A little knowledge is a dangerous thing. So it was for John Smith, experiencing his few years in England's newly opened colony of Virginia. He wrote:

"The Sommer is hot as in Spaine; the Winter cold as in France or England. The heat of sommer is in June, July, and August, but commonly the coole breeses asswage the vehemency of the heat. The chiefe of winter is halfe December, January, February and halfe March. The colde is extreame sharpe, but here the Proverbe is true, that no extreame long continueth."

"In the year 1607, was an extraordinary frost in most of Europe, and this frost was found as extreame as Virginia. But the next yeare, for 8, or 10 dayes of ill weather, other 14 dayes would be as Sommer."

I guess he thought he had experienced the best and the worst, and went on in opening the "Generall Historie of Virginia, new England and the Summer Isles" (1624) to summarize the climate for this region of North America in seven sentences, substantially less than a printed page.

"The temperature of this country," he says, "doth agree well with English constitutions, [they] being once seasoned to the country." Now, the devil is in the details here, because as historian Ed Hale estimates, this "seasoning" ultimately took the lives of about 85 percent of those arriving in the Colony in it's first decade or so!

We tend to think of John Smith as the authority on early Virginia, but one needs to remember that he was talking from the perspective of just a few years. He was experiencing weather, not climate. Weather can sometimes run through a sequence of many years of cold, or hot, or wet, or dry weather.

USGS research scientists Stacey Verardo and Tom Cronin suggest that the Chesapeake Bay may have been slowly warming for the better part of four centuries, judging from the preserved remains of minute planktonic dinoflagellates preserved in the Bay's sediments.

But, during this time there was both a Medieval Warm Period, well-documented by historians from the 12th through 14th centuries, followed by what scientists call the Little Ice Age in the 15th and 16th centuries, up until about the time of European contact with the Chesapeake environment. The USGS scientists count their upward trend in temperatures from that time forward, and suggest it may have been related to the progressive deforestation of the basin and its reduction to agricultural uses.

The effects of deforestation are substantial in other ways as well and in 1753, a Pennsylvanian wrote:

"Our runs [streams] dry up apace, several of which formerly would turn a fulling mill are now scarce sufficient for the use of a farm. The reason of which is this, when the country was covered with woods and the swamp with bush, the rain that fell was detained by these interruptions and so had time to insinuate into the earth and contribute to the springs and runs. But now the country is cleared the rain as fast as it falls is hurried into the rivers and washes away the earth and soil of our naked fields, fills and choaks the springs, and makes shoals and sandbanks in our creeks and rivers; and hence several creeks mentioned by Mr. [William] Penn to be navigable are no longer so."

Another writer, Lester DeCoster, indicated that 142 years later, in 1895, more than two-thirds of Pennsylvania's 27 million forest acres had been cleared. What a truly catastrophic impact this must have had on streams flowing with mud! A far cry from Count Zinzindorf's 18th century trip across the state when he remarked that one could see a pin on the bottom whilst wading chest deep.

I remember one weekend in November 1968: The boat had been hauled in for the winter and it had been a raw and blustery autumn with the leaves long driven from our trees back at my northern home. At the same time, while attending a scientific meeting at Solomons Island, MD, I stood, in shirt sleeves, outside the Chesapeake Biological Lab, where it was about 60 F, with a tranquil Chesapeake lapping the beach at Sandy Point. Willow oaks and imported (invasive and exotic) tropical bamboo were still in full leaf. A palm tree was nestled in the lee of a lab building, planted there years earlier. In the occasional, leafless, deciduous trees, I saw clumps of mistletoe. I thought I had truly come to the South.

Looking back now, from a perspective of three decades of familiarity with the Bay, I recall that nine years after my first encounter, the weather was very different. Edgar Woodburn, the local grocer, drove his car out onto the Patuxent atop the ice of a "Millennium Winter," stepped out to have his picture taken, then turned aghast to watch the surface crack menacingly and his shiny car sink slowly into the depths. The palm tree died and mistletoe vanished from the local flora.

