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Empirical evidence that humans are causing global warming

What the science says...

Select a level... Basic Intermediate

Less energy is escaping to space: Carbon dioxide (CO2) acts like a blanket; adding more CO2 makes the 'blanket' thicker, and humans are adding more CO2 all the time.

Climate Myth...

There's no empirical evidence

"There is no actual evidence that carbon dioxide emissions are causing global warming. Note that computer models are just concatenations of calculations you could do on a hand-held calculator, so they are theoretical and cannot be part of any evidence." (David Evans)

The proof that man-made CO2 is causing global warming is like the chain of evidence in a court case. CO2 keeps the Earth warmer than it would be without it. Humans are adding CO2 to the atmosphere, mainly by burning fossil fuels. And there is empirical evidence that the rising temperatures are being caused by the increased CO2.

The Earth is wrapped in an invisible blanket

It is the Earth’s atmosphere that makes most life possible. To understand this, we can look at the moon. On the surface, the moon’s temperature during daytime can reach 100°C (212°F). At night, it can plunge to minus 173°C, or -279.4°F. In comparison, the coldest temperature on Earth was recorded in Antarctica: −89.2°C (−128.6°F). According to the WMO, the hottest was 56.7°C (134°F), measured on 10 July 1913 at Greenland Ranch (Death Valley).

Man could not survive in the temperatures on the moon, even if there was air to breathe. Humans, plants and animals can’t tolerate the extremes of temperature on Earth unless they evolve special ways to deal with the heat or the cold. Nearly all life on Earth lives in areas that are more hospitable, where temperatures are far less extreme.

Yet the Earth and the moon are virtually the same distance from the sun, so why do we experience much less heat and cold than the moon? The answer is because of our atmosphere. The moon doesn’t have one, so it is exposed to the full strength of energy coming from the sun. At night, temperatures plunge because there is no atmosphere to keep the heat in, as there is on Earth.

The laws of physics tell us that without the atmosphere, the Earth would be approximately 33°C (59.4°F) cooler than it actually is.

This would make most of the surface uninhabitable for humans. Agriculture as we know it would be more or less impossible if the average temperature was −18 °C. In other words, it would be freezing cold even at the height of summer.

The reason that the Earth is warm enough to sustain life is because of greenhouse gases in the atmosphere. These gases act like a blanket, keeping the Earth warm by preventing some of the sun’s energy being re-radiated into space. The effect is exactly the same as wrapping yourself in a blanket – it reduces heat loss from your body and keeps you warm.

If we add more greenhouse gases to the atmosphere, the effect is like wrapping yourself in a thicker blanket: even less heat is lost. So how can we tell what effect CO2 is having on temperatures, and if the increase in atmospheric CO2 is really making the planet warmer?

One way of measuring the effect of CO2 is by using satellites to compare how much energy is arriving from the sun, and how much is leaving the Earth. What scientists have seen over the last few decades is a gradual decrease in the amount of energy being re-radiated back into space. In the same period, the amount of energy arriving from the sun has not changed very much at all. This is the first piece of evidence: more energy is remaining in the atmosphere.

 

Total Earth Heat Content from Church et al. (2011)

What can keep the energy in the atmosphere? The answer is greenhouse gases. Science has known about the effect of certain gases for over a century. They ‘capture’ energy, and then emit it in random directions. The primary greenhouse gases – carbon dioxide (CO2), methane (CH4), water vapour, nitrous oxide and ozone – comprise around 1% of the air.

This tiny amount has a very powerful effect, keeping the planet 33°C (59.4°F) warmer than it would be without them. (The main components of the atmosphere – nitrogen and oxygen – are not greenhouse gases, because they are virtually unaffected by long-wave, or infrared, radiation). This is the second piece of evidence: a provable mechanism by which energy can be trapped in the atmosphere.

For our next piece of evidence, we must look at the amount of CO2 in the air. We know from bubbles of air trapped in ice cores that before the industrial revolution, the amount of CO2 in the air was approximately 280 parts per million (ppm). In June 2013, the NOAA Earth System Research Laboratory in Hawaii announced that, for the first time in thousands of years, the amount of CO2 in the air had gone up to 400ppm. That information gives us the next piece of evidence; CO2 has increased by nearly 43% in the last 150 years.

