Science Writing

Why do leaves change color in the fall?


First, we must understand a bit about photosynthesis and chlorophyll. During the summer, the leaves use sunlight to make its food. Chlorophyll, which gives leaves its green color, absorbs the sunlight, which is used to turn water and carbon dioxide into sugars. As the days get shorter and the nights colder, trees stop their food-making process and the chlorophyll breaks down. What’s left behind are the other leaf pigments, like yellow, orange and red.


When I was in Telluride a month ago (where fall comes about six weeks earlier than the Front Range) I noticed the aspens on the hillsides were not that bright. They seemed drab. Kind of blah compared to previous years. Why do some year’s colors seem more vibrant than others? According to the Colorado State Forest Service, a wet growing season with a dry sunny fall with cool (but not freezing) nights are the best conditions for brilliant fall colors. Telluride had a wet late summer and early fall. Maybe that had something to do with it?

It’s a bird! It’s a plane! It’s a … UFO?


No, it’s a lenticular cloud. This stacked lens or saucer-shaped formation appeared over the foothills west of Boulder early the morning of Tuesday, Oct. 17. Illuminated by a spectacular sunrise, this particular lenticular even made the Boulder Daily Camera. 

Lenticular clouds are formed when air moves over mountains and cooling (and therefore condensation) take place. These clouds don’t move with the breeze like regular clouds, but hang out in the same place as they are continually reformed by the rising air. They usually form on the downwind side of a mountain (like Boulder!)

Hot air: NOAA’s Pieter Tans is certain about CO2


Although his ultimate boss, the President of the United States, might deny it, a Boulder climate scientist has no doubts at all that humans are causing climate change.

Pieter Tans, chief of the Carbon Cycle Greenhouse Gases Group at Boulder’s National Oceanic and Atmospheric Administration has a three-part message for climate change doubters and deniers. Human actions are 100 percent responsible for the increase in greenhouse gases. Greenhouse gases retain in the atmosphere 100 times more energy than all the energy humans produce. And some greenhouses gases will remain in the atmosphere for thousands of years, even after emissions stop.

“More CO2 will cause more warming – it’s a fact,” Tans said.


Tans can say these things with such certainty because he has spent his career measuring and analyzing greenhouse gases (primarily carbon dioxide, but also methane, water vapor and others) in the atmosphere. The Boulder lab is also home to a global reference network, which monitors air samples from around the world. The lab calibrates all the air collection canisters and measuring devices, which are subject to stringent quality control methods.


In 1972 Pieter Tans was a physics student in the Netherlands. Reading a book on humans’ impact on climate was enough to make him change his studies to earth science. More than 40 years later, Tans still cites that experience as life-changing.

“That was my conversion,” he said. “I’ve never doubted since then that mankind had a big impact on climate.”

Tans says he and his colleagues have three main goals. First is to meticulously document their methods and findings as a defense against climate change deniers.

“We have nothing to hide,” Tans said.

Second is to understand where greenhouse gases come from, where they are absorbed and how they cause the climate to respond.

And lastly, to help policy makers from different countries understand how effective their efforts are (or are not) at reducing greenhouse gas emissions.

“We need to keep more records than we did before,” Tans said. “We see that as our contribution we can make to get the world off carbon fuels.”

There is another thing Tans knows for sure: A particular sub type of carbon, known as isotope carbon 14, can enable scientists to much more accurately pinpoint the sources of CO2 from fossil fuel burning. And that could help cities and countries know whether they are meeting emissions reduction goals.

Tans’ number one scientific goal is to add carbon 14 measuring as a standard component of the global reference network. This could provide a number of additional valuable benefits, he says. This isotope, decays pretty quickly over time. Half will decay in 5,730 years. This means all the prehistoric dead plant matter we burn as fossil fuels does not contain any carbon 14 – because it has decayed and is long gone. As a result, the carbon in the CO2 produced from the burning of those fossil fuels consists of other isotopes, not carbon 14.

The upshot: If CO2 in the atmosphere is increasing due to emissions from fossil fuel burning, then we should see a decrease in the ration of carbon 14 to overall carbon. According to Tans, this is, in fact, just what’s happening. But a rigorous monitoring program would provide much more information.

It would make fossil fuel CO2 measurable and in Tans’ words “visible” in a sense. Among other things, scientists could monitor trends of emissions of fossil fuel CO2 from cities and states that have set reduction targets and thereby confirm whether they are meeting those goals.

The carbon 14 monitoring plan has been in the lab’s requested budget since 2012, Tans said. But Congress has always removed the funding. According to Tans, it would cost about $5 million a year, a seemingly paltry sum in the scope of the federal budget.

Tans is sure about many climate-related things, but he refuses to speculate about climate change impacts, saying these kinds of predictions are a form of “hubris.” He prefers to stick to the facts. For example, for every one-degree Celsius increase, the air holds seven percent more water vapor, creating the potential for stronger storms with more rain. Facts like it will take the atmosphere 100,000 years to return to the way it was before humans.

But these things don’t get him down.

“This is very painful for our future,” Tans said. “But I’m not really discouraged. I’ll just do the best I can.”

A skiable laccolith


I snapped this picture of Mount Crested Butte from my campsite in Washington Gulch. Yes, this was a beautiful sunset. But that’s not the point. See how the mountain (which kind of stands alone and is home to 1,547 skiable acres) is a pointy, dome-shape? That’s a laccolith. And Colorado has a lot of them.

Laccoliths are formed when magma intrudes up into sedimentary rock layers and creates a mountain with a domed top and flat bottom. According to the Colorado Geological Survey, the laccolith triangle extends from Carbondale in the north to Crested Butte in the south and is one of the best places to view the unique geologic formations. Other well-known mountains in the area are also laccoliths: Mount Sopris, Marcellina Mountain and Gothic Mountain to name a few.

The area around these laccoliths is also known for its high-grade coal. The heat from the intrusions baked the bituminous coal into anthracite coal. (Crested Butte was founded as a coal mining town!)

Study finds traffic is main cause of small airborne particles that contribute to climate change

A recent study has found that urban traffic is a major source of the smallest airborne aerosol particles, which could pose health risks and are a factor in human causes of climate change.

Up until now, the source of these tiny particles, known as nanocluster aerosol particles or NCAs, has not been studied extensively. It was thought that the natural atmosphere, (namely, cloud formation) was a primary source of these particles. But the study found that in urban air, these tiny aerosol particles make up a significant part of the total number of particles and are a direct result of traffic emissions.

Researchers from Tampere University of Technology in Finland conducted the study, which took place in Helsinki and other parts of Europe. Scientists measured the particles in three locations: next to a road, in a street canyon and with a mobile laboratory — a van that took measurements as it drove on highways from northern Spain, through France, Luxembourg, Belgium, the The Netherlands, Germany, Denmark, Sweden and to Finland via ferry.

The highest concentrations of NCAs were found in tunnels and the concentrations matched the daily traffic patterns at the measurement sites. The study also implies that atmospheric processes, like cloud formation, are not necessary for the formation of a large number of NCAs.

These findings help to understand the atmospheric processes that affect climate change, urban air quality and public health. The full text of the study can be found at