, floods, and storms piling up, one may wonder: Just how much warmer is the Earth going to get?
The answer hinges on two main factors: how much more heat-trapping gasses humans will emit, and how the planet will respond.
Whether humanity continues to dawdle or actually takes aggressive action to cut emissions is the biggest source of uncertainty in the future of the planet since the bulk of the warming we’re experiencing is due to the waste gasses from burning coal, oil, and natural gas. The Intergovernmental Panel on Climate Change (IPCC), the United Nations’s climate science team, has chalked out five scenarios with different levels of action needed from global leaders to curb climate change to plug into its climate models.
On the other side of the equation, scientists have been working to narrow down the scope of possible reactions that the planet has to all of this heating. They’ve been getting better measurements of the Earth’s behavior, refining their physical models of things like rainfall and ocean currents, and designing more sophisticated computer simulations to get a better sense of what complicated reactions could happen and the kind of events that could be put into motion as the planet heats up. With these inputs, they’ve come up with a range for how much further Earth will warm for a given amount of carbon dioxide in the atmosphere, a parameter known as equilibrium climate sensitivity.
If the amount of carbon dioxide in the atmosphere were to double relative to the era prior to the Industrial Revolution, the most recent major IPCC report finds the world will warm between 2 degrees Celsius and 5 degrees Celsius, with a best estimate of 3 degrees Celsius. It means that some of the more dire forecasts of warming from the past are much less likely, and so are some of the more optimistic predictions.
But, in assembling this report, scientists were surprised that a subset of climate models were producing warming estimates that were much hotter than others. In response, they changed how they factor these outliers into the overall estimate, reducing their influence rather than weighing them equally.
Last year, a team led by former NASA scientist James Hansen found that previous sensitivity estimates had vastly underestimated the role of aerosols, such as soot and dust, and that there may be more warming baked in than we realized. These fine particles suspended in the sky can have myriad effects on the global climate.
Figuring out the future of warming is not just an academic exercise. If you’re constructing a road, a home, a power plant, or if you simply have any stake in the world decades away, you need to start planning and building now for that future. If the world does go down one of the more extreme warming scenarios, curbing greenhouse gas emissions may not be enough to keep the planet livable for humans. We could be forced to employ more extreme and controversial interventions like geoengineering to rein in runaway warming.
So how exactly do we figure out whose vision of the future is most accurate before that future becomes the present? It’s an ongoing process. As scientists expand the boundaries of knowledge of the planet, they’re also coming up with ways to reconcile these diverging views.
The messy truth of climate modeling
The fundamental concept of climate change is fairly straightforward — more heat-trapping gasses in the atmosphere cause the planet to retain more heat — but the practical ways this plays out get extraordinarily complicated very quickly.
For example, warmer air can hold onto more moisture. Water vapor is itself a greenhouse gas, so that can create a feedback that accelerates warming. In addition, more moisture in the air can lead to extreme rainfall in some areas and less in others. It also forms clouds, which can reflect sunlight back into space, cooling the area below, or trap even more heat. Now calculate these effects over the entire planet and over the course of decades and you end up with models that demand the most powerful supercomputers in the world just to run.
Scientists are gradually filling in the blanks in these models with lab experiments and real-world measurements, but even the most sophisticated simulations have to make assumptions and judgment calls about which variables are the most important and how much they should shape the final calculations. That’s why climate researchers can come to different conclusions about how much the planet will warm. One scientist might think that cloud cooling effects offset more warming, while another might stipulate that melting ice caps will have a stronger feedback effect.
One of the more confounding variables in climate models is the effect of aerosols. Like carbon dioxide, aerosols are a by-product of fossil fuel combustion, but they also come from natural sources like sand and sea spray. Aerosols have a range of impacts on the climate.
“Some aerosols are light-colored and scatter more light, which means we get a cooling effect, and some aerosols, like soot, are dark and absorb light, which means we get a warming effect,” said Eliza Harris, a senior scientist at the Swiss Data Science Center. “And that effect also varies depending on how high it is in the atmosphere.”
As humanity burned more fossil fuels since the dawn of the Industrial Revolution, the concentrations of both greenhouse gasses and aerosols rose in the atmosphere.
Scientists like Hansen think that aerosols helped mask some of the warming caused by carbon dioxide and hypothesize that the global climate is more sensitive to greenhouse gasses than conventional estimates predict. Since many aerosols drive air pollution and are health hazards, efforts to limit air pollution have unintentionally reduced their cooling side effects, and further reductions will speed up warming further. Hansen has described this reduction in air pollution leading to more warming as a “Faustian bargain.” (Hansen declined to comment for this story).
There have actually been a number of historical examples of aerosols cooling the global climate. Major volcanic eruptions like the 1991 eruption of Mount Pinatubo in the Philippines inject so many gasses and particles into the sky that they dim the sun enough to cool the planet. After the Pinatubo eruption, global average temperatures fell by roughly 1 degree Fahrenheit (0.6 degrees Celsius) for more than a year.
But getting a full inventory of aerosols can be difficult. Satellite measurements can end up obscured by clouds while ground-level sensors don’t pick up what’s going on high in the atmosphere.
“There’s a lot of uncertainty on the abundance of aerosols today,” said Loretta Mickley, who co-leads the Atmospheric Chemistry Modeling Group at Harvard University. “That said, we are confident that anthropogenic aerosols certainly did increase in the late 20th century as industry ramped up in much of the developed world. There is a decline in aerosols in much of the developed world now, but increasing aerosols in places like India and China.”
More recently, a new international regulation drastically limiting sulfur aerosol pollution from global shipping went into effect. That sudden drop in pollution over the busiest shipping routes in the world led to a sudden warming in those regions, contributing to record-high temperatures in waters like the Atlantic Ocean. A study this year found that a major drop in air pollution over China contributed to warming in the Pacific Ocean.
How scientists reconcile their differences
Of course, a number of scientists dispute Hansen’s recent findings and think his team is overestimating the role of aerosols. The question then is how do you weigh results like this in the context of all the other climate research?
For the IPCC, the conventional method was to include all the major climate models and average out their findings, giving each one equal weight. Climate models are often evaluated by examining how well they match historical observations of warming using starting conditions from the past. On this front, the aggregate model tended to outperform most individual models.
But in the most recent assessment, the IPCC decided to change its approach. The models that skewed hotter did a poorer job of reproducing historical temperature patterns, so the IPCC gave them less weight in the aggregate in calculating the final range of sensitivity.
That makes sense if you’re mainly concerned with temperature, but that’s only one dimension of the climate.
Neil Swart, a researcher at the Canadian Centre for Climate Modeling and Analysis, developed one of the hottest models used in the latest IPCC assessment. He noted that climate models that may not be as good at predicting temperature might be better at forecasting other important variables like precipitation. By evaluating models solely on temperature, modeling groups could end up tuning their results to better fit within the selection criteria rather than letting the simulations run their course. So there is still a compelling argument for egalitarianism.
The debate highlights how even with the best measurements and models, scientists have to make some subjective decisions. For people who have to make decisions now that depend on the future climate, it adds to the frustration and can fuel distrust.
Still, it’s important to note that the vast majority of scientists agree on the broad contours of climate change and that it’s prudent to halt the relentless rise of carbon dioxide in the atmosphere.
“Look, the climate is sensitive to greenhouse gasses. We’re not exactly sure to the right decimal place what that sensitivity is, but we have no doubt that it’s sensitive,” Mickley said. “We do know if we cut greenhouse gasses to zero today, we would vastly improve the outlook going forward.”