How Do We Know That Humans Are Causing Climate Change? These Nine Lines of Evidence

While most Americans acknowledge that climate change is happening, some are still unsure about the causes.

They are often labeled “climate skeptics,” but that label can cause confusion or even anger.

Isn’t the nature of science to be skeptical? Isn’t it good to question everything?

Yes, but —

Here’s what is getting lost in the conversation:

Scientists have been asking these questions for nearly 200 years. The scientific community has been studying these questions for so long that collectively they have amassed an overwhelming amount of evidence pointing to a clear conclusion.

A similar situation is smoking and cancer. Nowadays, no one questions the link between smoking and cancer, because the science was settled in the 1960s after more than 50 years of research. The questions have been asked and answered with indisputable evidence.

We can think of the state of human activities and climate change as no different than smoking and cancer. In fact, we are statistically more confident that humans cause climate change than that smoking causes cancer.

Our confidence comes from the culmination of over a century of research by tens of thousands of scientists at hundreds of institutions in more than a hundred nations.

So what is the evidence?

The research falls into nine independently-studied but physically-related lines of evidence, that build to the overall clear conclusion that humans are the main cause of climate change:

  1. Simple chemistry that when we burn carbon-based materials, carbon dioxide (CO2) is emitted (research beginning in 1900s)
  2. Basic accounting of what we burn, and therefore how much CO2 we emit (data collection beginning in 1970s)
  3. Measuring CO2 in the atmosphere and trapped in ice to find that it is indeed increasing and that the levels are higher than anything we've seen in hundreds of thousands of years  (measurements beginning in 1950s)
  4. Chemical analysis of the atmospheric CO2 that reveals the increase is coming from burning fossil fuels (research beginning in 1950s)
  5. Basic physics that shows us that CO2 absorbs heat (research beginning in 1820s)
  6. Monitoring climate conditions to find that recent warming of the Earth is correlated to and follows rising CO2 emissions (research beginning in 1930s)
  7. Ruling out natural factors that can influence climate like the Sun and ocean cycles (research beginning in 1830s)
  8. Employing computer models to run experiments of natural vs. human-influenced “simulated Earths” (research beginning in 1960s)
  9. Consensus among scientists that consider all previous lines of evidence and make their own conclusions (polling beginning in 1990s)

(You can also see these nine lines of evidence illustrated in the graphic below)

Skeptics sometimes point to the last two supporting lines of evidence as weaknesses. They’re not. But even if you choose to doubt them, it is really the first seven that, combined, point to human activities as the only explanation of rising global temperatures since the Industrial Revolution, and the subsequent climate changes (such as ice melt and sea level rise) that have occurred due to this global warming.

The science is settled, and the sooner we accept this, the sooner we can work together towards addressing the problems caused by climate change – and towards a better future for us all.

 

(Click here for a pdf version of the graphic)

 

This entry was posted in Basic Science of Global Warming, Greenhouse Gas Emissions, Science, Setting the Facts Straight. Bookmark the permalink. Both comments and trackbacks are currently closed.

39 Comments

  1. Josh Smith
    Posted March 23, 2017 at 3:58 pm | Permalink

    This is terrific. Would you be willing to make it available as a PDF so that it can be printed at high resolution?

    • My Question
      Posted March 24, 2017 at 7:19 am | Permalink

      Why doesn't anyone ask the most important question on this subject. Who is causing the majority of the problem?

      If the US with all our regulations trying to control the pollution means we only produce a small portion of the planet's pollution, then what are we doing about the countries/people that cause the real problem? If we regulate ourselves into conditions that we strangle our economy in the US while other areas of the planet do nothi ng or little, does that make sense?

      I do not know the answer and possible we are the biggest problem on the planet but possibly we are the smallest contributors to the problem. Without the answer to this question it is like we have locked the doors to our expensive convertible in the highest crime area of the country yet we left the top down. What are we really accomplishing?

      • Kevin
        Posted March 24, 2017 at 7:52 am | Permalink

        So we should just start polluting more because other countries are doing it? That doesn't make much sense either.

        • Atul
          Posted March 24, 2017 at 8:16 am | Permalink

          I think rules should be same for every country on the planet in curbing the pollution otherwise its not gonna work out.

