The notion that human activity, or the activity of any one organism, can affect Earth on a planetary scale is still a hard one for many people to swallow. And it is this kind of disbelief that fuels much of the public skepticism surrounding global warming.
But in a meeting this week in Paris, officials from 113 nations have agreed that a highly anticipated international report will state that global warming was "very likely'' caused by human activity.
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The idea that biology can alter the planet in broad and dramatic ways is widely accepted among scientists, and they point to several precedents throughout the history of life.
The mighty microbes
Human-caused global warming — also called “anthropogenic” global warming — is the latest example of life altering Earth, but it is not the most dramatic.
That title probably goes to the oxygenation of Earth’s early atmosphere by ancient microbes as they began to harness the power of sunlight through photosynthesis.
Humans “are having a strong effect on global geochemical cycles, but it does not compare at all to the advent of oxygenic photosynthesis,” said Katrina Edwards, a geo-microbiologist at the University of Southern California. “That was a catastrophic environmental change that occurred before 2.2 billion years ago [which] wreaked its full wrath on the Earth system.”
Edwards studies another way life impacts the planet in largely unseen ways. She focuses on how microbes living on the murky ocean floor transform minerals through a kind of underwater alchemy.
For example, microbes facilitate a chemical process called oxidation, whereby oxygen in sea water combines with magma oozing up from the ocean floor to change, for example, one form of iron into another.
“These [microbes] are completely off radar in terms of global biogeochemical cycles,” Edwards told LiveScience. "We don't consider them as part of the Earth system right now in our calculation about what's going on, and we don't consider them in terms of how the Earth system will move forward into the future."
These reactions are strongly influenced by life and have been occurring for billions of years, for as long as the oceans have been oxygenated and there have been microbes inhabiting the seafloor, Edwards said.
On land, microbes, and in particular a form of bacteria called cyanobacteria, help keep soil in place and suppress dust.
“We’d certainly have way more dust storms, and it would not be anywhere as nice on Earth, if they weren’t around,” said Jayne Belnap, a researcher with the United States Geological Survey.
Scientists believe the tiny critters performed the same roles on early Earth.
“One of the big conundrums for geologists is that, OK, you have this big ball of rock, the soil is weathering out and you have these ferocious winds. What in the world is holding the soil in place as it weathers out of the rocks?” Belnap said in a telephone interview. “Cyanobacteria are also credited with that function.”
The microbes anchored soil to the ground; this created habitats for land plants to evolve and eventually for us to evolve.
“They literally created Earth in a sense,” Belnap said.
“Cyanobacteria are just like ‘it,’” she continued. “I’ve been telling everybody to make a small altar and offer sacrifices every night. We owe them everything.”
A snowball planet
The mighty microbes also triggered sudden climatic shifts similar to what humans are doing now.
Recent studies suggest that the proliferation of cyanobacteria 2.3 billion years ago led to a sudden ice age and the creation of a “Snowball Earth.”
As they carry out photosynthesis, cyanobacteria break apart water and release oxygen as a waste product.
Oxygen is one of the most reactive elements around, and its release into the atmosphere in large amounts destroyed methane, a greenhouse gas that absorbed the sun's energy and helped keep our planet warm.
Some scientists think the disappearance of this methane blanket plunged the planet into a cold spell so severe that Earth’s equator was covered by a mile-thick layer of ice.
Earth might still be frozen today if not for the appearance of new life forms.
As organisms evolved, many developed the ability to breathe oxygen. In the process, they exhaled another greenhouse gas, carbon dioxide, which eventually thawed out the world.
That was the first biologically triggered ice age, but others followed, said Richard Kopp, a California Institute of Technology researcher who helped piece together the Snowball Earth scenario.
A new leaf
When trees first appeared about 380 million years ago, they also disturbed Earth’s atmospheric equilibrium.
Unlike animals, plants breathe in carbon dioxide and expel oxygen.
Trees transform some of that atmospheric carbon into lignin — the major constituent of wood and one of the most abundant proteins on the planet.
Lignin is resistant to decay, so when a tree dies, much of its carbon becomes buried instead of released back into the atmosphere as carbon dioxide.
Less carbon dioxide in the atmosphere thins the blanket of gases that keeps Earth warm, and that cooling effect can trigger global cooling, possibly even an ice age.
“There was some glaciation that started around that period that was driven, at least in part, by the evolution of land plants,” Kopp said in a telephone interview.
Trees also affected the global carbon cycle in another indirect way. As they tunnel through the ground, tree roots break down silicate rocks into sediment and soil.
Silicate rock contains large amounts of calcium and magnesium. When these elements are exposed to air, they react with atmospheric carbon dioxide to form calcium carbonate and magnesium carbonate, compounds that are widespread on Earth.
The human difference
Though it might seem as if humans are mere fleas along for a ride on the back of an immense animal called Earth, our intelligence, technology and sheer numbers mean our species packs a punch that can shake the world in wild ways.
While we are not the first species to drastically alter our planet, our influence is unique in a number of ways, scientists say.
For one thing, humans have developed large-scale industry, said Spencer Weart, a science historian at the American Institute of Physics.
“We are capable of mobilizing things beyond our own biology,” Weart said. “I emit a certain amount of carbon dioxide, but my automobile emits far more.”
Another is the rate at which humans are warming Earth.
“Humans are the most common large animal to ever walk the planet,” said Kirk Johnson, a chief curator at the Denver Museum of Nature & Science. “Population, plus brain power and technology, is a potent combination and the result is that humans are effecting change at very high rates.”
Belnap agrees. “I don’t think we’ve fundamentally changed any process. We’ve just cranked up the speed,” she said. “We haven’t introduced anything new. We’ve just changed how fast or slow it happens, and mostly fast.”
But no matter how high humans cause the mercury to rise and how much damage we do to the planet, Earth and life will survive, scientists say.
It just might no longer be in the form we prefer, or the form that allows us to thrive.
“What we need to be thinking of as humans causing changes to the Earth system is what the consequences will be to us human beings,” said Edwards, the USC geo-microbiologist. “The Earth could care less. We will be recorded as a minor perturbation in the Earth system. The Earth will go on. The question is: Will we?”
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