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There is no consensus
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Climate Hustle
The significance of past climate change
Posted on 21 April 2010 by John Cook
A common skeptic argument is that climate has changed naturally in the past therefore humans aren't causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. When I try to explain why to people, I usually get blank, confused stares. I gave a presentation to a roomful of engineers this week and after explaining the significance of past climate change, complete with slides of climate sensitivity PDFs and examples of positive feedbacks, the result was a long, silent pause. I asked if anyone understood what I'd just talked about. A few asked some follow-up questions which made it clear they didn't. So I'm reworking my whole explanation of past climate change in an attempt to make it as clear and simple as possible. Comments, particularly on anything confusing or unclear, are welcome!
In the past, climate has changed, sometimes very dramatically. This has gone on long before SUVs and coal fired power plants. If climate can change on its own, couldn't current global warming be natural as well? To answer this, first you have to ask why climate has changed in the past. It doesn't happen by magic. Climate changes when it’s forced to change. When our planet suffers an energy imbalance and gains or loses heat, global temperature changes.
This can happen in a number of ways. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, all the particles suspended in the atmosphere reflect sunlight and the planet cools. These effects are referred to as external forcings because by changing the planet's energy balance, they force climate to change.
Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, we can work out past temperature change, the level of solar activity plus the amount of greenhouse gases and volcanic dust in the atmosphere. From this, we can determine how temperature has changed due to past energy imbalances. What we have found, looking at many different periods in Earth's history, is that when the Earth gains heat, positive feedbacks amplify the warming. This is why we've experienced such dramatic changes in temperature in the past. Our climate is highly sensitive to changes in heat.
What does that mean for today? Rising CO2 levels are an external forcing. They're causing an energy imbalance and the planet is building up heat. From Earth's history, we know that positive feedbacks will amplify the CO2 warming. So past climate change doesn't tell us that humans can't influence climate. On the contrary, the past tells us that climate is highly sensitive to the CO2 warming we're now causing.
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Comments 1 to 50 out of 87:
1. Few comments on some issues I think should be explained if you want this to be as simple as possible:
- The role of greenhouse gases in the past climate (or where they come to the atmosphere) might need an explanation.
- You should explain what positive feedback means and the factors which are positive feedbacks.
- The part about CO2 being an external forcing might call for additional explanation - I don't think it's readily clear for someone not familiar with the issue.
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2. I suspect that the argument has to proceed by easy steps. One step is to assert that it is not logical to exclude a new factor, without further examination. Before 1890, there were plenty of reasons for accidental deaths, but being involved in a motor car accident was not one of them - for obvious reasons. But we do not use that as an argument to say that car accidents should be ignored. In the case of CO2, it was not previously present in large quantities (unless you're going back before human history), but it now is and its projected increase in the time scale we're interested in will dominate other factors.
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3. You don't really believe this do you?
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Response: It's not a question of belief - this is the state of the science as explained in the peer-reviewed literature. I go into more detail with peer-reviewed references at Climate's changed before - this was an attempt at a simpler, easier-to-understand explanation.
4. VnceOZ
Which bit of 'this' are you referring to? And why do you use a word like 'believe'?
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5. John,
In the past . . .
What happened as Surface Temps rose, due to CO2 on occasion, with regards to the rates of convection and evaporation?
Did they increase, decrease, or stay the same?
If convection and evaporation rates increased as surface temps increased, did that cool the surface, heat the surface, or did the surface temps remain the same?
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Response: See, this is the thing. In my presentation to the engineers, I did up this gorgeous little schematic of positive feedbacks, showing increased evaporation, more water vapor causing an increased greenhouse effect, more clouds, etc. Very colourful, I was very proud of it :-) But I think I went into so much detail describing the positive feedback processes and the derivation of climate sensitivity, I think everyone had forgotten the point of the explanation by the time I got to the end.
The basic point was to say that there are a number of feedbacks - both positive and negative. Finding the net feedback by adding up all the individual feedbacks is a complex job. But you can cut through all that in one fell swoop by looking at past change. By just comparing temperature change to changes in the energy balance, you determine the net feedback without having to know all the individual feedbacks.
So I skipped all those details in this blog post in the effort of a simpler argument. But going through the explanation too fast leaves people wanting more details. Where to draw the line?
6. Every time I hear the "Climate changed naturally in the past, so humans can't be responsible this time around" I always respond by saying "so does this mean that 'because many forest fires start naturally, humans are *never* responsible for forest fires?'" The comments are equally nonsensical, yet it never ceases to amaze me how often the Contrarians try & push this illogical position!
