Without an atmosphere, cosmic rays and the solar wind would directly impinge on the surface of the earth.

Life on Earth needs the atmosphere to exist. There are three main reasons:

1. Life needs liquid water in order to do its chemical reactions. Water boils in empty space. In order for it to stay liquid, it needs to be under pressure. The atmosphere provides that pressure.

2. The atmosphere contains gasses that life needs to exist. These main gasses are carbon dioxide and oxygen. Life needs carbon dioxide because all living things contain carbon, and without a source of carbon to take from the air, we would run out. Life also needs oxygen with which to burn the carbon and produce energy - which, conveniently, recycles the carbon dioxide back into the atmosphere.

3. Water flows downhill. In order for life to exist on land, there needs to be something replenishing the water on land. This something is the weather, which happens in the atmosphere, evaporating water off of the oceans and then raining or snowing it back onto the land.

The atmosphere provides essential support for life in a number of ways. Obviously it contains the CO₂ that plants use as well as the oxygen that animals need. It is also an important component of the water cycle, holding water vapor which eventually falls as rain. Less intuitive is the role of the atmosphere in transporting dust which fertilizes different parts of the world, not to mention the seeds, pollen, etc. involved in plant reproduction.

The atmosphere also acts as an insulating blanket, keeping the temperature of the earth's surface in the range which can accommodate life. On one hand, the atmosphere is another mass of material which can take up energy coming in from the sun, thereby preventing the earth from getting too hot (the moon, which has no atmosphere, can get up to 250 Fahrenheit in the sun). On the other hand, the atmosphere also holds some of that energy, thus preventing the earth from getting too cold at night. This is basically the "greenhouse effect", which we commonly hear of in a negative context. (The detrimental effect is essentially too much of a good thing - if the earth heats too much, then it could negatively impact life rather than enabling it. More discussion on this is left for another question though.)

The atmosphere also protects life from radiation. A sunburn is caused by excessive exposure to UV radiation, much of which is blocked by the ozone layer. The ozone layer also absorbs other types of radiation and prevents them from damaging life on the surface. Shooting stars are evidence of the atmosphere protecting the surface from physical threats as well . When rocks and dust from space encounter the particles in the atmosphere, they experience high stresses and also heat up due to friction. These help to break down the incoming debris into harmlessly-sized objects before they reach the ground.

The earth's atmosphere helps serves multiple multiple functions in the protection of life on Earth. The ozone in the Earth's atmosphere helps block the harmful UV radiation in sunlight, thereby preventing damage to the cells in living organisms (UV radiation can harmfully and irreversibly change the DNA in living organisms and kill them). The atmosphere provides pressure (there is such that water can exist on the Earth's surface as a liquid, and liquid water provides essential nourishment to all organisms and regulates the Earth's climate. The atmosphere helps retain the heat provided by the sun , warming the surface and regulating the temperatures, such that no extreme highs or lows in temperature occur on the surface between day and night. Without a way to regulate heat, the surface of the earth would become extremely hot during the day and cool off rapidly as night approaches, leading to baking temperatures during day time and freezing temperatures at night and making the environment extremely harsh for living organisms. In other words, the atmosphere is crucial for life on Earth as we know it.

Can you think of ways the atmosphere helps life on earth? What do you think the temperature on earth would be like if we didn't have an atmosphere to be a blanket between us and the cold of outer space? How would we breathe on earth? I think we'd need to go around in space suits. Maybe you can think of more ways the atmosphere helps life on earth.