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Empirical evidence that humans are causing global warming

What the science says...

Select a level... Basic Intermediate

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.

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 line of empirical evidence that humans are causing global warming is as follows:

We're raising CO2 levels

Human carbon dioxide emissions are calculated from international energy statistics, tabulating coal, brown coal, peat, and crude oil production by nation and year, going back to 1751. CO2 emissions have increased dramatically over the last century, climbing to the rate of 44.25 billion tonnes of CO2 per year in 2019 (IPCC AR6, WG III Technical Summary 2022).

Atmospheric CO2 levels are measured at hundreds of monitoring stations across the globe. Independent measurements are also conducted by airplanes and satellites. For periods before 1958, CO2 levels are determined from air bubbles trapped in polar ice cores. In pre-industrial times over the last 10,000 years, CO2 was relatively stable at around 275 to 285 parts per million. Over the last 250 years, atmospheric CO2 levels have increased by about 130 parts per million (Figure 1). Currently, the amount of CO2 in the atmosphere is increasing by around 16 gigatonnes every year, while CO2 emissions from fossil fuels and industry today exceeds 35 billion tonnes of CO2 per year (Figure 2).

10,000 years of carbon dioxide concentrations

Figure 1: Atmospheric CO2 concentrations over the last 10,000 years from ice cores (dots) and direct air measurements (blue line). This clearly shows a rapid increase over the last few hundred years (source: Berkeley Earth).

Annual CO2 emissions

Figure 2: Annual CO2 emissions from fossil fuels and industry.  This has increased from virutally nothing in 1750 to more than 35 billion tonnes per year in 2021 (source: Our World in Data).

Humans are emitting more than twice as much CO2 as ends up staying in the atmosphere. Nature is reducing our impact on climate by absorbing more than half of our CO2 emissions. The amount of human CO2 left in the air, called the "airborne fraction", has hovered around 43% since 1958.

CO2 traps heat

According to radiative physics and decades of laboratory measurements, increased CO2 in the atmosphere is expected to absorb more infrared radiation as it escapes back out to space. In 1970, NASA launched the IRIS satellite measuring infrared spectra. In 1996, the Japanese Space Agency launched the IMG satellite which recorded similar observations. Both sets of data were compared to discern any changes in outgoing radiation over the 26 year period (Harries et al. 2001). What they found was a drop in outgoing radiation at the wavelength bands that greenhouse gases such as CO2 and methane (CH4) absorb energy. The change in outgoing radiation was consistent with theoretical expectations. Thus the paper found "direct experimental evidence for a significant increase in the Earth's greenhouse effect". This result has been confirmed by subsequent papers using data from later satellites (Griggs & Harries 2004, Chen et al. 2007).

Figure 3: Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature (Harries et al. 2001).

When greenhouse gases absorb infrared radiation, the energy heats the atmosphere which in turn re-radiates infrared radiation in all directions. Some makes its way back to the earth's surface. Hence we expect to find more infrared radiation heading downwards. Surface measurements from 1973 to 2008 find an increasing trend of infrared radiation returning to earth (Wang & Liang 2009). A regional study over the central Alps found that downward infrared radiation is increasing due to the enhanced greenhouse effect (Philipona et al. 2004). Taking this a step further, an analysis of high resolution spectral data allowed scientists to quantitatively attribute the increase in downward radiation to each of several greenhouse gases (Evans 2006). The results lead 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."

Figure 4: 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 planet is accumulating heat

When there is more energy coming in than escaping back out to space, our climate accumulates heat. The planet's total heat build up can be derived by adding up the heat content from the ocean, atmosphere, land and ice (Murphy et al. 2009, IPCC AR6 WG1, Chapter9). Ocean heat content was determined down to below 2000 metres. Atmospheric heat content was calculated from the surface temperature record and heat capacity of the troposphere. Land and ice heat content (eg - the energy required to melt ice) were also included (Figure 5a). This accumulation of heat also leads to sea level rise, both through thermal expansion of the ocean, melting of land ice, and changes in land water storage (Figure 5b). 

Global energy inventory and global sea level rise

Figure 5:Global energy inventory (a) and global sea-level budget (b) from 1971-2018 (IPCC AR6 WG1 Chapter 9).

From 1971 to 2018, the planet has been accumulating heat at a rate of just over 300,000 gigawatts with the vast majority of the energy going into the oceans. Considering that a typical nuclear power plant has an output of 1 gigawatt, imagine 300,000 nuclear power plants pouring their energy output directly into our oceans.  On average, every square metre of the climate system has been accumulating heat at a rate of 0.6 ± 0.11 Wm2, consistent with other determinations of the planet's energy imbalance (Hansen et al. 2005, Trenberth et al. 2009).

