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Empirical evidence that humans are causing global warming

Posted on 15 October 2009 by John Cook

It still surprises me when I hear skeptics claim there is no evidence that we're causing global warming. The evidence is there in the peer reviewed literature. What they're really saying is they haven't bothered to look. So to make things easier for everyone, here is the evidence that humans are causing global warming. It's not based on theory, climate models, faith or political ideology but on direct, empirical observations. The line of evidence is as follows:

Humans are raising CO2 levels

Is it arrogant to claim we humans could possibly affect something as large as the global climate? It's not a question of arrogance. It's merely a question of numbers.

The first measurements of atmospheric CO2 were conducted by Charles Keeling in 1958 at Mauna Loa, Hawaii. Currently, atmospheric CO2 levels are being measured at hundreds of monitoring stations across the globe. For periods before 1958, CO2 levels are determined from analyses of air bubbles trapped in polar ice cores.

In pre-industrial times, CO2 has been relatively stable at around 260 to 285 ppm. Over the last 250 years, atmospheric CO2 levels have increased by about 100ppm. Currently, the amount of CO2 in the atmosphere is increasing by 15 gigatonnes every year.

We can calculate how much CO2 we're emitting from international energy statistics, tabulating coal, brown coal, peat, crude oil and cement production by nation and year. What we find is fossil fuel emissions have continued to increase. In 2008, we were emitting CO2 at a rate of 29 gigatonnes per year.

Figure 1: CO2 levels (Law Dome ice core and Mauna Loa, Hawaii) and Cumulative CO2 emissions (CDIAC).

Humanity is emitting nearly twice as much CO2 as ends up remaining in the atmosphere. Oceans and plants are actually reducing our impact on climate by absorbing a large portion of our CO2 emissions. Our actions are having a significant impact on the composition of our atmosphere. It's not arrogant to say we can change global climate. On the contrary, the arrogance lies in thinking we can act as we like without consequences.

CO2 traps heat

How does CO2 trap heat? Sunlight passes through our atmosphere and warms the earth. The earth cools by emitting infrared radiation back towards space. As infrared radiation travels through the atmosphere, some is absorbed by greenhouse gases such as water vapour and CO2. This warms the atmosphere which then reradiates the infrared radiation in all directions. Some escapes to space while some radiates downwards and further warms the Earth.

With more CO2 in the air, we expect to see less infrared radiation escaping out to space.  To confirm this, satellite readings of outgoing radiation in 1970 were compared to measurements made from 1996 through to 2006 (Harries 2001, Griggs 2004, Chen 2007). They found a drop in outgoing radiation at the wavelengths that CO2 absorbs energy, consistent with theoretical expectations, thus finding "direct experimental evidence for a significant increase in the Earth's greenhouse effect".

Ground measurements also find an increase of infrared radiation heading back down towards Earth, confirmation of an enhanced greenhouse effect (Philipona 2004, Puckrin 2004Wild 2008, Wang 2009). By closely analysing the changes at different wavelengths, scientists can calculate how much each greenhouse gas contributes to the warming effect. The results are consistent with both theory and satellite measurements of the enhanced greenhouse effect, leading the authors 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)

Our planet is accumulating heat

We know we're raising CO2 levels. We have proof that more CO2 causes an enhanced greenhouse effect. What is the result? When you add up all the heat accumulating in the oceans, land and atmosphere plus all the energy required to melt glaciers and ice sheets, you find that the planet is accumulating heat at a rate of 190,260 GigaWatts (Murphy 2009). Considering a typical nuclear power plant has an output of 1 GigaWatt, imagine 190,000 nuclear power plants pouring their energy output directly into heating our land and oceans, melting ice and warming the air.

Figure 3: Total Earth Heat Content from 1950 (Murphy 2009).

What about claims that it hasn't warmed since 1998. Could we be experiencing global cooling? Not at all. How do we know? Think about what global warming really is. The planet is accumulating heat. More energy is coming in than is going out. Is this energy imbalance still occuring?

To answer this question, think about where most of global warming goes? Around 95% goes into warming the oceans. Measurements of ocean heat content find a warming trend through to the end of 2008 (Schuckmann 2009). There's strong evidence that the oceans are still accumulating heat. Global warming is still happening.

