How long is long enough? Do we need more climate data?

climate change data global warming warmest years on record2016 was the hottest year on record, making it the third year in a row to set a new temperature record. Indeed, the past few decades have been so warm, that anyone who is 31 years old or younger has never experienced a single month when the earth’s temperature was below average (that’s using the 1951–1980 average that is generally used for benchmarking global temperatures; NASA data). That’s pretty striking evidence that the planet is warming. Nevertheless, there are many who object to this and claim that we simply don’t have enough data to know what is really going on. They point to the fact that our global records only go back to 1880 and insist that 130+ years is nothing when you consider the entire history of planet earth. However, this argument ignores several key points that I will elaborate on. First, we actually do know a lot about the climate prior to 1880, and we have used those data to understand how the climate works. Second, by using our understanding of past climate, we have been able to test the current drivers of climate change and determine with an extremely high degree of confidence that the current warming is not natural.

How long is long enough?

Before I deal with the actual evidence itself, I want to point out a fundamental problem with this argument. If you think that 130+ years isn’t enough data, then how much will be enough? How long does the trend need to go to convince you? Think about it, if the argument is that 130+ years is nothing compared to the billions of years that earth has been around for, then there is probably no realistic amount of data that would convince you. For example, would 200 years convince you? I doubt it, because in the grand scheme of things, those extra 70 years are also minuscule. To put this another way, people have been saying “there isn’t enough data” for decades now, even as more data has continued to accumulate. Back in 2000, for example, tons of people said that we don’t have enough data and the trend probably won’t continue, but 16 additional unusual (and often record setting) years of data have been collected since then, and many of them still aren’t convinced. So how long does the trend have to last for before you’ll accept it?

My point is that this argument sets up a non-falsifiable situation that allows people to appear rational by demanding more evidence, when the reality is that no amount of evidence will convince them. To put that another way, people who use this argument are picking an arbitrary amount of data that is necessary, rather than actually looking at the quality and content of the data that we already have. This argument is really a cop-out. It is an excuse for blind ignorance, rather than a logical argument. So if you are prone to using this argument, I hope that you will set it aside for a minute and actually look at the data that we have gathered.

Past climate data

When we talk about global temperature records, we are generally referring to the period starting with 1880, because that is when we had sufficient, real-time measurements from around the world to confidently state the global temperature. However, we do in fact have climate data going back much, much further thanks to things like ice cores, the ratios of certain chemicals, etc. So although we may not be able to confidently state a precise global mean temperature before 1880 that is directly comparable to our modern measurements, we still have a really good understanding of what the climate was like. For example, Martin et al. (2005) used the Mg/Ca ratios of benthic foraminifera (a marine protist) to trace ocean temperatures back for 90 thousand years (which is substantially longer than 130+).

This is really important, because we can use those past climate data to understand things like how solar cycles and CO2 affect the climate. Scientists have, of course, done this, and here is what they found in a nutshell. The earth goes through prolonged cycles known as Milankovitch cycles that affect the earth’s eccentricity, precession, and obliquity (i.e., orbit, tilt, and axis) and this affects the amount of energy entering the earth or the location/season at which it enters. This (as well as other factors like solar activity) often causes a small amount of warming (often regional), and that warming causes the oceans to release the CO2 that is stored in them (Martin et al. 2005; Toggweiler et al. 2006; Schmittner and Galbraith 2008; Skinner et al. 2010). That CO2 then drives the bulk of the warming (this is why there is a brief “lag” in the climate data; Lorius et al. 1990 ; Shakun et al. 2012). So in short, past events of global warming occurred whenever something caused enough small-scale warming (often regional) for the oceans to release CO2, and that spike in CO2 caused most of the actual warming (more details here).

Our current warming

Now that we have established the drivers of past climate change, we can look at our current situation to see whether or not natural factors are causing the current warming (spoiler alert, they aren’t). You see, the thing that the “only 130+ years of data” argument ignores is that we already understand causal relationships when it comes to the drivers of climate change. In other words, scientists aren’t just sitting back and saying “well it’s been warming for 130+ years, therefore it must be us and it will keep on warming until we change.” Rather, they have carefully examined the cause of the warming, and it is the understanding of that cause that allows us to be so confident.

