Monday, June 23, 2008

Global warming and cooling - the view from 50-plus million years

kw: opinion, global warming

Body of a Letter to my Dad: All my brothers are politically liberal, and one in particular apparently believes "Global Warming is All Our Fault". My Dad and I are more conservative. I am probably more moderate, because I believe human activity is responsible for between one-quarter and one-half of the rise in global temperatures since the 1950s. However, I do not give much credence to the gloomy predictions found in an increasing number of books and "journalism" related to the subject.

I have gathered four charts found in the Wikipedia Commons, a great resource for materials that can be used freely, as long as their source is referenced. Click on any of these images to see a larger version, typically 600-700 pixels wide.

The first chart shows a composite summary of Holocene temperatures, a weighted average of many proxies, some of which are shown in various colors, to illustrate the uncertainty of such a record. A proxy is something we can measure today, such as Oxygen isotope ratios in tree rings or clam shells, that have a known relationship to global temperature. The Holocene period, also called "Recent", is the past 11,000 years, since the end of the most recent Ice Age.

As far as I can find out, the zero-temperature-difference line is pegged to the 1980 global temperature, about a quarter degree C (half a degree F) warmer than that in 1950. First pay attention to the inset, which covers the Christian Era. This began with a bit of cooling about 100AD, then warmed toward the Medieval Optimum which ended with the Little Ice Age (LIA) that ran from the 1400s to about 1880.

On a longer scale we find that, since just over 10,000 years ago, the Holocene has been both warmer and cooler than today, but overall it has been remarkably stable, within half a degree C of the 1980s value. On a particular day of the week, a degree or two is too small to notice, but the amount of energy in a one-degree shift in global average temperature is quite significant.

The period from 4,000 to 8,000 years ago, marked "Climatic Optimum?", marks the period when agriculture and citified civilization became widespread. All of the modern grains and other major crops were domesticated during this period. Funny thing...for the past 4,000 years, these crops have been "thriving" in a somewhat cooler world than that to which they are best adapted. It may be that the "2004" temperature suits them better!

This chart's scale is too small to show the "hockey stick" graph we often see, showing just the past 2,000 years, which were extra-stable (even including LIA), followed by a one-degree uptick beginning less than 100 years ago. Interestingly, that uptick almost matches the one that occurred 8,200 years ago. One human lifetime is such a short interval...

Let's expand the scale by a factor of forty, and look at the entire "Ice Ages" record, as it was thought of until recently. These proxy temperatures are from two ice cores extracted in Antarctica; the EPICA core goes back nearly one million years. I'd hate to be the guy that counted 850,000+ dust bands!

Concentrate first on the blue line. Four glacial periods, and the end of an earlier, fifth one, plus five interglacial periods, are evident. Both the black line and the blue line show that the modern interglacial period, the Holocene, is the coolest of the far. We'll get back to this later on. The most recent prior interglacial period is called the Eemian, and it began with a pulse that got as warm as five degrees C (9°F) above the 1950s zero line, for a thousand or two thousand years. The key element here is the roughly 100,000-year cycle of ice ages. Earlier on, they were more frequent, as shown in the next chart.

This chart is composed of data from several ice cores and a number of other proxies. It goes back halfway to the most recent glaciation of Antarctica, which was 12 million years ago. The primarily 100,000-year ice cycle began just over a million years ago. Prior to that, it was closer to 40,000 years. The gradually-increasing amplitude of icing events beginning 3 million years ago, late in the Pliocene era, is probably associated with the irruption of Panama cutting off the equatorial Pacific and Atlantic oceans, and their gradual adjustment to being connected only via the Southern Ocean.

The 2.5 million years prior to the Holocene comprise the Pleistocene, which began geologically when temperate-latitude soil profiles began to be interrupted by periodic glaciation. Prior to that, the Pliocene was almost always warmer than today. Now let's take one more scale expansion, to look at climate since the large Dinosaurs were wiped out and the smaller ones began to evolve into birds.

The whole prior chart fits into the blue rectangle at lower left. The bright blue scale shows that, prior to ten million years ago, the climate was, with one short exception, warmer than three degrees C (five or six deg. F) warmer than the 1950 zero line. Prior to the late Eocene, or prior to 34 million years ago, the warming, compared to today, was four to twelve (!) degrees C, or 7-22 degrees F.

Let's look briefly at the spike called PETM, in the yellowish area. This is the Paleocene-Eocene Thermal Maximum. It was apparently caused by a sudden gush of carbonic gases into the atmosphere when volcanoes began to erupt in a new area of the seabed, releasing billions of tons of methane clathrates. Methane gas oxidizes in the atmosphere to carbon dioxide, on a scale of a few decades. But here, for more than a thousand years, these two gases were put into the atmosphere at a rate that roughly equals the human-caused influx of today. So, a thousand years of excess carbonic gases caused a sudden temperature excursion of about two degrees C. That's it. And when the volcanoes cooled down, so did the planet...just as fast.

Based on all the forgoing, we can ask a few questions:
  • Predictions are being made that the globe will warm as much as six degrees C (11 °F) by the year 2100AD, or around 150 years of human emission of carbonic gases. How do such predictions square with the PETM of two °C after ten centuries?
  • Recent coral bleaching events have led to predictions that another degree or two of warming will cause the extinction of corals. Corals of all ages are found, including those that thrived during the "Eocene Optimum" when even polar oceans were bathtub-warm. Have corals become so fragile?
  • Antarctica froze over for the first time 34 million years ago, them melted off suddenly 24 million years ago. All without benefit of people burning coal. What happened?
  • It is known that many crops grow best when carbon dioxide is five times as abundant as it is today. Such levels were common in earlier times. Why would a rise in CO2 today be such a threat?
  • CO2 levels during the Pleistocene have been lower than in any earlier time. This is probably due to C4 photosynthesis evolving in the Pliocene or Miocene. If there were no C4 photosynthesis, isn't it likely that Carbon Dioxide would remain at a much higher level than today?
  • Based on the former questions, doesn't it make sense to talk about growing grass rather than trees to sequester the gas? Trees don't grow very efficiently at the low modern concentration of CO2 gas!
  • There are many claims of extinction, perhaps of a quarter to a half of all species. (Note, "all animal and plant species" is meant. Bacteria have much wider tolerance for thermal swings) Neither the PETM nor the sudden Antarctic melting prior the Miocene caused such levels of extinction. Why the hype?
  • A closer look at the second chart shows smaller wiggles that seem to cover 1,000-3,000 years each, with a height of a degree or less, mainly during the cooler, icy phases. These are harder to trace in the interglacial periods. However, Gerald Bond and his colleagues discerned eight "Bond events" since 11,000 years ago, the most recent of which is the LIA. We are in a warming phase betwen Bond events. How does this affect global warming predictions? This last is why I say our gases may be responsible for no more than one-quarter of the recent warming.
I am no "global warming denier", but neither am I an uncritical believer. As a Geologist, I am accustomed to thinking in tens to hundreds of millions of years. On such a scale, researchers (whether humans or super-roaches) of the distant future are likely to look upon our period as a mini-PETM at worst. We're going to run out of fossil fuels before we can spit out as much gas as the PETM event did. And it was a two-degree anomaly. We have no clue to what caused the ten degrees C of warming that it sat on top of!

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