It's unlikely that humans will stop Earth's temperature from surpassing 1.5 degrees Celsius above pre-industrial levels, a rise that would lead to devastating climate change.

To help cool the planet, some scientists are proposing the use of solar geoengineering, a method in which chemicals are sprayed into the atmosphere to reflect the sun's rays back into space.

A team of Harvard scientists plans on starting a real-world solar geoengineering study in early 2019.

The scientists will release balloons containing calcium carbonate, which they expect to help lower temperatures.

Some scientists are strongly opposed to solar geoengineering, saying the method could harm ecosystems and be weaponized.

To reverse rising temperatures and prevent catastrophic climate change, some scientists are turning to solar geoengineering, or the modification of Earth's atmosphere using tools like reflective aerosols, mirrors in the atmosphere, and controlled cloud formations.

A team of scientists at Harvard University is on track to become the first to test geoengineering methods outside the laboratory. As early as spring 2019, the team will launch an outdoor sky-modifying experiment in the United States, spraying particles into a small part of the sky to help reflect some of the sun's rays back into space.

Their project, the $3 million Stratospheric Controlled Perturbation Experiment (SCoPEx), will send two balloons into the stratosphere, which stretches from 6 to 31 miles above Earth's surface. The first balloons will be filled with ice to make sure the instruments work properly. Later ones will include calcium carbonate.

SCoPEx aims to determine how these chemicals interact with the stratosphere and whether they can help lower temperatures on Earth.

Read more: How a last-ditch ‘planet-hacking’ plan could keep Earth habitable for longer

In a report released in October, the Intergovernmental Panel on Climate Change said the world will be hit by some of the most severe effects of climate change after temperature levels exceed 1.5 degrees Celsius above pre-industrial levels.

While stopping this is still possible, it would require a coordinated shift in the global economic system, which is unlikely. Humans would have to reduce greenhouse-gas emissions by 45% in the next 12 years, ultimately reaching zero emissions by 2050.

Without a major change in the economic system, only a new approach could stop catastrophic climate change, and the Harvard scientists believe solar geoengineering could get us there.

The proposed geoengineering solution has already been observed in nature. According to Nature magazine, the 1991 Mount Pinatubo eruption in the Philippines released about 20 million tons of sulfur dioxide into the stratosphere. The planet's temperature went down about 0.5 degrees Celsius after the eruption, and for 1.5 years, Earth went back to temperature levels seen before the steam engine was invented.

Harvard's scientists have chosen calcium carbonate because they expect it to heat up less than sulfate and have less of an impact on ozone, according to Nature.

Opponents of the method say solar geoengineering could harm the ozone layer and one day lead to military use of weather-altering technology.

According to the Global Challenges Foundation, solar geoengineering could also disrupt ecosystems until food supplies are threatened. If we suddenly stopped using the method, global temperatures would quickly go back to what they are now, and scientists say many species would be unable to cope with such a rapid change.

Intergovernmental Panel on Climate Change, IPCC, Chair Hoesung Lee, center, speaks during a press conference in Incheon, South Korea, on October 8, 2018. Preventing an extra single degree of heat could make a life-or-death difference in the next few decades for multitudes of people and ecosystems on this fast-warming planet, an international panel of scientists said. Ahn Young-joon/AP

In late November, researchers at Harvard and Yale published their findings about the potential costs of implementing solar geoengineering methods. The researchers found that the most cost-effective way of releasing sulfate particles into the stratosphere would be using a high-altitude aircraft. Such a program would cost about $2 billion per year for the first 15 years of use, according to the study published in Environmental Research Letters.

Yale lecturer Wake Smith and Harvard lecturer Gernot Wagner, the study's authors, also determined it would be hard for anyone to deploy such a geoengineering program in secret.

The recent study was misreported by some news outlets that claimed Smith and Wagner were advocating for the use of solar geoengineering. The scientists, however, stressed that their paper is not an endorsement.

"We don’t make any judgment about the desirability of stratospheric aerosol injection," Wagner said in a press release. "But we do show that a hypothetical deployment program starting 15 years from now, while both highly uncertain and ambitious, would be technically possible from an engineering perspective."

For now, SCoPEx will start small, as the balloons will release up to 35 ounces of calcium carbonate, which is roughly equivalent to the weight of a dictionary. Frank Keutsch, the project's lead researcher, told Nature that the experiment will help scientists understand what the presence of calcium carbonate in the stratosphere would do, since the compound does not exist there. The project's website says the test will not endanger people or the environment in a significant way.



Based on existing science, the benefits of solar geoengineering could outweigh any potential drawbacks, experimental physicist David Keith told Nature. Keith, who is part of the Harvard team, said a still-unpublished study of precipitation and temperature shows that a solar geoengineering program done in moderation would benefit nearly every place on Earth.

"Despite all of the concerns, we can’t find any areas that would be definitely worse off," Keith told Nature. "If solar geoengineering is as good as what is shown in these models, it would be crazy not to take it seriously."