So we see a direct line of evidence that we're causing global warming. Human CO2 emissions far outstrip the rise in CO2 levels. The enhanced greenhouse effect is confirmed by satellite and surface measurements. The planet's energy imbalance is confirmed by summations of the planet's total heat content and ocean heat measurements.

For more evidence that humans are causing global warming, check out The human fingerprint in global warming.

Last updated on 16 July 2023 by Ken Rice. View Archives

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Denial101x video

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Comments 1 to 25 out of 419:

  1. What speculative large scale effect could stem from more insulation with unchanged energy input? Could it be similar to the effect of more energy input with unchanged insulation? That insulation's extent has been verified. Changes over time of outgoing long wave radiation have been measured and shown to have decreased by the amount expected from increased GH gases: Harries et al, Increases in greenhouse forcing inferred from the outgoing longwave radiation spectra of the Earth in 1970 and 1997, Nature. 2001 Mar 15;410(6826):355-7 Abstract here: "The evolution of the Earth's climate has been extensively studied, and a strong link between increases in surface temperatures and greenhouse gases has been established. But this relationship is complicated by several feedback processes—most importantly the hydrological cycle—that are not well understood. Changes in the Earth's greenhouse effect can be detected from variations in the spectrum of outgoing longwave radiation, which is a measure of how the Earth cools to space and carries the imprint of the gases that are responsible for the greenhouse effect. Here we analyse the difference between the spectra of the outgoing longwave radiation of the Earth as measured by orbiting spacecraft in 1970 and 1997. We find differences in the spectra that point to long-term changes in atmospheric CH4, CO2 and O3 as well as CFC-11 and CFC-12. Our results provide direct experimental evidence for a significant increase in the Earth's greenhouse effect that is consistent with concerns over radiative forcing of climate." It essentially means that there is more heat retained in the system in the bands of the mentioned gases. It is claimed that this additional heat is warming the climate. Is that such a bold claim? Now, I'm sure Will Nitschke will find objections. This is where the logic breaks down. When ample evidence has been gathered and critics of a theory continue to refute it, because no amount of evidence can ever be satisfying and they will not state what kind of evidence would be so, or define it in a way that they know is impossible to reach. Indeed, there is no such thing as absolute certainty.
  2. "here is a clear empirical evidence that CO2 is rising, CO2 causes warming and the expected warming is observed." -Yes, Yes and No. CO2 is rising and we expect that to cause warming. But "the expected warming" is not observed. If by expected you mean that we have seen warming that yes, but not from CO2 alone. In fact, the IPCC uses "aerosols" to explain the cooling from 1944 to 1975, and kindly explain that we do not know much about the climatic impact of aerosols. So to say that the "expected" warming is observed is to mislead: the expected warming from KNOWN factors (i.e. such the IPCC says we know lots about) would have been a steady increase from 1944, interupted by a few volcanoes and La Ninas. And to answer your question: "What is causing the warming if not CO2?" The suns irradiance, cosmic rays, a positive PDO, and a range of other factors, along with CO2. "Why isn't rising CO2 causing the warming?" Well, IT DOES, albeit not all of it, but from there, it's a long way to prove that a warming of half a degree until now will translate into an additional 5 degrees to year 2100.
  3. The following is actually pertinant to all these threads but this one seems the closest. "[ Global Warming: Has the Climate Sensitivity Holy Grail Been Found? by Roy W. Spencer, Ph.D. updated 7:00 a.m. CDT, June 30, 2008 (The following is a simplified version of a paper entitled "Chaotic Radiative Forcing, Feedback Stripes, and the Overestimation of Climate Sensitivity" I submitted on June 25, 2008 for publication in the Bulletin of the American Meteorological Society.) ABSTRACT: This article addresses new satellite and modeling evidence that previous satellite diagnoses of high climate sensitivity--which directly translate into predictions of dangerous levels of global warming--contain a