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 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 and , measure the brightness of the Sun from Earth orbit, where the dimming effects of our atmosphere are minimal. Another satellite, called , 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.