Simple Sam Learns About the 4 Lines of Evidence

Posted on August 26, 2010


The Earth’s climate has changed, Sam.

“I heard you say that before,” says Simple Sam.

Temperatures all over the world have gone up in the past century, the oceans are warming, glaciers worldwide are melting, the hurricanes are howling stronger.  And the evidence tells us that greenhouse gases put in the air by human activities are the main cause.  If we go on putting greenhouse gases in the air at business-as-usual rates, global temperatures will increase “swifter, higher, stronger” over the 21st century and beyond, and that’s the kind of go-to you want from an Olympic athlete, not the climate.

“What kind of evidence do “we” have?”

There are four main lines of scientific evidence, Sam.  Get out your pen and notebook; this will be on the test.

“There’s a test?” asks Sam.

Just kidding, Sam.  I could almost see the exclamation mark over your head.

“Is that teacher humour?”

I know teacher humour is far too subtle for most students.  Anyway, Sam…

The first line of evidence is physics.  I talked about the physicist Svante Arrhenius the other day when we discussed climate feedback cycles, remember?  He was the one who first came up with basic mechanism of the greenhouse effect.  (See here.) I should also mention John Tyndall, who invented the first ratio spectrophotometer.


A single-beam spectrophotometer measures the intensity of light, Sam.  A double-beam or ratio spectrophotometer compares the intensity of light passing along two different paths.  Spectrophotometers of any sort can distinguish between different wavelengths of light, and can measure the intensity of light we can’t see, like infrared.

A spectrophotometer is useful if you want to see whether a gas—let’s say carbon dioxide—absorbs light at certain wavelengths, and if so, how effectively it absorbs it.  John Tyndall, in

fact, began research into exactly such things in 1859.  He found that among the gases he studied water vapour, carbon dioxide and ozone were/are the best absorbers of heat energy.  Svante Arrhenius, when he was developing the theory of the greenhouse effect, relied on the work of John Tyndall and others.

So when we discuss the greenhouse effect, we are simply talking about the behaviour of gases, heat transfer problems, and so on.   Sheer physics, and fairly basic physics at that.

“I never heard about it before,” says Sam.

Perhaps you fell behind in your physics reading, Sam.  Anyway …

The second line of evidence is our record of the distant past. You look back a few million years and you realize that’s there is no such thing as a stable climate, not over the long term.  In the last million years, the Earth has gone through 10 major ice ages, for instance.

We’ve been lucky.  Since the development of agriculture and the rise of urban civilization, the climate has been unusually stable.  But what we’ve learned from studying past climates is that Earth’s climate can be sensitive to relatively small influences.  Like you with your clop-clop slippers, Sam.

“Do you always have to mention the slippers?  Why are you so obsessed with my slippers?”

That’s what I mean, see?  Sensitive. To continue …

The third line of evidence is the recent climate record beginning about 1880. We know the world is warming because a worldwide array of instruments has been telling us, and it has been warming in lockstep with the rising levels of greenhouse gases in the atmosphere.  A rising level oddly coincident with the fact that we’ve been putting these gases into the atmosphere for a long time.

If you believe in coincidence.

And there’ve been other environmental changes—expected by theory—which have also happened.

What are they?”

Droughts, floods, heatwaves, increasing extreme weather events, melting glaciers, melting sea ice, defrosting permafrost, rising heat content in the oceans.

“Oh, them.”

And the fourth line of evidence comes from climate models.


Yes, Sam, climate models, together with physics and what we’ve learn about past climates, tell us that, unless we slow down our relentless program of injecting greenhouse gases into the atmosphere, unless we stabilize greenhouse gas concentrations at a reasonable level, somehow lower than they are today, then global temperatures will continue to go up and up, perhaps uncontrollably.

If that is the case, if we continue our “business-as-usual” pace, then the killer heatwave that Russia recently experienced, the floods in Pakistan and China, the crop-killing droughts which are already reducing the Earth’s agricultural capacity, all these will be “business-as-usual” in a generation or two.