          • James
            Posted March 29, 2017 at 10:35 am | Permalink

            There are problems there as well though, which have to be resolved, in that smaller/weaker/not as advanced/etc countries cannot cut to the extent that a more developed nation can, if at all without support, but they too need to cut emissions. So they need support that isn't as simple as tying a rule on every country (that they could pretty much choose to ignore anyways)

            It gets even more complicated when politics and feelings of sovereignty comes into play.

            Not as simple as same rules for every country.

        • MY QUESTION
          Posted March 24, 2017 at 4:29 pm | Permalink

          Kevin, I never implied that we should pollute more. If we do not tackle this issue globally we are not going to get anywhere. Why has there been no answer to my question by anyone or does that answer make you uncomfortable in your position? If other countries create 90% of the pollution that causes global warming and they refuse to do anything about it what progress do you think we will make? Causing our economy to be stagnant like it has been for years will not give us the resources to make a meaningful change. We have to do something about this but lets plug the biggest hole in this boat before we all sink. If the US is that biggest hole in the boat then we have the most to accomplish but if we are only a small hole we need a different approach.

          • You Answer
            Posted March 25, 2017 at 11:59 am | Permalink

            There is substantial research on who is the largest contributor. It used to be the US, but China has now overtaken us.

          • Nameless
            Posted March 28, 2017 at 3:47 pm | Permalink

            China has also as of late increased their solar panel production threefold and we elected a moron who believes climate change is a hoax. So…

      • Odin
        Posted March 24, 2017 at 8:31 am | Permalink

        I fully agree that pollution should be minimized – plastics, hazardous chemicals, oxides of sulfur and nitrogen, mercury etc going into our landfills, ground water and oceans are a huge concern. However I do not view a naturally occurring, clear, colorless, odorless, essential for life on earth, trace gas (carbon dioxide) as a pollutant, nor do I think it is having much, if any, effect on global temperatures.

        • Posted March 24, 2017 at 1:51 pm | Permalink

          You are absolutely right in that CO2 is essential for life on Earth. In fact, without CO2 in the atmosphere, Earth would be entirely frozen and 60F cooler! This is basic physics that was identified in the 1800s. Therefore, even though CO2 is a trace gas, it is responsible for the comfortable temperatures on Earth, and therefore plays a major role in impacting global temperatures. An entire body of decades of scientific literature backs up this claim. However, the more CO2 we add into the atmosphere, the warmer it gets.

      • Posted March 24, 2017 at 10:02 am | Permalink

        OP wonders about the contribution of the US to the problem: "I do not know the answer and possible we are the biggest problem on the planet but possibly we are the smallest contributors to the problem."

        This is relatively easy to find out. Even if you count only current emissions, the US is the second highest contributor in total (China is first).

        If you measure emissions per person, the US is also in the top dozen or so countries, much higher than China. The US average per person is several times higher than what is sustainable.

        And if you count historical emissions, the US is the number one cause of the problem.

        References
        http://www.wri.org//blog/2014/11/6-graphs-explain-world%e2%80%99s-top-10-emitters
        https://en.wikipedia.org/wiki/List_of_countries_by_carbon_dioxide_emissions_per_capita

      • Posted March 24, 2017 at 1:43 pm | Permalink

        Hi there!

        We do have extensive data on country and sector contributions to climate change. The US is currently the second largest emitter of carbon dioxide emissions, responsible for around 16% of emissions from energy use. While China became the world's largest emitter around 10 years ago, responsible for around 28% of CO2 emissions from energy use, China is currently leading the way with actions to reduce emissions of greenhouse gases. For example, China is now the largest producer of wind and solar energy. It is therefore a misconception that the US is the only country taking action. China, the US, and the EU account for more than half of global CO2 emissions, and together can have a major impact on reducing climate change. All three are working towards a better future, and to date 139 nations have ratified the Paris Agreement to curb climate change.

        References:
        https://www.epa.gov/ghgemissions/global-greenhouse-gas-emissions-data
        http://cdiac.ornl.gov/trends/emis/tre_coun.html
        http://grist.org/climate-energy/7-signs-that-china-is-serious-about-combatting-climate-change/
        http://unfccc.int/paris_agreement/items/9485.php

      • Vance
        Posted March 25, 2017 at 3:26 pm | Permalink

        Where do you get the idea that the u.s. only produces a "small portion" of the world's pollution? And- a lot of the pollution from china comes from the manufacture of goods for u.s. and european markets- so it is still our lifestyle that's ultimately to blame.

