So you don't "believe" in climate change...
It's something you hear a lot - "Oh, I just don't believe in climate change." At a first glance it does look like a ridiculous notion. The human-industrial era has lasted for approximately 0.00001% of the planet's 4.5 billion year history. Our planet is enormous - the atmosphere weighs around 5,100,000,000,000,000,000 kgs. And there aren't that many people, relatively speaking. Trees outnumber people by around 420 to 1. Are we so self-important as to think we are having a measurable effect on such a system?
I have respect for those who profess uncertainty about climate change. Climate change goes against our experiences - everything else we've created has had a short time span. And the science behind global warming is complex - I don't claim to understand it all. But using the usual Bayesian framework, twenty years of new information has moved me from a skeptic to somebody who is fairly certain the scientific consensus is right.
This is all just a convoluted way to say that I often get in fights with climate skeptics. My vast experience in said arguments has taught me that there is a strong diversity of opinion as to why climate science isn't real. About half of the US population is not very concerned about climate change, but they shouldn't be lumped together into a single bucket. The different groups have forced me to build an argument which can reach people on a continuum of skepticism. My approach involves a series of questions, moving steadily up the climate change scale.
Question 1: Do you think the world is getting warmer?
At the base level of those against taking action on climate change are those who think that the climate is literally not changing. They may have just experienced a colder than usual summer or a snowfall in April. You only have to look back 40 years to see concerns about "global cooling." Even if this long-debunked view was never a consensus, it's easy to understand why non-experts would be troubled by the controversy.
Many people who believe the world is not warming are confusing weather with climate. Weather is what it is like outside right now; climate is the average of weather over a period of time. Some locations are cooler today than they were 50 or 100 years ago. But as the below series of average global temperatures shows, the larger trend is decidedly one way.
Once we are in agreement that yes, the planet is getting warmer, you can move to the next question.
Question 2: Do you think the current trend is just a normal fluctuation?
The Earth's climate has changed dramatically over time. The Paleocene-Eocene Thermal Maximum Era featured global temperatures at least 5 degrees Celsius warmer than now, and stayed that way for millions of years. The recent Ice Age had average surface temperatures 5-7 degrees cooler than today. It only warmed up around 10,000 years ago, a blink in the lifetime of our planet. The 1-3 degrees seems within the normal fluctuations.
The best illustration of this fallacy, as so often is the case, comes from xkcd. The image is too large to reasonably include here, so if you haven't seen it, you should go look. Here is a normal sized xkcd comic to fill the gap:
Welcome back. So, you see the issue. Yes, the world regularly experiences climate fluctuations. No, none of them have moved with nearly the pace of the current warming period. On to Question 3.
Question 3: Do you believe the unusual change is due to human behavior?
Questions 1 and 2 are dispensed with fairly easily - we have data that leaves little room for contradiction. Question 3 is trickier - proving a human cause uses raw data, but also requires an inference.
Datapoint #1: Human activity is producing an accelerating amount of greenhouse gases.
Unlike global average temperature, you have observed this with your own eyes: your car's exhaust, the factory on the edge of town, smog over Los Angeles or Beijing. While there are sources of greenhouse gases other than human activity, human activity is adding our fair share. Here is a chart of man-made carbon dioxide emissions over the last 150 years:
Yes, that is 30 billion tons per year. It's a lot of carbon.
Datapoint #2: The amount of greenhouse gases in the atmosphere is increasing.
So we are adding things to the air. But as we said above - the atmosphere is huge. Our annual emissions represent only around 1/200,000th of the atmosphere's total mass each year. The planet has various natural ways to counterbalance this change; maybe plants are thriving and consuming all that extra CO2?
Unfortunately, it seems the plants are losing the battle. Here are atmospheric CO2 levels going back 400,000 years (ppm = parts per million):