large spurious bias. It is shown that those exaggerated estimates were the result of faulty assumptions regarding clouds when analyzing variations in average global temperature and average reflected sunlight off of the Earth. Specifically, it has been assumed (explicitly or implicitly) that, for global averages on time scales of three months or more, temperature variations cause clouds to change, but that cloud variations do not cause temperature to change. But when properly filtered, the satellite data reveal evidence of cloud variations indeed causing temperature changes, and that this source of natural climate variability biases the estimate of climate sensitivity in the direction of a very sensitive climate system. The new interpretation suggests a very low sensitivity. If the new sensitivity estimate is accurate, it would suggest only 0.5 deg. C of manmade warming by the year 2100. The new sensitivity estimate also suggests that warming over the last century can not be explained by human greenhouse gas emissions alone, but instead might require a mostly natural explanation. ]" Changing the sensitivity number does change ecery argument all at once.
  4. "Rising CO2 levels are based not on one station but over 300 stations in 66 countries (World Data Centre for Greenhouse Gases). " My count from the WDC site is 298 stations of which only 159 actually measure CO2. The others measure different gasses. Being imprecise simply gives the sceptics another rabbit trail to run down. Current models have a VERY GREAT problem with clouds; even with modern satellite imagery it is beyond our current capability to assess with any reasonable degree of accuracy cloud distribution and density and thus the effect on insolation.
  5. Well, Beck's analysis seems to disprove the base line data used to start the whole issue, so what does that do for the AGW theory?? Even if you put a higher anomaly factor in than Beck, you still end uf with substantially higher CO2 figures than were used to construct the AGW argument. It will be interesting to see the responses .....
  6. Re #11 A skeptic would easily recognise that Beck's analysis is nonsense! see, for example, post #172 here: remember that this is a "skepticalscience" site...we should make at least a little effort to be skeptical!
  7. Let me see if I understand this. This seems to be the argument. First, CO2 levels are rising. Second, CO2 is a greenhouse gas. Third, during the last 30 years, global temperatures have been rising. Fourth, the things that ordinary cause temperatures to rise -- such as increased solar activity -- are not causing the current rise in temperature. Therefore, increased C02 is causing global warming. Of these four points, I believe that no one disputes one and two. Yes, CO2 levels are rising. Yes, CO2 is a greenhouse gas. I do not think anyone denies either of those two assertions. But, there are a number of points that I think an honest scientist would want to explore. How much is CO2 rising, compared to the past? Do we have more or less CO2 now than we did in various past times? When we put more CO2 into the air, what happens to it? Does it get absorbed by the ocean, or does it stay in the air forever? What other things in the atmosphere are increasing or decreasing? I would feel much more confidence in the global warming theory if they would address points of this sort, which seem like pretty obvious questions to me. But, for the moment, lets just say that points one and two are basically undisputed. I believe that is true. Point three is the pivot the argument. The whole theory stands or falls on it. It is basically saying that there is a huge, unexplained increase in global temperatures in the last 30 years, which we have to explain. But is that true? First off, during the last ten years, there has been no increase. Of course, one can say -- and it is true -- that a ten year period means little or nothing in climate science. But think about that. If ten years means nothing, why is 30 years of such huge importance? If the whole theory rests on 30 years, but 10 of those years are AWOL, how good is the theory? And how can we say that the usual suspects do not explain the increase, when there has been no increase for ten years? What we are basically talking about is a twenty year trend, which stopped ten years ago. So what? Excuse me, but I have read enough climate science to realize that 10 or 20 year trends do not mean much. Most climate cycles operate over 100s if not 1000s of years.
    Response: "CO2 is a greenhouse gas.  I do not think anyone denies either of those two assertions"

