The ozone layer is a layer in our atmosphere, named so because of the presence of ozone molecules (O3). It serves to protect us and the environment from UV radiation, which has the potential to cause cancer in humans and damage organic material.  Aquatic ecosystems are especially susceptible to these rays because of the damage done to plankton, which play an integral role in their food chains. As a result of this damage, loss of biodiversity and a reduction in fish yields are common.  Overall, the ozone layer is generally seen as a good thing we don’t want to destroy.
Unfortunately, chlorofluorocarbons (CFCs), chemicals used in a variety of consumer goods such as refrigerators, aerosol sprays and air conditioners, almost did destroy the ozone layer during their widespread use in the twentieth century.  This is done through a series of chemical reactions :
i) CFCs first rise to the stratosphere, where they are exposed to ultraviolet radiation and release a chlorine atom.
CFCl3 + UV rays -> CFCl2 + Cl or CF2Cl2 -> CF2Cl + Cl
ii) This net reaction is then produced:
i) Cl + O3 -> ClO + O2
ii) ClO + O3 -> Cl + 2O2
overall: 2O3 -> 3O2
As you can see, the net result is that ozone is broken down into regular, boring, useless-at-blocking-UV-radiation oxygen molecules. Furthermore, because the chlorine atom is regenerated at the end, one CFC molecule can break down thousands of ozone molecules.
Fortunately, the global community actually came together and solved this issue! This may come as a surprise to those who are paying attention to the sluggishness in which the global community has been solving global warming, but this time the problem was addressed in a swift and effective manner. First, a hole in the ozone layer above the South Pole was discovered in the early 1980s. In response, the Montreal Protocol, negotiated and signed by 24 countries in 1987, went into effect, which sought to phase out the use of CFCs in consumer goods by 2000. Following new scientific evidence that the damage was worse than what was previously thought however, this agreement was strengthened several times, before CFCs were finally banned completely in 1996.  Today, the amount of chlorine in the stratosphere is decreasing, though it may take up to another century for the ozone layer to completely heal.
I hope this example of how the international community came together and averted a crisis serves as a glimmer of hope for all of those who feel overwhelmed by the issues we face as a planet today.