Anyone who has tossed a log on a campfire has witnessed energy changing forms, from dead organic matter to heat.
Ironically, whether you’re using a beam of light through a magnifying glass, tree branches or a splash of gasoline, your campfire’s fuel technically originated with the sun.
The key difference, however, is that all these energy sources came to be through vastly different geological paths, which attaches different consequences to their use.
What Are Fossil Fuels?
In the case of a fossil fuel — typically coal, oil and natural gas — it took tens of millions of years to take its form.
That’s because all three of these fossilized elements were once prehistoric plants or other living organic matter. They grew up absorbing the sun’s energy, then died and began to decay.
While this type of matter would often burn up naturally or decompose near the Earth’s surface — slowly releasing carbon dioxide (CO2) back into the natural carbon cycle — sometimes, and during specific periods, this dead matter got buried.
Whether it was beneath oceans, swamps, soil or another force of nature, sufficient compaction and immense pressure trapped carbon within the Earth’s rocky layers.
Combined with heat and millions of years, this turned the fossilized carbon into oil, natural gas and coal, sequestered underground.
Like any precious commodity that requires aging, there is a limited supply of these biological resources — not to mention dire threats if they are misused.
This stands in contrast to renewable fuels like solar or wind, which are generated more directly from the sun or other abundant natural elements.
To gain a sense of the span of time and unique circumstances behind fossil fuels, just consider the ancient origins of the majority of the world’s coal.
Read More: How Ancient Forests Formed Coal and Fueled Life as We Know It
300 Million Years in the Making
Tens of millions of years before the dinosaurs, dense rainforests of towering trees and swamp vegetation covered much of Earth’s landmass.
Modern scientists have aptly named this span of time (between 359 and 299 million years ago) the Carboniferous period.
With the abundance of prehistoric plant life, atmospheric oxygen levels measured around 35 percent, compared to 21 percent today. Centipedes grew up to six feet long. And the wingspans of pigeon-sized dragonflies exceeded two feet.
Then, relatively suddenly (in geological terms), a series of events caused a major die-off that began roughly 300 million years ago.
This was shortly before tectonic activity shifted the planet’s land and oceans to form the Pangea supercontinent, which later broke up into our modern continents.
Read More: The Next Pangea: What Earth’s Future Supercontinent Will Look Like
Releasing Trapped Carbon
When the rainforests collapsed, it created an unfathomable amount of dead organic matter, much of which settled beneath swamps, oceans and soil.
As landmasses shifted, and pressure and heat accumulated, this decaying matter was sealed off from the air above. Trapped underground in rock, many of those carbon stores gradually took the form of coal.
Today, more than 300 million years after the minor extinction event, this buried carbon is being released at a rapid rate as humans extract the resource and burn coal for energy. The specific coal beds from the Carboniferous period still make up the primary source of coal throughout the world.
Other stores of fossilized crude oil and natural gas came about from similar geological events that occurred at least 60 million years ago and prior.
Read More: The 5 Mass Extinctions That Have Swept Our Planet
Biggest Contributor to Climate Change
The problem is that life above the surface on Earth is not accustomed to so much CO2 circulating in the atmosphere.
Though humans have been mining and burning fossil fuels on the industrial scale for only a couple hundred years, recent climate data reveals what NASA calls a sudden and “relentless rise of carbon dioxide,” which is a top contributor to climate change.
Put another way: “The annual rate of increase in atmospheric CO2 over the past 60 years is about 100 times faster than previous natural increases,” including tracking back to the end of the last Ice Age, according to NOAA Global Monitoring lab.
This is now a known contributor to what some researchers are calling the sixth mass extinction, currently unfolding on Earth.
Read More: How to Save Planet Earth
Narrowly Avoided 'Snowball Earth State'
One 2017 study published in Earth, Atmospheric and Planetary Sciences underscores the dramatic effect of keeping carbon stored — rather than releasing it into the atmosphere.
That work used multiple climate models to take a look at the end of the Carboniferous period and read the atmospheric levels during that transition.
Essentially, the researchers found a reverse greenhouse effect, where a significant drawdown of atmospheric carbon nearly led to “a fully glaciated snowball Earth state.”
“Ironically, the formation of the majority of these coal reserves during the late Palaeozoic [which includes the Carboniferous period] brought the Earth close to the threshold of global glaciation,” the authors wrote in the research article.
Atmospheric CO2 Levels
Their calculations estimate that atmospheric CO2 levels measured at less than 180 parts per million immediately after the Carboniferous period.
By contrast, Earth’s steadily climbing levels today are above 420 ppm, which surpasses any other mark in recorded history.
With this type of data on the books, the U.S. Environmental Protection Agency now recognizes that the burning of fossil fuels is the country’s largest source of human-caused greenhouse gas emissions, making transition to alternative energy sources a high priority.