A serpentine belt should be replaced as soon as it shows signs of fraying, is badly cracked or gets contaminated with oil. Most manufacturers also recommend replacement every four years or 50,000 miles as preventative maintenance. When a serpentine belt breaks, all drive power to whatever it turns is lost. This means the water pump stops circulating coolant through the engine, the alternator quits producing power, the power steering pump ceases to assist steering, and the air conditioner quits cooling.
Continued driving without power to your water pump will result in engine overheating and engine failure, requiring costly repairs and even engine replacement. Ignoring serpentine belt replacement has a high safety impact, as you will most likely be stranded with no warning when the belt breaks, and if your power steering fails you could lose control of your vehicle. There is no environmental impact to ignoring serpentine belt replacement.