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What was it like the last time CO2 levels were this high?

Posted on 16 April 2011 by John Cook

A vivid picture of our climate's future can be found in our past. Currently, atmospheric carbon dioxide (CO2) levels have reached 390 parts per million (ppm). The last time CO2 was that high was around 3 million years ago, during the Pliocene. Back then, CO2 levels remained at around 365 to 410 ppm for thousands of years. Consequently, the Pliocene gives us vital clues of the long-term effects of raised CO2 levels. New research has just been published that examines this period and confirms previous findings that  the Pliocene  was dramatically warmer than current temperatures.

The research, published in Csank et al 2011, uses two independent methods to measure Arctic temperature during the Pliocene, on Ellesmere Island. They find that Arctic temperatures were 11 to 16°C warmer (Csank 2011). This is consistent with other independent estimates of Arctic temperature at the time. Global temperatures over this period is estimated to be 3 to 4°C warmer than pre-industrial temperatures.  Sea levels were around 25 metres higher than current sea level (Dwyer 2008).

This tells us our climate is sensitive to changes in CO2. If we were to stabilise CO2 levels at around 400 ppm, we'd expect over the long-term a further warming of 2 to 3°C, which is significantly greater than the warming predicted by climate models. This is because climate models only include short-term feedbacks, such as increased water vapor and melting of sea ice. They are yet to take into account the long-term feedbacks from the melting of ice sheets and vegetation changes.

This also tells us that ice sheets are sensitive to sustained warmer temperatures. The Greenland and Antarctic ice sheets lost significant amounts of ice during these warmer temperatures. This sensitivity is apparent in current observations, with both Greenland and Antarctica losing ice at an accelerating rate.

The ice loss is particularly dramatic in Greenland. Two decades ago, the Greenland ice sheet was in approximate balance - ice loss at the edges as glaciers calved into the ocean was balanced by ice gain in the interior from increased snowfall. One decade ago, the ice loss at the edges had increased and Greenland was losing around 100 billion tonnes of ice every year. Currently, this ice loss has increased to around 300 billion tonnes of ice per year.

Earth's past history is sending us a strong message - current CO2 levels are not safe. This is echoed by movements like who tell us we need to stabilise CO2 levels at 350 parts per million. That means dramatically reducing carbon emissions and looking for ways to remove CO2 from the atmosphere or we may eventually face the potentially catastrophic conditions of the Pliocene once more.

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Comments 51 to 67 out of 67:

