The first thing is to determine whether they understand why the atmosphere stays warm. It can be best to ask them to explain why they believe that heat does not rapidly escape into space. Get them to think about that. Then explain to them about the greenhouse effect and how without it the temperature of the lower atmosphere would be about zero degrees F instead of its present 57°F; the temperatures on earth without CO2 would likely have never been warm enough for life. Then explain that the level of CO2 have varied throughout the earth's history, and the climate on earth has always varied with. It is really rather simple. Ask, since the atmosphere retains a certain amount of heat, isn't it common sense that changing the gases would change it's characteristics? If the person is at all interested, you can then explain how we know that the warming is due to human emissions: warming more at night than day shows that it is the greenhouse effect and not the sun, that the increase in energy reflected by the atmosphere to the surface has been directly measured (both from satellites and the surface) and how the increase is attributed to specific greenhouse gases by the frequency of radiation. Mostly people who do not believe humans are warming the environment simple cannot imagine it -- they are unscientific and unable to imagine how science works. The comparison to people learning that the earth is round is a good one: just imagine how hard it would be to unlearn your belief that the earth is flat even though it is so obvious now.

But be aware the climate change deniers generally don't believe scientists at all. They are mostly angered and paranoid uber-conservatives who fear that there is some vast conspiracy against them. Research shows that these people don't even trust their own doctor, much less scientists. So they will deny anything. Proof does not exist to them because they believe the physics departs of every university in the world have been taken over by they conspiracy of the leftest, Marxist United Nations. You can point out that the National Academy of Science has said that global warming is "indisputable" but that won't be important to them because they believe scientists are bad people. You are best to just ignore these people. Fortunately they are mostly very old and will die off; unfortunately they are delaying any changes to adapt to or mitigate warming and are causing heavy tolls to be paid by others.

If somebody is interested enough to learn, and to learn from the NAS which is the association of the greatest scientists in America, you can refer them to the NAS. There is both a downloadable booklet and a series of videos here that explains what scientists know and what to expect from the future. This take a little investment of time, but people who are simply unaware of how much we know about climate would likely find this information beneficial.

http://nas-sites.org/americasclimatechoi...

Of course, the JimZs of the world -- who believe physics was invented as a hoax and all scientists are part of the evil conspiracy -- won't care about this.

Start with Grimm's fairy tales and go from there.

Joseph Goebbel,

“If you tell a lie big enough and keep repeating it, people will eventually come to believe it. The lie can be maintained only for such time as the State can shield the people from the political, economic and/or military consequences of the lie. It thus becomes vitally important for the State to use all of its powers to repress dissent, for the truth is the mortal enemy of the lie, and thus by extension, the truth is the greatest enemy of the State.”

AGW is a political idea, not a scientific one. Notice that all the answers that the greenies have are political in nature? If it truly were a scientific problem, wouldn't you reasonably see scientific answers?

Quotes by H.L. Mencken, famous columnist: "The whole aim of practical politics is to keep the populace alarmed ― and hence clamorous to be led to safety ― by menacing it with an endless series of hobgoblins, all of them imaginary." And, "The urge to save humanity is almost always only a false face for the urge to rule it."

You can't reasonably explain AGW to a true scientist, but you can confuse someone who is naive enough.

So in direct answer to your question, use Goebbels' principle and lie, lie, lie.

Not easy most of the public is easily led, and believe what they are told, most people don't even understand how little co2 we actually have in our atmosphere, when I have asked people I get anywhere from 3% to 40% instead of 0.039%

If you tell a myth often enough and enough people say it, it becomes common knowledge.

Its the same as for a scientist, Just leave out the bullshit.

AGW is a theory

Scientists use soft ware to make their theories..Who designs the soft ware..the scientists of course..do you think that the scientists have an agenda..it's called money..funding..without it the scientists wouldn't have job

This works fairly well for adults: http://www.amazon.com/Rough-Guide-Climat...

There are links in it for more detail and more concise overviews.

At a somewhat more intellectual level, this covers it too: http://www.aps.org/publications/apsnews/...

As a scientist, geologist by degree, I would say it is a phrase that was invented to push a political agenda and not a scientific one. It is a way that leftist again attempt to use science to fool those who are ignorant of science. They want people to believe that the world climate was stable until humans began emitting CO2. They want people to be afraid of a couple of degrees of warming so they can push their anti-human anti-capitalist agenda. It is no more complicated than that.

Wikipedia is a great place to start.

lol, I think the scientists don't even know how to explain it.

Twenty-year hiatus in rising temperatures has climate scientists puzzled

http://www.theaustralian.com.au/news/fea...

because it is naturally it occurs in nature

The melting of the polar ice caps.