It's a cold fall day in north Georgia, which isn't unusual. But it's more unusual than it used to be.
Global climate change is, by definition, global. Local- or regional level data can't be used to prove or disprove it. But as human beings we witness the change, and experience the change, at the local or regional levels.
For example, if you think that spring is coming earlier and earlier these days, you're right. Here in Georgia, trees are reaching "first leaf" some four days earlier than they did from 1961-1980. It may not seem like much of a change, but it has come very quickly and shows every sign of continuing. By the middle of the century, spring on average may come as much as two weeks earlier than in the historical data, throwing off plants, animals, birds and insects.
And then there's the issue of temperature extremes. In a stable climate, record lows and record highs should occur in roughly equal numbers. I looked at the Atlanta data for two time periods -- since 2000, and since 2010 -- and this is what I found:
The Atlanta data offer a good explanation why local data can't be used to characterize a global phenomenon, because the change as we've experienced it and witnessed it is substantially greater than it is nationally or globally. Nationally, record highs have outnumbered record lows by roughly a 2-1 ratio since 2000; Atlanta's ratio of more than 3-1 is somewhat unusual.
Another way to document the change is through shifting plant-hardiness zones as issued by the U.S. Department of Agriculture. Certain plants are intolerant of too much heat; others of too much cold.
Rhododendrons, for example, need cold winter nights as part of their flowering process, and all of Georgia once provided suitable habitat for "rhodies". That's no longer the case. The same is true of an even more iconic plant, the Georgia peach.
Under maps updated by USDA in 2012, based on temperature data compiled through 2005, growing zones in Georgia were shifted 50 to 100 miles northward since the 1990 update. The Georgia coast now has a climate more akin to north-central Florida, and has become too warm for most varieties of rhododendrons or peaches to thrive. **
People who spend a lot of time outdoors -- gardeners, farmers, hunters and fishermen -- will often tell you that they've noticed these changes themselves. And that is pretty stunning. As skeptics of climate change often point out, the climate has never been stable; it is always changing. But according to scientists, it has never changed at a rate that would be noticeable within the range of a human lifetime.
Climate change at the end of the last glacial period, for example, took 5,000 years. But according to scientists at the National Oceanic and Atmospheric Administration, "the current rate of global climate change is much more rapid and very unusual in the context of past changes." In fact, if warming continues at the rate projected in climate models, "there is no evidence that this rate of possible future global change was matched by any comparable global temperature increase of the last 50 million years."
That's a long time. And no observable mechanism other than man-made changes is powerful enough to account for such a rapid transformation.
** The USDA's updated 2012 plant-hardiness map of Georgia, based on weather data through 2005, was significantly different than the 1990 map, based on data through 1986:
- Zone 9a, which in the 1990 map was typical of central Florida, appears for the first time in Georgia in the 2012 map and now dominates the Georgia coast. That puts it outside the growing range for most varieties of rhododendrons and peaches, which is zones 5 through 8.
- Zone 8b, once confined to counties right along the Georgia-Florida border and along the coast, now covers the entire southern third of the state.
- The border of Zone 7b, which once extended as far south as Macon, has moved 75 miles north to Atlanta.
Zone 7a, which once covered most of north Georgia, has retreated northward to a mere sliver of northwest Georgia and a bit of northeast Georgia.
USDA Plant Hardiness Map, issued in 2012 based on data through 2005