Earth's Unique Balance
So many rare conditions had to be just right for this ocean planet to be habitable for life. Our distance from the Sun determines the amount of solar radiation we're exposed to. Too close and we would melt, too far and we would freeze. Meanwhile gases in the atmosphere reflect, filter, and trap in solar radiation, protecting us from the temperature extremes of day & night. This atmospheric phenomena, incubating the life that grows within, is aptly named the Greenhouse Effect.
To understand the importance of the right mix of gases, we only need to take a look at two of our neighboring planets, Venus & Mercury. At 96% Carbon Dioxide, the atmosphere of Venus may be closer to what Earth once had back before it started to breathe. However it's atmosphere is also over ninety times denser. Since CO2 is a very strong greenhouse gas, it traps thermal radiation to such a degree that its average surface temperature is 860°F / 460°C, hot enough to melt lead. There is so little heat loss that there is almost no difference in temperature between the equator and the poles, nor between temperatures on the day & night sides of the planet. Compare this to Mercury, the planet closest to the Sun. It has four times more solar energy than Venus, but lacks an insulating atmosphere. And so Mercury's peak temperature (427 °C) is actually cooler than it's more distant neighbor, and the dark side of the planet swings to the frigid extreme of -297°F/183°C.
Fortunately for us, the same photosynthetic life that gave us oxygen to breathe also made Earth's climate bearable also absorbed most of the atmospheric carbon dioxide, leaving water-vapor as our planet's primary greenhouse gas. CO2 still plays a part, but water is the essential ingredient because reactive. It is the key element of Earth's self-regulating system that keeps our climate within cyclical norms. At play within this climatic system is a tension of positive and negative feedback loops, the former amplifying the effects of the greenhouse effect, the latter counteracting them.
A Self-Regulating Climate
The Sun's heat acts as the driver of weather. It is the engine behind the atmospheric circulation that pushes moisture throughout globe, upwards from the warm equator towards the poles, fueling the prevailing trade winds. Meanwhile the oceans act as giant heat sinks that can store and release the sun's energy over time, smoothing the daily & seasonal fluctuations in temperature.
The sun's warmth causes vapor to rise from the oceans into the atmosphere (a process that offsets some of the absorbed heat through evaporative cooling). The warming water vapor is transient, but inversely leads to more reflective cloud cover, limiting the solar radiation reaching earth's surface. On hot days the sun drives towering thunderheads soar up into the frigid heights of the troposphere, exploding in a cloud burst of rain or hail, cooling the land, quenching the thirst of forests. In the winter of the opposing hemisphere, this increased global moisture equates to more snow & ice, ready to counteract the heat of the approaching summer. And the ice that doesn't melt adds to titanic glaciers, to be released in future years.
Meanwhile photosynthetic life works in conjunction with this hydrological cycle. When direct Sun hits the warm ocean, it increases the blooms of algae. A primary atmospheric biproduct of this is Dimethyl Sulfide, a key aerosol which acts as the nuclei for condensation in the process of cloud creation. The rain that falls on the land allows photosynthesis to occur there too.
Intertwined with the hydrological cycle, the carbon cycle moves another critical element for life throughout Earth's systems. Increased heat from greenhouse CO2 triggers a further increase in atmospheric water vapor. Since both H2O & CO2 are the essential ingredients in photosynthesis, a Carbon Dioxide increase has historically been counteracted by increased plant growth. When Earth's ecological systems are flowing smoothly, this sequence of events consequently fuels greater biosequestration, a negative feedback loop that starts removing some of the CO2 greenhouse gases from the atmosphere and from the oceans, trapping them within vegetation, soil and sediment. In this way, photosynthetic life works to actively keep CO2 at a bearable equilibrium. During the 100,000 cyclical ice-ages, when the glaciers advance the from poles, covering boreal forests, this carbon sequestration slows. With re-uptake hampered, over time warming CO2 will builds in the atmosphere again, warming the poles until the glaciers again start to recede, and the process starts anew.
It is these intertwining systems keep Earth in balance. It is a climatic homeostasis, making the planet hospitable for life. Without this underlying foundation, it's unlikely there would be sentient life here to experience it.