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We surmise that during the first stage, our ancestors were able to interact safely with fire; in other words, instead of simply running from it, they had become familiar with how it works. To get a deeper understanding of this stage, we can look to research done on chimpanzees—our closest living relatives—by Jill Pruetz, a primatologist at Iowa State University, who has studied chimps’ interaction with wildfires in West Africa. Pruetz has found that chimps clearly understand the behavior of fire enough to have lost the fear of it that most animals typically possess. In fact, Pruetz has observed chimps monitoring the progress of a passing wildfire from a few meters away and then moving in to forage in the burned-out area. So while chimps cannot build or contain fires, they understand how fire moves across the landscape, and they use this knowledge to their benefit. It is not hard to imagine a similar scenario playing out among small groups of our own early ancestors, perhaps the australopithecines, who lived from around 4 million years ago until about 2 million years ago in East Africa. The first stage may have persisted throughout much of prehistory.

The second stage would be when people could actually control fire—meaning they could capture it, contain it, and supply it with fuel to keep it going within their living areas—but they were still obtaining it from natural sources like forest fires. It is difficult to establish when this stage occurred, for a couple of reasons. One is that some claims for very old fires were simply incorrect. For example, at the famous Chinese site Zhoukoudian, what were originally thought to be the remains of 700,000-year-old Homo erectus fires turned out to be natural sediments resembling charcoal and ash.

Second, and perhaps most crucial, is that some of the earliest fire residues have been found in open-air settings—not inside caves—and consist of isolated fragments, small scatters of burned bones, or patches of discolored sediments. While it is possible that these residues are the remains of hominin campfires, it is equally possible, if not probable, that they were produced by naturally occurring wildfires. Every year, lightning causes tens of thousands of wildfires across Africa, Asia, and Europe. In the past, some of these would have burned the remains of hominin camps, including bones, stone tools, and sediments. In such cases, the fire residues have nothing to do with hominin occupation of the sites.

During the final stage, humans learned how to make fire, but again, we are not yet sure when this happened. Starting about 400,000 years ago, we begin finding much better evidence for human-controlled fire, such as intact campfires, or “hearths,” that contain concentrations of charcoal and ash inside caves, where natural fires don’t burn. Furthermore, the number of sites with such evidence increases dramatically. So it is clear that by this time, some hominins in some regions could manage fire and thereby control it, but whether they could make it remains an open question.

Between 2000 and 2010, our research team—made up of three Paleolithic archaeologists who focus on stone tool technology and two geoarchaeologists who study how archaeological sites form—excavated two Middle Paleolithic sites, Pech de l’Azé IV and Roc de Marsal, in the Périgord region of southwestern France. Pech IV and Roc de Marsal are caves that were regularly used as campsites by small groups of Neanderthals from 100,000 to 40,000 years ago, which is about when Homo sapiens, modern humans, arrived in Europe.

The burning of fossil fuels refers to the burning of oil, natural gas, and coal to generate energy. We use this energy to generate electricity, and to power transportation (for example, cars and planes) and industrial processes. Ever since the invention of the first coal-fired steam engines of the 1700s, our burning of fossil fuels has steadily increased.  Across the globe each year we now burn over 4,000 times the amount of fossils fuels burnt during 1776.  The effects of the burning of fossil fuels, especially carbon dioxide, are having far-reaching effects on our climate and ecosystems.

The burning of fossil fuels is the primary cause of current climate change, altering the Earth’s ecosystems and causing human and environmental health problems.

Flares burn at sunset in the Bakken oil and gas fields in North Dakota Credit: Jeff Peischl/CIRES and NOAA

Fossil fuels form over millions of years from the burial of photosynthetic organisms, including plants on land (which primarily form coal) and plankton in the oceans (which primarily form oil and natural gas). To grow these organisms removed carbon dioxide from the atmosphere and the ocean, and their burial inhibited the movement of that carbon through the carbon cycle. The burning of this fossil material returns this carbon back into atmosphere as carbon dioxide, at a rate that is hundreds to thousands of times faster than it took to bury, and much faster than can be removed by the carbon cycle.  Thus, the carbon dioxide released from the burning of fossil fuels accumulates in the atmosphere, some of which then dissolves in the ocean causing ocean acidification.

The burning of fossil fuels affects the Earth system in a variety of ways. Some of these ways include:

• Releasing the greenhouse gases carbon dioxide (CO2) and nitrous oxide (N2O) into the atmosphere, which intensifies the greenhouse effect (the re-radiation of heat in the atmosphere), increasing the Earth’s average air temperatures. These greenhouse gases can remain in the atmosphere for decades to hundreds of years.
• Emitting an array of pollutants that reduce air quality and harm life, especially sulfur dioxide, nitrogen oxides, and airborne particles such as soot. Poor air quality can cause respiratory disease.
• The airborne particles also increase the reflectivity of the atmosphere, which has a slight cooling effect. The reason is that the airborne particles, such as soot and sulfate aerosols (from sulfur dioxide), reflect some sunlight back into space, increase cloud formation, and make clouds more reflective. The net effect of burning fossil fuels is warming because the cooling is small compared with the heating caused by the greenhouse effect, in part because airborne particles only stay suspended in the atmosphere for a few days to months, while greenhouse gases that cause warming remain in the atmosphere for many decades to hundreds of years.
• Changing patterns of snow and ice melt. Airborne particles (especially soot) that settle on snow increase the absorption of sunlight due to their dark color, heating the surface of the snow causing melting. In certain parts of the world, the presence of soot (in addition to global warming) has caused winter ice and snow melts earlier and faster today than in previous decades, which also changes local patterns of freshwater availability.
• Increasing the acidity of precipitation. Sulfur dioxide (SO2), nitrogen oxides (NOx), and carbon dioxide (CO2) react with water vapor, oxygen, and other chemicals to form acid rain. Acid rain can contaminate freshwater sources, resulting in harmful algal blooms that reduce water oxygen levels and harm fish populations and other wildlife. Additionally, acid rain increases chemical weathering of rocks, including manmade structures.
• Using large amounts freshwater. Power plants that burn fossil fuels cool their systems by removing freshwater from local rivers and lakes. The warm water returned to nearby ecosystems can cause stress for local species.

Can you think of additional cause and effect relationships between the burning of fossil fuels and other parts of the Earth system?

Visit the greenhouse effect, greenhouse gases, and temperature pages to learn more about how burning fossil fuels affects global climate and ecosystems.

Investigate

Learn more in these real-world examples, and challenge yourself to construct a model that explains the Earth system relationships.