      • Will Cerf
        Posted March 27, 2017 at 8:01 pm | Permalink

        That's the whole point of the Paris Climate agreement.

    • Sharyn Stein
      Posted March 24, 2017 at 1:21 pm | Permalink

      We have added a link to a PDF version of the graphic at the bottom of the post. Thank you.

  2. SHERI BORIS
    Posted March 23, 2017 at 4:20 pm | Permalink

    This information needs to be sent to the skeptics in the White House

  3. Robert White
    Posted March 23, 2017 at 7:44 pm | Permalink

    Please with the PDF request above. Knowledge is indeed power but knowledge shared is POWER SQUARED!

    • Sharyn Stein
      Posted March 24, 2017 at 1:21 pm | Permalink

      We have added a link to a PDF version of the graphic at the bottom of the post. Thank you.

  4. Labillyboy
    Posted March 24, 2017 at 5:52 am | Permalink

    How does AGW explain the ice age of only 12000 years ago? Seas were 300 ft lower and North America was under 300 ft. of ice. It warmed up enough for humans to habitate the continent with zero help from AGW… the slightest increase in CO2 is a bad joke. The concensus is only among "scientists" paid for that opinion. This article is pure nonsense.

    • Aloof Wolf
      Posted March 24, 2017 at 7:43 am | Permalink

      The point isn't that AGWis the cause of all warming throughout time. It is that the current warming trend is happening faster than in any previous perios in so far as scientists can determine from historical evidence.

      • Odin
        Posted March 24, 2017 at 8:06 am | Permalink

        That's not true. There have been many times in earth's past history when global temperatures have changed much more sharply than current. Since the 1850's it is predicted that we have had a temp increase of about 1 C. This is trivial compared to past events. About 25 climate shifts, called Dansgaard-Oeschger cycles, have been identified in the ice core records. These cycles occurred about 11,500 years ago and averaged annual temperatures on the Greenland ice sheet warmed by around 8 °C over a 40 year time frame, in three steps of five years where a 5 °C change over 30–40 years is more common. Or the Paleocene-Eocene Thermal Maximum, timed at 55 million years ago, which may have been caused by the clathrate gun effect (increases in sea temperatures, and/or drops in sea levels which may can trigger the sudden release of methane from methane buried in seabeds), although alternative mechanisms have been identified. This was associated with rapid ocean acidification. The Permian-Triassic Extinction Event, also known as the great dying, in which up to 95% of all species became extinct, has been hypothesized to be related to a rapid change in global climate. The Carboniferous Rainforest Collapse occurred 300 million years ago, at which time tropical rainforests were devastated by climate change. The cooler, drier climate had a severe effect on the biodiversity of amphibians, the primary form of vertebrate life on land.

        • Aloof Wolf
          Posted March 24, 2017 at 9:37 am | Permalink

          Regardless, it is happening at a faster than "normal" rate. Are we, as the whole of humanity, prepared to handle the effects of a rapid change?

    • Posted March 24, 2017 at 2:17 pm | Permalink

      "AGW" refers to anthropogenic global warming, which is a term for post-Industrial Revolution warming. Therefore, AGW has nothing to do with the Ice Age 12,000 years ago.

      Earth's climate is determined by a balance between incoming sunlight and outgoing heat. Any factor that can change these properties (like changes in the Sun's energy output) can change the climate. While today it is human activities that are changing the amount of greenhouse gases in the atmosphere, which controls how much heat escapes to space, other changes throughout Earth's history have subsequently altered the climate. We have many ways to determine these past changes and the causes. The history of Earth is characterized by many swings from glacial to interglacial periods. From another blog of mine:

      "Ice ages often begin when there is less sunlight in the Northern Hemisphere during summer, causing less ice to melt and more ice to accumulate – which, in turn, causes more sunlight to be reflected, and so on.These changes in sunlight depend on the shape of the Earth’s orbit around the sun, the angle of tilt of the Earth on its axis, and whether the tilt of the Earth is pointing toward or away from the sun – all of which vary on different timescales of tens of thousands of years. When the conditions align in favor of a colder Northern Hemisphere summer, glaciers grow." (see the second link below)

      Eventually, more intense summer sunlight in the Northern Hemisphere triggers an interglacial, of which warm conditions are quickly exacerbated by internal feedbacks like the release of stored carbon dioxide in the oceans.

