Antonyms for the phrase "slope upwards" could include phrases such as "fall downwards," "descend," or "decline." These phrases all suggest a downward or negative movement, rather than an upward or positive one. Other antonyms might include "level out," "plateau," or "remain constant," which suggest a lack of upward or downward movement altogether. Opposing phrases like "climb upwards," "ascend," or "rise," could suggest alternative means of upward movement, but still maintain the idea of moving or increasing upwards, rather than downward or remaining stagnant. Overall, these antonyms provide different ways of expressing the opposite of "slope upwards," while maintaining a focus on movement, direction, and impact.