Graphic: How We Know We’re Causing Global Warming

by John Cook, in a Skeptical Science cross-post

In 1859, physicist John Tyndall ran an experiment demonstrating the greenhouse effect. Visible sunlight easily passes through our atmosphere to warm the Earth. However, invisible heat rays rising from the Earth’s surface, otherwise known as infrared radiation, don’t easily escape back to space. What Tyndall showed by shining heat rays through tubes filled with different gases is that certain gases like water vapour and carbon dioxide block the heat rays. These became known as greenhouse gases.

Tyndall also made several predictions of what we should expect to see if greenhouse gases were causing warming (Tyndall 1861). In fact, we expect to see a number of distinctive greenhouse patterns in global warming. Observing these patterns strengthens the evidence that humans are causing global warming, as well as eliminates other possible natural causes. Let’s have a look at the many human fingerprints on climate change:

Humans are raising CO2 levels

The first point to establish is that humans are the cause of the rise in atmospheric CO2 levels. This fact is common sense. The amount of CO2 in the atmosphere is going up by around 15 billion tonnes per year. Humans are emitting around twice that much! On top of this, there are a number of lines of evidence to confirm that we’re the cause of rising CO2 levels.


When we measure the type of carbon accumulating in the atmosphere, we observe more of the type of carbon that comes from fossil fuels (Manning 2006). As you burn fossil fuels, you take oxygen out of the atmosphere. Measured oxygen levels are falling in line with the amount of carbon dioxide rising (Manning 2006). There’s been a sharp rise in “fossil fuel carbon” in corals (Pelejero 2005) and sea sponges (Swart 2010). Anthropogenic CO2 is penetrating even to the ocean depths (Murata 2010). Measurements of radiocarbon in tree-rings confirms human activity is the cause of rising CO2 (Levin 2000). Even the pages of ancient books trace the rising effects of fossil fuel pollution going back to beginnings of the industrial revolution (Yakir 2011).

So many independent lines of evidence (and common sense) confirm that yes, we are responsible for the recent rise in atmospheric CO2.

The extra CO2 is trapping heat

Our understanding of the greenhouse effect provides a number of verifiable predictions. If carbon dioxide is trapping more heat, we should see less heat escaping to space. Satellites measuring infrared radiation coming from Earth find less heat escaping to space over the last few decades, at those exact wavelengths that carbon dioxide absorbs energy (Harries 2001, Griggs 2004, Chen 2007). The researchers who analysed this data described this as:

“…direct experimental evidence for a significant increase in the Earth’s greenhouse effect”.Harries 2001

If less heat is escaping to space, there’s only one place it can go — back to the Earth’s surface. Scientists check this by measuring infrared heat coming down from the atmosphere. These measurements confirmed the satellite data — more heat is returning to the Earth’s surface (Philipona 2004, Evans 2006, Wang 2009). This extra piece of evidence upon the existing body of evidence led scientists to conclude that:

“This experimental data should effectively end the argument by skeptics that no experimental evidence exists for the connection between greenhouse gas increases in the atmosphere and global warming.”Evans 2006

Unfortunately the scientists underestimated the human capacity to ignore evidence staring us in the face.

Global warming has a distinct greenhouse signature

As far back as the mid 1800s, Tyndall predicted that greenhouse warming should cause nights to warm faster than days. This is because at night, the Earth’s surface cools by radiating heat out to space. Greenhouse gases trap some of this heat, slowing the night-time cooling. It took over 130 years before Tyndall’s prediction was confirmed. Over the last few decades, surface measurements have observed nights warming faster than days (Braganza 2004, Alexander 2006, Zhou 2009).


Tyndall made another prediction of what greenhouse warming should look like. Just as greenhouse gases slow down nighttime cooling, they also slow down winter cooling. So Tyndall anticipated winters warming faster than summers. Again, recent analysis of temperature trends over the last few decades bear this out (Braganza et al 2003, Braganza et al 2004). Both thermometers and satellites find winters warming faster than summers.

And the evidence continues to build. Another distinctive greenhouse pattern can be found in the atmosphere. With heat being trapped, we expect to see the lower atmosphere to warm. But with less heat escaping to space and more carbon dioxide in the stratosphere, we also expect to see the upper atmosphere cool. Satellites and weather balloons both observe this curious contrast between upper cooling and lower warming (Jones 2003).

With the lower atmosphere (the troposphere) warming and the upper atmosphere (the stratosphere) cooling, the boundary between the troposphere and stratosphere, otherwise known as the tropopause, should rise as a consequence of greenhouse warming. This has been observed (Santer 2003). An even higher layer of the atmosphere, the ionosphere, is expected to cool and contract in response to greenhouse warming. Satellites measure this effect (Laštovika 2006). We are changing the very structure of our atmosphere.

