03/02/2005
Farming for Better or Worse
Here on Marco Island, February and March are the peak months of “the season”, the time when the snowbirds in Florida number in the zillions. At least it seems that way when one ventures out on the main roads and highways. A safe weather forecast that would be correct 90 percent of the time here would be “sunny and warm”. There are exceptions, as was the case Sunday, when our Venetian friends (from Venice, Florida) were to come down for a visit. Venice is about a two-hour drive from here and they called to say they were not coming because of predictions of heavy rain and possible tornadoes! Fortunately, the latter did not materialize but in Venice it did rain all day with 5 inches total. Here on Marco, it was relatively sunny without a drop of rain.
Why does Florida have “the season” and why does New Jersey have winter and summer, not to mention spring and fall seasons? Let’s examine how the earth orbits the sun and pretend that the earth’s axis is perfectly perpendicular to the plane of the earth’s orbit. In other words, if you drew a straight line between the earth and the sun and you had a rod through the earth that passed through the North and South Poles, the rod would be at right angles to that line. If that were the situation, as the earth made its yearly orbit around the sun, the northern and southern hemispheres would receive the same amount of radiation from the sun. No seasons.
We know that’s not true. When it’s winter in the Northern Hemisphere it’s summer in the Southern Hemisphere. The axis of the earth is tilted, not perpendicular to that line we drew form the sun to the earth. It’s now winter in New Jersey. The earth is tilted so that New Jersey in the Northern Hemisphere is tilted away from the sun while Australia in the Southern hemisphere is tilted towards the sun. Australia gets more of the sun’s rays than New Jersey. Six months from now, the earth will be at the opposite point in its orbit and the situation and the Northern Hemisphere is tilted towards the sun and it will be summer.
I’ve left out two important facts. First, the earth spins around its axis like a top slowing down and starting to wobble. What this means is that the ends of that imaginary rod through the poles are tracing circles as the earth wobbles. But this a very slow wobble. It takes some 22,000 years for the ends of that rod to trace complete circles. Let’s take the point on the earth’s orbit around the sun where we are today. The earth is tilted at a certain angle to the sun. It will be 22,000 years from now when the earth is once again at precisely the same angle at this point in its orbit.
I also neglected to say that the earth doesn’t literally “circle” the sun. Its orbit is not a circle, but an ellipse; there are opposite points where the earth is closest to and farthest from the sun. Combine this fact with our 22,000-year tilt cycle and we get an interesting situation. If we start at a time when the earth is closest to the sun and the tilt is such that the Northern Hemisphere is most exposed to the sun’s rays, what does that mean? This is the situation under which the Northern Hemisphere receives the most solar radiation and hence should be the time of the warmest northern summer. This won’t happen again for 22,000 years. But, 11,000 years later, when the earth is tilted in the opposite direction, summer will occur when the earth is at the farthest point in its orbit around the sun, and this should be a time of the coolest northern summer.
If you plot the solar radiation in the Northern Hemisphere over the years, it rises and falls with the peaks and valleys 11,000 years apart and the distance between peaks of 22,000 years. Why am I taking you through all this? It has to do with what may be a major misconception about when and what started global warming and a radical claim that global warming began much earlier and for a different reason than the common wisdom would have it. The claim is detailed in an article by William Ruddiman titled “How Did Humans First Alter Global Climate?” in the March Scientific American. Ruddiman is a marine geologist, former chair of the department of environmental sciences at the University of Virginia and experience as an oceanographer and research scientist at the U.S. Naval Oceanographic Office and Columbia’s Lamont-Doherty Earth Observatory.
Ruddiman claims that global warming began, not with the industrial revolution in the 1800s, but thousands of years earlier. In fact, he claims that humans have been affecting global climate for at least 8,000 years! There were no cars or smokestacks pouring out greenhouse gases, but Ruddiman cites some very intriguing histories of two greenhouse gases, carbon dioxide (CO2) and methane in the atmosphere.
Let’s concentrate on the history of methane in the atmosphere over a period of hundreds of thousands of years. How do we get such a history? Let’s go down to Antarctica, where the ice has been laid down over millennia and piled up to amazing depths. Intrepid researchers have managed to drill down deep enough to obtain a core of ice 3 kilometers (almost two miles) long. (I don’t know if this was one long core or a bunch of cores covering a total depth of 3 kilometers. Either is impressive!)
When the ice in the core was laid down over a 400,000-year period, it trapped tiny bubbles of air. By analyzing the bubbles for methane and CO2, scientists obtained plots of the amounts of these gases over these 400,000 years. They found that the concentrations of both gases rose and fell in regular patterns over this time. Ruddiman presents a plot over the past 250,000 years or so and the concentration of methane cycles up and down within a range of about 400 and 700 parts per billion (ppb).
And, what do you know? The methane peaks and bottoms out along with the solar radiation, the time between peaks being 22,000 years! The correlation is remarkable. The plots in the article cover 10 peak-to-peak cycles, with the last peak occurring 11,000 years ago; this means we should now be at or near the coolest northern summer period.
The methane content at the last peak was over 700 parts per billion (ppb). Sure enough, for the next 6,000 years the methane content fell along a normal curve to below 600 ppb. Then, 5,000 years ago, something happened. The methane departed from the normal curve and started increasing, as it has to this day, when it’s back up over 700 ppb. Without going into detail, the carbon dioxide in the atmosphere also took off from its normal curve 3,000 years earlier than for methane, or 8,000 years ago.
What happened? Ruddiman proposes that it was a new human endeavor – farming! Farming started about 11,000 years ago in Mesopotamia and China. About 8,000 years ago, our Stone Age counterparts started clearing forests in Europe to plant crops. Did this burning of forest and additional farming fuel the rise in CO2? And the methane? It was 5,000 years ago that Chinese farmers started flooding land near rivers to grow rice. A big source of methane is the decay of vegetation and the stagnant water in rice paddies promotes such decay.
With more deforestation and farming, the CO2 and methane concentrations continued to rise. But in the past 1,500 years or so, the CO2 levels fell markedly on some occasions. Ruddiman points out that three episodes of falling CO2 happened at times when there were major plagues that killed off significant numbers of people. For example, there was a long period of plagues that culminated in about 500-600 AD with the deaths of 25 to 40 percent of all Europeans. The CO2 rose after that only to fall again, when the Black Death killed about the same percentage of Europeans. Another, more precipitous fall in CO2 happened in the period of 1500 to 1700 when diseases introduced by the Europeans killed 90 percent of native populations in the Americas.
Each of these incidents would have reduced the amount of farming activity in the affected areas and the abandoned farms would be headed back to their natural forested state before the population recovered and farming commenced in earnest once again. A few weeks ago, we talked about global dimming of the sunlight reaching the ground and its possible holding back of global warming. Ruddiman suggests that greenhouse gases from farming counteracted the normal cooling trend expected from the fact we’re in the bottoming out phase of the 22,000-year solar radiation cycle. Could the farmers be responsible for preventing a new ice age?
As for me, I’m more confused than ever and will concentrate on whether to wear my heavy or light sweatshirt on tomorrow morning’s beach walk here on Marco Island. It’s supposed to be in the 40s and a brisk day for sunny Florida.
Allen F. Bortrum
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