“The sky is falling.”

We have to take it seriously.  According to climate models, Sam, by 2100, the average global temperature will have gone up between 2°C and 7°C higher than it used to be just a century ago.  That’s a 3.6° to 12.6° temperature rise in Fahrenheit.  The difference between 2 and 7, between 3.6 and 12.6, depends on what we do about greenhouse gas emissions, firstly, but also on the ways different models weigh the sensitivity of climate to the things we’re throwing at it.  There are suggestions in the long historical record, as I said, that small disturbances can lead to large effects.

“You’re going to have to give me some evidence of that some time.”

I will, Sam. –Of course, nobody is suggesting that climate change is going to stop in 2100.  It will certainly continue, and it may go on becoming more extreme as slow-moving responses in the climate system rev up and kick in over time.  Climate models can’t even deal with slow-motion responses, which is kind of scary since there are bound to be some. –But sufficient unto the century are the troubles thereof, eh, Sam?

“Did somebody say that?”

Didn’t you just hear me?  –So what would the world look like with 2°C of warming?  Not the same as now, that’s for sure.  More heat waves, and more intense.  Fewer cold spells. And since more heat means more evaporation means more water vapour in the air, we can also expect more rainfall on a global scale.

There’ll be changes in rainfall patterns.  More here, less there.  Usually more where you already have it, and less where you already don’t have enough.  Higher plant productivity in some places, maybe, but decreases in others.

Ecosystems both on land and water will be disturbed.  As species are stressed, biodiversity will suffer.  Food production will be endangered in some regions.  There’ll be rising sea levels, and continuing decreases in Arctic sea ice.

Have you seen that sign in a local window repair business?  No, I guess you don’t get out much, Sam, being fictional and all.  Anyway, it says, ‘Thank goodness for little boys with slingshots.’ Yes, some parts of climate change might benefit some people here and there—though I wouldn’t bet on it—still I think, all in all, nobody’s going to like it much.

Of course, liking it or not liking it will not matter at all if the world warms by 7°C (12.6°F).  Then the survival of entire societies will be at issue. At 7°C of warming the world and its climate will have Jekyll and Hyde’d so much, it will hardly be recognizable.  And everything I have already mentioned will be worse and more extreme.

The world has seen climates like the ones envisioned under 7°C of warming, as some of your denier brethren have pointed out, Sam, but human civilization hasn’t, and almost never has the transition into different climates been so rapid.  The rapidity and size of the transition might overtake the ability of many countries and species to adapt.

“Wow, we know a lot, don’t we?”

Is that sarcasm, Sam?  –Yes, there are uncertainties in what we know, in climate science itself.  We don’t know, really, how much warming will result in a given case because we’ve never threatened our civilization with climate disaster before.  But we can get some idea by studying past climates.

Also, it’s hard to achieve detail in climate models, as for instance over small regions.  And predicting rainfall is particularly tricky.

We also don’t know exactly where the tipping points are.  Scientists were surprised recently by the rapid melting of the Arctic sea ice cover, which, because that melting altered the albedo of the Arctic Sea, has greatly accelerated warming in the Arctic.

“So why are we supposed to act, and tax, and cut back, when we don’t really know for sure?”

We know more than enough to be frightened, Sam.  Scientists are frightened.  Some scientists have moved past fear to cynicism.  And anyway, not knowing works in both directions.

Let me put it this way.  We know an elephant has escaped from the zoo, but we don’t know whether the lions have as well.  A guy you don’t really like says they did.  Maybe you should pay attention to the guy, because the zoo is just down the street, and you just now heard a rustling out back in the bushes.

The rustling might be a lion.  There were lions in the zoo, and the zoo has been breached, and somebody said they escaped.

But you’re not really sure.

Would you send your grandchildren out back to check?


Read further about John Tyndall @

Posted in: Climate School