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Today’s temperatures and levels of heat-trapping gases are already unusual in the context of the last one hundred and fifty years. Using “natural thermometers” such as tree rings, coral reefs, and ice cores, it’s possible to go back thousands of years. And when we do so, the picture doesn’t change one bit.

Over the last six thousand years, the earth has been gradually cooling— until the last century, that is. But there have been no major natural cycles in Earth’s temperature and greenhouse gases over that time, and no conditions that compare to today.

It is only with the beginning of the Industrial Era in the 1800s that we see sudden, large increases in carbon dioxide, followed by temperature increases. So the idea of a younger Earth is not compatible with the argument that it’s just a natural cycle. There are no natural cycles over the last six thousand years anywhere near the magnitude of what is happening today.

Well, what if we are comfortable with the idea of an Earth that is millions of years old? Then, does the “it’s just a natural cycle” argument work?

Actually— no, it doesn’t even work then.

Over time scales that assume an older Earth, records show that over at least the last 800,000 years the earth has undergone very large natural cycles, from frozen ice ages to warm interglacial periods. But today, our temperature is at the upper end of the range of what is normal for any of those cycles. And even more telling, since the 1800s our levels of carbon dioxide and other greenhouse gases in the atmosphere have soared far beyond any levels seen during these natural cycles.

So even the natural cycles tell us that the conditions we are seeing today are extremely unusual in the context of the earth’s history, whether young or old.

Here are temperatures and carbon dioxide levels over the last 6,000 years:

Natural thermometers extend our temperature records back many thousands of years. Average temperatures based on natural records (red line) show that temperatures have cooled slightly over the last 6,000 years, and that carbon dioxide levels in the atmosphere (blue line) have remained relatively constant. Not until the beginning of the Industrial Era do carbon dioxide levels and temperature begin a startling rise.

Here are temperatures and carbon dioxide levels over the last 800,000 years:


And here’s where you can find the ice core and “natural thermometer” data yourself, from NOAA.

Want more? Here's an Encyclopedia of the Earth explanation of the long-term natural cycles that are thought to have caused the ice ages and warm interglacial periods like the one we're in now, over tens of thousands of years; and the short-term cycles like El Niño, that vary from year to year.