You may not pay much attention to the seasons beyond how they affect your closet. But even if you haven't noticed much difference in the fall season from year to year -- other than trendy fabrics and hem lengths -- scientists certainly have.
The primary difference scientists have observed is that it's getting warmer. Autumn temperatures in northern latitudes have risen 1.1 degrees Celsius (2 degrees Fahrenheit) in the past two decades [source: ScienceDaily]. In the Netherlands, scientists recorded a mean autumn temperature of 3.4 degrees Celsius (38.1 degrees Fahrenheit) above the long-term average in 2006 -- it was the highest temperature ever recorded [source: Wageningen University].
These abnormally high temperatures could mean only one thing: a shorter fall and winter. A number of studies indicate that in many areas, leaves are returning sooner than they ever have historically. In parts of Europe and Asia, spring has been arriving about a week earlier than usual for the last two decades [source: ScienceDaily]. Elsewhere, the flowering season for some plant species began an entire month early.
Not only are leaves returning sooner, they're also falling later. But unlike the return of leaves, which is directly linked to increased temperatures, scientists suggest the delay in leaf fall is tied instead to rising levels of atmospheric carbon dioxide. Trees exposed to elevated levels of this greenhouse gas stay green longer since they use it to produce sugars through photosynthesis. Over the past 30 years in Europe, atmospheric CO2 has risen by 13.5 percent, causing a delay in leaf fall of 1.3 to 1.8 days each decade [source: ScienceDaily].
While this may seem like good news -- plants are being more productive, after all -- it's also worrisome, because they need to set buds in order to prepare for the coming seasons. If they wait too long, plant life will be vulnerable to frost and other weather events.
The correlate of a shorter autumn is a longer growing season. The 2006 Netherlands study found 440 plant species flowering in December, when normally, the number of flowering plants is only 2 percent of that amount [source: Wageningen University]. In parts of Europe and Asia, the growing season is now 18 days longer; in North America, the season has lengthened by 12 days [source: ScienceDaily].
The downside of higher temperatures is that it could spell trouble for those fall vacationers who look forward to a show of color this time of year. This is because the changing colors -- and their intensity -- rely on the early fall weather. Of critical importance are the cool nights: without them, the leaves simply continue about their business producing chlorophyll and photosynthesizing. As long as high temperatures persist, the chlorophyll doesn't go away to reveal the underlying red and orange colors characteristic of autumn. Global warming would also affect autumn color if it brought cold and rainy weather, severe drought or early frost to the fall season. Read Why do leaves turn red, in particular? to better understand why leaves change color.
Of course, as we mentioned earlier, not all parts of the world will respond to global warming in the same way -- autumn in northern Europe may get shorter, while Costa Rica may see a fall season it previously never really had. Climate change works in mysterious ways, so cherish whatever autumn brings this year -- you may never see another quite like it.