Figure 3: Time series of global mean heat storage (0–2000 m), measured in 108 Jm-2.

So why have we experienced surface cooling in recent years? It's not unusual or unprecedented for surface temperatures to show cooling over short periods. As the ocean contains much more heat than the atmosphere, relatively small exchanges of heat between the ocean and air can cause significant changes in surface temperature.

In 1998, we experienced the strongest El Niño on record. This moved massive amounts of heat from the Pacific Ocean into the atmosphere, leading to an abnormally warm year. Conversely, the last few years have seen the strongest La Niña conditions in over 20 years which had a cooling effect on global temperatures. It's not unusual or unprecedented for surface temperatures to show short term cooling during a long term warming trend.

So we have a clear line of evidence. Our CO2 emissions far outstrip the observed rise in CO2 levels. Surface and satellite observations confirm an enhanced greenhouse effect. And ocean heat observations tell us the planet is accumulating heat. Science tells us we need to reduce our CO2 emissions to stop global warming. But like any issue, we'll never come close to resolving it unless we admit there is a problem.

NOTE: There is a more detailed, technical version of this post for the more eggheaded readers. This post was written as part of Blog Action Day.

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Comments 51 to 71 out of 71:

  1. I can't find the "earth total heat content Figure 3 in (murphy 2009) are you referring to the paper "An observationally based energy balance for the Earth since 1950"?
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    Response: Yes, the data from my Figure 3 (Earth's Total Heat Content) comes from the energy storage element of Figure 6b in "An observationally based energy balance for the Earth since 1950" (Murphy 2009). Dan Murphy very generously sent me his data.
  2. I also got a question about the (Harries 2001, Griggs 2004, Chen 2007) papers on radiation observations. The clearest fig to my mind is Figure 3. This figure is a subtraction 1970 radiation from 2006 data. I see the drip (negative values) in radiation at the wavelength for CO2 so energy at that wavelength is being retained more in 2006 compared to 1970. But I see positive readings at other wavelengths. My understanding would be that relatively more energy is being lost from the earth at those wavelengths in 2006 compared to 1970. Why is it possible to ignore the the energy at these different wavelengths and only focus on the wavelengths associated with CO2?
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  3. Correction to above "The clearest fig to my mind is Figure 3 from chen 2007"
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  4. I believe if you are going to talk about humans creating CO2 we should also look at the large human CO2 sink ... crops. Looking at easy to find crop data for the US and the world; it is easy to calculate that the carbon we put into the atmosphere (based on your 29 gigaton CO2) is (27% Carbon) 7.8 gigatons of Carbon. Just looking at world wheat production, 21.8 gigatons of Carbon is taken out of the atmosphere (based on ~40% carbon in whole plant and the plant weighing ~100 times the seeds harvested). Heck even if the plant weighs 50X more than the seeds harvested that 10 gigatons of carbon from the atmosphere. What if you added in all the other human planted food crops? bet you come up with the fact that we have taken much more CO2 out of the atmosphere than we ever created by 10,000X. No something else is at play with the increase in temperature we are seeing ....
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  5. Usmar, the conclusion should be that there's no more CO2 in the atmosphere given how much we take out each year. Clearly this is not the case. You should follow the whole carbon cycle, not just one single step. The carbon stored in crops is recycled quickly, it averages to zero. Trees, on the contrary, live and continuosly take out CO2 from the atmosphere for centuries before their carbon is recycled. Not a subtle difference indeed.
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  6. Regarding CO2 saturation in our atmosphere (post#1), isn't it also true that over-saturation would cause MORE heat to be trapped at lower and lower altitudes in our atmosphere, where the effect actually matters (since surfaces are where H2O is evaporated, ocean temps are affected, and much of what constitutes 'weather' is determined)? I mean, who cares if the stratosphere is hot or not?
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  7. Nobody reads peer reviewed science literature except for other scientists. So then, whose fault is when the average person doesn't have the information. Theirs, for behaving like they always have and you should have expected them to, or yours for waiting this long to spell it out for them. For years all we've heard is politicians telling us about global warming and climate change, but not giving us the evidence. Just because some slick talker in a suit says something is true, doesn't mean it is. If that were the case then there wouldn't be so many of them saying so many different things. If you want people to believe you then it is your responsibility to give them reason to. Otherwise you have no right to complain.
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  8. There is little doubt that the burning of fossil fuels by humanity has caused concentrations of CO2 in Earth's atmosphere to rise. The graph of CO2 concentration versus time is a "Hockey Stick". Can anyone show that global temperatures over the last 1,000 years correlate with the CO2 concentrations without using tree ring temperature proxies?
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  9. gallopingcamel, one reason people get frustrated with you is that you ask questions or (more usually) make strong assertions without bothering to look for the answers with the tools built into this Skeptical Science site. In this case, you could have clicked the "View All Arguments..." link under the thermometer at the top left of every page, or typed "hockey stick" into the Search field. Either method quickly would have led you to the page Hockey stick is broken, where you would find a bunch of hockey stick graphs made from data having nothing to do with tree rings.
