Part 3 of 5: Causes of Past Climate Change

This is the third installment of a five-part series by Bill Chameides on How We Know Humans Cause Global Warming.

1. A 175-year-old Puzzle
2. What Chemistry Tells Us
3. Causes of Past Climate Change
4. The Medieval Warming Period
5. The Only Explanation Left

An argument I hear frequently from climate change skeptics goes like this: "Climate has undergone warming and cooling cycles for millennia. This is no different. It’s just another naturally occurring warm cycle."

It’s true that climate has undergone warming and cooling cycles for millennia, but it’s not true that this is just another naturally occurring warm cycle. Here’s why.

Scientists have made a study of the history of climate change (the field is called "paleoclimatology"), so we know quite a bit about past shifts in global temperature and what caused them. Carbon concentrations and temperatures can be tracked back hundreds of thousands of years by looking at ice cores, and millions of years using other proxies.

When using ice cores, CO2 concentrations are measured from air bubbles in the ice, and temperature is estimated from hydrogen isotopes in the water. (See my previous post in this series for an explanation of isotopes.)

The graph below, which is based on ice core data, shows CO2 concentrations and temperature from 400,000 years ago to the present. You can’t see the 1°F to 3°F differences in global temperature that we are concerned with today because the temperature estimates shown in this graph are local for Antarctica, which has experienced less warming than the global average. Still, it shows that temperature has varied greatly over time, and this variation is strongly correlated with CO2 concentrations.

Data Sources for CO2: Vostok ice core, Law Dome ice core, and Mauna Loa air samples.

Data Source for Temperature: Vostok ice core.

The major dips in the graph are ice ages, caused by shifts in the Earth’s orbit around the sun. The warming from the change in how sunlight hits the Earth is amplified by an increase in CO2 concentration. The sequence goes like this:

  1. Orbital changes trigger an initial, relatively small temperature change on Earth.
  2. Melting permafrost and other changes caused by the warming release CO2 into the atmosphere.
  3. Since CO2 is a greenhouse gas, increased CO2 concentrations cause more global warming.

You also can see smaller ups and downs before modern times. These were caused primarily by variations in solar output and volcanic activity.

Now look at CO2 and temperature in the recent past – the far right of the graph. See how CO2 is rocketing up, and higher today than it’s been in the last 400,000 years? Actually, CO2 concentrations are higher today than they’ve been in the last 650,000 years – this graph just doesn’t go back that far.

Let’s focus in on more recent history so we can get a better look. The graph below zooms in on just the last 1000 years, using temperatures for the whole Northern Hemisphere, not just Antarctica.

Data Sources for CO2: Law Dome ice core and Mauna Loa air samples.

Data Source for Temperature: NOAA.

The natural phenomena that caused climate change in the past cannot account for what we see today. We’re already in a warm period orbitally, as you can see from the first graph, and now suddenly CO2 concentrations and temperatures are skyrocketing.

This break in the pattern – even if you had no other information – strongly suggests something different is happening today. And as the other posts in this series show, something different definitely is happening. Today’s abnormally high temperatures and CO2 concentrations come from the burning of fossil fuels. As you’ll see in the last article in this series, nothing else can account for it.

Next up: the Medieval Warm Period. Stay tuned!

This entry was posted in Basic Science of Global Warming. Bookmark the permalink. Both comments and trackbacks are currently closed.


  1. K.C. Weber
    Posted June 30, 2007 at 9:41 am | Permalink

    Dear Dr. Chameides,

    On many occasions you have discounted the idea that the sun may be contributing to global warming. It’s true that there are other causes, and mankind is definitely a contributor, but the sun’s contribution to global warming goes far beyond the fact that we are in a warm period orbitally, as you state above.

    For the last 60 to 70 years, the sun has been going through a period of increased solar activity that is greater than it has been for the last 8000 years. This is because of increased sunspot activity, solar flares, and other factors. Solar activity affects the climate either by variation in the sun’s output, or by an indirect effect on the amount of cloud formation.

    During this time, other things could happen as well other than warming. If a huge solar flare occurred on the sun, and from the Earth’s point of view this flare appeared in the center of the solar disc, the radiation from that flare would be headed straight toward our planet. When this radiation reaches the Earth, it would not only affect the temperature at the time, it would also disrupt satellite communication and electrical power over large areas.

    The sun cannot be discounted in any discussion on global warming. In short, the sun has a tremendous impact on the Earth. It is, after all the source of all climate on the Earth.

    For further information on this and other discussions, look under the title Solar Activity under:

    K. C. Weber

  2. jbosanko
    Posted July 3, 2007 at 11:04 am | Permalink

    K.C., Bill sent in the following response from his treo:

    Thanks KC for your ever vigilant monitoring of our site. However you are wrong.

    It should not come as a surprise that over the decades of investigating global warming, scientists have actually thought of the possibility that the sun might be the cause of the warming trend. We have eliminated that possibility through direct observation – your claims and citations not withstanding. I have discussed this subject in past posts & will cover it again in my last post in this series.

