I hope you played with your food when you were young. Perhaps you experimented at some point with pushing a drinking straw through Jello. If you twisted the straw as you removed it from your food, you could sometimes trap a column of gelatin in the straw. You then had the choice of either blowing the Jello at a sibling or, if your parents were at the table, gently squeezing the gelatin out of the straw onto your plate with your fingers.
Geologists take samples of ancient muck and mire in a way similar to kids playing with Jello. We bang pipes down into the soft Earth of lakebeds or peat bogs, pull them up, and push out narrow columns of mud inside. The muck is composed of many, many layers that go back in time. We geologists call this activity “coring,” and although it’s physically tough work, it’s no more complex than jamming straws into Jello.
The reason geologists make cores of mud is that low spots on the Earth can record the climate of Earth’s past. Evidence geologists get from coring lakebeds and peatbogs has taught us just how frequently both regional and global climate changes.
A Scandinavian geologist got the coring and climate story started. His name was Lennart von Post, he lived and worked around 1900, and he was the first geologist to carefully investigate what cores of muck could reveal about past climates.
Of all the places where geologists can core the Earth, our favorite spot is peat bogs. That’s because peat is the first step in the long geologic process of producing coal, and geologists are inordinately fond of all fossil fuels. So it was quite natural that von Post started coring the ancient remains of plants and mud layers that make up the peat of southern Sweden.
Little fragments of twigs and leaves can be preserved in peat, and if you can identify the species of plant that produced such material you have your first clue about past climate in a region. Von Post went to work identifying such bits of old plants, but he also had the wit to look at the ancient mire through a microscope. What he discovered was that he could identify ancient pollen in the layers of peat he was cataloging.
Pollen is surprisingly sturdy stuff. It will remain intact for literally thousands of years, lying in a layer of muck, waiting for a geologist to come along, core it, and identify the plant that produced it.
If you have allergies, you know pollen is blown around on the slightest breeze. That’s the basic fact that makes pollen much better than twigs or leaves for telling us past climate. Pollen reflects all the plants in a whole region.
If you know the identity of the whole range of plants in a region, you know pretty well what the climate must have been like, both in terms of temperature and precipitation. (Think of gardening “zones.”) And once you’ve described the pollen from a core, you can make a carbon-14 date of a twig and assign a specific age to the climate you’ve been able to deduce.
Ancient pollen makes it crystal clear that climate varies again and again over whole regions on Earth. Just for example, in northern Europe where von Post first worked, there have been ten major climate intervals in the last 15,000 thousand years. Each of these shifts was substantial.
The warmest era – when oak forests covered the lowland of Sweden – was what we geologists call “the Optimum,” the balmy times of about 6,000 to 8,000 years ago. That era was much warmer than today.
Some of the great shifts in climate were global in scope, some were only regional. And just to give us all nightmares, some of the biggest shifts in temperature occurred in just 20 years or so – well within a single human lifetime.
Studying past climates demands strength in the field, patience in the lab, strong eyes for microscope work – and plenty of courage, too. The simple but brutal fact is that major and minor climate change is woven into the fabric of the Earth itself.