Harrow and Hillingdon Geological Society
Dr Dave Waltham
Dept of Geology, Royal Holloway College .
The Earth is possibly unique within the universe. Its climate has changed very little over 4 billion years and it is this climatic stability, with an average global temperature of 15º C today that has made it an abode for intelligent life forms. Three features affect the climate:
1. Brightness of the sun
2. Reflection from the Earth
3. Production of green house gases.
Solar luminosity has become 40% brighter in 4 billion years but the Sun has expanded slightly by about 15% and hence there has been very little change in actual temperature. The Earth’s main reflectivity is about 10% from the sea, 70% from ice or clouds and so there is a lot of scope for changes in this.
If ice had covered the early Earth and if there had been no greenhouse effect the temperature would have been -90ºC whereas a modern Sun plus an atmosphere similar to that of Venus would generate temperatures up to 400ºC. So variations COULD have been 100s of ºC. However, the temperature on Earth has actually varied only by about 10ºC. Why has the temperature change been so small? The Sun has been emitting more heat, but the concentrations of CO 2 and CH 4 have been falling therefore maintaining the equilibrium. Carbon dioxide plus rain, falls as acid rain on to volcanic rocks to produce clay, sand, and carbonate which are washed into the sea where it forms limestone. Hence, CO 2 from the air is locked in the limestone and this is temperature dependant. When the Earth is hot, CO 2 is sucked out quickly but when temperatures are lower the process is slower. This is known as weathering feedback. However, the land area has increased and this plus evolution of land plants has improved the weathering efficiency. As a result, the average temperature has fallen from perhaps 80ºC 4 billion years ago to only 15ºC today, concluding that fast land growth produces temperature decrease.
Our climate has become more stable in last 0.5 billion years. Earlier fluctuations were at least 20ºC but why have these fluctuations dropped?
Gaia or Goldilocks?
Global Warming and Life
Imagine a time when: 1. CO 2 concentration is doubled
2. Average temperatures have risen by 8ºC
3. Polar temperatures are 20ºC and all polar ice has melted.
Methane is released from the sea floor and the permafrost consumes the oxygen. Ocean currents have stagnated and rainforests disappeared. 75% of all land life and 95% of marine life are extinct. This is not a current prediction but actually occurred in the Permo-triassic extinction which was caused by a temperature rise of just 8ºC.
There are extremophiles, organisms which exist in a range of temperatures from -60ºC to +120ºC, but all are micro-organisms. Large complex life forms prefer temperatures from -10ºC to +40ºC. Biodiversity is also greatest in stable environments like rainforests and ocean floors.
A more variable climate = NO people.
The Earth’s morphology = different climate but why?
Gaia – James Lovelock – Life itself maintains an optimal environment for life.
Goldilocks – The visible universe contains billions and billions of stars but most planets are too hot or too cold to support life. Humans could only evolve on a “lucky” planet where by chance conditions were suitable.
Gaia – a complex biosphere produces climate stability.
Goldilocks –climate stability is necessary for a complex biosphere.
Has Gaia gone wrong?
Oxygen levels rise but carbon dioxide levels drop = less greenhouse gases = temperature drop = ICE AGES.
Testing Goldilocks – Climate stability requires obliquity stability, NB Ice ages are driven by a 2ºC change in temperature however on Mars it varies by 40ºC.
The obliquity of the Earth’s axis is affected by the Moon and Giant planets within the Solar System. The Earth’s rotation is not fixed in space and a change of 1º per 71.6 years in its axis is known as precession. Currently the rotation of the Earth is 24hr and precession is 26,000 yr but if the Moon were removed this would increase to 75,000 yr. Oscillation of the planets will produce resonance and the Earth’s axis will become unstable but there is no resonance. Obliquity is chaotic if Earth’s precession resonates due to orbital variation of a major planet. Increased tidal drag slows rotations and decreases precession. Hence, the Moon is a two-edged sword, increased tides decrease precession but 4.5 billion years ago, the Earth’s rotation was just 8hr.
The Moon was formed about 4.5 billion years ago by the impact of two planets. The outer shells formed the Moon and their cores formed the Earth.
Obliquity Stability in Earth-Moon-like Systems
Model assumptions, the Moon-forming collision produced a system with a large moon but it is not necessary for the earth to have a large moon. In fact stability is more likely with a small moon.
Precession rate v Mass
A moon 17km more in diameter would make the earth very unstable.
If the moon was x2 in size the Earth’s axis would be more unstable but big moons generate good tides which could help life. The Moon’s mass is 99% of the critical mass needed for climate stability.
Planetary Separation and Obliquity Stability
The limiting factor was Saturn’s orbital variation at 26sec/year.
Currently there are 218 known exoplanets (planets around other stars).
A process known as gravitational lensing has detected 4 planets so far. The brightness of a star is bent by the presence of another star which focuses and intensifies the light. This also allows detection of planets.
Imagine that there are 9 independent requirements for a planet to be habitable and each of these is met on 10% of the planets therefore one planet in a billion will have all these requirements. This would give us 100 suitable planets within our galaxy. What if 1% of these planets evolve complex life and 1% of these then produce advance civilizations. This calculation gives an approximation of there being another planet similar to Earth in the universe.
The Earth is a rare and beautiful place and the luckiest in the universe. There could be bacteria in the universe but these are not regarded as complex.
The continuing increase in solar warmth predicts that in 500x 10 6 years time the Earth will become too warm for life.