Dismissed a decade ago as far-fetched and dangerous, schemes to tame the effects of global warming by engineering the climate have migrated from the margins of policy debates towards centre stage.

"Plan A" remains tackling the problem at its source. But the UN's top climate science body has made it clear that slashing carbon pollution won't be enough to keep Earth from overheating.

That has opened the door to a host of geoengineering schemes, from building underwater walls to shore up an Antarctic glacier the size of Britain to injecting a giant sunscreen into the stratosphere.

Here is a menu of "Plan B" geoengineering solutions, along with their potential drawbacks:

- Direct CO2 capture -

Experiments have shown it is possible to suck planet-warming carbon dioxide directly from the air, converting it into fuel pellets or storing it underground.

A company backed by Microsoft co-founder Bill Gates launched a pilot facility in Canada in 2015, and another company operates one in Iceland.

DRAWBACK: The technology is currently prohibitively expensive and might take decades to operate at scale.

- Solar radiation management -

Unlike other strategies, solar radiation management does not target CO2. The goal is simple: prevent some of the sun's rays from hitting the planet's surface, forcing them back up into space.

One idea is to inject or spray tiny reflective particles into the stratosphere -- possibly with balloons, aircraft or through giant tubes.

Nature sometimes does the same: Debris from the 1991 eruption of Mount Pinatubo in the Philippines lowered the planet's average surface temperature for a year or two afterwards.

Sixty-six million years ago, a ten-kilometre wide asteroid strike threw up so much debris that it wiped out land-based dinosaurs built for steamy tropical climes.

In April, a balloon test flight in Sweden for the Harvard-led project SCoPEx, short for "Stratospheric Controlled Perturbation Experiment", was postponed amid concerns over the implications for the environment and people in the country.

In Australia, trials of a controversial technique to spray microscopic salt crystals into the air above the threatened Great Barrier Reef to make the clouds brighter began last year.

Australian scientists say if successful and combined with other climate and protection measures this could help slow the decline of the reef.

DRAWBACKS: Even if it works as intended, solar radiation management would do nothing to reduce atmospheric CO2, which is making oceans too acidic. There is also the danger of knock-on consequences, including changes in rainfall patterns, and what scientists call "termination shock" -- a sudden warming if the system were to fail.

- Afforestation -

Extensive planting of trees could significantly slow the concentration of CO2 in the atmosphere, which currently stands at more than 510 parts per million, 50 percent more than 150 years ago.

DRAWBACK: Even if deforestation could be reversed -- more than 100,000 square kilometres of tropical forests have disappeared each year since 2013 -- the huge number of trees needed to put a dent in CO2 emissions would clash with food and biofuel crops.

- BECCS -

Bioenergy with carbon capture and storage (BECCS) marries a natural process with a high-tech one.

The first step is to plant rapeseed, sugarcane, corn or "2nd-generation" biofuel crops such as switchgrass, which pull CO2 from the air while growing. The second step is while burning the harvested plants for energy to sequester the CO2 produced.

In theory, the result is less CO2 in the atmosphere than when the process started. Virtually all climate change models projecting a future consistent with the Paris Agreement's temperature targets assume a key role for BECCS.

DRAWBACK: Studies calculate that up to twice the area of India would need to be given over to biofuels, putting BECCS in conflict with food crops. Such schemes would also require vast amounts of fresh water.

- Ocean fertilisation -

Microscopic ocean plants called phytoplankton gobble up CO2 and drag it to the bottom of the ocean when they die. Their colony size is limited by a lack of natural iron, but experiments have shown that sowing the ocean with iron sulphate powder creates large blooms.

DRAWBACKS: Scientists worry about unintended impacts. Die-offs of plankton, for example, use up oxygen, which could create massive "dead zones" in the oceans, something already on the rise.

- Enhanced weathering -

Natural weathering of rocks removes about one billion tonnes of CO2 from the atmosphere every year -- about two percent of total man-made C02 emissions.

Spreading a powdered form of a greenish iron silicate called olivine across certain landscapes can mimic that process, experiments have shown.

DRAWBACKS: It would be expensive to mine and mill enough olivine to make a difference.

- Biochar -

Biochar is charcoal made by heating plant waste -- rice straw, peanut shells, wood scraps -- over long periods in low-oxygen conditions. It can store CO2 for long periods, and also enriches soil.

DRAWBACK: The scientific jury is still out on how quickly this method could be scaled up, and on the stability of biochar used as a fertiliser.

mh-klm/wai

MICROSOFT

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