This was 1976 and 1977, while we were in the grips of what the press touted as the "Millennium Winter." Ice cover on the creek where I live was continuous for two months; its thickness, at more than 1 foot, was about equal to the theoretical maximum for this latitude (38 degrees North). It was heralded as the potential start of a new Ice Age. After all, it had been 10,000 years since the last one, and the pendulum was swinging. It still is.

Weather is what you get, this week, this season, year to year, for a run of years. Climate is the long-term average of all available data, like the 40-year series published some years ago for Solomons Island. It is a surprisingly stable set of numbers, despite a few unusual years. It will, for example, tell you when the average date of first and last frosts are for a location, but won't protect your spring peas or lingering fall tomatoes against individual years.

Back in the 1970s, when Andrew McErlean, then a University of Maryland biologist with 17 years tenure on the Chesapeake, was asked if he'd experienced any unusual years, he responded emphatically, "Oh yes, 17 of them."

Where does one turn for perspective in judging one's personal experience? Is the historical literature perspective enough: the chronicles of the Chesapeake colonies by John Smith and those that followed him?

Is a lifetime enough? Consider Capt. Charlie Thompson, who came here from Scandinavia near the turn of the century. As a immigrant lad in New York, he'd been a stevedore helping to carry the mast of Sir Thomas Lipton's classic America's Cup challengers down the docks on South Street. He then moved to the Chesapeake and spent his adult life fishing on the Patuxent River.

When I met him, he'd been gill-netting striped bass there each spring for more than half a century. In the Millennium Winter, we trotted him down to the dock to comment on the estuary, which was locked in ice. The kid from Hallowing Point whom we called "Radio Sam" (one was grafted to his ear much of the time) had ridden his horse across the river ice to Benedict. We'd been skating on it for weeks during our lunch hour. Charlie took a draw on his old pipe and said: "Nope, seen this before." He drew again on the pipe. "In the '20s, steamers couldn't get up the river here for weeks on end."

So, was that the second "Millennium Winter" in 50 years? In fact, it could be, in climatological terms, because the statistical likelihood of getting such conditions, even if only one-in-a-1,000, is the same each year. It's unlikely, but you could get two such years back to back. The winters of both 1976 and 1977 were, in fact, pretty cold.

We have, in further fact, just come through several extraordinarily wet years and mild winters. A couple of those, as the Bay Journal has reported, were the wettest on record, but our records are a small segment in geologic time, a small segment in the 10,000 year time line of Chesapeake Bay.

It's only within the last few centuries that climatic variability has begun to interact with human-induced change. Palaeoecologist Debra Willard and her colleagues at the U.S. Geological Survey are studying the Chesapeake's history by reading sediment cores taken from deep in the Bay floor. Four meters (more than 12 feet) covers a lot of time, as much as 7,000 years. Willard reports that in seven millennia, we have cycled from drought to wet periods many times.

At the end of the 19th century, though, very wet conditions coincided with our new nation's aggressive agricultural expansion. The removal of forest coupled with deep tillage and heavy rains pounded the Bay with a vast flux of erodible soils, which we see in sediment cores and call the "Agricultural Horizon." Seaports vanished from in-filling; stream beds and flood plains all over the basin were reshaped. Those effects were unprecedented during our tenure here and have left us with conditions that still perplex ecosystem management and recovery more than a century later.

Be prepared in the coming years, prepared to be surprised, to hear cries of coming desertification of the Eastern Seaboard, to hear cries of the millennium winter again, and the millennium drought (which will be extraordinarily serious because of our profligate use and abuse of water). Be prepared, again, for floods, and for storms that demonstrate the turbulence of global warming.

If all this is confusing, don't be surprised or discouraged. In every year, (Listen to the monthly National Oceanic and Atmospheric Administration climate summaries.) individual records for high and low temperatures are broken on individual dates. One of the reasons these records are kept is that over the long run of years, this monitoring builds a database and a series of rolling average temperatures for every month, every day of the year. It's against this distribution of dates, sometimes fortuitously augmented by wonderful people who kept records 100 or 200 years ago, that we can judge the slow drift of temperatures upward and conclude that we are, in fact, in a period of global warming.

Just remember this winter, even if it is the mildest or most severe in decades, that we're experiencing weather not climate.