What’s fascinating about all these greenhouse signatures is they also rule out a number of other potential causes of global warming. If the sun was causing global warming, it would cause summers to warm faster than winter, days to warm faster than nights and the upper atmosphere to warm. Observations rule out the sun.

Similarly, the pattern of ocean warming rules out ocean cycles as the driver of global warming. The world’s oceans have been building up heat over the past half century. This isn’t a case of heat shifting around due to ocean cycles but the entire global ocean system building up heat. The specific pattern of ocean warming, with heat penetrating from the surface, can only be explained by greenhouse warming (Barnett 2005).

If it walks like a duck and quacks like a duck…

Current global warming shows all the distinctive signatures of greenhouse warming. To be skeptical that humans are causing global warming, you must believe two things. Something unknown is causing warming that happens to mirror the greenhouse effect. And something unknown is somehow suppressing the well understood (and well observed) greenhouse effect. So we can accept what we know to be true (greenhouse warming) or we accept two unknowns.


The saying goes if it walks like a duck and quacks like a duck, then it must be a duck. But climate skeptics are trying to convince us it’s some other, undefined animal impersonating a duck that’s also mysteriously hiding the real duck.

H/T to James Powell, Scott Mandia and Lou Grinzo whose words inspired this post. The “How we know we’re causing global warming” graphic has been added to the Climate Graphics resource and with a Creative Commons Licence, is free to be published elsewhere.

John Cook

Related Post:

Below are old comments from the earlier Facebook commenting system:

It truly amazes me that I keep seeing the same weak argument over and over. “The Earth warmed naturally in the past, so humans can’t be causing it now”.

Let’s break this down into the very simplest terms. If you put some food on a rock in the hot sun, it will warm up. That is natural warming. If you build a fire and put your food over it, it will also get warmer. That is man-made warming.

The existence of natural warming does not deny the existence of man-made warming. Cavemen understood this principle 100,000 years ago when they learned to make fire and cook food. They knew that just as it is possible to have natural warming, it is also possible to have man-made warming.

So now it is the 21st century. How come today’s deniers cannot reason out what cavemen understood 100,000 years ago? Are people really that dense?

6 · Like · Reply · Subscribe · August 10 at 2:14pm

Rakesh Malik · Top Commenter · Photographer/Owner at White Crane Photography

“So now it is the 21st century. How come today’s deniers cannot reason out what cavemen understood 100,000 years ago? Are people really that dense?”

Sadly, dense seems to be more the rule than the exception these days. Even in the face of empirical observational evidence, people point to the chilly Pacific NW summer and say, “Where’s global warming?” because they’re apparently oblivious to the severe heatwave that’s covering the rest of the country…

1 · Like · Reply · August 10 at 4:09pm

Jim Balter · Top Commenter · Santa Barbara, California

“global warming” does not mean that it’s warm globally (everywhere), it means that the *average* temperature of the globe is rising. People need to be educated, but it’s hard to do that when most are plugged into corporate media.

1 · Like · Reply · August 10 at 7:35pm

Mark Dolce

I would say CO2 emitted by burning fossil fuels and other human activity contributes to global warming — not causes it, because the earth goes through it’s own warming/cooling cycle without any help from anyone, right?

4 · Like · Reply · Subscribe · August 10 at 11:16am

Bart Laws · Top Commenter · Assistant Professor at Brown University

Well yeah, except that right now it wouldn’t be warming except for human emissions of greenhouse gases. So no, that isn’t right.

Like · Reply · August 10 at 11:23am

Joan Savage · Top Commenter · SUNY-ESF

Fossil combustion causes the net increase in temperature because combustion gases inhibit normal cooling. It’s not gently adding on, it’s snuffing out part of the normal process.One of the markers on the graphic is the increase in night time temperature, “nights warming faster than days.”

Like · Reply · August 10 at 11:55am

Nicholas Berini · Hoboken, New Jersey

Mark — “is the main contributor to current global warming” I guess would be more precise language.

John — I think this post is absolutely fantastic. The one thing I don’t love is the use of the word “curious” for the stratosphere cooling fingerprint. As Tyndall predicted this in 1859 it is far from curious….

Like · Reply · August 10 at 12:04pm

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Susan Manning Blanchard · Duke University

Very understandable description of what global warming is and how/why it is happening.

1 · Like · Reply · Subscribe · August 13 at 6:10am

Robert Cooper Manning Jr · Georgia State University

Is the scientist from Max Planck a lost cousin?

Like · Reply · August 13 at 9:15am

Susan Manning Blanchard · Duke University

I did notice the citation and wonder a bit.

Like · Reply · August 13 at 2:00pm

Damian Newell · Residential Sales at BAYLEYS REAL ESTATE DUNEDIN

love love love.

