Lesson 27: Earth as a living planet
In this lesson, we will discuss four features that
make life possible on Earth, including the presence of water and oxygen and the
linked roles of plate tectonics and climate stability. We will then talk about
the effect human activity has had on our planet and the consequences of global
warming. Finally, we will summarize what ultimately makes a planet habitable
based on our study of the five terrestrial worlds.
There are four unique features of Earth that
allow it to support life as we know it:
The first is surface liquid water. Earth has a surface temperature and
atmospheric pressure that allow liquid water to be stable on the surface.
Second is atmospheric oxygen, the presence of oxygen in the atmosphere and
an ozone layer.
Third is plate tectonics,
which is responsible for most of the Earth's surface features.
Finally, the last feature unique to Earth is its climate stability,
the fact that
the climate has been stable enough over billions of years to
allow for liquid water.
The third and fourth features -- plate tectonics and
climate stability -- are
closely linked. Climate stability would not exist without our unique system of
tectonics.
Life as we know it requires water, and animal life requires oxygen.
As we know, water came from volcanic outgassing,
and the correct distance from
the Sun and the moderate greenhouse effect allowed it to
form into oceans without freezing or evaporating.
The oxygen we breathe (O2), however, could not have come from geological
processes. Also, oxygen gas is not a stable molecule; it is a highly reactive
gas that would gradually disappear from our atmosphere if it were not
continually replenished. How, then, does it make up 21% of our atmosphere?
Plants and many microorganisms release oxygen through photosynthesis. Basically
all of Earth's oxygen was originally released by photosynthetic life, and they
continue to produce oxygen to maintain its level in the atmosphere.
Some of that ordinary oxygen (O2) becomes ozone (O3) which makes up our ozone
layer that is critical to protecting life on Earth.
The Earth's lithosphere is broken into more than a dozen plates that slowly
move about in a process known as plate tectonics. Where these plates meet and
collide, some of the seafloor crust is pushed under continental plates in a
process called subduction. This crust that gets pushed under can melt and sink
into the mantle.
This process of subduction acts to recycle the ocean floor, as old ocean floor
descends into the mantle and gets melted down, and then eventually erupts back
to the surface as molten rock.
Climate stability has clearly been critical to life on Earth. Venus is too hot
for life as the result of a runaway greenhouse effect while Mars is too cold
due to a lack of atmosphere.
The amazing mystery of Earth's climate stability is that
the Sun has brightened about thirty percent in the last 4 billion years, while
Earth's temperature has remained relatively stable.
The answer to that
mystery is a mechanism by which the Earth's greenhouse effect adapts to
changes in temperature. This mechanism is called the carbon dioxide cycle, or
CO2 cycle.
In this cycle, carbon dioxide enters the atmosphere through volcanic
outgassing. Increased carbon dioxide in the atmosphere means an increased
greenhouse effect which increases temperatures. As temperatures increase, more
water evaporates and there is greater rainfall. The rainfall pulls the CO2 out
of the atmosphere, reducing the greenhouse effect and causing the planet to
cool down.
The dissolved CO2 forms into carbonate rocks which then get pushed down into
the mantle through the process of subduction. The whole cycle starts over as
these carbonate rocks melt down and the CO2 is recycled back into the
atmosphere through volcanic outgassing.
Now you can see how Earth's unique system of plate tectonics and
subduction makes the carbon dioxide cycle possible.
The planet can self-regulate climate over long time scales, but in
the short term, human activity can increase greenhouse gases rapidly enough to
dramatically change Earth's climate.
Global warming can be linked to human activity instead of purely natural causes
due to three reasons:
First of all, the greenhouse effect has reliably explained surface temperatures
on other planets, so we can be sure that the greenhouse effect would have the
same effect on Earth's temperatures.
Second, human activity, particularly the burning of fossil fuels,
has increased the amount of carbon dioxide in
the atmosphere by 40% over what it was before the industrial revolution. We can
be sure this added carbon dioxide is the result of fossil fuels since it
carries a distinct ratio of CO2 isotopes that is only found from the burning of
fossil fuels.
Finally, climate models have been developed that
take into account the enhanced greenhouse effect from human production of
greenhouse gases, and these models match up with the observed rise in global
temperatures.
Global warming does not affect all regions equally. Some areas experience more
dramatic increases in average temperature.
Increases in temperature can result in many unexpected consequences.
Global warming can cause changes in rainfall patterns,
leading to
more periods of prolonged drought and increased occurrence of wildfires.
Increased energy in the atmosphere and increased evaporation from
the oceans may result in more frequent and more intense storms.
Global warming can also lead to a rise in sea level as ice melts.
Finally, about thirty percent of the carbon dioxide released into the
atmosphere dissolves in the oceans, undergoes chemical reactions, and makes the
oceans more acidic, harming ocean life.
Our study of the terrestrial worlds tells us that
there are two crucial ingredients to making a habitable planet.
First, the planet needs to be large enough to retain internal heat and remain
geologically active. This allows volcanic outgassing to supply the water and
gases to form an atmosphere, and it also allows the core to generate a
protective magnetic field, or magnetosphere.
Second, the planet needs to be at the right distance from
the Sun to allow for water vapor to condense and rain down to form oceans,
creating a carbon dioxide cycle to regulate the climate and the greenhouse
effect.
Knowing these criteria about a planet's size and distance from
its star allows astronomers to study distant terrestrial worlds around other
stars and make predictions as to whether they could support life.