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September 28, 2007

Pictures Trump Words


National Geographic has always been better at pictures than words. I particularly like the big foldout charts and maps that come with the magazine. Couldn’t find this one online so had to fold, scan, and reassemble. It’s about the relationship between global warming, CO2 content in the atmosphere, and sea level over the last 400,000 years. The interesting thing is that the picture DOESN’T show what the accompanying words say. This post is a practical exercise in the skepticism recommended in the last post – especially around critically important issues like global warming and rising sea levels where we must take some action or act by not acting.

To put this in context (not shown in the picture) the earth has been in a glacial epoch (a period during which there is extensive year-round ice somewhere on the planet) for the last 40 million years although, for most of its history, it has apparently been ice-free even at the polls. During a glacial epoch the amount of earth covered by ice varies periodically and, for at least the last 400,000 years (not a big sample) there has been a regular 100,000 year cycle to global warming and cooling. There are usually about 10,000 noticeably warmer years (the interglacial period) in each 100,000 year cycle; the rest of the time it’s cold. Each panel above represents about 100,000 years cropped so that the warm period is always on the right.

The burnt orange at the bottom is seal level. If you look at the far right, you see the level today and note that the sea is generally much lower during the colder periods when a lot of water is tied up in ice sheets on land. The lighter orange is average global temperature (subject to some debate); the temperature is shown in yellow where it exceeds current temperatures. The light blue line is atmospheric CO2 concentration as measured from Antarctic ice cores (except for last 50 years which are measured directly). This line leaps in recent years to concentrations more than 30% higher than previous observed peaks during the last 400 thousand years.

The words on the chart say:

“The warm spells… occur every 100,000 years or so and last about 10,000 years, driven by changes in earth’s orbit and orientation. Historically, temperatures rose first, then CO2 increased, accelerating temperature rise. Sea levels followed in turn…”

Well, that all makes sense according to the current dogma; but it’s not what the pictures show in three of the panels. Look at the left-most panel: note that the sea did all of its rising BEFORE either temperature or CO2 increased from their periodic lows and that the sea levels started down before the peak of the warming. Things that come after can’t cause things that come before.

In the fourth panel, the one we live in, and in the third panel, sea levels start to increase before warming begins although both then proceed together. Only in the second panel do rising sea levels and rising temperatures appear to co-occur.

It could be that the rising sea level cause warming. We can easily tell ourselves a story to explain that. When temperatures are low and there is little ice-free ocean, there is little evaporation from the sea. Snow fall decreases. Glaciers do sublimate (evaporate into the air) as well as tumble into the sea. If there isn’t enough snow to balance the ice loss, the extent of glaciation goes down, the earth is less reflective, it absorbs more sunlight, and it heats up. No reason to believe this story either – or think it is the “only” explanation; but we can tell from the pictures that neither warming nor rising CO2 levels are responsible for triggering the rise in sea levels (although they may accelerate it) since the sea levels “usually” (in our small sample) start to rise first.

Let’s take a closer look at our own time. Here it is bigger below:



Note that temperatures have been on a plateau for the last eight thousand years or so, unlike the other three periods where there was a sharp peak. This is unlikely to have been caused by our ancestors who were scarce and not yet indulging in agriculture let alone industry. Also note that during much of the last eight thousand years, temperatures have been HIGHER than today. The words on the chart say “11 of the 12 warmest years on record have occurred since 1995.” Yeah, but we haven’t been keeping records for very long. Temperatures are shown significantly higher (and sea levels and CO2 concentrations lower) about six thousand years ago.

This interglacial period, known as the Holocene, IS different than the three which preceded it: the temperature and sea level graphs are flat rather than peaked and it’s already lasted two thousand years longer than usual. This departure from recent history is NOT anthropogenic even though the very recent rise in CO2 concentration probably is.

It’s dangerous to conclude anything at all by looking at just 400,000 years of data from earth’s 4 plus billion year life. It’s possible that ALL the apparent patterns we see here are random. But even a small amount of data can be enough to disprove a hypothesis. This data, for example, shows that it is incorrect to say or imply that rising sea levels are always caused by or always follow (two different statement if you read them carefully) rising temperatures. So lowering the temperature (even if we were able) might not make the sea stop rising and higher temperatures could lead to lower seas – perhaps because of more snowfall.

The data also shows that temperatures today are NOT historically high, even by Holocene standards and that temperatures have been higher when CO2 concentrations are lower. That DOES NOT invalidate the prediction that the historically high CO2 concentrations will lead to higher temperatures and higher sea levels; it just makes the forecast less certain and should make us all more determined to question assertions that a specific increase in CO2 will certainly lead to a known increase in global temperature and a known increase in sea levels. Climate is too chaotic for such predictions; too little is known about the cause of past dramatic changes in all three variables.   

See the previous post for whether skepticism means inaction.

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