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  10. climateresponse at 16:22 PM on 22 March, 2010 Well, here you are, after all, at a site dedicated to explanation of the topic. No need to complain! Your concern has also already been answered by a very excellent writer and scientist by the name of Dr. Spencer Weart. Do take a few hours to read his book The Discovery of Global Warming, published free to read on the Web.
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  11. Poptech, I'm sure you don't realise the consequence of what you said. I can not immagine any measurements without a model, a simulation, some data analysis but the simplest ones. No modern experimental apparatus, no high tech gadget can be immagined without models and simulations of some sort. But maybe you don't know, if you shout the alarm in bold characters for a trivial background subtraction. It looks like you think that it's enough to push a button and magically the "measurements" come out. The age of Galileo and Newton is over, get real.
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  12. Poptech, Following up on Riccardo's reply, you need to realize that a model is inherent in even an old-fashioned liquid-filled thermometer that is a glass tube attached to a wooden board on which are painted numbers. The numbers are the model.
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  13. Poptech - you might like to comment on the other 4 papers which also followed the idea of lets measure directly the radiation from the atmosphere since you think Evans is invalid? (And you are sure the conference didnt require review?). Now all of these papers involve measurement of the downward infrared. Do you contest that this measurement was made? Or that the measurement is invalid? The "model" used for interpretation is the RTE. Are you seriously implying that the equation derivable from QM and verified in lab experiments cant be trusted? Actually I would be interested to know what emperical evidence would change your mind. I am interested in how you understand "empirical evidence".
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  14. you seem to forget that there have been periods in history much warmer than today with no adverse effects, there have also been periods much much cooler than today, two examples: medieval warm period, the little ice age. warming and cooling are a trend, as for co2 it tends to rise with increased solar activity which you can see not just on our planet, the sun burns hotter, we receive more heat, our ice begins to melt, same thing happens on Mars which is further from the sun, i find it hard to believe our SUV's are causing the ice on Mars and moons of Jupiter to melt. most of the earth's Co2 is in the sea, with increased temperature comes more evaporation allowing more co2 in to the atmosphere.
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  15. Re: transjasmine (65) Welcome to Skeptical Science! In all honesty, the majority of your comment shows a lack of understanding of the physical processes of our planet and its temperature control system. That's not intended as a slam, just an observation. We all start at one point lacking an understanding of things. It is the hallmark of sentience to acknowledge that lack and seek to redress it. As I'm sure you will. For a greater understanding, I would suggest investigating the following: 1. Start here, find your level of interest and understanding and increase it. 2. Read Spencer Weart's History of the Discovery of Global Warming. 3. Watch Richard Alley's talk on how CO2 functions as the global temperature control knob. When you have questions, and you will, come back here & look in the upper left corner of the page to find the Search function. Search for an appropriate thread post for your answers to your questions. If you don't find an answer or an appropriate thread, pick the closest or most appropriate thread and post your question there. Someone will help you at that point. The Yooper
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  16. Dear me, transjasmine : so many misunderstandings in such a short post. Please look at these links to skeptical 'arguments' before you go any further : Climate's Changed Before It's Not Bad It's a 1500 Year Cycle We're Coming Out Of The Little Ice Age Medieval Warm Period Was Warmer It's Not Us It's The Sun It's Cosmic Rays Mars Is Warming Other Planets Are Warming There's No Empirical Evidence Neptune Is Warming Jupiter Is Warming Pluto Is Warming Solar Length Cycle Proves It's The Sun The Sun Is Getting Hotter Solar Cycles Cause Global Warming It's Global Brightening CO2 Lags Temperature It's The Ocean CO2 Is Coming From The Ocean Warming Causes CO2 Rise Hope that helps.
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  17. Regarding CO2 measurement prior to Mauna Lau: Ernst-Georg Beck’s ‘180 Years of Atmospheric CO2 Gas Analysis by Chemical Methods’ in Energy and Environment, Vol. 18 No. 2, 2007 will no doubt be of interest to some.
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  18. Carbon500 @67, Beck took a large number of historical CO2 measurements from a large number of sites, including sites contaminated by local emissions of CO2. He made no effort at quality control, and treated contaminated samples as though they measured background CO2 levels. That makes his paper worse than useless, the reason, no doubt, it was published in Energy and Environment rather than in a scientific journal.
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  19. The real problem with this great hypotheses is that has no experimental proofs. It's any better than Henrik Svensmark's hypotheses for the influence of the cosmic rays in the cloud formation, in fact is worse than Svensmark's hypotheses because the CO2 domiance doesn't explain how our glacial eras existed. I like to think of this CO2 hypotheses as the Newton's theory for gravity, it works kind well when we discribe punctual phenomena but is not good if we look at the hole picture, but Svensmark's hypotheses is more like Einstein's relativity theory, because it is more dynamic and takes into account the influence of the hole Milky Way through our hole Solar System, instead of think of it as one isolated point that doesn't interact with anything. Fortunately it is science and it needs to be proved, but to claim that CO2 cause global warming without any experimental proof it's either not science or at least very irresponsable.

