2010-08-11 22:21:12Basic rebuttal 2: Climate's changed before - REVISION 2
James Wight


I know Robert Way has already "called dibs" on this argument, but he's apparently changed his mind. So I thought I'd have a go at it.

The following is basically a slightly edited version of John Cook's post on 21 April, "The significance of past climate change".

The skeptic argument: Climate's changed before

What the science says: Climate reacts to whatever forces it to change at the time; humans are now the dominant forcing.

A common skeptic argument is that climate has changed naturally in the past, long before SUVs and coal-fired power plants, so therefore humans cannot be causing global warming now. Interestingly, the peer-reviewed research into past climate change comes to the opposite conclusion. To understand 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.

There are a number of different forces which can influence the Earth’s climate. When the sun gets brighter, the planet receives more energy and warms. When volcanoes erupt, they emit particles into the atmosphere which reflect sunlight, and the planet cools. When there are more greenhouse gases in the atmosphere, the planet warms. These effects are referred to as external forcings because by changing the planet's energy balance, they force climate to change.

It is obviously true that past climate change was caused by natural forcings. However, to argue that this means we can’t cause climate change is like arguing that humans can’t start bushfires because in the past they’ve happened naturally. Greenhouse gas increases have caused climate change many times in Earth’s history, and we are now adding greenhouse gases to the atmosphere at a increasingly rapid rate.

Looking at the past gives us insight into how our climate responds to external forcings. Using ice cores, for instance, we can work out the degree of past temperature change, the level of solar activity, and 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 and timescales 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. We can even quantify this: when you include positive feedbacks, a doubling of CO2 causes a warming of around 3°C.

What does that mean for today? Rising greenhouse gas levels are an external forcing, which has caused climate changes many times in Earth's history. 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 greenhouse warming. So past climate change doesn't tell us that humans can't influence climate; on the contrary, it tells us that climate is highly sensitive to the greenhouse warming we're now causing.

2010-08-11 23:26:15
John Russell


Generally excellent -- my kind of answer ('cos I can understand it!).

A couple of points.

1) As well as your point that some sceptics say "climate has changed many times in the past", meaning "therefore we can't be changing it now", many, if not more, mean, "so it doesn't matter; we -- the planet -- adapt!"

Is it therefore worth making the point that, yes, climate has changed many times in the past but as long as modern humans have been around, CO2 concentrations have been stable. In previous times when CO2 concentrations did change, we humans were not around to experience the problems it caused. Now we are causing these problems.

2) Scientists have always known that the Earth's climate has been forever changing. In the past though, these changes have been caused by natural processes and the effects have been relatively slow to happen -- often over thousands of years. This has allowed plants and animals to migrate or adapt and/or evolve.

The important bit. However, the changes than humans are now causing, by taking all the locked-up carbon out of the ground and ejecting it into the Earth's atmosphere, is happening at a rate never experienced since life appeared on the planet. What we are doing is therefore unprecedented and highly dangerous to life on Earth.

Best wishes,


2010-08-11 23:37:45Little Note
Robert Way

A doubling of CO2 gives a warming of 3 degrees?
I'm not sure about that (though i'm no expert) I think a doubling is guaranteed to contribute 1 degree (even agreed to by skeptics) but that the remainder occurs through feedback mechanisms.

Another thing you might want to note but its not necessary is you could say that changes in the ocean water currents too. That way you would cover a lot of the natural variability associated with the PDO, AMO and NAO that people often throw  out as an argument.

I really like this write up though. It is very easy to understand and readable.
2010-08-12 23:41:02Doubling of CO2
John Cook


James, great to have you involved. Can you go ahead and claim this argument on the rebuttal list? This will enable you to edit the rebuttal, add your final text.

Robert's basically right - doubling of CO2 causes direct warming of 1°C, water feedback adds another warming of 1°C and other feedbacks like ice albedo changes add an additional 1°C - all adding up to roughly 3°C. So "a doubling of CO2 means a warming of around 3°C" is essentially correct. However, you could also say "a doubling of CO2 means direct warming of around 1°C". So it's easy to get confusing with loose terminology. So what about this wording:

We can even quantify this: when you include the effects from positive feedbacks, a doubling of CO2 causes a warming of around 3°C.

