How do we know global warming is not a natural cycle?


If the Earth’s temperature had been steady for millions of years and only started rising in the past half century or so, the answer would be obvious.

But the temperature has not been steady at all. Twenty thousand years ago, the planet was going through an ice age, with glaciers covering much of Europe and North America. A hundred million years ago, the Earth was so warm that crocodile-like creatures prowled through tropical vegetation not far from the North Pole.

In other words, climate variations are not at all unusual. So how do we know this one is caused by humans?

Several reasons. First, we know that burning coal, oil, and gas releases heat-trapping carbon dioxide, or CO2. And we know, thanks to careful measurements that started in the late 1950s, that CO2 levels in the atmosphere have been steadily climbing as we burn more. Not only that — chemists can tell the difference between the CO2 released naturally by plants and animals and the CO2 from burning fossil fuels. About a quarter of the CO2 now in the atmosphere is the result of human activity.

The one obvious natural suspect — the Sun — can pretty much be ruled out. It is an obvious suspect because astronomers know that the Sun can vary in brightness. When it does, the amount of heat it sends to the Earth varies too. It would not take much brightening to cause the increases in temperature we have seen. But satellites have been monitoring the Sun since the 1970s — when the fastest warming has been taking place — and the brightening just is not there.

There is plenty of other evidence — the pattern of warming, for example, which is greatest in the Arctic, and the pace, which is faster (as best we can tell) than prehistoric warming episodes. The upper atmosphere has actually cooled, because so much heat has been trapped below.

The bottom line is that nobody has come up with a natural explanation for the current warming episode that fits the observations. At the same time, the un-natural explanation—that our industrial civilization is a big part of the cause—fits the evidence.

That’s how we know it is not natural. One last thing. If the Earth has survived earlier warming episodes, what is so bad about this one even if it is not natural? The problem is that our civilization — where cities are located, where we grow food, where we get fresh water—is all based on the climate we have experienced for the last 10,000 years. So are many of the world’s ecosystems. If the climate changes, many of those things will suddenly find themselves in the wrong place.

How do we know?

Two NASA satellites, known as ACE and STEREO, measure the brightness of the Sun from Earth orbit, where the dimming effects of our atmosphere are minimal. Another satellite, called Aqua, looks down with an instrument called AIRS at the atmosphere from above to measure how much carbon dioxide is in it. And, as we discussed in the answer to the first question, we are measuring the Earth’s radiation budget from space.