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7. I think its important to mention the speed of climate change today vs the past.
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8. I agree with pretty much all of what you say above except for one major point-the issue of the time scales involved.
The past geological record indicates that changes on a global scale are invariably very slow. We're talking tens of thousands, to hundreds of thousands and even millions of years in the vast majority of cases. For something like acidification of global oceans by volcanoes, and indeed the majority of mass extinction events (except in rare cases of eg bolide impacts), that is the sort of time frame the geological record indicates is invariably required.
To take a few examples, the output of greenhouse and other gases by Siberian Traps volcanism at the end of the Permian occurred on a major scale over several hundred thousand to several million years, and that is how long it took to cause major climate changes such as (possibly) acidifying the oceans, the collapse of coral reef ecosystems, and mass extinction. It was not a ‘rapid process’ when compared to the scale of human lifetimes. The break-up of the Gondwana supercontinent is likely implicated in eg the Mid-End Triassic mass extinction-continents do not break up 'rapidly'. This occurred over millions of years - with possible stress-related tipping points etc, as rift-related volcanism increased over very long time periods. Many other examples from the geological record indicate much the same thing, (eg oceans don't ‘acidify’ within short human timescales when similar amounts of c02 have been added to the atmosphere in the sort of time scales involved as is currently the case).
This is also the major reason people such as the gradualist Charles Darwin were so skeptical of the presence of 'mass extinction' events in the geological record in the first place, and also I suspect why the person on the street is with current climate change and the extreme predictions around it as well. It's like worrying about continental drift changing the climate within this century.
This is one of the major skeptical arguments, that major global changes such as those predicted by the IPCC to occur within the next century by human emissions of greenhouse gases will NOT occur, not so much because the concepts and theory is wrong, but because the scale of time involved is far too short, and that the geological record provides very good support for this contention. Academics and other pro AGW advocates get the concepts largely right, but get the time scales involved largely wrong.
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9. I would say that in my experience the key point to get across to sceptics is that during past climate changes there were no humans around to experience the effects. Consequently from today's human viewpoint past changes do not seem such a big deal. But the truth is they were huge. Although climate change of three, four degrees or more over tens of thousands of years would allow most plant and animal species to move, adapt or evolve to cope with that change; the more serious and/or sudden climate changes would cause huge extinction events. The problem, in explaining this, of course, is that such events seem no big deal to us today, looking back over millennia. So what, if large mammals -- which we're aware of only through scant fossil evidence -- once died, to be replaced by other equally unfamiliar species?
On the other hand the current rapid Climate Change -- when humans are now one of our planet's larger animal species -- is a highly risky for both us and most of the other living species; though as an extinction event it will probably be no worse than many in the past. Arguably it's made worse by the unprecedented fact that humans are responsible for creating the problem this time.
It would be interesting to conjecture (is it possible to know for sure?) the extinctions and changes to the nature of life on the planet as a result of past climate changes, and then extrapolate how the current climate change being instigated by humans could alter the nature of life on the planet today.
Of course one of the problems in doing this is that global heating is only one of many serious issues now being caused by human influences on life's existence on our planet. Pollution; deforestation; industrial agriculture; fresh water diversion; resource depletion; over-population -- to name just some of the most obvious -- are all key components in the environmental timebomb we're creating.
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10. It may seem a bit off-beat, but I personally think paleoclimate more often than not is about red-herrings.
While a number of false arguments try to use paleoclimate to deny AGW, I must admit I have never been convinced that the paleoclimate record is terribly useful to the debate. Fundamentally, there is nothing it gives us that is not more clearly communicated by contemporary data.
For example, the "Hockey Stick" shows that the contemporary rate of warming diverges enough from the last few thousand years to raise a smoking gun type of question, but the graph itself is not an explanation -only a way of flagging an anomaly.
Even if the hockey stick was proven wrong, it wouldn't matter - because arguments about medieval warming, past CO2 levels, higher ancient temperatures etc. all miss the point:
In the contemporary world, we can identify the rate of global warming, the mechanisms of warming and the predicted (and confirmed) effects.
Until there is a better explanation for the colossal body of evidence of the last 100 years, the opponents of the AGW thesis will try and drag our attention into the distant past, where the ground under any given argument is weaker.