      References:
      http://the-toast.net/2014/07/04/gal-science-earth-historical-temperature/
      https://www.edf.org/blog/2016/02/11/human-emissions-just-cancelled-next-ice-age-heres-why-we-should-care

  5. Odin
    Posted March 24, 2017 at 7:53 am | Permalink

    Yawn – another poorly informed , poorly researched media article. Everyone agrees that CO2 levels have risen and that at least most of this increase can be attributed to man. Nevertheless, there are many points not mentioned in the article. First – CO2 only absorbs and emits energy at very cold temperatures (-70C and colder) and to use physics properly one would wonder how CO2 could transfer any significant amount of energy back to the warm earths surface – second law of thermodynamics – heat transfer is from warmer to colder bodies. Second – water vapour is an efficient greenhouse gas and is much more abundant in the atmosphere and absorbs and emits energy at temperatures in line with earths surface and lower atmosphere. Water vapour is a much more powerful greenhouse gas. Third – the ice core data, published in several high quality science journals, clearly shows that increases in global temperatures PRECEDE increases in atmospheric CO2 levels i.e. warming generates CO2 and not the other way around (a warming ocean degasses CO2). Furthermore, the latest report from the IPCC acknowledges the following: the climate sensitivity previously ascribed to CO2 was greatly overestimated most likely resulting in the complete failure of the climate models (on which AGW theory is built). The IPCC further acknowledges – Chapter 2 of their recent report – that current datasets indicate no significant trends in the frequency of global tropical cyclones, tropical storms, hurricanes and major hurricanes over the last 100 years. They also state that there is no trend in the frequencies of floods on a global scale. Guess what people – bad things happened before the 1900's – we just don't have any data for this. So what has caused the warming since the Little Ice Age which ended mid 1800's. The sun – a Maunders Minimum caused the Little Ice Age and the end of the Maunders Minimum ended the Little Ice Age and temperatures started to increase – thank goodness for that. Solar activity has continued to increase- according to the Max Planck Institute, and has reached an 8000 year high in the mid 1980's (published in Nature). I fully acknowledge that increased CO2 levels may have some effect on global temperatures but the overall effect is likely to be small but beneficial to increased plant growth and crop yields.

    • Charlene
      Posted March 24, 2017 at 8:12 am | Permalink

      Nicely said.

    • Posted March 24, 2017 at 3:37 pm | Permalink

      Hi there,

      Thanks for reading the article and for your thoughtful response. I am actually a PhD scientist – not a journalist – and have spent my entire career studying the physics of climate change. The purpose of this post was to summarize our state of understanding, so I apologize for not being able to mention all of the relevant physics.

      For your first point, this is exactly how the greenhouse effect works. Because the CO2 is at higher elevations and thus colder temperatures, when the radiation from the surface at a warmer temperature is absorbed in the layers above which are at a colder temperature, it is re-emited at lower wavelengths and with less energy. The colder layers therefore emit less radiation so that there is a net loss of radiative energy escaping to space from the atmosphere-surface system as a whole. This is Kirchhoff's law of thermal radiation. "How the CO2 transfers any significant amount of energy back to warm Earth's surface" is through your second point: through water vapor. Water vapor considerably amplifies CO2's impact on surface temperature because of its abundance and strong radiative efficiency. For your third point, CO2 and temperature are in a tight feedback relationship with each other. Increased temperature can trigger more CO2 in the atmosphere, which yields more warming due to more CO2. Same is true for the opposite, cooler temperatures can take CO2 out of the atmosphere and into the ocean, which further cools climate because of less CO2 in the atmosphere. In the past, temperature changes (likely from changes in solar activity) triggered the start of the cycle. But today, the cycle is being triggered by changes in CO2 first.

      Further, the IPCC certainly does not acknowledge "a complete failure of climate models," nor is that the case, and the reduced lower end of the climate sensitivity range in the most recent report does not mean that higher climate sensitivities are unlikely, and thus we still have serious concerns about future warming. Climate sensitivity is only one metric, and considering that we've already seen negative impacts on ever continent and in every ocean from the warming so far, we have real reason to be concerned about the future regardless of the exact climate sensitivity number.