2010-08-15 04:35:33Oops corrected

Quick one to point out I'd missed this when scanning available arguments to tackle in the list and so claimed it. Unclaimed now, and sorry!
2010-08-15 15:59:12About emphasis
Graham Wayne

Hi James - I think there is a 'clear text' aspect to this you might include if you thought it useful, exposing a certain illogic to the way the argument is sometimes deployed outside of the science. It is this:

"In the past, natural forcings have raised the temperature of the earth first, followed by an increase in CO2 (which creates a positive feedback that speeds up the heating process). It is argued that because a rise CO2 has not preceded historic warming events, it cannot do so now. This is to argue that because a thing hasn't happened in the past, it cannot happen in the future".

2010-08-16 22:06:44Response to feedback
James Wight

Thanks for the feedback, everyone. I have reworded the “doubling of CO2 causes 3 degrees warming” bit above, and added an extra paragraph about the PDO.

Graham, “CO2 lagging temperature” is a more specific argument about past climate change, which has already been claimed by Anne-Marie Blackburn. However, this argument does rely on the same sort of logic, so I suppose I could add something along these lines:
“It is obviously true that past climate change was caused by natural forcings. However, to argue that this means we can’t cause climate change is like arguing that humans can’t start bushfires because in the past they’ve happened naturally.”

John Russell, your suggestion seems more appropriate to the “It’s not bad” argument than this one. It didn’t occur to me to talk about adaptation because John’s intermediate rebuttal doesn’t mention it at all. I do think you have a point that this is often what contrarians mean when they talk about past climate change, but I’m not sure this is the right place to address it.

Maybe the “Climate’s changed before” argument should be split into two separate arguments? One would be something like “Climate’s changed before so we can’t be causing it now”, and the other one “Climate’s changed before so we can adapt like we did in the past”. John Cook would need to create the page for the new argument, obviously. Any thoughts, anyone?
2010-08-16 23:58:07Comment
Robert Way

The PDO may be a re-organization of heat but the AMO is not so be careful what you indicate there. Natural oscillations such as the AMO and NAO are well correlated with warming periods when in certain phases. This includes during the MWP and the early 20th century.
2010-08-17 07:43:15
Anne-Marie Blackburn
Anne-Marie Blackburn
It's an excellent post. I'm just wondering whether we need to go into the external vs internal forcings - it seems unnecessary when you consider who the basic rebuttals are addressing. Maybe your post could be simplified by saying that the climate changes in response to changes in several factors? Though I wouldn't object to this being published as it is. I don't know enough about the AMO and NAO but Robert seems to know what he's talking about so perhaps his point needs to be addressed.
2010-08-17 14:08:03Nice

I like this for how it not only addresses the novelty of the current dominant forcing but also tells us that contrary to being some well of apathy to draw on, previous climate changes helps us to understand how our new forcing method will change the climate today.
2010-08-17 17:23:32Bushfires - up to you
Graham Wayne

Hi James - I'm giving your excellent post the thumbs-up now. I like the bushfire analogy but I don't feel strongly about it's inclusion either way...

2010-08-17 17:53:12Thumbs up
John Cook

Good post (but I would say that, wouldn't I?). I could take or leave the 2nd paragraph in brackets, it upsets the line of logic and I'm not sure it's necessary to keep it. The whole point of the plain english rebuttals is they don't need to dot every I and cross every t but speak in broad strokes. I think part of the reasons scientists have trouble relating to a general audience is they try to include everything. But having multiple levels frees us up to not have to address every nook and cranny, but defer it to higher levels. But either way, is fine by me.
2010-08-17 23:43:38Have revised the above rebuttal
James Wight

Alright, I’ve taken out the bit about internal causes and have added the bushfire analogy. Let me know how it reads now.

For the record, here is the paragraph I’ve deleted:
(It is true that the climate also has a number of internal cycles such as the Pacific Decadal Oscillation, but these cycles basically just move heat around the climate system. In the case of the current global warming, satellites are measuring an energy imbalance at the top of the atmosphere, and we’ve observed heat building up over decades in the oceans. So we know the cause must be external, not internal.)
2010-08-18 09:08:56

Looks good to me!
2010-08-19 16:44:34
Ari Jokimäki

This is good.
2010-08-19 17:29:28


This is a very good, clear and concise article easily accessible to the general public.

2010-08-20 11:43:23Should this have been posted in my name?
James Wight

John C, I see you've posted this in my name. I hope you don't mind that most of it is blatantly plagiarized from one of your posts?