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The human fingerprint in global warming

What the science says...

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Multiple sets of independent observations find a human fingerprint on climate change.

Climate Myth...

It's not us

'What do the skeptics believe? First, they concur with the believers that the Earth has been warming since the end of a Little Ice Age around 1850. The cause of this warming is the question. Believers think the warming is man-made, while the skeptics believe the warming is natural and contributions from man are minimal and certainly not potentially catastrophic à la Al Gore.' (Neil Frank)

When presented with the overwhelming evidence that the planet is warming, many people react by asking "but how can we be sure that we’re causing the warming?" It turns out that the observed global warming has a distinct human fingerprint on it.

In climatology, as in any other science, establishing causation is more complicated than merely establishing an effect. However, there are a number of lines of evidence that have helped to convince climate scientists that the current global warming can be attributed to human greenhouse gas emissions (in particular CO2). Here are just some of them:

10 Indicators of a Human Fingerprint on Climate Change

The first four pieces of evidence show that humans are raising CO2 levels:

  1. Humans are currently emitting around 30 billion tonnes of CO2 into the atmosphere.
  2. Oxygen levels are falling as if carbon is being burned to create carbon dioxide.
  3. Fossil carbon is building up in the atmosphere. (We know this because the two types of carbon have different chemical properties.)
  4. Corals show that fossil carbon has recently risen sharply.

Another two observations show that CO2 is trapping more heat:

  1. Satellites measure less heat escaping to space at the precise wavelengths which CO2 absorbs.
  2. Surface measurements find this heat is returning to Earth to warm the surface.

The last four indicators show that the observed pattern of warming is consistent with what is predicted to occur during greenhouse warming:

  1. An increased greenhouse effect would make nights warm faster than days, and this is what has been observed.
  2. If the warming is due to solar activity, then the upper atmosphere (the stratosphere) should warm along with the rest of the atmosphere. But if the warming is due to the greenhouse effect, the stratosphere should cool because of the heat being trapped in the lower atmosphere (the troposphere). Satellite measurements show that the stratosphere is cooling.
  3. This combination of a warming troposphere and cooling stratosphere should cause the tropopause, which separates them, to rise. This has also been observed.
  4. It was predicted that the ionosphere would shrink, and it is indeed shrinking.

(References for all of these findings can be found here.)

Often one hears claims that the attribution of climate change is based on modeling, and that nobody can really know its causes. But here we have a series of empirical observations, all of which point to the conclusion that humans are causing the planet to warm.

Basic rebuttal written by James Wight

Update July 2015:

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


Last updated on 8 July 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

Argument Feedback

Please use this form to let us know about suggested updates to this rebuttal.

Further reading

Professor Scott Mandia has a detailed explanation of why more CO2 causes stratospheric cooling that is well worth a read.


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Comments 101 to 113 out of 113:

  1. David Kirtley and MA Rodger

    Thank you both for the responses.  David, the Bloomberg link did not get me to the article, and neither did their search option.  Do you happen to have a copy?

  2. Hmmm...that's odd. The link works for me. Maybe try google?

  3. The Bloomsberg link is not to an "article" as such but to a set of graphs beneath which is set out the methodology behind the graphs. And within that methodology is a link to what could be construed as the underlying "article" - Miller et al (2014) 'CMIP5 historical simulations (1850–2012) with GISS ModelE2' although it has broader coverage.

  4. For a purely empirical fit of ENSO, Volcano, solar to temps since 1950, try here. Unlike other curve-fitting exercises which try to show warming is due orbit of jupiter, rise of irrigation etc, this uses half the data as training set and then uses the result to model the other half.

    Benestad & Schmidt 2009 challenged Scafetta nonsense with naive empirical analysis of effects of forcing which I think may be closer to what you want.

  5. richieb1234 - As MA Roger pointed out, volcanic activity adds complexity to any kind of curve-fitting. Climate response to volcanic aerosols is quite fast compared to other forcings. 

    Tamino did an excellent article on using a combination of a simple 2-box model (one fast response, one slow) along with ENSO, and and showed an excellent fit to the observed climate response, see Once is not enough. Two time constants (2 and 26 years) and one large oscillation appear to be sufficient parameters to map the climate response to forcings. 

  6. David Kirtley:  I did get into the Bloomberg link using Google.  Very intereting.  But I was surprised that it shows a negative trend on temperature effect of aerosols all the way to 2005.  The information I have shows worldwide aerosol concentrations dropping by about 30% between 1980 and 2010.  Shouldn't that have caused somewhat of a reversal in the temperature contribution?

  7. :As I mentioned earlier, I am a trainer.  As I read and reread the original post, I think a good analogy to use is the way in which police organizations identify the source of an illicit drug.  A drug that's intercepted on the street could come from anywhere: Colombia, Bangladesh, a rooftop in New York.  "You just can't tell for sure."  But international law enforcement agencies have developed the forensic methods to identify the source by examining its physical and chemical signatures.  It works for natural and synthetic drugs.  One can even identify where drugs have been "cut" by the signatures of the chemicals used.  The same approach is used to distinguish natural causes of warming from man-made causes.  I think this is a good analogy that most adults can identify with.  Once they understand the approach, their minds will be much more open to hearing about the 10 signatures of anthropogenic warming.

