Climate change isn’t new. Our planet is restless and its environment rarely stays still for long. There have been times in the distant past when carbon dioxide levels were much higher than they are today and Antarctica was a tropical paradise. There have been others when carbon dioxide levels were much lower and even the equator was encrusted with ice.
But over the past ten thousand years, the time during which human civilization has existed, Earth’s climate has been unusually steady. We humans have become used to a world where the way things are is more or less the way they will be, at least when it comes to temperature. In other words, we have been lucky.
Now our steady reliable climate is changing, and this time nature isn’t to blame. But how do we know for certain that the world is warming, and how can we identify the culprit?
The Heat Is On
When you’re trying to determine whether the world’s temperature is rising, the biggest problem is picking out a signal from the background “noise.” Even in our relatively stable times, temperatures lurch up and down from one day to another, from season to season, from year to year and from place to place. To be sure that the underlying trend is changing, you need to take precise measurements from many different places around the world, and do so for an extremely long time.
We do have a few long temperature records, thanks to certain individuals who decided to make the measurements just in case they ever proved useful. The world’s longest is the Central England Temperature Record, which is a tribute to the obsessive data-collecting habits of seventeenth-century British natural scientists. It covers a triangular region of England from London to Bristol to Lancashire and stretches back to 1659. This impressive record shows clear signs of warming, especially toward the end of the twentieth century.
However, the record covers only a tiny part of the globe. Changes in England don’t necessarily reflect changes in the United States, say, or Brazil. It also doesn’t go back far enough to reveal just how unusual our recent warm temperatures really are. How do they compare, for instance, to the apparent warm period in medieval times when the Vikings settled a verdant, pleasant “Greenland” and there were vineyards in northern England? Or to the so-called Little Ice Age in the midcenturies of the last millennium, when the River Thames in London froze over completely so that frost fairs were held on its solid surface?
To answer these questions, scientists have come up with ingenious ways to expand the records geographically and extend them backward in time. Some people have tried to interpret written archives that didn’t quote actual temperatures,1 but the best way is to look at records written not by humans, but by nature.
Every year, the average tree grows a ring of new wood around its trunk. In a good year the ring will be thicker, in a bad year, thinner.2 Researchers drill a small core into the side of the tree, about the diameter of a wine cork, extract the wood, and then count and measure. By examining trees that are different ages, and even some trees that are long dead but have been preserved in peaty bogs, they have come up with a temperature record spanning more than a thousand years and from regions across northern Europe, Russia, and North America.
For more tropical regions, corals can play a similar role since they, like trees, grow a new ring every year. And in the frozen north and south (and the snowcapped peaks of tropical mountains), ice also contains a record book of past climate. Each year’s snowfall buries the previous one. If temperatures are cold enough, the snow stays around long enough to be squeezed into ice, clearly marking out the annual layers because summer’s snow crystals are larger than winter’s, or because more dust blows in each year with the winter winds. The amount of snow that fell in a given year, and especially the changing nature of the oxygen atoms bound up in the ice,3 gives clues as to how warm it was then.
Another clue comes from changing plant life, as written into the record of mud at the bottom of lakes. As temperature rises and falls, different plants flourish and each one sheds its pollen into passing currents of air. Some of this lands on the surface of a nearby lake, before slowly sinking into the mud beneath. Drill a hole in this mud, collect and analyze the pollen grains each layer contains, and you have yet another record of temperature changes over time.
Researchers have now used a host of different ways like these to analyze and splice together these different measures, and all come to strikingly similar conclusions for temperatures over the last thousand years.4 The eleventh century was indeed relatively warm, corresponding to the Medieval Warm Period. (“Verdant” Greenland turned out to be more of a marketing exercise than the truth. Ice cores drilled into the heart of Greenland’s ice cap show that a substantial quantity of ice has been present on the island for hundreds of thousands of years. Any Vikings who fell for the hype must have had an unpleasant shock when they arrived.)