Harvard scientists will attempt to replicate the climate-cooling effect of volcanic eruptions with a world-first solar geoengineering experiment set for early 2019.

The Stratospheric Controlled Perturbation Experiment (SCoPEx) will inject calcium carbonate particles high above the earth in an attempt to reflect some of the sun's rays back into space.

It will likely mark the first time the controversial concept of dimming the sun — more scientifically known as stratospheric aerosol injection (SAI) — will be tested in the real world.

Existing understanding of SAI comes from computer modelling and also from observing the natural effects of volcanoes, which create a haze of sulphate particles that effectively cool the planet.

The 1991 Mount Pinatubo eruption in the Philippines, for instance, caused the average global temperature to drop by about 0.6 degrees Celsius in the following 15 months.

Harvard team plans to launch experiment in early 2019

The Harvard team, led by scientists Frank Keutsch and David Keith, has been working on the SCoPEx project for several years and revealed in a recent article in Nature that it would launch the first phase of the experiment as early as the first half of 2019.

"This experiment will help us learn more about the efficacy and risks of solar geoengineering," said the SCoPEx team.

"Computer modelling and laboratory work tell us some very useful things about solar geoengineering, but as with all other aspects of environmental science, computer models ultimately rest on observations of the real environment," said the team.

"Measuring the ways that aerosols alter stratospheric chemistry can, for example, improve the ability of global models to predict how large-scale geoengineering could possibly disrupt stratospheric ozone."

Geoengineering among plans to reverse global warming

While geoengineering has long been controversial, the recent United Nations' Intergovernmental Panel on Climate Change (IPCC) report highlighted its potential as a Plan B if global warming can't be capped at a safe level of 1.5 degrees Celsius.

One of the panel's chief concerns about solar geoengineering was the absence of global field experiments, so the progress of SCoPEx, the first such early stage test, will be closely watched.

The method employed by the SCoPEx team will be to spray the stratosphere with calcium carbonate — the compound used commonly in cement, and medicinally as an antacid.

Philanthropically-funded team will not seek to file patent

The experiment will take place in a small area 20 kilometres above the south-west of the USA. A balloon will release small amounts of calcium carbonate, about 100 grams at a time, then circle back to record the results.

According to the Nature article, Keutsch and Keith are aiming to launch the experiment in the spring of 2019 but want an external advisory committee to review the SCoPEx project before the experiment goes ahead.

The team is philanthropically funded, and they "strongly discourage" the commercialisation of solar geoengineering technology in order to ensure a necessary level of transparency for informed public decision-making. They will file no patents related to the technology.

While the SCoPEx team hopes their findings can help to build more informed public and academic debate on the topic of solar geoengineering, critics argue that any attention garnered by the nascent science is a distraction from a real global effort to cut greenhouse gases.

Geoengineering masks the real problem, say critics

At best, solar geoengineering only masks the effects of such pollution rather than reducing it. Related worries are the potential "termination shock" if the practice is ever ceased, unforeseen side effects and the difficulty of governing it across borders.

With all of this in mind, the IPCC only gave very qualified approval to the practice in October.

This is in contrast to the more tried and tested practice of carbon-dioxide removal, such as carbon capture and storage, which it has incorporated into almost all of its modelling for safe pathways forward where global warming is limited to 1.5 degrees Celsius.