 

Atmospheric CO2 levels (Green is Law Dome ice core, Blue is Mauna Loa, Hawaii) and Cumulative CO2 emissions (DOE Data Explorer). While atmospheric CO2 levels are usually expressed in parts per million, here they are displayed as the amount of CO2 residing in the atmosphere in gigatonnes. CO2 emissions includes fossil fuel emissions, cement production and emissions from gas flaring.

The Smoking Gun

The final piece of evidence is ‘the smoking gun’, the proof that CO2 is causing the increases in temperature. CO2 traps energy at very specific wavelengths, while other greenhouse gases trap different wavelengths.  In physics, these wavelengths can be measured using a technique called spectroscopy. Here’s an example:

Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapor is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

The graph shows different wavelengths of energy, measured at the Earth’s surface. Among the spikes you can see energy being radiated back to Earth by ozone (O3), methane (CH4), and nitrous oxide (N20). But the spike for CO2 on the left dwarfs all the other greenhouse gases, and tells us something very important: most of the energy being trapped in the atmosphere corresponds exactly to the wavelength of energy captured by CO2.

Summing Up

Like a detective story, first you need a victim, in this case the planet Earth: more energy is remaining in the atmosphere.

Then you need a method, and ask how the energy could be made to remain. For that, you need a provable mechanism by which energy can be trapped in the atmosphere, and greenhouse gases provide that mechanism.

Next, you need a ‘motive’. Why has this happened? Because CO2 has increased by nearly 50% in the last 150 years and the increase is from burning fossil fuels.

And finally, the smoking gun, the evidence that proves ‘whodunit’: energy being trapped in the atmosphere corresponds exactly to the wavelengths of energy captured by CO2.

The last point is what places CO2 at the scene of the crime. The investigation by science builds up empirical evidence that proves, step by step, that man-made carbon dioxide is causing the Earth to warm up.

Basic rebuttal written by GPWayne

Addendum: the opening paragraph was added on 24th October 2013 in response to a criticism by Graeme, a participant on the Coursera Climate Literacy course. He pointed out that the rebuttal did not make explicit that it was man-made CO2 causing the warming, which the new paragraph makes clear. The statement "...and humans are adding more CO2 all the time" was also added to the 'what the science says section. 


Update July 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial

Last updated on 12 July 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 176 to 200 out of 417:

  1. Question: For these predictive models, what is their range of predictions? And what are their assumptions? Do we get access to those assumptions and get to question them?
  2. einhverfr, "assumption" is a loose word in science (about as loose as "consensus"). If you want to learn about what goes into IPCC modeling, you could go directly to the source (noting, of course, that this is AR4 from 2007). Go here for discussion on range.
  3. @einhverfr further to what DSL wrote, the source code for several GCMs are in the public domain, so if the journal papers that explain the assumptions are not sufficient, you can always download the code and find out for yourself exactly how they work and test out the sensitivity of the projections to those assumptions by altering them and running some simulations. If you can show something interesting then there is nothing to stop you from submitting a journal paper questioning the assumptions. It is interesting to note there has been no attempt by the skeptics to make a GCM that explains the observed climate without CO2 (they seem to much prefer statistical models). I suspect that there is a good reason for this, which is essentially that it can't be done without making obviously unrealistic assumptions about the physics.
  4. Within chapter "CO2 traps heat" is a pdf download link to: Philipona, R., B. Dürr, C. Marty, A. Ohmura, and M. Wild (2004), Radiative forcing - measured at Earth's surface - corroborate the increasing greenhouse effect, Geophys. Res. Lett., 31, L03202, doi:10.1029/2003GL018765. The pdf is not (directly, May 2012) reachable through the link (agu.org). I did not check wether it is paywalled or requires registration/login before reading. Link to the abstract page on agu.org Currently (May 2012) Dr Marty provides the pdf over his personal page on slf.ch (WSL Institute for Snow and Avalanche Research SLF). Since this is one of the "directly measured" evidence papers cited by many (IPPC 2007, Rahmstorf&Schellnhuber...), there should be a safer and better accessible place for it.
  5. According to this thread, " What the science says... Direct observations find that CO2 is rising sharply due to human activity. Satellite and surface measurements find less energy is escaping to space at CO2 absorption wavelengths. Ocean and surface temperature measurements find the planet continues to accumulate heat. This gives a line of empirical evidence that human CO2 emissions are causing global warming." I have several quesitons: 1. What percentage of the energy in the absorption bands of CO2 are being absorbed today? 2. Isn't there information from NOAA that indicates that although Global Heat Content has risen over an extended period, it has recently done an abrupt levelling? 3. Isn't the information in the argument a collection of items that indicate a correlation, but not a causal relationship? What are the experiments that have been run to prove or disprove the causal relationshio? Dave
    Response:

    [DB] Please document the paternity of your graphic (the data sources used and any changes made).