So what is the cause? You probably guessed it; it’s CO2. The image to your right shows the earth’s atmospheric CO2 concentration over the past 800,000 years. You’ll notice that it is substantially higher now than it has been during any point in that time period (surely 800,000 years is long-term enough). Also, remember that we know from our studies of past climate change that CO2 is a major factor in driving planetary warming. At this point, the situation is really quite simple: we know that CO2 traps heat and is important for regulating our climate, we know that CO2 has been a major driver of past climate change, and we know that we have greatly increased the CO2 in our atmosphere, therefore, the only logical conclusion is that we are causing the climate to change. Please note that this conclusion is not based on 130+ years of warming. Rather, it is based on understanding physics and the factors that drive climate change. Indeed, scientists predicted that burning fossil fuels could change the climate way back in 1896. So the recent warming is simply the confirmation what scientists predicted over 100 years ago.

To put this another way, we can demonstrate that we are the cause using a simple syllogism.

  1. In the past, large increases in CO2 have caused the planet to warm
  2. We have caused a large increase in CO2
  3. Therefore, we are causing the climate to warm
This figure from Hansen et al. 2005 shows the effect of both the natural and anthropogenic drivers of climate change. Notice how only anthropogenic sources show a large warming trend. Also, see figure 2 of Meehl et al. 2004.

This figure from Hansen et al. 2005 shows the effect of both the natural and anthropogenic drivers of climate change. Notice how only anthropogenic sources show a large warming trend. Also, see figure 2 of Meehl et al. 2004.

Nevertheless, you may protest and say, “what about volcanoes, the sun, etc.” Well, first off, volcanoes actually only emit a very tiny amount of CO2 (less than 1% of what we produce Gerlach 2011). Second, scientists have carefully looked at volcanoes, the sun, and other natural drivers of climate change, and they simply cannot explain the current warming without including our greenhouse gas emissions (Stott et al. 2001; Meehl et al. 2004; Hansen et al. 2005; Allen et al. 2006; Lean and Rind 2008; Imbers et al. 2014).  In other words, we know that the current warming isn’t natural, because we have tested the natural causes of climate change, and they all come up short (even when combined together; more details here). Output from the sun, for example, simply doesn’t correlate with our current warming, therefore we know that it isn’t the sun that is driving it (note: the actual tests are far more complicated than simple correlations, because they both control for confounding variables and combine the effects of variables, but it is often convenient to think about it as simple correlation to start with, then build complexity on top of that). To be clear, these tests have also shown that the current warming is not 100% human induced. There is some natural background fluctuation, but the bulk of it is man-made, and it will continue if we don’t reduce our fossil fuel use.

It is probably also worth mentioning that we are certain that the increase in CO2 is from us because of isotope ratios. I explained this in more detail here, but in short, carbon has two stable isotopes (C13 and C12), and the ratio of those two isotopes in fossil fuels is different from the ratio that is in the atmosphere. Therefore, if the CO2 is from us burning fossil fuels, we would expect that atmospheric ratio to shift to be more like our fossil fuels, which is exactly what has happened (Bohm et al. 2002; Ghosh and Brand 2003;Wei et al. 2009). These ratios are like fingerprints that demonstrate beyond any reasonable doubt that the CO2 is from us.

Confirmation via models

At this point, you may be tempted to think that all of this is well and good, but we can’t really know what is happening without more data. I disagree, but there are still additional pieces of evidence that we can bring to bear. For example, we can be highly confident that our current understanding is correct thanks to models (as well as other data that I’ll talk about later). Scientists are a pretty clever bunch, and what they realized is that we can construct models based on our current understanding of how climate works, and use those models to make predictions about the future. If our understanding is correct, then the models should make accurate estimates, and if our understanding is wrong, the models should make incorrect estimates (science thrives on testable predictions like this). Granted, there is a certain amount of uncertainty around this, because the models are reliant on the skills of their programmers, as well as natural factors that are out of our control (e.g., if the output of the sun changes and is different from the model input, that will cause the model to be wrong, even if our basic understanding of the climate is correct). Nevertheless, these models have actually done a remarkably good job of predicting the current warming (Hansen et al. 2006; Frame and Stone 2012; Rahmstorf et al. 2012; Cowtan et al. 2015; Marotzke and Firster 2015). Now, yes, I know, you have read on the internet that the models have all been wrong. Well, I hate to break it to you, but not everything on the internet is true. The claims that the models have been wrong are based on cherry-picking and misrepresenting how the models work. The actual scientific analyses have found that they have done a good job of predicting the current warming (more details here and here). Also, it is worth mentioning that the models have been refined over time because our understanding of the climate has improved over the years. However, those refinements simply improve the accuracy of the models, they don’t change the overarching trend or predictions.

