Of all the assaults humanity is inflicting on the earth, nitrogen pollution is one of the most devastating. In more than 400 coastal areas worldwide, agricultural runoff is sapping oxygen from large swaths of oceans, killing nearly all marine life. In the Midwest, much of the synthetic fertilizer used in farming washes into the Mississippi River and out to the Gulf of Mexico where every year it creates a “dead zone” up to 8,000 square miles.

So imagine the reaction in 2007 when one company reportedly approached the governments of the Philippines, Malaysia, Chile, and Morocco with a plan to do much the same by dumping up to 1,000 tons of nitrogen-rich urea into their offshore waters. The Australian-based Ocean Nourishment Corporation was looking to test its patented technology to cultivate oceanic gardens of phytoplankton that would suck up carbon dioxide from the atmosphere, a major factor in global warming. It theorized that once the microscopic plants die, they carry some of the carbon to a watery grave on the ocean floor. This could allow Ocean Nourishment to sell credits in carbon-trading markets. While public opposition forced the company to scrap its plans, it has been able to carry out smaller tests. (Another ocean fertilization firm, Planktos, which filed for bankruptcy in 2008 after environmental groups thwarted its plan to dump more than 50 tons of iron ore about 200 miles from the Galapagos Islands, also built its business model on selling carbon credits.)

Ocean Nourishment is one of a new class of companies developing saleable “geoengineering” technologies to counter the effects of global warming. Geoengineering is sometimes defined as “the deliberate large-scale intervention in the Earth’s climate system in order to moderate global warming.” An indication of the complexity involved can be gleaned from the geoengineering experiment humanity has been conducting since the mid-18th century. Having dumped more than 200 billion tons of carbon into the atmosphere since then, we’ve known for decades that we’re dramatically altering the biosphere. Hundreds of institutions and thousands of scientists have been trying to understand global warming for years, yet the effects are constantly surprising, whether it’s methane (a potent greenhouse gas) pouring out of the arctic permafrost, oceans suddenly absorbing less carbon dioxide, the rapid spread of invasive species and pathogens, or mountain glaciers melting faster than expected.

Many geoengineering enthusiasts are undeterred by the uncertainties and risks. They aim to create a human-controlled “planetary thermostat” and discuss how to remake the Earth’s atmosphere and climate by artificially brightening clouds, mimicking volcanic plumes, or constructing a space-based sunshade, all of which would cool the planet by reflecting more sunlight back into space. Others are testing carbon-filtering artificial trees, methods to enhance oceanic carbon sinks, speeding up natural processes of turning carbon into rock, or sequestering liquid carbon dioxide deep underground. Some suggest reforesting the planet, genetically engineering trees to be carbon hungry and crops to be reflective-while others suggest cutting down heat-absorbing boreal forests as climate-control measures.