    [Albatross] Please also familiarize yourself with the comments policy. Your questions have been addressed elsewhere at SkS, so your above comment constitutes "sloganeering" which is in breach of the comments policy.

    As for the causal relationship between CO2 and global temperature, statistical tests have determined that changes in CO2 Granger cause changes in temperature (e.g., see here).

  6. matzdj... You're not presenting a chart of "Global Heat Content" as far as I can see. You're presenting a chart of surface temperature. Don't forget that most of the heat energy is retained in our planet's oceans.
  7. matdj... Here is the page I find for NOAA's Global Ocean Heat Content... LINK Note specifically the second panel in the series. 0-2000m. I don't think you're going to see any "leveling off."
  8. matzdj, why do you post the same graph as you posted on the 17th May on another thread? Did you read the responses you were given (several detailed responses including at least nine links to follow from myself and DB)? Did you do any analysis to determine whether there has actually been a significant change in trend? Or are you just here to repeat tired old debunked talking points and not listen to the responses you were given? The graph you provide is not [ocean] heat content from NOAA - I provided you with the link to NOAA ocean heat content (the same as Rob's above) on 17th May. Until you do the analysis to determine whether there has been a change in trend, your comment is empty of substance in any case.
  9. matzdj's questions @180 are either nonsensical or grossly misinformed. Take his first question as an example. His question assumes, falsely, that CO2 has a constant "absorption band" regardless of its concentration in the atmosphere. On the contrary, as the CO2 concentration increases, the current absorption band widens, and new absorption bands form, as can be seen by this comparison of modtran emission spectra for 375 and 999999 ppmv of CO2 in the atmosphere, and with no other greenhouse gases: 375 ppmv 999,999 ppmv The Modtran model is reasonably, but not entirely accurate. In this instance it introduces a large inaccuracy by preserving atmospheric temperatures as if there was no change in CO2 content. If temperatures were allowed to adjust, the large W shape between wavenumber's 400 and 800 in the 99.99% example would be a large V or U shape instead, indicating much larger "absorption". The reason for this is that the outgoing spectra is not a product just of absorption of surface radiation, but also of radiation from the atmosphere. For empirical comparison, here are the IR spectrums of the Earth, Mars, and Venus. Note that the absorption from Venus is from the surface radiation with a blackbody spectrum for 735 degrees K, not the 260 degrees K shown: In his second question, matzdj assumes the Earth is just 700 meters deep by using the OHC record for only that portion of the ocean. He would have done far better to use the OHC record for the first 2000 meters of the ocean, as below: Better yet, he should have used the data for the entire Earth, as in figure 4 above. So, "Isn't there information from NOAA that indicates that although Global Heat Content has risen over an extended period, it has recently done an abrupt levelling?" Umm. No! Not unless you ignore the available evidence from NOAA that you find inconvenient. Finally, in his third question, matzjd simply ignores the fact that the correlation between CO2 forcing and temperature was first predicted from physical principles around 200 years ago. The correlation of the Earth's temperature to large changes in CO2 forcing throughout Earth's history for at least the last 600,000 years (the period with any available data) did not come as a surprise to anyone. It was a predicted consequence of the observed fact that CO2 is transparent to visible light, and absorbs Far IR radiation, the observed fact that the Earth re-emits the energy received from the Sun, primarily as visible light and Near IR radiation in the Far IR spectrum, and that energy is conserved. I have discussed extensively the experimental observations that make us confident of our understanding of the transmission and emission of CO2 in the atmosphere, and consequently make us certain that there is a greenhouse effect. As matzjd seems determined to ignore that evidence, here I shall simply show the correlation shown by the inexact (because of a lack of a precise temperature and humidity profile), and obsolete (1987 vintage) modtran model and observations:
  10. My apologies for what is likely an off-topic comment, but I wasn't sure of a better place to put the question. Has the Woodfortrees website moved? Google claims it should be at woodfortrees.org, but there's nothing there but an apache tomcat setup page. I hope nothings happened to the site - it's very handy.
  11. Many of us are wondering what's up w/Woodfortrees, Steve. It's much cited and well-liked by folks of all stripes. Hopefully the proprietor is having a nice vacation somewhere and simply hasn't noticed it's down. (OK, that's sounds pretty unlikely but I guess we may as well be optimistic.)
  12. Folks, apologies for the WFT downtime - it was caused by an IP allocation cockup at my hosting provider, and as Doug guessed, I didn't notice for a day! Should be fixed now in DNS but it may take a while to propagate. Best wishes Paul
    Response: [DB] Thanks, Paul. If we can help in any way down the road, let us know.
  13. Good to hear! Thanks for the work in keeping your website going.
  14. Aye, thanks Paul -- it's an obviously highly useful tool and easy enough that people who've never engaged the science can get a toehold for their motivation.
  15. I have a couple of suggestions/questions about the text:

