Yesterday’s Weather

Recently I found myself relaxing, full of good food, after a graduation party for a friend of mine.  Considering that she qualified for her degree long after she qualified for AARP, it was something of an occasion.  She is not dumb.  Far from it.  She just has a short attention span.  (There is an upside to it, though; six more classes and she will have her masters…assuming she doesn’t change her major again.)

So the bunch of us were sitting around enjoying the smell of pine trees and the whine of mosquitos when someone opined that he didn’t believe in this Global Warming stuff.  To my surprise, a chorus of agreement went around the circle without a single challenge.

Until, of course, it got to me.

Mind you, I am not so in love with the science of meteorology that I accept anything they put out.  I think there is solid sense in the argument that asks if their supercomputer models fail so dismally in forecasting tomorrow’s weather, how well can they predict the next century?

So about tomorrow’s prognosis I have my doubts.  But I am just fascinated by the ingenuity they have shown in reconstructing yesterday’s weather.

If there is anything ephemeral, it is the weather.  A dying animal might leave his bones in the earth to allow us to infer who he was, how he lived, and even how he died.  But what trace does a sunny day leave behind?  Or a whole season of sunny days?

Why, there wasn’t even any way to reliably measure temperature until early in the eighteenth century.  And thermometers weren’t commonly available until the mid-nineteenth century.  So our first method of reconstructing the past, the historical record, becomes a purely qualitative tool.  We are reduced to recording how often someone said, “Boy, it sure is hot!” and alternating it with how often someone said, “Boy, it sure is cold!”

Hardly the model of scientific precision.  But what else can we do?

Quite a lot, as it turns out.  Instead of simply abandoning the written record as hopelessly qualitative, some people who think outside the box have found that with a little creative logic, much that is worthwhile can be interpolated.  Okay, so nobody was recording temperature.  But they were writing down when the crops came in.  And you can certainly conclude that if the grapes were ready early, it was a warmer than normal year.  Likewise if someone said the river stayed frozen till after Easter, some (tiresome) lunar calculations of when Easter fell that year will tell you if that year was colder than normal.

Compared to what, you may ask?  Since nobody but us folks takes notes about the ephemeral climate, how can one evaluate the relative accuracy of someone’s journal or farm ledger?  Is there anything else we can compare it to?

I’m glad you asked, because this is where the human ingenuity really comes in.  It’s a little like one of those junkyard challenges: Can you construct a working clock using only lumber, coral, muck & dust, and ice?

First, the lumber.  The science of dendrochronology (wonderful word) takes advantage of the fact that trees lay down one growth ring for every year of life.  The width of the annual rings is simply a function of weather.  Widely spaced rings indicate good years; narrow, crowded rings dry or cold years.

The oldest living trees, the bristlecone pines, can be over 4000 years old.  So they neatly overlap human recorded history.  But we don’t have to find the oldest living tree to create the longest record.  Dendrochronologists have figured out how to create one longer than the life span of any species of tree.

Like this: if you have a living tree that is (say) 1500 years old and you find a beam in some Scandinavian long house that was 1500 years old when it was cut down 1000 years ago, you have a 500 year overlap of a time when both trees were living and reacting to the same climate.  You align the overlapping patterns of thick versus thin rings.  Now you have one scale of the last 1000 years from your living tree and a second period with two scales for 500 years where their rings overlap.  Coordinating those two allows you to add the final section of your long house log to take your record back another 1000 years.

Then, you go to a peat bog and dig up a tree that was 1500 years old when it died 2000 years ago.  You now have another 500 year overlap and can add yet another 1000 years to your record.  By this process, the scientists have managed to create continuous, yearly climate records for dozens of areas around the world that reach back something over 5000 years.  (The longest single record is over 10,000 years.)

Next, let’s look at our next material, coral.  Coral is the skeleton secreted by marine polyps.  Two points about it are useful for measuring climates: First, the polyps deposit material at different rates in winter and summer, creating layers rather like the rings of trees.  Second, the coral is composed of calcium carbonate.  Since the carbon is extracted from the ocean which gets it from the atmosphere, coral can be dated using standard Carbon-14 techniques.  As an added bonus, the ratio of oxygen isotopes changes with changing ocean temperature.  After separating out a lot of other contributing factors, the O16/O18 ratio gives us the ambient temperature when the coral grew.

Now what was that about “muck & dust?” Well, it turns out that the sediment on the bottom of oceans and lakes is nicely laid down in layers that reflect the climate.  Some of those sediments have lain undisturbed for millions of years.  The muck of the sediment contains dust particles, microscopic animal and plant fossils, and the critical (from a paleoclimate point of view, anyway) pollen particles.

The size of the dust particles tells the strength of the winds that were blowing.  The fossils tell what plants and animals were flourishing, reflecting the climate.  But the most useful indicator is the pollen.  The amount of pollen and the mix of types is a direct reflection of the flora and hence  a measure of rainfall and temperature.  Sediment cores have been taken that give a climatic record extending back over two million years.

Finally, let’s look at what ice can tell us.  Ice at the poles, in glaciers and on mountain tops, is also laid down in layers, year by year.  By drilling a core down through the ice, we get yet another striated chronicle of events on earth.  In this case, we get multiple benefits: Obviously, the relative thickness of the layers is a function of the weather when they were laid down.  In addition, gases are trapped in the layers, allowing those same oxygen isotopic studies to be done on ambient temperatures.  Dust is found in these layers, too, as are insects and that all-important pollen.  The wealth of cross-correlated climate data in these cores is just mind-boggling.

Most importantly, the ice sheets in the polar regions have grown, undisturbed, for millions of years.  Unlike the coral and sediment layers, they have been deposited during both glacial and interglacial periods, guaranteeing a truly unbroken record.  The half-life of radioactive elements in the corals and sediments allow them to be dated, but it is the ice cores that provide the all-important cross-check.

So while we may debate endlessly about what our supercomputers are telling us about the future of earth’s climate, a staggering wealth of human ingenuity is quietly building up a detailed record of all the variables that made up its past.  Personally, I am confident that record will someday allow us to finally calibrate those wonderful weather models.

Presuming, of course, that we still have that much time.

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