(Part of the How to Talk to a Global Warming Skeptic guide)

Objection: Even the scientists don’t know that the climate is changing more than normal and if it’s our fault or not. If you read what they write it is full of “probably,” “likely,” “evidence of” and all kinds of qualifiers. If they don’t know for sure, why should we worry yet?

Answer: Probability is the language of science. There is no proof; there are no absolute certainties. Scientists are always aware that new data may overturn old theories and that human knowledge is constantly evolving. Consequently, it is viewed as unjustifiable hubris to ever claim one’s findings as unassailable.

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But in general, the older and more established a given theory becomes, the less and less likely it is that any new finding will drastically change things. Even the huge revolution in physics brought on by Einstein’s theory of relativity did not render Newton’s theories of classical mechanics useless. Classical mechanics is still used all the time; it is, quite simply, good enough for most purposes.

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But how well established is the greenhouse effect?

Greenhouse effect theory is over 100 years old. The first predictions of anthropogenic global warming came in 1896. Time has only strengthened and refined those groundbreaking conclusions. We now have decades of very detailed and sophisticated climate observations, and super computers crunching numbers in one second it would have taken a million 19th century scientists years with a slide rule to match. Even so, you will never ever get a purely scientific source saying “the future is certain.”

But what certainty there is about the basic issue is close enough to 100 percent that for all practical purposes it should be taken as 100 percent. Don’t wait any longer for scientific certainty; we are there. Every major institute that deals with climate-related science is saying AGW is here and real and dangerous, even though they will not remove the “very likely” and “strongly indicated” qualifiers. The translation of what the science is saying into the language of the public is this: Global warming is definitely happening and it is definitely because of human activities and it will definitely continue as long as CO2 keeps rising in the atmosphere.

The rest of the issue — how high will the temperature go, how fast will it get there, and how bad will this be — is much less certain. But no rational human being rushes headlong into an unknown when there is even a 10 percent chance of death or serious injury. Why should we demand 100 percent certainty before avoiding this danger? Science has given the human race a dire warning with all the urgency and certainty we should need to prompt action.

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We don’t have time or reason to wait any longer.