    What about changing:

    Figure 3: Spectrum of the greenhouse radiation measured at the surface. Greenhouse effect from water vapor is filtered out, showing the contributions of other greenhouse gases (Evans 2006).

    To:

    Figure 3: Spectrum of the greenhouse radiation messured at the surface.  Greenhouse effect from water vapor is filtered out but showing the contributions of other more evenly distributed greenhouse gases (Evans 2006).

     

    Something is missing at questionmark between figure 4 and 5:

    ... accumulate heat to the end of 2008 at a rate of 0.77 ± 0.11 Wm?2

  16. Answering James Madison from another thread. James, you appear to claiming that observed temperature rise is not in keeping with the model predictions. The reason I asked what was your model, was because it appears your broad model assumes linear increase in CO2 means near linear increase in surface temperature. In fact, if you look at an individual GCM run, (not ensemble mean), then no such prediction is made. To me, it seems you are attacking predictions that were never made. That might indeed be a problem with communicating science, but it is not a problem with the science. Surface temperatures have a very large component of internal variability. This is well known and reasonably well understood. That is the reason why I pointed you to total OHC - a better diagnostic as temperature imbalance.

    If you think global warming is outside predictions, perhaps you would note that we have had a long string of La Nina/neutral phase in ENSO. This strongly influences surface temperatures. Care to make a prediction on what the surface temperature will do when the next 1.8 or higher El Nino occurs or do you think such an event wont happen again. (you can find an historical record of ENSO here).

  17. Hi fellas sorry for my usual blockhead question in this mostly quite technical discussion, but I'm wondering whether any/all of the empirical evidence outlined tells us about the amount of the warming for which we are responsible as well as simply identifying C02 as one of the culprits.  

    The reason I'm asking is because I've been arguing quite a bit with some guys who claim that they accept C02 is responsible for some warming, but the extent to which it is responsible can't be or at least hasn't been proven.  I had a look at the "human vs. natural contribution" page, but it seems the studies there are mainly using models, which of course gets the inevitable eye-rolls from skeptics.  Thanks in advance for any help with this blockhead question!

    Response:

    [DB] In addition to the fine advice already rendered to you, this post is a treasure-trove of information:

    Climate Change Cluedo: Anthropogenic CO2

  18. dvaytv @192

    There is a logical argument that you can make:

    Since all the physico-chemical properties of CO2 are well-established, and we can measure CO2's greenhouse (i.e. IR absorption) activity in the laboratory, and we have worked out both the math of its action in the atmosphere, and done the actual outgoing IR radiation reduction measurements -- spectrally resolved -- from space, we are very sure that CO2 is he most relevant greenhouse gas at this point in time. As we have, at the same time, overwhelming evidence that the increase in atmospheric CO2 since preindustrial times is entirely man-made, it follows logically (from these two lines of evidence) that man must be -- directly and indirectly -- 100% (with respect to CO2) responsible for the climate crisis that awaits us.

    Ask them what contrary evidence they have.

  19. dvaytw - Point them to the RealClimate page The CO2 problem in 6 easy steps, which is perhaps the most succinct and numerically supported explanation I have come across. 

  20. Hey all, I don't have any preconceived skeptical notions.  Found myself on this site as I'm legitimately seeking scientific evidence so I can feel informed.

    Can someone help me reconcile the following sentence from the article....

    "In June 2013, the NOAA Earth System Research Laboratory in Hawaii announced that, for the first time in thousands of years, the amount of CO2 in the air had gone up to 400ppm."