  1. Dr. Hansen has a new paper out discussing the use of Aerosols, and how they seem to be reflecting a good deal of the light and warming- thus far. Anybody have more to add regarding this?
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  2. 51. I linked to a video of a talk around the new paper and more on RC where you can find a link to the paper here...
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  3. new c#51. I've seen it, but it's 50+ pages of reasonably heavy going. (Though I would have said the production, rather than the use, of aerosols.) I won't pretend to understand the whole thing even when I'm done with it. But I do need to read it through a couple of times just to get the general ideas straight. (Then I'll have to decide whether I follow up any references to get the full flavour of any interesting details.)
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  4. Note to mods: Please notice that Gilles did in fact mostly succeed in his goal of wrecking this thread. More to the point, you helped him do it via your tolerance and catering (by responding extensively to his absurdities). There's much of interest about Pliocene climate that might have gotten discussed otherwise. I'm all in favor of free speech, but when the proverbial theater is on fire, shouts of "What smoke? If there is any smoke, you haven't proven it's from in here, that it's necessarily a result of fire, or that your hypothesized smoke and fire would be harmful!" (lather, rinse, repeat) are not only wholly counter-productive but boring. Put another way, this is a learning site, and people can't learn much in threads like this. My advice: Put Gilles in moderation and rigorously delete anything OT or trollish.
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    Moderator Response: [DB] OK.
  5. Gracias, Daniel!
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  6. Let me get this straight, the preindustrial levels of CO2 are determined by ice core and other proxies, not by direct chemical analysis. The 90,000 preindustrial chemical analysis were thrown out as unreliable. The ice core proxies, and others, were then accepted at about 280 ppm. Yet, the leaf stomata proxy, a study that did show the CO2 levels were about the same as today, is also unacceptable. Why is this? Could it be that the CO2 levels were, in fact, nearly the same as today? How is global warming real if the preindustrial levels of CO2 are the same as today?
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  7. If the CO2 levels rise following a warm period, could today's rising levels of CO2 be the result of a warming period 800-1000 years ago? Tree rings evidence do show a warming period at this time in the past. So, today's rising CO2, if this is true, could be the delayed response from centuries before.
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    Moderator Response: Post your comment on the thread CO2 lags temperature.
  8. henryj#56: "Yet, the leaf stomata proxy, ... is also unacceptable. Why is this?" Because the leaf stomata data are terrible: The error bars around stomata reconstructed CO2 values tend to be +/-50ppm. See the plant stomata thread for a reference and further comments.
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  9. Measuring the CO2 trapped in an air bubble in the ice is a pretty direct analysis if you ask me. You dont "chuck out" analyses, you examine their error bars and sampling technique (which is a more common reason to reject a sample as being representative of atmosphere at the time).
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  10. Henry Justice @56 even modern CO2 measurements show CO2 levels much higher than 390 ppm. To get those values, all you need to do is to take your samples near a local source of CO2. We know it is contamination from a local source because if you move away from it by gaining altitude (first image), or if the effect of the local source is diluted by high wind speeds (second image), the measurements converge on the values obtained for CO2 by measurement at remote locations free from contamination. The problem with the CO2 measurements you claim to have been thrown out is that they are known to have been located near or in major sources of CO2 such as towns, industrial plants, roads and forests. Forests are interesting because during the day trees draw CO2 from the atmosphere, diluting it, while at night they add CO2 to the atmosphere. That pattern can be clearly seen in CO2 measurements from forests, although it is weaker the higher the wind speed. Contrary to your claim, CO2 measurements from ice cores are not measurements of a proxy, but of atmospheric CO2 trapped in ice bubbles. There are no trees or factories in the antarctic, so no contamination. Hence the measurements are accurate. So, why should we take contaminated measurements with no quality control (as Beck does) over uncontaminated measurements?
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  11. Henry, I have replied on your CO2 question here as requested by moderator.
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  12. "This tells us our climate is sensitive to changes in CO2". No. This tells us our climate is sensitive to the formation of the Isthmus of Panama and the associated changes in ocean circulation.
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  13. Berényi Péter @62, so your theory is that the Arctic froze over because the warm current from the Gulf of Mexico to the Barents Sea was strengthened? Well it's so obvious when you think about it.
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  14. #63 Tom Curtis at 19:39 PM on 1 May, 2011 so your theory is that the Arctic froze over because the warm current from the Gulf of Mexico to the Barents Sea was strengthened? Well it's so obvious when you think about it. It is not my theory, it is the standard one. When the Isthmus of Panama is removed, tropical high salinity surface waters can escape directly from the Atlantic to the Pacific. The remaining branch of the warm current does not have sufficient salinity to go down to the bottom before it reaches the Arctic Ocean (as it does today), so it just goes around and heats the entire region up. Also, the additional warm surface water entering the Pacific via the Panama Strait maintains near-permanent El Niño conditions there.
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  15. 62, BP, You comment seemingly came out of nowhere. Is it from another thread? What exactly is your point?
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  16. BP @ 62 No. This tells us our climate is sensitive to the formation of the Isthmus of Panama and the associated changes in ocean circulation. No, this tells us the Earth's climate is sensitive to perturbation full stop. CO2, ocean circulation, global vegetation (talking the arrival of flowering plants here), global orography (mountain chains) all have had significant impact on the climate. And there was no ocean acidification during the Pliocene.
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  17. BP's comment seems on topic to me. His question is "How much of the climate difference between the Pliocene and today is due to the closure of the former Panama seaway?" Step one in answering the question: find out how global ocean circulation changed. BP found a good 1998 paper on that. Next step: integrate this with modern climate models. Closure of the Panama Seaway during the Pliocene: implications for climate and Northern Hemisphere glaciation from ten years later is a good start.
    Abstract/Summary The "Panama Hypothesis" states that the gradual closure of the Panama Seaway, between 13 million years ago (13 Ma) and 2.6 Ma, led to decreased mixing of Atlantic and Pacific water Masses, the formation of North Atlantic Deep water and strengthening of the Atlantic thermohaline circulation, increased temperatures and evaporation in the North Atlantic, increased precipitation in Northern Hemisphere (NH) high latitudes, culminating in the intensification of Northern Hemisphere Glaciation (NHG) during the Pliocene, 3.2-2.7 Ma. Here we test this hypothesis using a fully coupled, fully dynamic ocean-atmosphere general circulation model (GCM) with boundary conditions specific to the Pliocene, and a high resolution dynamic ice sheet model. We carry out two GCM simulations with "closed" and "open" Panama Seaways, and use the simulated climatologies to force the ice sheet model. We find that the models support the "Panama Hypothesis" in as much as the closure of the seaway results in a more intense Atlantic thermohaline circulation, enhanced precipitation over Greenland and North America, and ultimately larger ice sheets. However, the volume difference between the ice sheets in the "closed" and "open" configurations is small, equivalent to about 5 cm of sea level. We conclude that although the closure of the Panama Seaway may have slightly enhanced or advanced the onset of NHG, it was not a major forcing mechanism. Future work must fully couple the ice sheet model and GCM, and investigate the role of orbital and CO2 effects in controlling NHG.
    The next step is to look at papers that cite this one, using google scholar.
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