    Sadly, there are many who deny the assertion that CO2 is a greenhouse gas, even despite observations of an enhanced greenhouse effect. Much of the discussion on this website of late have been on that topic alone.

    "during the last ten years, there has been no increase"

    Statistically, this isn't true, the trend in global temperature has still been increasing. Don't fall into the trap of comparing single points in a noisy signal - that is not the way to determine a long term trend. More importantly, the physical reality is that the planet is still accumulating heat. There is still an energy imbalance. Satellites and ocean heat measurements find more energy is still coming in than going out.
  8. I think that this post was different in the past and included a list with some fingerprint evidence (rise of the tropopause, expansion of the hadley cell...) with links... Where can I find this?
    Response: Yes, sorry about removing that information. As I added in more info to this page, I simplified the presentation lest it get too long and unwieldy. So I've temporarily removed all the other bits and pieces until I get organised and restructure it to a subpage somewhere. In the meantime, here is other evidence of warming: Note - as you say, some of the evidence listed above are unique to CO2 warming - the carbon "fingerprint".
  9. Loads of thanks, and good new article! We have a rather good grasp of the current climate system. The only thing that has change enough to warm the planet is GHG (mainly CO2) and all the observations are consistent with that. Some people still insist that there might be some misterious unknown thing doing it (and some other unknown thing cancelling out the CO2 warming effect) even though there isn't any evedence at all. It seems incredibly twisted to me. It is false that there has been no warming during the last ten years. Why do you cherrypick CRU instead of GISS, Rick? Do you just choose the sources that fit your bias?: What happens if you begin one year earlier or later? A remarkable change in your trend? That's why your 10-year trend is not significant, and that's why 30-year trends are significant, because they don't change much if you add or remove some years: Are short-term cooling trends unusual under the current global warming? No: If you want to disprove global warming just on a statistical basis you will need more than a statistically insignificant short-term cooling trend.
  10. Thanks for the thoughtful presentation of evidence in favor of AGW. I'm concerned with the logic of how you put your argument together on this page, however. Your first point is that CO2 is increasing. The second is that CO2 traps heat. OK so far. The third point is that the earth is warming. You lose me there. The reason we know that humans are causing the earth to warm is that the earth is warming? I just don't see how that logically follows. I'd really like to see a clear argument for the AGW hypothesis that separates evidence of warming from evidence for the cause of that warming. That's what I was hoping to find here, but did not.
    Response: The cause of global warming is outlined above in point 2: the enhanced greenhouse effect from increasing CO2. Point 3 (warming is happening) is the logical consequence of Points 1 (we're raising CO2) and 2 (CO2 traps heat). To accept the first two points, that human CO2 emissions are causing heat to be trapped, is to accept that humans are causing the planet to accumulate heat. With more heat in our climate, temperatures will rise.
  11. David Rourke, the first point is not simply that CO2 is rising. The point instead is that humans are causing the rise of CO2. Since that rise of CO2 increases heat retention, it does indeed follow that humans are causing a rise of heat retention via the mechanism of increasing CO2.
  12. Sorry if I seem dense. Allow me to make an analogy to evolutionary theory. It is observed that species change over time (evolution). Natural selection is a theory that explains how that happens. The fact of evolution doesn’t prove natural selection. Natural selection explains evolution. If evolution did not happen (species did not change over time) then natural selection would be in trouble, but that doesn’t mean that a scientist can say, “Look at how these species have changed. Natural selection is therefore proven!” Evolution is consistent with natural selection, but evolution doesn’t prove that the natural selection hypothesis is correct. Instead, we accept that evolution happens (based on considerable evidence) and compare different explanations for what causes it (natural selection vs. Lamarckism, for example). Natural selection makes many correct predictions, while Lamarckism makes many incorrect predictions. Therefore, natural selection is accepted as true (as true as anything in science). In the same manner, once we accept global warming, it does nothing to support AGW vs. any other explanation for increased global temperatures. What testable predictions does AGW make? What testable predictions do alternative theories (such as increased solar activity) make? How do those predictions hold up when compared with facts? That’s really the case that needs to be made here, I think.
    Response: I had hoped that I had made a clear case above not only for global warming, but more importantly, that humans are causing global warming. If humans are causing global warming, we expect to see an enhanced greenhouse effect. More specifically, we expect to see the enhanced greenhouse effect at the wavelengths that CO2 absorb energy. This has been observed both by satellite measurements observing less infrared radiation escaping to space and surface measurements observing more infrared radiation returning back to the Earth's surface.
  13. This web site and it's articles is the best (most thorough most civil, most compact, most interesting) I have ever come across. If we agree that human-caused CO2 has has caused the increase in global temps, the more difficult problem is what to do about it. Does anyone really think the answer is electric cars? More bumper stickers? Without hyper-analysing energy issues, it seems that the underlying belief is that we need to substitute non-Co2 releasing energy with non-CO2 producing energy. Is this even POSSIBLE on a global scale? And if so? Will the surge of population over the next hundred years erase those efforts? And, is there enough time? Even if our solutions were implemented on a a world-wide scale over the next 20 years, how long would it take for the volume of arctic ice to re-establish itself and confirm our "fix" has succeeded?
  14. oops, in the last commet the first mention of "non-CO2 releasing energy" should have been "CO2-releasing energy"
