Clouds and Climate Change

New research on cirrus clouds from an international research team, including Purdue University scientists, could help make climate change models more accurate.
November 3, 2020

“I bring fresh showers for the thirsting flowers, /From the seas and the streams;/I bear light shade for the leaves when laid/In their noonday dream.”

A poem by 19th-century English poet Percy Bysshe Shelley captures what most of us know about the clouds we see in our skies.

But the same clouds that have helped Indiana feed the world could also be speaking volumes about the effect our actions have on the earth’s climate.

New research from an international team of scientists, including from Purdue University, provides insights into how natural and human-made particles affect precipitation and climate change.

Professor Daniel Cziczo, head of Purdue’s Department of Earth, Atmospheric and Planetary Sciences, was part of the team that looked into how emissions from trees, plants and other organic materials affect the creation of cirrus clouds.

Cirrus clouds are made of ice and look like wispy strands of hair. They are found in the highest level of the troposphere.

“I think a lot of folks have heard about global warming, and the thing that we’re all trained to think about is we’re putting these gases into the atmosphere like carbon dioxide, and they act as this blanket. They trap heat and the earth gets warmer,” said Cziczo. “That’s all true, and scientists have been doing that research for over a hundred years now, so it’s pretty well known. But one of the things that people don’t realize is that we, as humans, are also putting particles into the atmosphere that act as the seeds on which clouds form.”

Cziczo said particles come from natural and human-made sources. Depending on how they reach the atmosphere, they’re known as primary and secondary particles.

Primary particles are released directly into the atmosphere through processes like burning or combustion. They can originate from natural sources like forest fires and dust storms, or from human-caused sources like the burning of coal and other fossil fuels.

Secondary particles are created when gases released into the atmosphere react with other chemicals there to transform into particles.

Cziczo joined scientists from other American universities, including the Massachusetts Institute of Technology, and from French and German universities to research how secondary organic aerosols, or secondary particles from natural sources, affect the formation of clouds.

He said humans have the ability to affect cloud formation through particle and gas emissions. More particles can lead to the formation of ice, which can have a cooling effect. Although fossil fuels and carbon dioxide emissions cause warming, Cziczo said it’s important to account for this cooling mechanism also.

“If we’re going to get our projections for climate right, then we have to understand how we’re changing particles, how we’re changing clouds,” he said.

The researchers focused primarily on cirrus clouds, the wispy, hair-like clouds found high in our region of the atmosphere.

Cirrus clouds are made up of ice crystals, but can trap some heat.

“That’s actually when they become important for climate. They’re really the only cloud type that we know of that tends to have more of a warming effect than a cooling effect,” said Cziczo. “They act like a little bit more like a blanket than they do like a sun shade, and so understanding cirrus, where they are, how abundant they are, how they’re changing, is really critical for understanding Earth’s climate.”

The team collected data at an observatory atop the Puy-de-Dôme, an extinct volcano in central France. They began seeing more of the particles that can form cirrus clouds in cases where there was a high abundance of secondary organic particles.

Cziczo and MIT researchers created secondary organic particles in a laboratory chamber to test the field findings.

They fed the particles into a cloud chamber, a laboratory device that can be used to mimic the atmosphere and basically form clouds in a lab. The team found that the particles they used to look like secondary organic particles in the atmosphere were creating ice.

“We realized that what we were seeing in the field was really cause and effect. It wasn’t just two observations, and so that led us to do some of the data analysis and start considering how important this effect could be,” said Cziczo. “That’s when we got to the paper stage and realized that this was a sort of previously unrealized way of making these ice crystals in the atmosphere.”

The finding will help scientists understand more about how human activities like deforestation, urbanization and reforestation could influence cloud formation, which, in turn, affects where and how it rains.

Climate change has already affected how people live and work in Indiana.

Over the last century, the average annual temperature for the state has warmed by 1.2 degrees Fahrenheit and is projected to rise from 5 to 6 degrees more by mid-century.

The average annual precipitation in the state has increased 5.6 inches since 1895, and it is falling in heavier downpours. That change will increase flood risks, potentially polluting water as combined sewer systems overflow.

Those and other changes are caused by human activity, a vast majority of which is due to the emission of greenhouse gases like carbon dioxide and methane.

Cziczo said scientists have studied greenhouse gases and global warming for more than a century, but there is still more to learn.

“What we’re still trying to do as scientists is become more certain about what we know. We think we’ve got all of the building blocks, but getting the story exactly perfect, making sure we understand where the uncertainty is and being able to project it into the future, that’s what we’re really trying to do,” he said. “Clouds are this aspect of climate change that’s less well understood than greenhouse gases, so if we’re going to improve our forecast and our projections of climate in the year 2050 or 2100, this is one of the areas that we really have to study more.”

Cziczo said that besides the applications on Earth, the knowledge gained from this and other research can be applied to other planets in the solar system and beyond.

Clouds and Climate Change