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):
Now, it is possible that the CO2 we are adding to the air isn't the thing causing the amount of CO2 in the air to increase. But that is akin to saying the sweetener I add to my coffee isn't the thing making it sweeter.
Datapoint #3: Increasing greenhouse gases will cause temperatures to increase.
This is the most difficult to "prove" - we can't get there with a simple chart. It is possible that 1) the greenhouse gases we are emitting do not affect the climate AND 2) temperatures are rising for another, unrelated reason.
Unfortunately, the chemistry that starts with greenhouse gases and ends with higher temperatures is fairly simple. In addition, the theory by which greenhouse gases cause increased temperatures would also result in several other effect. Scientists have a strong record of being able to predict these before they are seen. The ability to make predictions is the hallmark of a successful scientific theory.
To the second point, alternative explanations for climate change have also been tested. However, Most of the obvious ones rejected. The most obvious culprit is the Sun - Solar activity can indeed vary significantly over time. Unfortunately for skeptics, the last thirty-five years have generally featured slight solar cooling.
This evidence is strong - but it is admittedly circumstantial. The consensus view, expressed by the Intergovernmental Panel on Climate Change, is that we are only 95-100% certain that humans are the dominant cause of climate change.
Question 4: Do you believe humans can't/shouldn't try to stop climate change?
First, let's dispense with the "can't." If you agree that humans are causing climate change, then we can halt it if we choose to. Nothing physically prevents us from stopping all emissions, which would prevent the worst effects of climate change. But of course stopping all emissions immediately would lead to many problems in a modern society - mass starvation to start with. So it is a fair question to ask whether the benefit of actions needed to contain climate change are worth the cost.
Most climate scientists agree that if we can limit temperature increases to around 2 degrees above a benchmark, we will avoid most of the worst effects of climate change. And there are plans which most believe will meet this goal. These plans aren't easy to achieve, but they are in the realm of possible.
Vox recently put out a good summary of the high-level changes needed; here are the headlines:
CO2 emissions from energy and industry must decrease by 50% each decade until the 2050s.
Net emissions from land use must fall to zero in the next 40 years.
We need to develop the technology to remove 5 billion tons of CO2 from the atmosphere each year.
Yeah - this won't be easy. But we should also note the progress that is already being made. Prices for renewable energy have fallen to where they are competitive with fossil fuels. There is no reason to think this trend will stop as technology improves. Emissions for electricity production may already be peaking. Transportation is also becoming steadily more energy efficient. Carbon capture technology is still nascent, but is improving. We aren't yet on pace to limit warming to 2 degrees. We are starting to see what's possible when economics is combined with governmental-enforced regulatory incentives. But neither of these factors alone will be enough.
What is the cost of aggressive action against climate change? It seems like it should be a lot. For the first 200 years after the start of the Industrial Revolution, economic growth came only with increased energy usage, hence increased carbon emissions. But signs show this relationship is breaking down; we can now envision steady growth without steady fossil fuel use growth. Perhaps most importantly, we can protect ourselves against rampant climate change while still allowing developing countries to develop. It's not just me saying this - New Climate Economy and the IMF agree. Harm to growth from climate adaptation will likely be small enough as to not be noticed.
And compare this to the cost of doing nothing. Climate change models are famous for their uncertainty, and we don't know for sure what will happen if we continue on our current path. But so far, most global warming studies have tended to underestimate the impact. Maybe you think the probability of catastrophic climate damage is low - say 50%, even 25%. The problem is, the potential cost is so enormous that even probability weighted, the economics demand action.