Nature is has an amazing internal clock. We have found we can predict, almost to the hour, when certain things will happen during the year. We can track when flowers open and leaves bud in the spring. We can predict how crops will grow during the summer and when to harvest them in the fall. And, we can measure when plants go dormant in the fall. Of course, we can also measure the animal activities associated with all of these milestones, such as when animals migrate.
What we have found is that this timetable is changing. Events that occur in the spring are occurring at progressively earlier times. And, the fall time table is occurring progressively later. This means spring is coming earlier and winter is coming later. This is a sure sign of global warming.
What we want to be able to do is predict how this timetable will change with future changes to the global environment. What we find is that changes actually occur much faster than we thought.
A study published in Nature magazine found that plants are actually responding to global warming much more than predicted. This is a real problem because if plants flower too early then the insects that feed on those flowers won't be there. When the insects arrive, the flowers will be gone. Then, the animals up the food chain that depend on those insects will suffer. And, this will progress right on up the food chain.
People are fond of saying, "So what?" when it comes to global warming. This is an example of the 'what'. You cannot change one part of the equilibrium of nature and not expect to see repercussions.
It is just a law of nature. And, we need to understand that.