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    Moderator Response:

    [PS] Scientific verification is taking hypothesis/model and comparing prediction against observation. This article has many such verifications. Perhaps you should actually read the linked papers? Perhaps you could explain what you mean by the strange statement" the CO2 domiance doesn't explain how our glacial eras existed."?? This makes no sense - climate theory is entirely consistent with glacial eras. Perhaps read the articles here and here?

  20. jpalombarini @69, the emperical evidence that there is a greenhouse effect is the fact that the IR emission to space from the Earth is significantly distorted from that of a black body, and more importantly, from that of the surface - as shown in this graph from a 1970 paper:

    (Details here, particularly the last two sections).

    Since 1970 numerous observations have shown the same effect, and provided detailed confirmation of the theory:

    Caption: Figure 1. Scatterplot of 134,862 measured values of OLR against OLR calculated by the Fu-Liou model, both in units of W/m2. The solid line is the one-to-one line. (Details)

    Were you claim "there are no experimental proofs" there are in fact well over 130,000 direct observations showing unequivocally that it exists.  Further, that is no surprise given that its existence is a directly predicted consequence of the laws of radiation together with the laws of thermodynamics and the composition of the Earth's atmosphere.  Claims that "there are no experimental proofs" of the existence of the greenhouse effect fall into the same category as claims that "there is no experimental proof" that the Earth orbits (a point within the surface of) the Sun.

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  21. jpalombarini @69.

    The analogies you present to explain yourself are as dire as your understanding of the operation of greenhouse gases.

    Newton's laws act across the entire known universe so do indeed apply to "the whole picture." You are wrong to assert otherwise. The failing of Newton's laws is solely that they need amending in certain circumastances, eg by relativity when speeds or accelerations are high.

    Your suggestion that Svensmark's hypothesising in some way over-rules greenhouse gas physics is most odd. The exotic nature of cosmic rays has no impact on the evaluation of the physics. And the science you place such faith in shows "that cosmic rays play a minor role in cloud formation, and have not contributed in any significant way to the global warming over the past 50 years," to quote another SkS article you would do well to read.

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