1 · Like · Reply · Subscribe · August 10 at 8:37pm

Ross Ronald · Kingswell High School Sorry bout that, but it appears some science is sneaking in

Like · Reply · August 11 at 12:42am

Thomas Jamison · Top Commenter

Spencer is not science. He said so himself, that his motives are political. He is arguing nonsense, that clouds cause the ENSO. Completely bunk.

Like · Reply · August 11 at 4:32am

Jim Balter · Top Commenter · Santa Barbara, California

Funny how the denialists reject thousands of peer-reviewed climate science journal articles that disagree with their beliefs, but if they find just one news article that seems to support their beliefs, they completely uncritically accept it and announce it as “some science sneaking in”.

1 · Like · Reply · August 11 at 8:39pm

John Borstelmann · Stanford University

Pretty obvious conclusion if you are willing to be rational. It may be hard to wrap your head around the fact that humans have so multiplied ourselves and our impacts as to shrink the world and change the climate, but we have. Species extinctions are at an all time peak as well (the sixth great extinction event, ongoing, is caused by us); habitat is rapidly shrinking for wild critters. And scientists such as James Lovelock (the Gaia hypothesis) and James Hansen have been saying this for decades now. The evidence keeps mounting. We’re way past the point of debate. Time to get serious and do something, individually and collectively!

Like · Reply · Subscribe · August 10 at 10:47pm

David McMahon · University of New Mexico

It’s interesting that this website hasn’t mentioned Salby’s dramatic presentation on CO2 ( Of course it won’t be published until next year, but it’s going to rock a lot of boats. I wonder if he will be trashed like Spencer, even though Salby’s climate credentials are very solid. Apparently Salby’s work calls into question every single point made in this article.

Like · Reply · Subscribe · August 10 at 1:11pm

Doug Percival

Spencer’s recent paper was “trashed” because it was bad science. Likewise, Salby’s “dramatic” lecture has been “trashed” because it was bad science (and based on his lecture I would be surprised if it is ever published in the peer-reviewed climatology literature). In neither case does the “trashing” have anything to do with whether someone’s “credentials” are “solid” — it has to do with the scientific merit, or lack thereof, of the work. Of course, regardless of the merit of the science, both Spencer’s paper and Salby’s lecture are being relentlessly promoted by the corrupt Murdoch organization and the rest of the denialist media as a “proof” that AGW is a “hoax”, and indeed even Spencer himself has stated that the media accounts of his paper are making claims that are not supportable by his actual paper.

1 · Like · Reply · August 10 at 1:34pm

David McMahon · University of New Mexico

Salby is not out to prove AGW is a hoax, he is following the evidence — and actually you’re wrong, he has gotten it accepted in a peer-reviewed climatology journal and its coming out early 2012. On what basis is it “Bad science”? At this point Salby’s talk is only suggestive — we have to wait for the actual paper to come out to determine if it’s bad science. Judith Curry wasn’t so quick to dismiss it. I guess “bad science” amounts to anything that contradicts your point of view.

Like · Reply · August 10 at 1:50pm

Doug Percival

What peer-reviewed climatology journal has accepted Salby’s paper? Are you saying it has already been peer-reviewed and accepted for publication, or that it has been submitted and accepted FOR peer review?

As for “following the evidence”, when told by an attendee at his talk that his claims are falsified by overwhelming paleoclimate evidence, Salby reportedly responded that he “doesn’t believe” the paleoclimate evidence. Why? Because it contradicts his claims. “Point of view” has nothing to do with it — Salby’s lecture is bad science because it is contrary to known facts and is full of methodological errors.

Judith Curry NEVER dismisses ANY “contrarian” claim, no matter how absurd.

1 · Like · Reply · August 10 at 2:33pm

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Jack Enright

This article is full of fallacies. 1st. There is no Greenhouse effect over earth. 2. Co2 does not trap heat in the atmosphere. 3. Observations about the climate can not be directly tied to rise in CO2 levels. The climate change is due to changes in cosmic ray flux and phytoplankton growth vs. organic carbon in ocean.

Like · Reply · Subscribe · August 10 at 9:25pm

Thomas Jamison · Top Commenter

Whoever told you these things was not giving you the truth, not even close.

1 · Like · Reply · August 11 at 4:42am

Jack Enright

Ok John define the “greenhouse effect”.

Like · Reply · August 11 at 1:42pm

Jack Enright

What happens in the laboratory with CO2 is not duplicated in the atmosphere.

Like · Reply · August 11 at 1:55pm

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janumakonda (signed in using AOL)

Good post on Global warming.There is evidence that Global Warming is the cause and Climate Change is the Effect.

Dr.A.Jagadeesh Nellore(AP), India.E-mail:

Like · Reply · Subscribe · August 10 at 10:10pm

Peter S. Mizla · Top Commenter · Vernon, Connecticut

When someone tries to feed me the line ‘earths climate has always gone through cycles’- my retort is, Yes, but has caused those cycles’ inevitably I hear silence.


