Next month, researchers in the U.K. will start to pump water nearly a kilometer up into the atmosphere, by way of a suspended hose.

The experiment is the first major test of a piping system that could one day spew sulfate particles into the stratosphere at an altitude of 20 kilometers, supported by a stadium-size hydrogen balloon. The goal is geoengineering, or the "deliberate, large-scale manipulation of the planetary environment" in the words of the Royal Society of London, which provides scientific advice to policymakers. In this case, researchers are attempting to re-create the effects of volcanic eruptions to artificially cool Earth.

The $30,000 test, part of a project called Stratospheric Particle Injection for Climate Engineering (SPICE), is inspired by the 1991 eruption of Mount Pinatubo in the Philippines. That volcano spewed 20 million tons of sulfate particles into the atmosphere, cooling Earth by 0.5 degree Celsius for 18 months. If the British feasibility tests are successful, the balloon-and-hose contraption could be used to inject additional particles into the stratosphere, thereby reflecting more of the sun's energy back into space, and hopefully curbing some of the effects of global warming.

"This is one of the first times that people have taken geoengineering out of the lab and into the field," lead scientist Matthew Watson said Tuesday during a press conference in London. "We are still decades away—and I do mean decades—from doing real geoengineering." Watson said his team still needs to determine which substances would work best at reflecting light, how much is needed to have an effect, and the possible unintended consequences of injecting the particles into the atmosphere, such as acid rain, ozone depletion or weather pattern disruption.

October's tests will mainly focus on whether the balloon-and-hose design could be an effective method to deliver the sunlight-reflecting particles. At an airfield in Norfolk, England, that is no longer in use, a helium blimp will hoist a regular pressure-washer hose one kilometer off the ground. An off-the-shelf pressure washer will pump up 1.8 liters of tap water per minute, to a maximum of 190 liters, says Hunt, which will evaporate or fall down to the ground locally. The researchers will monitor the performance of the system, and use the data to design the larger 20-kilometer-high setup.

In the past scientists have proposed similar atmospheric delivery methods using guns, airplanes, rockets and chimneys. In 2009 Russian scientists even tested airplane delivery on a small scale. But Hugh Hunt, a SPICE engineer at the University of Cambridge, said the balloon-and-hose design appears to be the most cost-effective option. Even when scaled up, the team expects the simple design to cost around $5 billion, in comparison with the $100 billion needed to launch thousands of high-altitude aircraft.

The water tests are expected to be harmless, but several environmental groups have criticized the plan—and geoengineering in general. Last year, the United Nation's Convention on Biological Diversity issued a statement forbidding geoengineering research that may impact biodiversity. The U.K. accepted that statement, but the SPICE experiment does not violate any international agreements due to its small scale, says Jason Blackstock, a physicist at Canada's Center for International Governance Innovation.

Nevertheless, the Canada-based Action Group on Erosion, Technology and Concentration (ETC) is calling the tests internationally irresponsible. In a written statement, they called on the British government to shut down the project, adding: "This experiment is only phase one of a much bigger plan that could have devastating consequences, including large changes in weather patterns such as deadly droughts."

Alan Robock, a Rutgers University meteorologist, shares some of those concerns. He has created computer simulations indicating that sulfate clouds could potentially weaken the Asian and African summer monsoons, reducing rain that irrigates the food crops of billions of people. It is premature to conduct such field experiments, Robock says. More computer modeling should be done first, he adds, to determine how injected particles might interact with the ozone layer and the hydrologic cycle.

Whereas Hunt agrees that such research is lacking, he said that the team needs real measurements in order to see if the tethered balloon design is viable. "If not now, then when would you start?" he asks. "This year, next year? Or maybe wait until a large block of ice falls off of Greenland? My choice is to have all the tools carefully thought through, so that we don't have to rush into anything."

To avoid dangerous climate change, some scientists estimate that global CO2 emissions must be cut by at least 80 percent by the end of the century. Geoengineering will not help achieve that long-term target, but the cooling effects of large sulfate clouds are nearly instantaneous, making geoengineering potentially valuable in the event of acute climate crises such as the melting of Arctic sea ice, which could further accelerate global warming over the decades.

The researchers made it clear in Tuesday's press conference that they do not advocate using geoengineering as an excuse for humanity to continue recklessly emitting carbon dioxide and other greenhouse gases. "[Geoengineering] should be considered as an emergency remediation while we wean ourselves off carbon," Watson said. "The question you have to ask is, is it worse without mitigation or with it? And that answer isn't obvious yet."