The question seems to be asking for the limits or thresholds within which life as we know it can be sustained on Earth. These limits can be considered in various ways including temperature, atmospheric composition, and water availability among others. Here are some general parameters:

1. Temperature: Most life forms on Earth thrive between -20 degrees Celsius and 50 degrees Celsius. Beyond these extremes, life becomes increasingly difficult, but there are exceptions such as extremophiles, which can survive in hotter or colder conditions.

2. Atmospheric composition: Earth's atmosphere is composed mostly of nitrogen (around 78%) and oxygen (about 21%), with small amounts of argon, carbon dioxide, and other gases. A significant shift in these proportions (especially oxygen and carbon dioxide) could make the planet uninhabitable for many forms of life.

3. Water: Water is critical for life as we know it. A severe lack of fresh water would make survival for many species (including humans) impossible.

4. Sunlight: Sunlight is necessary for photosynthesis, the process used by plants and some other organisms to convert light energy, usually from the Sun, into chemical energy that can be used to fuel the organisms' activities. Without sunlight, this process cannot occur, potentially disrupting the base of most food chains on Earth.

5. Magnetic field: Earth's magnetic field protects the planet from solar radiation. If it were to disappear, the increased radiation could lead to higher mutation rates and could threaten many forms of life.

These are just a few examples of the thresholds necessary for life on Earth. It's also worth noting that these parameters are interconnected and changes in one can lead to changes in others. For instance, a significant increase in the Earth's average temperature (global warming) could lead to shifts in atmospheric composition, alterations in the water cycle, and other cascading effects.