      Regarding trends in extreme events, there is certainly a lot of evidence about how extreme events are changing and are more frequent/intense because of climate change. The IPCC mentions a lot of this research in Chapters 10, 11, and 12 of the the IPCC AR5 WGI report. For example, the IPCC AR5 concludes that human influence has very likely affected frequency and intensity of daily temperature extremes and likely doubled the probability of heat waves. There is medium confidence that human influence has contributed to intensification of heavy rainfall and increased hurricane activity since the 1970s in the North Atlantic, and low confidence in changes in drought conditions and any long-term increases in hurricane activity. However, there is increasing consensus that storms are increasing in strength, and it evident that there is extra energy in the system due to human influence. New statistical techniques enable quantification of extreme event attribution to manmade climate change; individual events that have been attributed to manmade climate change include the 2000 Flooding in England and Wales, 2003 European Heat Wave, 2010 Moscow Heat Wave, and 2011 Texas Drought. BAMS has special reports that are more specific and current, and the World Weather Attribution service is rapidly analyzing events in near real time.

      While the Sun plays an important role in Earth’s climate by providing the primary source of energy, monitoring the variations in solar radiation using direct measurements from satellite sensors show that during the last few decades, there has been virtually no change in the total output of the Sun. In fact, the 11-year solar cycle was close to its minimum in 2010, yet Earth’s surface temperature set a record high that same year. Another indicator that today's climate change isn't caused by the Sun is that land surface temperatures and night temperatures are also warming. These combined observed effects in temperature warming in the troposphere are exactly what we would expect to happen with increased greenhouse gases. Further, the upper atmosphere would be warming up if it was the Sun. However, it is cooling. Again, this is consistent with the greenhouse effect.

      You mention that bad things happened before the 1900s. Absolutely. The difference now is that because we know we are the cause, we can be the solution. We are in control of what is happening, and so we could prevent a lot of negative outcomes. This is incredibly powerful information, and why I devote my life to solving this problem.

      References:
      https://www.ametsoc.org/ams/index.cfm/publications/bulletin-of-the-american-meteorological-society-bams/explaining-extreme-events-from-a-climate-perspective/
      https://wwa.climatecentral.org/
      https://www.skepticalscience.com/solar-activity-sunspots-global-warming-advanced.htm

    • Posted March 28, 2017 at 7:11 pm | Permalink

      "CO2 only absorbs and emits energy at very cold temperatures (-70C and colder) and to use physics properly one would wonder how CO2 could transfer any significant amount of energy back to the warm earths surface – second law of thermodynamics – heat transfer is from warmer to colder bodies."

      False. Try reading.

      The key process is absorption of longwave (heat) radiation coming upwards from the (heated up by the sun) ground, and then emission longwave (heat) radiation in all directions (since CO2 molecules have an equal chance of emitting in any direction the radiation they just absorbed).

      E.g. wikipedia:

      "Each layer of atmosphere with greenhouses gases absorbs some of the heat being radiated upwards from lower layers. It reradiates in all directions, both upwards and downwards; in equilibrium (by definition) the same amount as it has absorbed. This results in more warmth below. Increasing the concentration of the gases increases the amount of absorption and reradiation, and thereby further warms the layers and ultimately the surface below."

  6. A scientist
    Posted March 25, 2017 at 1:12 pm | Permalink

    Consensus (#9) is not, nor ever should be considered a line of evidence. As a 'scientist', the author should know this.

    • Posted March 27, 2017 at 11:27 am | Permalink

      I absolutely agree that consensus itself is not evidence. I included it rather as the culmination of all other lines of evidence, as I consider it as "available information" indicating that humans are largely responsible for climate change; it is important for people to know that thousands of scientists from all over the world have reviewed the previous lines of evidence for themselves, and came to the same conclusion.

      • A scientist
        Posted March 29, 2017 at 10:25 pm | Permalink

        Disappointing answer. If you agree then you should change the article. But you won't because you believe that 9 lines of evidence makes a stronger case and since you are motivated to 'sway' the masses instead of informing them (or providing the truth), you'll keep it the way it is. Therefore, you should consider yourself a journalist or an 'activist' that knows some science, but not a scientist.

