The year is 2050, the global average temperature has soared past the 2-degree Celsius (3.6-degree Fahrenheit) threshold, and the planet is besieged by drought, rising sea levels, and extreme rainfall.

Unrest has broken out in South America over companies grabbing vast tracts of land for a carbon dioxide removal (CDR) technique that uses bioenergy crops. This technique is widely favored by the European Union, whose member states' ongoing use of coal means they've failed to meet their emission reduction targets.

Tensions between the United States and China are mounting over the latter's plans to counter drought and crop failure by spraying large quantities of sulphate aerosol into the stratosphere, thereby reflecting sunlight into space to reduce global temperatures.

The American public fears the impact of this form of solar radiation management (SRM) on the global climate system, while a Chinese-led coalition hopes it will negate the worst effects of warming.

Back in the here and now, CDR and SRM are in actual fact still in their experimental infancy but some believe they will become necessary if we are to stay below the 2-degree warming limit set under the Paris climate agreement.

"One thing that has really changed the way this [climate engineering] is talked about, is the Paris agreement," said Stefan Schäfer, who is leading a project assessing the risks and opportunities of climate engineering for society at the Institute for Advanced Sustainability Studies (IASS) in Potsdam.

Once seen as desperate measures humans may turn to in the face of political failure, under "ambitious" Paris objectives, these technological interventions in the climate now seem less abstract.

"That's kind of a hyper-Anthropocene vision," said Schäfer, referring to the geologic epoch we have entered due to humanity's profound impact on Earth.

Unprecedented scales

Bioenergy with carbon capture and storage (BECCS) is the carbon removal strategy built into some of the United Nations Intergovernmental Panel on Climate Change (IPCC) pathway scenarios for reducing global emissions.

It's often described by proponents as a "negative carbon emissions technology," because they say it takes CO2 out of the atmosphere.

As described in our future scenario, the method involves planting trees and crops that remove carbon from the atmosphere as they grow, and are ultimately used as biomass. Emissions resulting from burning the biomass for energy are captured at the source, then stored underground.

"A lot of people see it as very benign," said Miranda Boettcher, a research associate at the IASS told DW. "People think, planting trees — that's not a problem, but there are a lot of problems associated with that."

According to most modeled scenarios, we will need to remove several billion tons of CO2 from the atmosphere annually to stave off catastrophic global warming. To achieve that using BECCS would require monocultures on an unprecedented scale — some computer simulations suggest the need for a land area 1.5 times the size of India.

Not only does that leave potential for conflict over which land where, but it could also have devastating implications for biodiversity and food production.

"At those scales, if you want to have significant drawdown of carbon, then you have to build up an industry from scratch — and everything associated with that, from the energy input to the impacts on biodiversity and food production," Schäfer said.

Adventures in the stratosphere

Solar radiation management in the form of spraying reflective aerosols into the stratosphere is not included in IPCC's emissions pathways. But the UN body has described it as a way "to some degree offset global temperature rise and some of its effects."

Other SRM technologies, such as placing mirrors in space to divert the sun's rays, are being researched but are thought to be too expensive and complicated to implement.

Aerosol spraying, on the other hand, is viewed as cheap and viable. Still, it is probably the more controversial of the climate engineering technologies.

Besides involving direct intervention in the earth's system with largely uncertain consequences, it doesn't deal with the root causes of climate change. It also raises big questions, such as: Who would get to set the global thermostat, and how can we avoid conflict between nations over the knock-on effects?

Certainties and uncertainties

It's fairly certain that spraying sulfate or other kinds of aerosols at least 19 kilometers (12 miles) into the stratosphere to reflect solar radiation into space would reduce global temperatures.

Such cooling was observed after the violent volcanic eruption of Indonesia's Mount Tambora in 1815, when inclement weather and plunging temperatures caused widespread crop failures and the "year without a summer."

To test how aerosols behave in the stratosphere and what other ramifications they could have would require a field experiment on a scale tantamount to actual implementation.

Because that's not an option, Harvard scientists led by David Keith are planning a minute-scale trial to measure some of the aerosol microphysics and effects on atmospheric chemistry.

As their findings won't reveal anything about global climatic effects, researchers are left with little choice but to rely on mathematical computer models that imperfectly replicate the real world.

Some show that reducing incoming solar radiation would trigger a drop in rainfall and water availability in the tropics; while others indicate delayed recovery of the Antarctic ozone.

"The uncertainty is quite high," said Ulrike Niemeier, a solar radiation management researcher at the Max Planck Institute for Meteorology in Berlin. And that uncertainty is not just in the science.

If, for example, the climate were subjected to a manmade act of manipulation, and a given country were then to suffer drought, that country could take legal action. Or worse.

"My main worry is international relations," said Niemeier. "This could cause trouble, and even be a reason for war."

To avoid conflict, countries would need to establish decision-making and voting procedures on whether and how to manipulate the global climate; as well as liability and compensation regimes, even when an extreme weather event couldn't be traced back to aerosol spraying.

The lesser of two evils?

In the meantime, natural and political scientists, ethicists, and economists from the IASS and other German institutions are collaborating to navigate these questions and assess whether climate engineering is an appropriate path to take  — and under which conditions such a path would be defensible.

Researchers are also working on a non-binding, voluntary code of conduct for carrying out experiments that could apply to individual scientists, institutions or countries.

Still, some, like Lili Fuhr, who heads the ecology and sustainable development department at the Heinrich Böll Foundation, say climate engineering can and should be avoided in favor of "much more transformative options where technologies exist."

These include a managed phaseout of fossil fuel production, reducing input of energy and resources by using a circular economy approach, and getting carbon out of the atmosphere by protecting and restoring natural ecosystems and forests.

However, the political will for such a radical transformation "is missing," she added.

That's a sentiment echoed by those assessing climate engineering. Schäfer says techno-fixes could end up diverting attention from the need to transition to a carbon-free economy. Climate engineering is seen as a "lesser evil" that nobody is particularly excited about, he added. In other words, it's a bad idea whose time may or may not come.

But if it does, being prepared will be key. Otherwise, said Schäfer, we could find ourselves on a slippery slope, and "in a situation where the deployment is closer than what otherwise might be desirable without us having actively engaged and reflected on this."

This is the 2nd report in a series that looks at the constructive steps Germany is taking to combat climate change. Global Ideas and the Grimme-nominated online medium Perspective Daily will bring you in-depth stories each week in the run up to 23rd Climate Conference taking place in November 2017 in Bonn.