Each time you encounter someone trying to claim human-induced global warming is a myth (e.g. because “Mars is warming too!”), you can save a lot of time and energy by just saying, oh yes, that’s myth #16 on the standard list of misunderstandings about climate change. Here’s the list, lovingly and painstakingly put together by John Cook.

Once you’ve got that out of the way, you can then challenge your assailant to identify a safe level of carbon dioxide in the atmosphere, and to get them to give evidence to justify that choice. If they don’t feel qualified to answer this question, then you get to a teachable moment. Take the opportunity to teach your assailant the difference between greenhouse gas emissions and greenhouse gas concentrations. That’s the single most important thing they have to understand. Here’s why:

  • We know that the earth warms by somewhere between 2 and 4.5°C (with a best estimate of about 3°C) for each doubling of CO2 concentrations in the atmosphere (this was first calculated over 100 years ago. The number has been refined a little as we’ve come to understand the physical processes better, but only within a degree or two)
  • CO2 is unlike any other pollutant: once it’s in the atmosphere it stays there for centuries (more specifically, it stays in the carbon cycle, being passed around between plants, soil, oceans, and atmosphere. But anyway, it only ever goes away when it eventually gets laid down as a new fossil layer, e.g. at the bottom of the ocean).
  • The earth’s temperature only responds slowly to changes in the level of greenhouse gases in the atmosphere. That means that even though we’ve seen warming of around 0.7°C over the last century, we’re still owed at least that much again due to the CO2 we have already added to the atmosphere.
  • The temperature is not determined by the amount of CO2 we emit; it’s determined by the total accumulation in the atmosphere – i.e. how thick the “blanket” is.
  • Because the carbon stays there for centuries, all new emissions increase the concentration, thus compounding the problem. The only sustainable level of net greenhouse gas emissions from human activities is zero.
  • If we ever manage to get to the point where net emissions of greenhouse gases from human activities is zero, the planet will eventually (probably, over centuries) return to pre-industrial atmospheric concentration levels (about 270 parts per million), as the carbon gets reburied. During this time, the earth will continue to warm.
  • Net emissions is, of course, the difference between gross emissions and any carbon we manage to remove from the system artificially. As no technology currently exists today for reliably and permanently removing carbon from the system, it would be prudent to aim for zero gross emissions. And the quicker we do it, the less the planet will warm in the meantime.
  • And 3°C global average temperature is about the difference between the last ice age (which ended about 12,000 years ago) and today’s climate. In the last ice age there were ice sheets 0.5km thick over much of North America and Europe. Now imagine how different the earth will be with a further 3°C of warming.

Okay, so that might be a little bit too much for just one teachable moment. What we really need is a simple elegant tool to illustrate all this. Anyone up to building an interactive visualization? John Sterman tried, but I don’t rate his tool high on the usability scale.


  1. Personally I would prefer to see a game based simulation.

  2. Ok, I’ll bite.

    challenge your assailant to identify a safe level of carbon dioxide in the atmosphere

    Somewhere between 10k and 14k ppm.

    I hope you don’t consider me an ‘assailant’ though…

    [Ah, yes. Looks like I didn’t define ‘safe’ well enough. Thanks for pointing it out -Steve]

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