Describe how the present-day atmosphere and surface temperature of the Earth might be different (a) if carbon dioxide had never been released into the atmosphere; (b) if carbon dioxide had been released, but life had never evolved on Earth.

The greenhouse effect is a natural process that warms the Earth's surface. It works similarly to a greenhouse for plants, trapping the sun's heat within the atmosphere. Gases in the Earth's atmosphere, such as carbon dioxide (CO2), methane, water vapor, and nitrous oxide act like a blanket, absorbing infrared radiation and preventing it from escaping into space. This allows for life on Earth to thrive in a relatively stable climate.

Imagine the Earth without this natural insulation; our planet would be about 33 degrees Celsius cooler, making it inhospitable for most forms of life as we know them. Thus, the greenhouse effect is crucial for maintaining the temperature that sustains ecosystems and human activities. However, when the concentration of greenhouse gases increases beyond natural levels, primarily due to human activities like burning fossil fuels and deforestation, this leads to an enhanced greenhouse effect, causing global temperatures to rise unnaturally, which is a central concern in current climate change discussions.

Carbon dioxide (CO2) plays a significant role in Earth's climate. As one of the primary greenhouse gases, its levels in the atmosphere dictate much of the global temperature. With the advent of industrialization, human activities have sharply increased the concentration of CO2, primarily through the burning of fossil fuels such as coal, oil, and natural gas, as well as changes in land use.

When CO2 levels rise, more heat is trapped in the atmosphere, leading to a warming effect known as global warming. This has numerous consequences, including melting ice caps, rising sea levels, and extreme weather patterns. The Paris Agreement and other international efforts are centered around reducing CO2 emissions to mitigate these effects. Understanding the integral role of CO2 in climate systems is essential for developing strategies to address climate change.

Photosynthesis is the process by which green plants, algae, and some bacteria convert light energy, usually from the sun, into chemical energy, creating glucose and releasing oxygen as a byproduct. This process plays a vital part in the atmospheric balance of Earth.

Through photosynthesis, plants act as natural carbon sinks, absorbing CO2 from the atmosphere and thereby reducing the greenhouse effect. They essentially help to regulate atmospheric CO2 levels, which has a direct impact on climate change. In the absence of photosynthesis, the Earth's ability to balance CO2 levels would be greatly diminished, likely leading to an accumulation of CO2 and a consequential increase in greenhouse effect and global temperatures. Therefore, preserving and expanding vegetation cover through reforestation and reducing deforestation are critical actions for maintaining the balance of greenhouse gases in the atmosphere.