The following reviews the scientific evidence of human impacts to demonstrate how humans are not only capable of global impact but have already transformed most Earth systems. The review draws on a recent (paywalled) article in Science.
One important point relates to timing: although the global geologic record contains evidence of some human activities like lead smelting and cement production since several thousand years ago, the presence of these and other human materials in the geologic record dramatically increases in the 1900s with the advent of organizational forms like the modern corporation and the adoption of industrial production processes.
The following types of evidence demonstrate how human activities have and continue to change Earth at a global level:
- new human-made materials found in the archaeological record
- altered processes of sedimentation
- new chemicals in ice and soil
- radioactive material in ice and soil
- changes in CO2 and methane concentrations
- climate change
New human-made materials
“Technofossils” are human-made materials that are found in the archaeological record. They include materials up to several thousand years old like pottery, glass, cement and metal alloys but also include recently created materials like plastics, highly-refined aluminum, and synthetic fibers.
The production of technofossils has accelerated in the 1900s, with growth rates becoming exponential around 1960 for some materials like concrete, plastics, aluminum, and synthetic fibers. For example, although concrete was invented centuries ago, half of the concrete humans have ever produced was produced since 1995.
Altered sedimentation processes
Sedimentation processes are the ways in which soil moves across the Earth. In order to create new human-made materials, humans have constructed resource extraction industries that have altered natural geologic processes and now reflect another aspect of human impact on Earth. By itself, the mining of minerals moves nearly 3 times as much sediment per year as all the rivers on Earth combined.
In addition to mining, human activities that alter sedimentation include landfills, the construction of cities, coastal reclamation projects, trawler fishing, deforestation, livestock grazing, and cropland development.
Of particular note is dam construction. Since about 1950, humans have built 1 large dam per day, each of which has a lifespan of 50 – 200 years. Dams interrupt the transport of sediment by rivers, changing the nature of river systems by depriving downstream environs of sediment. Deltas are sinking faster than in the past, and coastlines are retreating. Such processes threaten coastal settlements in delta regions like New Orleans, Cairo, and residents of Bangladesh.
Despite dams slowing the transport of sediment to lakes and oceans, chemical fertilizers used in agriculture flow through water. Delta regions thus face two major effects: loss of sediment and an overload of plant fertilizer. The former causes the loss of land while the latter causes the loss of species diversity in the water by creating “dead zones” where decomposing algae remove oxygen from enormous areas of lakes and oceans.
Chemicals in ice and soil
Chemicals that travel by air can be find everywhere on Earth’s surface, including places that are permanently covered with ice. Of particular note are polyaromatic hydrocarbons and polychlorinated biphenyls. These materials appear in the geologic record around 1945, immediately after their invention by materials scientists and chemists.
Human smelting of lead has occurred for centuries, but the amount of lead present in soils dramatically increased in the middle of the 1900s with the global burning of leaded gasoline.
Increased fertilizer use has doubled the amount of nitrogen and phosphorus in soils since 1900 and disrupted oxygen environments in lakes.
Industrial production and consumption of metals like cadmium, chromium, copper, mercury, nickel, lead, and zinc have left traces of these metals in soils across the globe. The adoption of platinum-filled catalytic converters in automobiles as an air-pollution reduction solution has increased the presence of platinum, rhodium, and palladium in soils next to roads.
Radioactive material in ice and soil
Nuclear weapons testing in the mid-1900s disbursed radioactive plutonium across the Earth. This plutonium will be identifiable in the geologic record for the next 100,000 years, eventually decaying into uranium, which will in turn decay into lead.
Changes in CO2 and methane concentrations
Burning of fossil fuels has changed the concentration of carbon dioxide (CO2) and methane in the Earth’s atmosphere. Increases in the amount of these gases in the atmosphere has increased the capacity of the atmosphere to capture heat energy from the sun, increasing global temperatures. Because CO2 from fossil fuels has a different chemical signature than CO2 produced through natural processes, there is high confidence that increased atmospheric CO2 is primarily from human burning of fossil fuels.
The most dramatic atmospheric change has been the increase in CO2 concentration since about 1850. Atmospheric CO2 has increased by approximately 30% in that time period.
Increased atmospheric CO2, methane, and other greenhouse gases have contributed to changes in the Earth’s climate. Geologic records show large changes in climate have happened in the past–such as ice ages–but climate change has never before been directly linked to human activity as it is today. Further, climate is changing more rapidly due to human processes than did past periods of natural climate change. By the year 2070 Earth’s climate will likely be hotter than at any time since the emergence of modern humans some 200,000 years ago, posing potentially intractable adaptation problems for our species.
Another major consequence of climate change is rising sea levels. Coastal ecosystems like the Everglades, cities like New Orleans, and countries like the Seychelles and Bangladesh face severe harm and possible extinction from rising sea level. It will be difficult for these places to successfully adapt to sea level change on the expected order of 3-5 feet in the next 80 years and even more in the next centuries.
The evidence presented above provides compelling evidence that human activity in the 1900s achieved a scale that had and continues to have global environmental impacts.