  8. richieb124 - the IPCC reports each have a chapter on attribution which is what the literature calls it. I agree that your analogy is a good one but doing real-world attribution is a very complex task. The "fingerprints" in this post I think is about as good as it gets without using a physics-based model. Each individual one is ambiguous, but the sum is telling.

  9. richieb1234 @106,

    It is correct that there has been a substantial reduction on SO2 emissions over recent decades. The graphic on this webpage suggests 30% reduction 1980-2010 and other data suggests reductions do continue but less strongly. Yet when the aerosol forcing is shown (eg IPCC AR5 AII Table 1.2), we see a peak in 2000 and a very minor decline.

    One reason that this may be so is that aerosols are very short lived and thus their effects dependent on the region of their emissions. NASA data (since 2005) allows the latitude of emissions to be easily quantified. Since 2005 thre have been small reductions in higher northern latitudes (eg USA & Europe), quite large reductions in lower northern latitudes (eg China) while emissions in the tropics and the southern latitudes have remained roughly constant. I would suggest that there is little relationship between global totals for SO2 emissions and the global size of the resulting climate forcing. The location of the emissions has a big effect on the overall forcing levels.

  10. MA Rodger

    Thanks for the response.  I will look into the IPCC and NASA references.

    "The graphic on this webpage suggests 30% reduction 1980-2010 and other data suggests reductions do continue but less strongly."

    Yes.  If anything, particulate concentrations may rise for the rest of the century.  The graphic shows a very small contribution from Africa.  U.N. Population projections for Africa show 4 billion people by the end of this century, comparable to Asia.  That could ramp up worldwide average particulate concentrations and spread them more equally across the globe.  Qualitatively, the competition between GHG warming and particulate cooling could be the biggest uncertainty in the future.

    "One reason that this may be so is that aerosols are very short lived and thus their effects dependent on the region of their emissions"

    I see your point, but wouldn't the total amount of solar reflection be the most important variable, no matter where it occurs?  Wouldn't the resulting cooling eventually even out across the globe?  I will take your suggestion and look at the IPCC reference.     --Richieb

  11. richieb1234 @110,

    The reflectiveness of cloud is the method by which SO2 provides climate forcing, this by providing aerosols to make the forming clouds shinier. But there is much complication. SO2 is not the only aerosol agent and such mechanisms are short-lived and local. Thus, say, SO2 emissions causing shiny clouds at high latitudes, especially during winter with long nights, will not have as much sunlight to reflect. And the longevity/altitude of the aerosol will differ by location.

    The resulting cooling will be less localised.

    And globally there is an unresolved issue here. The reductions in SO2 emissions which will follow from eliminating FF use and the resulting loss of its negative climate forcing has got some to worry that there will be a boost to AGW as we de-carbon our economies. Some work is saying this is not really much of a concern (eg Shindell & Smith 2019 [Abstract]) while on the other side of the debate we find Cowern (2018) who argues that such a boost is already happening and the strong warming in recent years is due to the reducing SO2 emissions.

  12. One fingerprint seems wrong to me
    Fingerprint 'Less heat escaping to space'

    Assuming that the starting point of the energy balance is that the incoming radiation (IN) is equal to the outgoing radiation (OUT) (without taking into account the small part that is recorded as biomass in the lithosphere per year).

    If this is not the case and more energy is left behind on Earth every second, the temperature on Earth will continue to rise. This is not the case. More CO2 iterates to a new equilibrium at higher temperature.

    When more CO2 enters the atmosphere, an iteration takes place to a new equilibrium of IN = OUT, as follows: more radiation back means a higher surface temperature and therefore more blackbody long IR of the earth's surface, that means in a next step a little more LW IR back to Earth until it iterates to a new balance. The temperature of the earth's surface has then risen to such an extent that it compensates for the delay of the extra CO2.

    But seen from space, in the new equilibrium, the amount of radiation coming from the Earth per unit time is again equal to that from the sun, because the increase in the temperature of the Earth's surface has compensated for the delay by more CO2.

    If this reasoning is correct, then one of the fingerprints is not correct. If the reasoning is wrong, where is it wrong? You can easily calculate the iteration to a new equilibrium yourself with a simple formula or in an Excel spreadsheet.

  13. Marcel:

    The sequence of initial equilibrium --> CO2 added, equilibrium upset --> system changes to adapt and new equilibrium is reached more or less the way you describe it.

    In the Hansen et al 1981 paper examined here, figure 4 shows the modelled response to an instantaneous increase in CO2. The left side shows the immediate response, with the increased downwelling IR and a reduction in the loss to space. That continues in panel b), which is the state after the atmosphere has responded but before land an ocean can increase in temperature. The right panel shows the final new equilibrium after everything has warmed up.

    Hansen et al 1981 figure 4

    (Clicking on the image source should give you a larger version. I've scaled the display to fit the page.)

    We're a long way from reaching that final equilibrium, though, so we're still in panel b, more or less. We won't reach equilibrium until we stop adding CO2 and things can stabilize. If we only had to heat the ocean mixed layer (60-100m deep), then we'd still see heating for a few decades after CO2 stops rising. With deep oceans involved,  we're looking at centuries.

    The "fingerprint" relates to the current warming, not future equilibrium state.

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