  3. J. Allen
    Posted July 5, 2007 at 5:48 am | Permalink


    I looked back at the previous post cited by Bill in regards to solar contributions to global warming and the resulting comments. The citations Bill used in support of his claim support his argument yet your rebuttals have yet to show support for your claim regarding trends in solar activity. Be wary of a hasty generalization fallacy, which in short comes from the fact that the time period of observed solar activity may not represent the whole in either increased, normal, or decreased activity. I personally would conclude from the solar data that the last 30 years of activity show no overall increase and therefore is not significant contributor to the observed temperature data.

    I would agree that there are instances of solar aberrations like flares and sun spots, but can we conclude this is outside of nominal activity? I don’t think we can. We may also be perceiving increased activity because of better reporting and informational access, but don’t let perception detract from the facts and what observations have shown.

    I don’t want to beat a dead horse, but unless new findings show otherwise, its not seemly to suggest solar activity is a significant contributing factor.

  4. Posted July 5, 2007 at 4:02 pm | Permalink

    The people on this planet are in a state of denial and this will prove fatal for the human race.

  5. Tom W.
    Posted July 6, 2007 at 1:30 am | Permalink

    KC, show me the data. Show me the calculations. Vague ideas and claims do not warrant the same consideration as the broad, in depth, meticulous, quantitative, decades-long collective study by thousands of scientists trained in the world’s finest universities. Dedicate years of effort, do the studies, and publish the data and calculations that support your thesis.

    I think scientists do themselves a disservice by trying to grossly simplify the presentation of their work. Yes it is easier to follow, but it does not demonstrate the depth, and may give the mistaken impression that it is all just some simple ideas no more valid than anyone else’s. I like this series of little articles in that it does present data and gives some references for those interested in digging deeper.

  6. Posted July 6, 2007 at 12:26 pm | Permalink

    I do agree that the evidence shows we are experiencing a global warming and no doubt mankind is contributing to this problem. I want to ask. When is our problem with pollution going to overcome the wall of monetary committment. If we can produce electricity by solar, or drive a car on water, or filter the coal burners, why dont we? So we spend the money, may not even get the return, but so what. Do we want our childred and wildlife to enjoy our plant or die on it? Wake up people.

  7. Posted July 6, 2007 at 12:34 pm | Permalink

    Good question, Brian! Cap-and-trade legislation would provide the market incentive to make that happen. Let’s hope Congress passes it soon.

  8. Posted July 11, 2007 at 12:02 pm | Permalink

    Sheryl, Senator Feinsteins bill seems nice but it is no solution. Why do you think all these power plants are on board? I am sure there is a huge surplus or possible surplus of these “extra points” that these power plants can buy from eachother to cover their butts.
    Besides they are projecting around 110 billion dollars generated from these “auctions” of extra points by the year 2036. Come on, 29 years away and we only have 110 billion to use on research the whole time? That wont cut it, considering our government spends hundreds of billions PER YEAR on defense. I think we can squeeze a little money from some places to provide research and development. Did you know that clear back in the 1970’s there were cars developed that were more gas efficient than a lot of the cars we have on the road today. What happened? Are our automakers on board with oil companies? I sound like a radical, but if man was able to get to the moon almost 40 years ago, I am confident we can do better than we are now on technologies. Maybe the big companies are shelving the breakthroughs.

  9. Posted July 11, 2007 at 4:54 pm | Permalink

    The argument that anything big business supports is by-definition evil doesn’t get you very far. To make change happen, big business has to be on board.

    Here’s a post by Annie Petsonk – International Counsel at Environmental Defense – that explains how carbon markets work, using the market in Europe as an example:

    What’s Really Going On in the European Carbon Market

    As you’ll see if you read that, the cap-and-trade model is already working. This is do-able. It can work. We just have to get our legislators to act.

  10. Dave
    Posted October 6, 2007 at 6:32 pm | Permalink

    Global warming skeptics often say that, when viewed at a finer scale than that shown above, CO2 does not lead temperature increases historically, but instead lags it — the opposite of that expected if CO2 is the main factor causing the increased temperatures. Is this claim accurate? If so, it strongly suggests that CO2 and temperature are both being driven by other factors. There is a suggestion of a weakness in the connection between CO2 and temperature in the graph above — CO2 levels at present are way, way up compared to historical levels, but temps are only a bit higher comparatively.

  11. Posted October 10, 2007 at 3:56 pm | Permalink

    We addressed this in a previous post:

    Swindle: Ice core data shows that higher CO2 concentrations follow, rather than precede, temperature increases, therefore higher CO2 concentrations don’t cause higher temperatures.

    The Truth: The ice core observation is correct, but it doesn’t mean higher CO2 concentrations don’t cause higher temperatures. It just means that other things can cause warming, too.

    Human impact on climate change has been statistically discernable for only the last 50 years. Prior to that, change was initiated by natural factors such as variations in solar energy output. But this warming then caused the release of CO2 into the atmosphere, leading to more global warming. What the ice core data actually show is an amplification effect. Warming (however triggered) leads to the release of CO2, which causes more warming. (For more, see Bill’s previous post on “Causes of Past Climate Change“.)