Can we stop the Earth heating up?
The SCoPEx high-altitude balloon mission will investigate a controversial way to reduce global temperatures
A high-altitude balloon mission will investigate a controversial way to reduce rising global temperatures
In 2021, Earth hits a bleak milestone. The concentration of carbon dioxide (CO2) in the atmosphere will hit 150 per cent of its value in preindustrial times. That excess will be the consequence of human activities, the direct cause of the current climate emergency. If global temperatures are to remain no more than 1.5 degrees Celsius (2.7 degrees Fahrenheit) above preindustrial levels, according to the United Nations’ Intergovernmental Panel on Climate Change (IPCC), the world needs to decrease net emissions of CO2 to zero by 2050.
But even this won’t put a sudden brake on the temperature rise, because it takes time for CO2 reductions to take effect. The negative impacts of global warming will continue for decades to come, but is there anything else we can do to help bring temperatures back down to normal sooner?
A research group at Harvard thinks it might be possible to achieve a temporary reduction in global temperatures by making small tweaks to the composition of Earth’s upper atmosphere. Researchers are hoping to test some of that technology – and the viability of their theory – later this year in what they call the Stratospheric Controlled Perturbation Experiment, or SCoPEx.
The ultimate source of Earth’s heat is the Sun, which bathes the daytime side of the planet in a constant flow of infrared radiation. About 30 per cent of this is reflected back into space by the atmosphere, while the rest of it warms the planet during the day and is reradiated back into space at night. In the delicate balance that prevailed in preindustrial times, the incoming heat was exactly offset by the amount lost to space, ensuring global temperatures remained constant.
The problem today is that CO2 emissions disrupt this balance by absorbing some of the heat that should be radiated back into space, trapping it inside the atmosphere. The more
CO2, the higher the temperature rises. The only permanent solution is to reduce the amount of CO2 in the atmosphere. But there are other processes that can produce short-term reductions in global temperature on a much faster timescale than reversing the greenhouse effect.
Volcanic eruptions are one naturally occurring example. The eruption of Mount Pinatubo in the Philippines in June 1991 caused the average temperature in the Northern Hemisphere to drop by more than half a degree over the following 15 months. This sudden, dramatic drop had
“In simulations, stratospheric aerosol injection (SAI) appears to be a viable concept”
“The goal is to improve our models of the way aerosols form in the stratosphere”
David Keith
nothing to do with CO levels or the greenhouse effect, which continued to trap heat inside the atmosphere unabated.2Instead there was simply less heat to start with. A volcano blasts a cloud of dust particles high up into the stratosphere, forming a protective shield that prevents a fraction of the Sun’s heat from reaching Earth’s surface.
Temporarily lowering temperatures by injecting particles into the upper atmosphere is what the SCoPEx team believes we should be using in the fight against climate change. The basic idea is simple. A high-flying aircraft or helium balloon would dispense batches of microscopic particles, referred to as aerosols, into the stratosphere at altitudes of 20 kilometres (12.4 miles) or more – much higher than planes usually fly. The aerosols would remain suspended in the air, too tenuous to be visible from the ground, but enough to reflect a fraction of the Sun’s energy back into space.
In simulations, stratospheric aerosol injection (SAI) appears to be a viable concept. In a 2018 report, the IPCC concluded that a fleet of highflying aircraft could deposit sufficient aerosols to offset current levels of global warming. But it’s not a permanent solution – the aerosols would have to be replenished every few years – and it only tackles one of the symptoms of climate change, rather than addressing its root cause, the greenhouse effect. At best it’s a stopgap measure, countering rising temperatures while other efforts are made to reduce CO2 levels.
So far the research carried out on SAI has been theoretical, supplemented by a limited amount of real-world data from volcanic eruptions. The aim of SCoPEx is to make further measurements under carefully controlled conditions, allowing better calibration of the computer models. As Frank Keutsch, the project’s principal investigator, puts it: “If we are to provide decision-makers with useful information about whether this could work, we need to ground-truth our models.”
The natural aerosols ejected by volcanoes are mainly composed of sulphur compounds, but there’s a downside to using these in SAI. As well as cooling the atmosphere, sulphides can damage Earth’s protective ozone layer, which shields us from harmful UV radiation. For this reason the SCoPEx team is focusing on a more benign aerosol, calcium carbonate – chalk dust, in other words – which researchers hope will produce the desired cooling effect without harming the ozone layer.