Satellite measurements

Finally, I want to draw your attention to one final piece of evidence which, in my opinion, is the single most conclusive demonstration that we are causing the planet to warm. When energy from the sun hits the earth’s surface, much of it is radiated back of as long-wavelength, low-energy infrared radiation (IR, aka heat energy), and CO2 in our atmosphere is capable of absorbing those long wavelengths. Thus, it traps some of that heat energy before it leaves the earth. This is a scientific fact that no one disputes, and it is actually a good thing, because the earth would be inhospitably cold without CO2 and the other greenhouse gases. Nevertheless, it can be a problem, because if that CO2 increases, then more heat is trapped and the earth warms. Again, we know that this is largely what drove past climate changes.

Now that we have those facts in place, we can make another testable prediction. If our CO2 is causing the planet to warm, then the amount of energy entering the earth should be roughly constant, but the amount of energy that is leaving the earth should decrease significantly because some of it is being trapped by our CO2. Scientists have actually measured this using satellites, and just as expected, the energy from the sun is basically unchanged, but less energy is leaving at the specific wavelength that CO2 traps (Harries et al. 2001; Griggs and Harries. 2007; Huang et al. 2010). This is extremely clear evidence that we are causing the planet to warm, and if you want to say that the warming isn’t from us, then I want to know where that IR is going. We know that the roughly same amount is coming in, but less is going out, so if it’s not our CO2 that is trapping it, what is it? (note: recently, the sun’s output has decreased slightly, but it is not enough to explain the decrease in IR, and the pattern hasn’t been consistent; i.e., the sun’s output does not correlate well with the decrease in IR, but our CO2 does).

In conclusion, let’s put all of the pieces together. We know that CO2 traps heat and warms the planet, we know that increases in CO2 caused past climate change, we know that we have greatly increased the CO2 in the atmosphere, and we know that CO2 is currently causing the planet to trap excess heat. The conclusion from that is inescapable: we are causing the planet to warm. We do not need more data to be confident in that conclusion.

Note: To be clear, I’m not suggesting that we should stop studying the climate, we certainly should and there is still much to learn, but the evidence that we have so far is sufficient to be highly confident that we are causing the climate to change. To use an analogy, there is still a lot to learn about the human body and we should keep studying it, but we have plenty of data for me to confidently state that your liver isn’t the organ that is responsible for pumping blood.

Literature Cited

  • Allen et al. 2006. Quantifying anthropogenic influence on recent near-surface temperature change. Surveys in Geophysics 27:491–544.
  • Bohm et al. 2002. Evidence for preindustrial variations in the marine surface water carbonate system from coralline sponges. Geochemistry, Geophysics, Geosystems 3:1–13.
  • Cowtan et al. 2015. Robust comparison of climate models with observations using blended land air and ocean sea surface temperatures. Geophysical Research Letters 42:6526–6534.
  • Frame and Stone 2012. Assessment of the first consensus prediction on climate change. Nature Climate Change 3:357–359.
  • Gerlach 2011. Volcanic versus anthropogenic carbon dioxide. EOS 92:201–202.
  • Ghosh and Brand. 2003. Stable isotope ratio mass spectrometry in global climate change research. International Journal of Mass Spectrometry 228:1–33.
  • Griggs and Harries. 2007. Comparison of spectrally resolved outgoing longwave radiation over the tropical Pacific between 1970 and 2003 Using IRIS, IMG, and AIRS. Journal of Climate 20:3982-4001.
  • Hansen et al. 2005. Earth’s energy imbalance: confirmation and implications. 308:1431–1435.
  • Hansen et al. 2006. Global temperature change. Proceedings of the National Academy of Sciences 10314288–14293.
  • Huang et al. 2010. Separation of longwave climate feedbacks from spectral observations. Journal of Geophysical Research 115:D07104
  • Imbers et al. 2014. Sensitivity of climate change detection and attribution to the characterization of internal climate variability. Journal of Climate 27:3477–3491.
  • Lean and Rind. 2008. How natural and anthropogenic influences alter global and regional surface temperatures: 1889 to 2006. Geophysical Research Letters 35:L18701.
  • Lorius et al. 1990. The ice-core record: climate sensitivity and future greenhouse warming. Nature 139–145.
  • Marotzke and Firster 2015. Forcing, feedback and internal variability in global temperature trends. Nature 517:565–570.
  • Martin et al. 2005. Role of deep sea temperature in the carbon cycle during the last glacial. Paleoceanography 20:PA2015.
  • Meehl, et al. 2004. Combinations of natural and anthropogenic forcings in the twentieth-century climate. Journal of Climate 17:3721–3727.
  • Rahmstorf et al. 2012. Comparing climate projections to observations up to 2011. Environmental Research Letters 7:044035.
  • Schmittner and Galbraith 2008. Glacial greenhouse-gas fluctuations controlled by ocean circulation changes. Nature 456:373–376.
  • Shakun et al. 2012. Global warming preceded by increasing carbon dioxide concentrations during the last deglaciation. Nature 484:49–54.
  • Skinner et al. 2010. Ventilation of the deep Southern Ocean and deglacial CO2 rise. Science 328:1147-1151.
  • Stott et al. 2001. Attribution of twentieth century temperature change to natural and anthropogenic causes. Climate Dynamics17:1–21.
  • Toggweiler et al. 2006. Mid-latitude westerlies, atmospheric CO2, and climate change during the ice ages. Paleoceanography 21:PA2005.
  • Wei et al. 2009. Evidence for ocean acidification in the Great Barrier Reef of Australia. Geochimica et Cosmochimica Acta 73:2332–2346.
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6 Responses to How long is long enough? Do we need more climate data?