  15. thereisaidit, we already have the technology and it's improving rapidly; the only big question is time (or probably I should say the will to act). We will never see the arctic, or any other impact we already caused, recover. It's even worst, indeed; even if we act promptly we will see more degradation of our environment. We are not even thinking of turning back to the pre-industrial climate, we're only trying to avoid what is considered too dangerous to afford.
  16. I think the Skeptical Argument is improperly presented. It should rather be presented "There is no empirical evidence or proof that humans are [the primary causes of current global warming trends]. It's all based on theory and computer models.
    Response: Ideally, rather than use my own wording, I like to quote an excerpt from a skeptic article - usually one that sums up the argument succinctly and eloquently (lest I be accused of setting up a strawman argument). I'll get around to it for this argument sooner or later.
  17. More to the point, are there other, more prevalent sources of CO2 outside of industrial or non-natural based carbon emissions and further, are there other elements at play in increasing the global heat capacitance currently perceived?
  18. Re: #16 thereisaidit This isn't really skeptical statements i'm saying here but: I have two scenarios in my mind: 1) If renewable energy is to succeed there needs to be a massive price reduction in the price per kilowatt hour. My boss held up a $0.10AU coin and said that is the necessary goal for renewables to compete/ensure the economy can adjust. This site i've listed below has some figures though it might have company bias but as you can see in order for non CO2 producing forms of energy to flourish there needs to be something (ie.a heavy amount of research/investment etc) to get it there. 2) This second scenario isn't so pretty. This addresses the population issue you raised. I heard an interview with Paul Ehrlich who is a Professor in population studies. He is the author of The Population Bomb back in 1967 has some very dire predictions about world population levels. On your point on the ice caps: This is something I don't know but even if we could magically turn off the "CO2 emissions switch" I would think that the Artic ice loss will continue ie no matter what we do now global warming is happening and it is only a case of damage control reducing the extent in the rise of average temperatures in the future.
  19. It is wonderful to find such a detailed debate on this important topic. Can you confirm that my understand, as outlined below, is correct, or at least not nonsense. As life on Earth evolved, the Earth’s atmosphere was slowly changed. The atmosphere changed quite profoundly once the process called photosynthesis evolved. Much of the carbon dioxide in the atmosphere was sucked up by photosynthesis and transformed into oxygen and carbon. Carbon became the building material upon which life was based, and oxygen, as we all know, became essential for sustaining life. When a living thing died, the carbon that formed the building blocks of life was often reabsorbed by the soil. This fertilised the soil, making life even richer. Some of the carbon left when plant and animals died was subjected to enormous pressures, and over the aeons was turned to oil and coal. Life itself acted as means of storing carbon. In particular forests are like massive carbon storage facilities. Over millions of years, as life had the effect of reducing the amount of carbon dioxide in the atmosphere, the Earth cooled down. This was not a smooth process, and was subject to other variables. The question is this: by burning up carbon deposits that have stayed in the ground for aeons, and also through deforestation, is man reversing a process which took place over millions of years in a blink of the evolutionary eye.
  20. The section "CO Traps Heat" is suffering from a logical fallacy of Composition: The assertion is made that increasing CO2 and other GHG's absorb more long wave radiation and prevent it from escaping to space. This is well supported by Harries 2001 Figure 3C. Clearly assertion this is supported by both theory and measurement. The fallacy comes into play when applying this to the WHOLE of the outgoing long wave spectrum. Looking at Harries 2001 Figure 1B (not shown), the range 800-1000 cm-1 shows an increase in outgoing long wave that exceeds the decrease due to GHG's. This observation is also present in Griggs 2004 and Chen 2007. Harries 2001 attributes this increase over the range 800-1000 cm-1 to 'small residual ice crystals' not fully removed from the data due to field of view differences between the detectors. This assertion is not supported by the addition of the NASA AIRS satellite. Additionally, satellite measurements of total outgoing long wave radiation show an increasing trend with time (and CO2, although not causative). This is critical because it calls into question the causative relationship between "CO2 traps heat" and "Our planet is accumulating heat" in the block diagram. This relationship must be solid if the alarming predictions of the climate science community are to be believed and the mitigation of climate change can be achieved through CO2 emission reduction.
  21. What are peoples thoughts on Gerlich & Tscheuschner's paper "Falsification Of The Atmospheric CO2 Greenhouse Effects Within The Frame Of Physics"? A quick rundown can be found here:
  22. samantha, it's a more than two years old story and has been refuted so many times that there should be no need to pull it back. Give a quick look here and at the links provided.
  23. Riccardo Thanks for the link. I will look over it. BTW. The original paper is two years old, this paper is Jan2009.
  24. samantha, it's basically the same, they just found it hard to get it through peer review.
  25. I have a question. I live in West Texas where just a few generations ago fires used to burn up our whole world. Before the White Europeans stole the land from my ancestors the prairies were purified and cleansed of brush and trees by fires Native Peoples and lighting set. Millions of acers burned uncontrolled every time it turned off dry and especially after a wet year when we grew a lot of grass. Since hiways, roads, cultivated lands stop wildfires today not much of the prairies burn anymore. In fact the pasture land is not grass it is trees, cactus and brush. I still hear on the news of concern about co2 and other gasses wildfires emmitt into the sky. So how can the nominal co2 emissions of industry cause the earth to warm when since fires have always been a part of the enviornment? Ranger Texas

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