SCoPEx will use a large, uncrewed helium balloon, similar to a standard weather balloon except that it is fitted with propellers to allow the team on the ground to manoeuvre it in a controlled way. With assistance from the Swedish Space Corporation, scientists will launch the balloon near Sweden’s Kiruna. On its first flight, pencilled in for this summer, no actual material will be released into the stratosphere. The balloon will simply ascend to 20 kilometres (12.4 miles) altitude, where the team will try out the manoeuvring system and check that all the scientific instruments and communications function correctly.
Assuming the test run is successful, a second flight will perform a controlled release of one or two kilograms of calcium carbonate at the same altitude. The idea is that the balloon will be moving steadily in a straight line during the release, so the aerosol particles will form a narrow plume around a kilometre (0.6 miles) in length.
The balloon will then turn back through the plume, observing how the particles disperse over time and the extent to which they reflect sunlight.
As valuable as SCoPEx will be for our understanding of SAI, it’s important to see the project in perspective. “The goal is not to change
the climate or even to see if you can reflect any sunlight,” one of the project scientists, David
Keith, explains. “The goal is simply to improve our models of the way aerosols form in the stratosphere.” Keith says that at least another decade of research will be needed before the large-scale implementation of SAI could be contemplated. This, he says, “might involve injecting around 1.5 million tonnes into the stratosphere per year ... roughly a hundred aircraft would need to continuously fly payloads up to about 20 kilometres (12.4 miles) altitude.”
Given the daunting amount of work still to be done, it’s surprising to learn that SAI isn’t a new idea. It’s been around since the 1970s, and Keith himself has been working in the area for more than 20 years. But the research has progressed at a snail’s pace, partly due to a lack of official funding. A project as ambitious as SCoPEx would have been impossible without funding from private investors. On top of that, there are other issues that have slowed down research and caused repeated delays to the SCoPEx mission itself, which was originally proposed back in 2015.
There are two major controversies associated with SAI. The first is the gut reaction that tampering with nature is a bad idea. We created the climate crisis in the first place by pumping greenhouse gases into the atmosphere, so how can we be sure that pumping aerosols into it will make things better? Although computer modelling has indicated that SAI is safe, there’s still the possibility that it might have unforeseen side effects. People have speculated that it could disrupt weather patterns, harm crops by reducing the amount of sunlight they receive and – if sulphide aerosols are used – damage the ozone layer.
Arguments like these have led to a number of scare stories in the media, such as a far-fetched headline in the Daily Mail in 2019 that read: Bill Gates wants to spray millions of tonnes of dust into the stratosphere to stop global warming, but critics fear it could trigger calamity. Even some scientists are wary of following the SAI route. “That we might actually try to control the entire climate is a pretty terrifying idea,” as Douglas MacMartin, a professor at Cornell University in New York, put it. And the IPCC, in a 2018 discussion of what it refers to as Solar Radiation Modification, or SRM, concluded that “the combined uncertainties, including technological maturity, physical understanding, potential impacts and challenges of governance, constrain the ability to implement SRM in the near future”.
But none of this detracts from the need for research. SCoPEx aims to collect data in order to improve our scientific understanding. The real danger would be for some maverick organisation to go ahead with a large-scale implementation of SAI without that kind of scientific understanding.
“What would be best,” Keith says, “is for countries not to just start doing it, but to articulate clear plans and build in checks and balances”.
The second big objection to SAI research is harder to brush away. Rather than any imagined physical danger it might pose, the worry here is that governments and corporations that are already reluctant to reduce CO2 emissions will latch onto SAI as proof that such reductions are unnecessary.
It’s a situation that could negate any benefits that SAI might offer, and one that the Harvard scientists are doing their utmost to avoid. “We really do need to reduce emissions to zero if we want to address climate change,” their program director Lizzie Burns said in 2019. “One of the biggest taboos around this technology has been the fear that interests vested in fossil fuels and other processes would benefit from the world slowing down the transition to a zero-carbon energy system.”
However, the reality is that even if the SCoPEx mission is successful and SAI eventually proceeds to full implementation, it will only ever be a supplement to CO2 reduction, not a replacement for it. On another occasion, Burns offered a vivid analogy: “It’s like a painkiller. If you need surgery and you take pain medication, it doesn’t mean you no longer need surgery.”