Events occur in nature according to a complex series of triggers. For instance, the amount of daytime has a bearing on the growth of many plants and the elevation of the Sun in the sky affects the timing of animal migration. Another trigger is temperature.
We can measure the onset of spring and fall with good precision and have been doing so for decades. Some of the events are the migration of birds, the first and last leaves, the emergence of butterflies, and the opening of flowers. The conclusion of those studies is that spring is coming earlier and fall is coming later. Over the last 30 years, spring has been arriving between 2.3 and 5.2 days earlier per decade, depending on the biological event.
The angle of the Sun in the sky and the amount of daytime have not changed over that period of time, so those cannot be the causes of the change. We have to find something that has changed and the trigger that has experienced the most significant amount of change is temperature. As temperature rises, spring comes earlier and fall later. Many of the changes we see are in events we know are triggered by temperature. The evidence is pretty overwhelming that the Earth is getting warmer.
But, as to be expected, large and rapid changes have consequences. One of the consequences in this instance is that the timing isn't changing the same for everything. This is causing mismatches and damage. For instance, we can look at Henry David Thoreau's notebooks and see the difference in blooming time for different flowers. Some of those flowers have adapted and are blooming earlier and are thriving. Others have not adapted and are disappearing. Also, while spring may be coming earlier, the last frost is still coming at about the same time. Plants and animals that would have been still asleep and protected are now becoming vulnerable.
Just more evidence that the world is changing and not all of it is for better.