If you live in a warm or mild climate like most climate researchers, a cloud is just a cloud. Clouds bring rain and cooler temperatures. For the climate researchers, a cloud is of course a much more complicated matter. But the “knowledge” most of us have is that clouds bring cooler weather and rain.
Those of us who grew up in harsher climates have different knowledge. We know that clouds bring temperature change, but which direction depends on the season. On the wind swept Canadian prairie, when you wake up to blue sky and sunshine streaming through your bedroom window on a January morning, you had best go back to bed. It will be 30 below zero outside, perhaps worse. Better to sleep in… and pray for clouds so that it warms up.
The reasons aren’t that complicated. Clouds are mostly water vapour which is in turn a greenhouse gas which has (depending on whose numbers you use) 30 times or more the greenhouse effect of CO2. The concern is that a small CO2 increase will result in a temperature increase that allows the atmosphere to hold more water and cause an even bigger temperature increase. But greenhouse effect is a relative term. Insulation would be a better term.
Water vapour reflects heat. It cares about frequency, but not direction. In a northern climate, the sun is almost directly overhead in summer. Far more energy strikes the ground than can be radiated back, and the temperature goes up. Stick a cloud in between though, and the amount of energy reflected back at the sun is greater than the amount of energy that the cloud reflects back to earth, and so it cools down. In the winter, the sun hangs low in the sky, its rays striking the ground at a sharp angle. The amount of energy from the sun is far less than what the earth is radiating back to space, so it gets cold. Stick a cloud in between, and less energy from the sun gets through, but the amount of heat radiated from the earth and reflected back goes up even more, so the warmest winter days are cloudy ones. Which is how the clouds taught the tree rings to lie.
In theory tree ring width ought to be a good indicator of climate. Warmer temperatures and a longer growing season result in thicker rings and imply a warmer climate. There are other factors of course, but those are the basics. A lot of the early tree ring studies were very successful in showing a close correlation between tree ring data and annual temperature readings. But they were done in warm or mild climates where a tree ring is just a tree ring and a cloud is just a cloud. What if they were done in Yellowknife? Or Siberia?
At the centre of the current climate controversy is tree ring data from Siberian larch trees which imply that the earth was much cooler hundreds of years ago. But the clouds are locked in a conspiracy with the trees to fool the researchers. Siberia is a harsh climate with large temperature swings between winter and summer, and one of the shortest growing seasons on the planet. In brief, the part of the year that the tree ring measures is far smaller than the part it doesn’t. Worse, the researchers only tested the efficacy of their data by testing it against local weather station readings from April through September. http://www.ncdc.noaa.gov/paleo.....a2001.html
They already “knew” that tree rings tracked climate, why test further? Had they been in Alabama or Greece where a cloud is just a cloud, they might have been right. But they were in Siberia where a cloud is either welcome shade or a much needed blanket depending on the season. Less moisture in the air means less clouds. Hotter summers and colder winters. More moisture means cooler summers… but warmer winters. Any climate reconstruction based on a short growing season in a harsh climate is missing half the data or more. Does it matter?
I don’t have access to Siberian weather station data. But Environment Canada makes much of their data available on line at http://climate.weatheroffice.e.....ada_e.html including a considerable amount of data from Yellowknife in Canada’s far north. It was fairly easy to graph the average temperature from April through September (the part of the year the tree rings measure) against the following October through March (the part of the year they don’t measure) and compare them to their long term averages. If the tree rings are indicative of climate, then the two graphs should oscillate in tandem. But if the oscillations are in opposite directions, then the tree rings aren’t telling the whole story. Following is the graph of both seasons compared to their long term averages from 1947 until 2006. The results are startling:
With exceptions in only a handful of years, warm summers were followed by cooler winters. Cool summers by warm winters. So if the researchers had modeled the climate records in Yellowknife based on tree ring data only, they would have produce a graph like this:
The tree rings measure really warm years in which the summer was 3 degrees warmer, but the year as a whole was 8. In really cold years, the summers were down as much as 2 degrees but the annual was 4. Worse, if we look at summer variance on the same graph as annual variance, we can spot multiple instances where the tree rings would have said it was warmer than normal, but the annual temperatures say it was less:
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