Submitted by DRI

Disastrous wildfires and prolonged droughts; stronger hurricanes and cyclones; severe flooding and sea level rise: These are just some of the impacts of a warming climate occurring all around us. 

Scientists warn that climate change will continue to increase the frequency and intensity of extreme weather events, with impacts ranging from flooded communities and ocean acidification to food shortages.   

To prevent the worst climate change impacts, experts say that humanity needs to rein in global carbon emissions and limit planetary warming to 1.5 degrees Celsius above pre-industrial levels. 

Crossing this seemingly minor threshold could mean a world without coral reefs, a meter in sea-level rise, and frequent ice-less summers in the Arctic. More than 100 countries have committed to this goal by signing the international climate compact, the Paris Agreement, but studies now show that this threshold is likely to be surpassed within the next decade.

With governments struggling to effectively reduce emissions of planet-warming greenhouse gases, some scientists have proposed exploring ways to actively cool the planet. Sometimes referred to as “geoengineering” or “climate engineering,” these methods seek to enhance the atmosphere’s natural ability to reflect solar radiation back into space, remove greenhouse gases from the atmosphere, or release some of the energy heating up the planet.  

“Once we reach the threshold laid out in the Paris Agreement, we’re in a more dangerous climate situation,” said David Mitchell, an atmospheric scientist at DRI. “There are feedback cycles in the climate system that could potentially guarantee the world will get warmer, and if so, it would be hard to get back to a more stable climate at that point. That’s the rationale for climate intervention research.” 

Mitchell is part of a team of DRI researchers examining the potential for climate intervention strategies to reduce global warming impacts. 

Cirrus cloud thinning

Cirrus-cloud thinning (CCT) is an idea first proposed in a 2009 ground-breaking study published in Environmental Research Letters. Cirrus clouds are wispy, high-altitude clouds that occur around the world. Unlike other clouds, they have an overall warming effect on the climate, trapping thermal radiation near the surface that outweighs the solar radiation they reflect.  

Thinning the clouds would allow more heat to escape Earth’s atmosphere, like cracking the windows in a car on a sunny day. This involves using cloud-seeding strategies much like the ones already commonly used around the world to increase precipitation, whereby dust is released into targeted clouds to facilitate ice formation. 

These dust particles form the nucleus of an ice crystal in a cloud, which can pull together tiny water droplets to form larger, heavier ice crystals that gravity can bring to Earth’s surface. This is the natural phenomenon that produces all precipitation, but for decades, scientists have used this process to supplement natural rain and snowfall. By converting some of the moisture in the cloud into precipitation, the remaining cloud is thinner, trapping less heat below. 

“The idea is that you would seed the Arctic when the sun angle is low in the sky, during the winter and fall,” Mitchell said. “Then, when the sun angle reaches a certain threshold, you would switch to seeding in the southern hemisphere, where it’s fall, and repeat this process.”  

The proposal is far from being ready for implementation, as it would require a massive commitment of time and money from societies around the world. Clouds in the polar regions would need to be seeded continuously during the darkest months, using airplanes or drones to distribute the dust particles to targeted regions. 

This method also doesn’t address the problem of growing amounts of carbon dioxide in the Earth’s oceans, which makes the water more acidic and creates challenges for sea life. Despite these obstacles, Mitchell said that recent scientific and technological advances have enabled him to understand the potential for CCT better than ever.  

“Reducing greenhouse gas emissions is still the number one most important thing to do,” he added. “But a lot of people have a misunderstanding, because even if we get to zero carbon emissions now, that won’t cool the planet. It will just keep things from getting worse.”  

Reducing heat in urban communities 

Cities across the world experience enhanced warming because of the way man made surfaces, like asphalt, pavement and buildings, absorb and radiate heat. 

Known as the “urban heat-island effect,” this warming is particularly acute in southwestern cities such as Las Vegas and Phoenix, creating hazardous conditions for everyone, but particularly for outdoor workers. Altering urban surfaces to reflect, rather than absorb, the sun’s energy could help reduce local temperatures.  

A subset of the research team led by DRI climatologist John Mejia is assessing the ability of lighter-colored, reflective rooftops and pavement to cool urban communities, as well as how this cooling could bring wider benefits. 

Conventional roofs can reach temperatures of 150°F on a summer day, trapping and radiating heat into the home beneath. The cumulative effect of heated roofs can also increase air temperatures in the community. According to the U.S. Department of Energy, reflective roofs can stay 50°F cooler under the same conditions.  

Moving forward 

DRI research will help answer questions about how each of these climate interventions could work, helping to prepare humanity for an uncertain future.  

 “All … of these research areas are interlinked,” said Naresh Kumar, DRI’s executive director of atmospheric science, “because they all deal with climate intervention. Cirrus cloud-thinning is on a more global scale, whereas cloud-seeding and heat island impacts are very local, but they are interrelated. 

“Because there are a lot of things that are still unknown about these potential climate mitigation strategies, we wanted to combine them into a single project to investigate some of the remaining questions.”