(CNN)In the battle against climate change, efforts to reduce emissions of harmful gases like carbon dioxide have consumed most of the headlines.
But many scientists say that simply minimizing man-made pollution is not enough to limit global warming to two degrees Celsius, as agreed in the 2015 Paris climate agreement -- humans must also devise other means to help cool the planet.
There's no shortage of imaginative proposals to achieve this goal. Some involve sucking carbon dioxide out of the atmosphere; others, including a giant space-based solar shield, would reduce the amount of sunlight that reaches Earth.
While many of these solutions are considered radical and impractical, others have been operational for years. But given their often high costs, significant side effects, vast scale and international impact, there's plenty of skepticism about the viability of the existing options.
"There's no way around reducing emissions," said Stefan Schaefer, climate engineering program leader at the Institute for Advanced Sustainability Studies in Potsdam, Germany. "Without that, none of the techniques can do anything useful."
"None of them can be a silver bullet," he added. "But investing some money to research these approaches makes sense."
Here are some of the existing technologies and a look at how realistic they are.
Scrubbing the air
While some projects remove carbon dioxide at its source -- such as power stations or gas fields -- before it enters the atmosphere, direct air capture (DAC) involves taking CO2 out of the air and storing it, often in underground reservoirs.
Chemical techniques for capturing CO2 -- such as "scrubbing" (using an alkaline to absorb CO2) -- are already being used in power stations and could be transferable to DAC projects.
Other suggested methods include using artificial trees to suck up carbon or building towers that suck in air and filter out the CO2.
- Far-reaching: DAC can combat emissions from disparate sources including homes and vehicles, not just single points (such as power stations).
- Mobile: Since the technology targets CO2 in the atmosphere, DAC facilities can be set up anywhere on Earth.
- High costs: Factoring in the building of plants and assembling new infrastructure, DAC is very expensive, both financially and in terms of energy input.
- Storage risks: Some scientists are worried about storing CO2 underground because of the possibility of leaks and the question of who would be responsible for monitoring the gas over time.
How realistic is it?
Unlike some other technologies, DAC has made it beyond the drawing board.
The world's first commercially operational DAC plant opened in Switzerland in May. Fans suck air into filters that absorb carbon dioxide. The gas is then piped into greenhouses and used as fertilizer to grow crops.
But Schaefer is skeptical about the scope of this technology: "The question with direct air capture is: how much can it actually achieve? How much CO2 can it actually remove from the atmosphere? And to what extent would you have to scale it up to impact the global climate?"
Building hundreds or thousands of DAC plants would take vast amounts of energy and materials and produce CO2 in the process, contributing to the problem the plants are meant to help solve.
Richard Darton, emeritus professor and co-director of the Oxford Geoengineering Programme at the University of Oxford, is more hopeful. He said that if the plants themselves use renewable energy to run, their carbon footprint could be very small.
Good old-fashioned trees
It may seem outdated, but tree planting is a vital tool in the battle against climate change and one of the simplest possible solutions.
While reforestation reintroduces trees to land only recently deforested, afforestation means planting trees in areas that have been without forests for a long time (or have never supported forests).
Trees take in and store carbon dioxide, reducing the concentration of the gas in the atmosphere.
- Low cost: Afforestation is one of the simplest and cheapest ways to remove CO2 from the atmosphere.
- Feasible: Some countries already have small or short-term afforestation projects, which could easily be expanded or replicated elsewhere.
- Side effects: Alongside the positive effect on the climate, planting trees brings plenty of other benefits too, including job creation and enhancement of soil fertility.
- More expensive food: Mass afforestation could lead to food price rises through competition for land.
- More warming: Changing grassland into forest darkens that area of the Earth's surface, meaning more of the sun's energy is absorbed, which could help raise (rather than reduce) Earth's temperature.
- Side effects: Habitats are destroyed and ecosystems disrupted with unpredictable long-term consequences.
How realistic is it?
"A useful first step is to stop cutting down trees in the first place," Schaefer said. "That's still happening on a large scale."
Darton agrees, and he sees great value in encouraging afforestation on land that's suitable.
But the new areas of forest have to be vast to influence global climate -- generating the problems described above -- and the positive effects are not permanent.
"There's always the problem that over the lifetime of the forest, carbon will go back into the atmosphere," said Darton. "It may gain us the decades we need, but over the long term it's not a solution."
Just add lime
Adding more lime or other alkaline materials into the ocean could be another way of removing CO2 from the air. This technology is known as ocean alkalinity enhancement. Rocks such as limestone or silicates would be ground up and dispersed in the ocean to increase its ability to store carbon.
The limestone is first quarried then broken down into quicklime at high temperatures in a process that also produces carbon dioxide. But if that quicklime is then dumped into seawater, scientists say it can absorb around twice as much CO2 as was released in the first reaction.
- Permanent: The gas reacts with the rock underwater and becomes inorganic, which means it stays in the ocean permanently.
- Double win: Adding lime to the oceans could also help fix rising ocean acidity -- a dangerous side effect of rising CO2 emissions -- as well as reducing CO2 levels in the atmosphere.
- Logistics: To achieve any significant impact on the climate, huge amounts of limestone would need to be quarried, transported and broken down.
- New CO2 produced: While reducing CO2 levels in the long term, the process itself generates carbon dioxide.
- Side effects: The effects on existing ecosystems -- which have developed to adapt to an increasingly acidic environment -- are unpredictable and could be severe.
How realistic is it?
More and more people are becoming interested in this technology, explained Schaefer, who said that tackling ocean acidification is something that few other geoengineering options can do.
But both Schaefer and Darton are fearful of the effects on the world's seas. "Many of us are quite nervous about distributing chemicals into the maritime environment," said Darton. "We don't know an awful lot about the sea and how it will react to changes in chemistry."
To make any significant inroads in carbon reduction, ocean alkalinity enhancement would have to be deployed on a very large scale. Darton said more research was needed into the longer term effects of this approach.
"I'm doubtful that this is something we could or should do in the long term," he said.
Parasols in space
Back in 1989, American James Early suggested putting a giant sunshade in space to offset the effect of greenhouse gases in the Earth's atmosphere. He envisioned a glass refractor so large and heavy it would have to be built on the moon.
More recent proposals designed to reduce the amount of sunlight reaching our planet include a swarm of thin metallic reflecting discs, a Saturn-like ring of dust particles injected into space from Earth, or a superfine mesh of aluminum threads.
Scientists estimate that reducing the amount of sunlight reaching Earth by just 2% could offset the effects of CO2 levels in the atmosphere doubling.