How do shifting seasons occur?

Shifting seasons are directly linked to warmer global temperatures. A slight change in temperature is enough to push the spring thaw earlier, and delay the first frost until later in the fall. ... The US EPA uses leaf and bloom dates reflecting national phenology trends to indicate climate change.

  The Earth's tilted axis causes seasons. During different parts of the year, the Earth recieves the sun's rays directly. It's summer in the Northern Hemisphere.  When the South Pole tilts toward the Sun, it's winter in the Northern Hemisphere.

Explanation:

the answer is b.) hydrogen bonds between bases.

answer: substrate concentration. enzymes work best when there is a high enough substrate concentration for the reaction they catalyse.