  1. Nice post! Question: What kind of resolution do these paleoclimate reconstructions of temperature have? I’m curious how past rates of temperature increases compare to that of the last century or so.

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  2. Mark Ryan says:

    I enjoy your blog and think you are one of the best science communicators on the net, FM.

    I also like your use of a simple syllogism -I have used this approach with students too, but I think you could use a more robust first term than the history of climate changes. The first term I use is the understanding of the greenhouse effect, specifically the way in which diatomic molecules like CO2 respond to radiation. Not only is this phenomenon well understood in nature, it is even the physical basis for the microwave ovens we all have in our homes.

    I prefer this as the first term of my syllogism because it is not subject to the same roundabouts of obfuscation that paleoclimate temperature records can be subject to. “CO2 is a greenhouse gas” can slot straight into your position number 1, and steps 2 and 3 remain unchanged.

    Liked by 1 person

  3. Aidan says:

    To be clear, I do not dispute that the climate is changing, that warming is global, or that it is mankind related. I have never personally studied it with enough energy to really hold a conclusive POV on it. I generally just accept it as factual prima facie, just as I would expect conclusions in my realm of study to be given benefit of the doubt, short of scientific evidence to the contrary. I’m an Environmental Health focused biologist (toxicology, hazardous chemicals, general ecology, general genetics, biostatistics) by education, and environmental scientist / safety guy by trade.

    I do not dispute your logic, as presented here, or that carbon dioxide traps heat. My question is only for clarification, as you seem to be ‘stuck’ on the idea that only fossil fuel carbon dioxide is the source for the trapped heat.

    What about deforestation? And more broadly, over-development of our natural gas cycling areas with strip malls, parking lots, subdivisions, etc.? What of the effects of paving over our watersheds, substituting wild grasses with ornamental varieties that are kept under an inch in height, and decreasing the volume of gases that nature can absorb and exchange into oxygen, relative to the bio burden placed on it from more people inhabiting those paved over places?

    http://www.climateandweather.net/global-warming/deforestation.html

    https://www.carbonbrief.org/deforestation-in-the-tropics-affects-climate-around-the-world-study-finds

    To what extent does each factor – all of which are still mankind origins – contribute to the amount of carbon dioxide that is trapping the heat? ie: is it 70% fossil fuels, 30% other sources of which deforestation is one; or is it 25% fossil fuels, 50% deforestation and 25% other sources? Again, random numbers for illustration only – absolutely no basis in selecting them.

    When framing the discussion with deniers, the important thing to remind them is this, in my experience:

    Whether or not you think that mankind has caused the climate change, you can’t deny that the climate is warming on a global scale. Trapping heat with carbon dioxide or other greenhouse gasses is definitely detrimental on a global scale. Deforestation is definitely detrimental, as are current non-sustainable development projects the world over.

    So no matter what is the initial cause (man made is backed by the current science), we are still in our best interests as a species to do whatever we can to minimize our effects, and to avoid becoming the catalysts for the eventual extinction of our species, as it currently exists.

    But, I would still like to know how much deforestation and urban development, by the numbers, plays into the carbon dioxide heat shield, versus fossil fuel emissions, etc.

    Cheers.

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    • Fallacy Man says:

      Hi, I actually didn’t mean to imply that anthropogenic greenhouse gasses were the only factor. Indeed, the studies that I cited show that natural factors are actually playing a role as well, but they can only explain a very small percentage of the warming, with greenhouse gasses being necessary to explain the bulk of it. Having said that, I’m not honestly sure exactly how much other human factors play into this, but I expect that they are fairly minor because otherwise the greenhouse gasses would not produce tight fits in the models. Also, specifically regarding deforestation, it is important to realize that one of the big problems with it is actually that it causes the release of carbon that had been sequestered by the trees. Thus, it also contributes to the increases in greenhouse gasses.

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