Like · Reply · Subscribe · August 10 at 6:39pm

Shaheer Cassim · Victoria, British Columbia

Hah. I usually do that too. Explain the cycles, a-hole.

The other is that catastrophic climate change has occurred many times before, so we should not be worried, because Earth has undergone many extinctions and catastrophic climate change. Gotta repeat it twice.

Like · Reply · August 11 at 7:04pm

Jack Enright

Good point Peter i.e. 14500BP there was a Warming Cycle during the Ice Age. There were no factories back then and People were not burning up the planet with toxic CO2 gas.

Like · Reply · August 11 at 9:19pm

Jack Enright

Shaheer so I see if someone disagrees with your point of view, you insult them without any provocation.

Like · Reply · August 11 at 9:22pm

Joan Savage · Top Commenter · SUNY-ESF

This is a very clean educational tool to get the basics straight.I’d like to see the graphic as a poster in libraries, schools, and post offices and as a page in earth science textbooks.

Like · Reply · Subscribe · August 10 at 10:39am

Wesley Rolley · Top Commenter · Northwestern University

Joan, I agree with that, but then any effort to do so will be met with resistance by those who intend to use the schools to push the anti-science ideology. The demands of some to validate all science by whether it fits their ideology is much like the Stalinist demands that science match his view of the world. The difference is that now we can no longer afford to wait for history to play out.

That, in a nutshell, is why I became a Green.

Like · Reply · August 10 at 4:24pm

Jack Enright

It is all a desperate attempt in trying to defend an old laboratory theory by saying that CO2 “traps” heat. There are legions of scientists and billions of dollars thrown into the fray without thought given to keeping an open mind and being objective about the evidence. I do not deny that pollution and the degradation of land cause climate change but simply have another theory similar to James Lovelock. Lovelock theorized that phytoplankton make clouds above their blooms which keep the planet Cool. But he did not finish his theory as to how CO2 interfered with this process. That is where my theory comes in.

Like · Reply · Subscribe · August 11 at 8:59pm

Jack Enright

So is this the same John Cook who studied physics at the University of Australia? If so you should be aware that there are forces more powerful than what the Sun can provide. Have you looked into the effect of Galactic Cosmic Rays on #1 bacteria and #2 coccolithophores. If so you will find that #2 are protected by a calcium shell. Interesting also that this arrangement occurred by the action of GCR millions of years ago and changed life on planet Earth. This should be very exciting news!

Like · Reply · Subscribe · August 11 at 9:52pm

Jack Enright

It would be better to point the empirical evidence toward a more likely candidate which is long chain Carbon in the form of soot but more so as organic carbon molecules. They have a far greater heat capacity and energy source for bacteria. It is a biological process that is changing the climate. But your article does have some merit with the evidence but goes astray with conclusions.

Like · Reply · Subscribe · August 11 at 2:25pm

Jim Balter · Top Commenter · Santa Barbara, California

There are thousands of peer-reviewed articles in climate science journals that provide the empirical evidence of greenhouse gases causing global warming. But no, all the scientists are wrong and internet crank Jack Enright is right. We know this because … he says so.

Like · Reply · August 11 at 8:37pm

Jack Enright

Say Jim if you were a Peer would you always agree with your fellow scientists? What if one of your Peers had a new idea? Maybe you should look into the theory before putting your foot where it dont belong.

Like · Reply · August 11 at 9:58pm

Jack Enright

Thomas that is an interesting theory that could explain the disappearance of the large mammal species like Mammoths at the end of the last Ice Age. Actually I believe this to be the case that large scale fires changed the climate but it can get very complex with the ecosystems.

Like · Reply · Subscribe · August 11 at 9:38pm

Jack Enright

I have looked into the so called “heat trapping” by CO2 in the atmos. There is no mechanism that could explain this. Perhaps supercooled water would have supersaturation of CO2 but still it would be insufficient. Convection currents kill the greenhouse theory.

Like · Reply · Subscribe · August 11 at 2:21pm

Rimu Atkinson · Top Commenter · Wellington, New Zealand

This is great also —

Like · Reply · Subscribe · August 12 at 4:08am

Jack Enright

Rakesh there was a nice heat wave during the Civil War, would you say it was because of all the fire and brimstone, shot and shell? So in your way of thinking cooking a Mammoth over a fire warms the planet up.

Like · Reply · Subscribe · August 11 at 9:56pm

Jake Morrison · Palmerston North, New Zealand

THIS is what science needs more of: clear, simple explanations of scientifically verifiable facts.

Like · Reply · Subscribe · August 12 at 12:02am

Jack Enright

So Bart you take the position that only “greenhouse gases” cause global warming. Is that the position of the IPCC?

Like · Reply · Subscribe · August 11 at 9:41pm