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 Nordic Special

Through a Glass Darkly
Organisms colonize glass in order to extract energy, “eating” metals such as iron or manganese contained within.
By Leslie Mullen

Plucking Daisyworld
On the hypothetical planet Daisyworld, flowers control the climate. Black daisies absorb sunlight and warm the planet.
By Leslie Mullen

Seeing Life in Viruses
We all try to avoid viruses due to the havoc they can wage on our health. Some viruses do more than create temporary discomfort.
By Leslie Mullen

Iceland Brings Astrobiology Down to Earth
If you want to learn about the role of water on Mars and Europa, Iceland is a good place to start.
By Simon Mitton

Mini-Sub for Tight Spaces
The water locked underneath icecaps or glaciers can tell us about our planet's past and its possibly warmer future. Similar environments on distant worlds could tell us whether life can originate in these harsh conditions. To study the icy depths, a Swedish team of researchers is designing a tiny submersible that can slip down a narrow borehole.
By Michael Schirber

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Can a Biosphere be Selfish?

A Question of Climate

Plucking Daisyworld

By Leslie Mullen

On the hypothetical planet Daisyworld, flowers control the climate.  Black daisies absorb sunlight and warm the planet.  The black daisies prefer to grow in cooler conditions, so the steadily increasing temperature kills them off.  White daisies, which prefer warmer weather, start to proliferate.  Since the white daisies reflect heat rather than absorb it, as they spread they cause the climate to cool.  This leads to the fall of the white daisy and the resurgence of the black.  This ecological see-saw can reach a more harmonious state if a planet maintains a balanced ratio of black and white daisies.

Archaea Cell

Daisyworld scenarios.

Daisyworld is a thought experiment spawned to illustrate the Gaia hypothesis.  The idea behind Gaia is that the Earth is a self-sustaining organism, with the Earth’s atmosphere, chemistry and geology working together to maintain an environment suitable for life. 

Peter Ditlevsen, a climatologist at the Niels Bohr Institute at the University of Copenhagen in Denmark, thinks Daisyworld is an interesting idea, but he says it doesn’t make sense as an analogy for the climate of the early Earth.

If you look at the change in the temperature of Earth through geologic time, says Ditlevsen, you have to account for more than just the heat coming from the sun.  During the Hadean era (4.6 to 3.8 billion years ago), the faint young sun was 30 percent less bright than it is today.  The Earth had water back then, and with so little solar radiation available, our planet should have frozen.  Most scientists think the early Earth was not a snowball, however.  One possible reason is that the Earth was kept warm by greenhouses gases such as carbon dioxide and methane. 


The thick atmosphere of Venus traps heats.  Image Credit: European Space Agency.

Our neighbor Venus provides an example of how effectively greenhouses gases can heat a planet.  Venus has a carbon dioxide-rich atmosphere that is almost 100 times more dense than our own. As a result, the surface temperature of Venus exceeds 400 degrees C (800 F) – hotter than the surface of Mercury, which is twice as close to the sun. 

The more traditional models for the climate of the early Earth take into account the effect of an atmosphere, but Ditlevsen says they do not consider the role that life can play in regulating the planet’s climate.

“The usual idea is that the geochemical balance of carbon dioxide is maintained between a source such as volcanism, and a sink such as weathering,” says Ditlevsen.  The addition of a biosphere would provide another sink for carbon dioxide, further altering the climate. 

“Photosynthesizing organisms eat carbon dioxide, and carbon dioxide is a greenhouse gas,” says Ditlevsen.  “The biota changes the climate by changing the atmospheric chemical composition.”

Ditlevsen has created a model for what he says is a more realistic view of the evolution of Earth’s early climate and biosphere.  His model echoes the balancing act of Daisyworld, but instead of having life regulate the amount of solar heating a planet receives, life regulates atmospheric gases.  Photosynthesizing organisms suck up carbon dioxide from the atmosphere, and the overall reduction of carbon dioxide leads to a cooler climate.  Eventually, the climate becomes so cold that the organisms die off.  Without that bioactivity, carbon dioxide levels rise again and lead to global warming. 

Archaea Cell

Life has altered the Earth’s atmosphere in complex ways throughout history.
Image Credit: NASA Goddard Space Flight Center

Ditlevsen says his model applies to the first few billion years of Earth’s history, when carbon dioxide was the major atmospheric gas, and before the sun became hotter and oxygen became such a dominant part of the atmosphere.  The lessons of such a model have applications for modern times, however, especially as humans daily add tons of carbon dioxide to the atmosphere through the burning of fossil fuels.  In our case, biological (human) activity is resulting in an increase rather than a decrease in atmospheric carbon dioxide.

This increase of carbon dioxide, and the inevitable global warming that will result from it, would not be the end for life on Earth, says Ditlevsen, but it will present major challenges to human civilization, “limiting sustainability for people in many parts of the globe."

“The problem is not that there is warming, it’s that it is happening at a speed that we cannot accommodate to,” says Ditlevsen. 

Some may look at Gaia to save us, hoping that planetary feedback mechanisms will eventually kick in and scrub all that carbon dioxide out of the atmosphere.  Ditlevsen agrees that such processes will occur, but only on very long timescales. 

“The carbon dioxide that we release into the atmosphere will eventually go away,” says Ditlevsen.  “It will become fixed in the oceans or the biosphere and so on.  This is something that will take a long time, so global warming is something we need to be concerned about for our own sake.”

By Leslie Mullen