  7. Posted March 28, 2017 at 10:15 am | Permalink

    I'm just a simple person and have no degree in any scientific matters. I'm just an certified Integrative Health Coach and love to see people thrive and live in an environment that will keep the species healthy and alive. I just wonder if we stopped the excessive amount of burning carbon-based materials and lived a life that was innocuous and less polluted, would the human species and all the other wonderful species that live on this planet benefit? I'm saddened that the focus of these argumentative replies are more focused on right and wrong and less focused on humanities betterment. I guess I'm just not savvy enough to understand the mechanics of keeping our planet and its species at its best.

  8. Jeff Aldrich
    Posted March 28, 2017 at 10:30 am | Permalink

    I have respect for EDF, most of their work is very grounded in solid science and not subject to hyperbole, there are issues with this Blog. Specifically Line #7 is just plain wrong and #9 is a misstatement.

    #7 dismisses natural causes of global warming yet the atmosphere has been steadily warming for 21,000 years, in this cycle, proving that man is not the driving mechanism. Geologists have documented hundreds of warming and cooling cycles (the earth can get really hot and cold!!) due to natural causes. Also, the hockey stick curve of Mann that they reference in #6 has been proven to be wrong and de-bunked many times over.

    #9 is a misstatement: yes most scientists that are informed or knowledgeable on climate science agree that the climate is warming and the majority agree that human activity acts as a heating force. Census has not been reached at all – including within the IPCC report- on the amount of the effect or if it even significant. It is one thing to get scientists to agree on which way the data is trending, it is another to get them to agree on the significance.

    What the EDF blog does not mention is the bigger question which is "Are the net effects of Global Warming a negative or positive?" Negatives include raising sea levels but positives include increased crops and much lower death rates (annual deaths due to cold are about 10x that due to heat).

    • Annie GYG
      Posted March 29, 2017 at 2:05 pm | Permalink

      The "dismissal of natural causes" is not a total disregard for natural causes – the meaning is that the level of warming is considerably more than can be explained by natural causes, but when you factor in human activity the answer becomes quite obvious. Human beings are causing a rapid escalation of climate change. Rising oceans are a huge negative since a majority of people on earth live near the ocean, causing huge disruptions in economy and massive relocations of people. The "positive" assumption that crops will increase is very pie-in-the-sky optimism indeed. The greater likelihood is crop failures due to erratic and violent weather shifts, resulting in economic upheaval and widespread famine. Deaths due to heat are also common, and will increase with not only rising temperatures, but also more violent storms and flooding. Let me ask this – what carries the greater risk: developing a clean, sustainable energy future by supporting research and offering incentives – or continuing to use fossil fuels indefinitely, thereby risking that we run out of useable deposits &/or destroy our ecosystem? I simply don't understand the controversy.

  9. Posted March 28, 2017 at 7:01 pm | Permalink

    Good stuff! But the chemistry that burning stuff in an oxygen atmosphere releases CO2 didn't start in the 1900s! Joseph Priestly was probably the most important figure, that was 1700s, but you can see more history here: https://en.wikipedia.org/wiki/Carbon_dioxide#Background

  10. FozziG
    Posted March 29, 2017 at 12:03 pm | Permalink

    My main question, which has never been adequately answered in anything I have read, is when looking at that CO2 graph and population growth graphs, they show STEADY increase but the temperature graph does not. In fact, there are some periods where slight cooling happened. There is SOME factor that is not being taken into consideration or mentioned. I know that climate is a complex system and we cannot simplify the issue to one key factor but that seems to be the focus.

    • Annie GYG
      Posted March 29, 2017 at 2:10 pm | Permalink

      One of the reasons for periodic cooling is the melting of ice sheets, which cools the ocean it melts into and causes cooler weather for a while, usually just regionally, but it would impact the overall averages. I'm sure there are other reasons due to the background natural shifts and changes. This isn't a closed system with only one thing happening at a time. The important thing you noticed is the overall trend and the overall rate of change, which is accelerating.

  11. Jefe
    Posted March 29, 2017 at 3:16 pm | Permalink

    How about we have 200 bureaucrats fly jumbo jets to Davos and stroke their beards and say hmmm?

    That will surely fix the problem.

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