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11. Thingadonta-how can you make such claims when global warming of +0.6 degrees has *already* occurred over the past 60 years, with nearly +0.5 degrees of that warming occurring in just the past 30 years?!?! The fact that virtually *every* natural climate change event in the past (with the exception of those caused by extremely massive natural disasters) has occurred at a rate of between 1/2 to 1/10th of the rate of recent change is simply further proof that nature is not the cause of the most recent climate change event. What humans are doing, by burning the accumulated & compressed CO2 storage of tens of millions of years of tree & plant life, is to effectively compress millions of years of geological activity into the space of just 250 years. To suggest that geo-engineering on this scale could *not* be the cause of rapid climate change simply shows how *weak* the contrarian argument actually is!
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12. re #6 and #9:
I get that argument sometimes and my response is that it wouldn't matter if there was either a more rapid or larger warming at some point in the distant past - there weren't billions of people living near the sea then. "The Planet" is not endangered - it is a small percentage of the planet's flora and fauna, plus a big percentage of its human beings. Of course, it's like asking for anesthetic at the dentist and being told you shouldn't have it because the dentist's grandfather had to suffer more than you!
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13. I think part of the problem is that you are really conflating two different issues... which if separated might be phrased as;
1: 'Climate has changed in the past so how do we know humans are causing it now?'
2: 'How do we know that the climate change we are causing is going to be significant?'
I think you can really handle the human causation question using CO2 alone (past correlations of CO2 and temperature, satellite and ground measurements confirming energy imbalance in CO2 absorption spectra, various proofs of increased CO2 levels being human caused, et cetera)... possibly following that up with explanation of why it ISN'T the Sun, cosmic rays, volcanoes, martian death rays, or whatever. All of which can be handled by showing lack of correlation - some match past changes, but none match the current.
Once you have established CO2 as a major 'control knob' in determining temperatures then you can get into feedbacks amplifying that effect and what the historical record tells us about those.
Basically, handle one issue at a time. Don't start talking about positive feedbacks before they've bought into CO2 increasing temperatures at all.
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14. How about this:
Rather than list all the possible primary drivers (forcings), start by describing a couple of periods when the climate changed in the past from different forcings. For example, Milankovitch and volcanoes as the initial forcing.
For each of the two periods in turn, describe in simple terms what the earth was like before the change (eg ice free Antarctica) and what it was like afterwards (eg lots of ice everywhere). Describe the forcing that led to the change (and define the terms - engineers most probably use slightly different jargon from climate scientists).
Then move to today and show the evidence that the climate is changing (temperature, ice, oceans, sea levels etc). And explain that the forcings that caused the prior changes you discussed are absent, and the only primary driver or initial forcing operating today is CO2 / greenhouse gases.
Then show how greenhouse gases also acted as a positive feedback, amplifying the changes you discussed in the two first examples (warming or cooling).
You can deal with the 'but couldn't it be due to ...' in the question time / discussion - keep your detailed slides on hand for when specific questions are asked.
If there are particular points you want to elaborate on (eg role of water vapour/clouds which some often ask about), you can probably do this during the discussion by saying, 'someone often asks me xyz' and then whip out your extra slides :).
(As a broad (over?) generalisation, engineers are often clever, but their thought processes tend to be linear rather than lateral. Simple works best. One step after another, with a bit of repetition and reiteration of key points along the way. No offense meant to engineers, it's just an observation from working with groups of engineers in a variety of contexts.)
The following publication is rather good, and will no doubt be more useful than my off the top of the head idea: CRED guide
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15. I concur with #13. I think the first principle is explain what is causing warming now, then climate history comes in to a supporting role.
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16. An excellent presentation of past climate change is available online: Richard Alley, "The Biggest Control Knob: Carbon Dioxide in Earth's Climate History" at the Fall AGU meeting, see To be sure this is for a scientist audience but the main points can be adapted for others... admittedly not easy. And of course, bear in mind that people have a hard time grasping knowledge that conflicts with what they want to believe!
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17. The thing with this line of argument that your tackling is that it fails basic logic: Human activity of course didn't cause past climate changes, but that’s no evidence that it doesn’t now.
Try that line of argument in a court of law against a arsonist, by saying that forest fires have always happened naturally; it won’t fly.
Indeed, GHG had an important role to play in many past climate changes (even though their concentration changed without human involvement). Looking at the past actually strenghtens the evidence for a climatic effect of GHG.
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18. I think you have the facts well explained, but there are a lot of them. Is an analogy helpful? Eg:
Climate change history is like precedent in law - you have a good chance that a previous finding or two might establish where things stand 'when deciding subsequent cases with similar issues or facts'.