    If CO2 has been at 400ppm levels thousands of years ago, doesn't that imply that humans might not be the only causal factor of high CO2 levels?  There seems to be a gap in correlation/causality and I'm just legitimately curious.  Thanks for any insight. 

  21. Gmoney, please see the Climates Changed Before (no 1 of on the myth list) and also In brief, there are many natural causes of climate change - just that they are not operating now. There are natural processes that gradually remove (over million year time scales) CO2 from atmosphere. We can tell that CO2 is the atmosphere from our emissions from two ways. One  is mass-balance - add up our emissions and compare with concentration in the atmosphere. From this we learn that the oceans are still cleaning up nearly half our emissions (and pH is reducing as a consequence). At some point in the future as the ocean warms, this will stop and CO2 will be emitted instead.

    The second method comes from looking at the isotopic composition in the atmosphere. Different carbon sources have different signatures, and the increased CO2 is consistant with a fossil fuel source.

  22. GMoney @195, when NOAA anounced CO2 levels of 400 ppmv "for the first time in thousands of years", it was an example of understatement:

     

    In fact, the last time CO2 levels rose to 400 ppmv or above was around 24 million years ago.  That is nearly 80 times the age of the human species, and 2,400 times longer than the existence of any civilization on Earth.

  23. Tom Curtis @197,
    I think 'around 24 million' years is a bit too long for the last time CO2 was at present levels.
    Certainly the comment - "highest in the last 800,000 years" - that is commonly heard is badly underselling the ocasion of reaching 400ppm. During that 800,000 years it never managed much past 300ppm (if it even managed that). And Hönisch et al (2009) extends that out to 2.1 million years by examining ocean ooze.
    I have crossed swords with those who feel passionately that CO2 was above 400ppm during the Pliocene 3.5 million years ago (during the closure of the seas between N & S America & the broadening of Drakes Passage between S America & Antarctica to full deep ocean proportions). As I see it, the evidence shows that it is 'possibly' so but not 'probably' so that CO2 was 400ppm at that time.
    The next candidate is the mid Miocene 13 million years ago associated with the uplifting of the Himalayas. And I would say that is a strong enough candidate to rule out an earlier date. Still, I note some of the graphics in this Yale compendium on Cenozoic CO2 do suggest an earlier date, so I may be wrong.

    So what is a 'safe' position? Certainly by 2100 we will be seeing CO2 unseen since 24 million years ago.
    And it is also 'safe' to say that humans as a genus have never experienced CO2 this high. And also C4 plants have never been 'ecologically significant' with CO2 this high - they are quite significant today comprising about 25% of the planets biomass.

  24. gmoney there are multiple lines of evidence that show that the rise in atmospheric CO2 is anthropogenic, see Tom's excellent blog post "Climate Change Cluedo: Anthropogenic CO2" for details. 

    This issue is pretty much a touchstone for assessing the sanity of climate blogs.  We can be very, very sure that the rise in CO2 is anthropogenic, it is one scientific question that really is settled beyond reasonable doubt, so blogs that argue the rise is natural are demonstrating a fundamental inability to face the facts.

  25. Few deny CO2 is a greenhouse gas, or that the atmosphere traps heat, or that people produce CO2 (i don't care if some do it doesn't matter in analyzing the science). Thats not the question. It seems to me what is presented here is justification to propose the hypothesis, not emperical proof that man is causing climate change. I think this is all basic science of many years ago and all the research in global warmimg would not be necessary if this rebutttal was indeed proof. The question is 'what impact does man's CO2 producction have' and thus 'emperical evidence of his impact on climate change'. So the rebuttal doesn't answer the question.

     

    A couple other points, the graph shows methane as not being less significant. I have heard otherwise recently; that CH4 is 10 times worse than CO2. But Im a little confuced about the methan arguement, becasue methane is heavy, and would stay near  the surface.

    The question also implies the issue of quality of numerical models. Saying they are mathematically representative of interactions in the climate, is far to simple a statement. As I understan they numerically integrate Navier Stokes Equations (NSE). While NSE are quite complete, integrating them is no simple minded task.

     

    It also been all over the news now that temperatures have not risen in the last 15 years.I realize the oceans are storing heat, and their are trade winds, and that this post started 4 years ago, neverthess average tempratures are not rising.

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