What Does 400 ppm Look Like?

Richard Norris holds a cast of a Pliocene-era walrus skull found in San Diego, Calif.

As atmospheric carbon dioxide levels rise, scientists look back four million years for answers on what to expect from climate

The Pliocene is the geologic era between five million and three million years ago. Scientists have come to regard it as the most recent period in history when the atmosphere’s heat-trapping ability was as it is now and thus as our guide for things to come.

Recent estimates suggest CO2 levels reached as much as 415 parts per million (ppm) during the Pliocene. With that came global average temperatures that eventually reached 3 or 4 degrees C (5.4-7.2 degrees F) higher than today’s and as much as 10 degrees C (18 degrees F) warmer at the poles. Sea level ranged between five and 40 meters (16 to 131 feet) higher than today.

As for what life was like then, scientists rely on fossil records to recreate where plants and animals lived and in what quantity. Pliocene fossil records show that the climate was generally warmer and wetter than today.  Maps of Pliocene vegetation record forests growing on Ellesmere Island in the Canadian Arctic, and savannas and woodlands spreading over what is now North African desert. Both the Greenland and Antarctic ice sheets were smaller than today during the warmest parts of the Pliocene.

In the oceans, fossils mark the spread of tropical and subtropical marine life northward along the U.S. Eastern Seaboard.  Both observations and models of the Pliocene Pacific Ocean show the existence of frequent, intense El Niño cycles—a climatic oscillation that today delivers heavy rainfall to the western U.S. causing both intense flooding but also increasing the river flows needed to sustain salmon runs. The absence of significant ocean upwelling in the warmest part of the Pliocene would have suppressed fisheries along the west coasts of the Americas, and deprived seabirds and marine mammals of food supplies.  Reef corals suffered a major extinction during the peak of Pliocene warmth but reefs themselves did not disappear.

Richard Norris, a geologist at Scripps Institution of Oceanography, UC San Diego, said the concentration of CO2 is one means of comparison, but what is not comparable, and more significant, is the speed at which 400 ppm is being surpassed today.

“I think it is likely that all these ecosystem changes could recur, even though the time scales for the Pliocene warmth are different than the present,” Norris said.  “The main lagging indicator is likely to be sea level just because it takes a long time to heat the ocean and a long time to melt ice. But our dumping of heat and CO2 into the ocean is like making investments in a pollution ‘bank,’ since we can put heat and CO2 in the ocean, but we will only extract the results (more sea-level rise from thermal expansion and more acidification) over the next several thousand years.  And we cannot easily withdraw either the heat or the CO2 from the ocean if we actually get our act together and try to limit our industrial pollution–the ocean keeps what we put in it.”

Scientists can analyze the gases trapped in ice to reconstruct with high accuracy what climate was like in prehistory, but that record only goes back 800,000 years. It is trickier to estimate carbon dioxide levels before then, but in 2009, one research team reported finding evidence of carbon dioxide levels ranging between 365 and 415 ppm roughly 4.5 million years ago. They based their finding on the analysis of carbon isotopes present in compounds made by tiny marine phytoplankton preserved in ancient ocean sediments.

That estimate made Earth’s last experience of 400 ppm a much more recent event than scientists have commonly thought. There has been broader consensus that carbon dioxide concentrations have been much higher than today’s but not for tens of millions of years. The assertion that Earth passed the 400 ppm mark a mere 4.5 million years ago has been supported by other analyses, many of which also concluded that the temperatures at that time were higher than previously estimated.  These studies suggest that the traditional way scientists currently rate Earth’s long-term sensitivity to extra doses of CO2 might not sufficiently take into account the slower effects of climate change on the sunlight-absorbing properties of the planet, such as ice sheet melt and changes in plant cover on land.

What that means is that Earth might react even more strongly to the increases in CO2 measured by the Keeling Curve. Several prominent questions remain to be answered, though, before accurate scenarios can be created. The extreme speed at which carbon dioxide concentrations are increasing is unprecedented. An increase of 10 parts per million might have needed 1,000 years or more to come to pass during ancient climate change events. Now the planet is poised to reach the 1,000 ppm level in only 100 years if emissions trajectories remain at their present level.

“Our grandchildren will inhabit a radically altered planet, as the ocean gradually warms up in response to the buildup of heat-trapping gases,” said Scripps Institution of Oceanography, UC San Diego geoscientist Jeff Severinghaus.

– Robert Monroe


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84 thoughts on “What Does 400 ppm Look Like?”

  1. Factual basis of the Keeling Curve is also the basis of its reputational distinction.
    Well done to all concerned. This a service and a legacy of which you should feel very proud.

    Following on from that, however: –

    [a] Scientists produce ‘climate-models’ and
    [b] Social Scientists produce ‘economic-models’.

    Neither are really transparent and neither can really challenge the others work.

    Consequently, the continuing omission of major feedback mechanism from the output of [a] makes the work of [b] very speculative at best. Moreover, ‘exemplary’ legislation [like the UK Climate Act for example] is actually just a science/policy hybrid that doesn’t really address either dilemma: – which emissions are which and how do you sensibly integrate science/policy decision-taking in the liught of that?

    Precuationary policy does seem to require that we must now make an effort to measure [c] budget-emissions and [d] feedback-emissions – however difficult – *separately* . We can [theoretically] control [c], but we cannot control [d]. Especially as the planet warms, if we respond with mitigation efforts to control [c] too slowly, these can be overwhelmed by [d].

    Here’s is a link to a conceptual effort that seeks to trigger process moe alive to that dilemma: – http://www.gci.org.uk/CBAT/swaps/Domains.swf

  2. A 2009 paper by Tripati et al indicated that the last time CO2 levels were as high as they are currently, and were sustained at those levels, was about 15 to 20 million years ago. Is there a more recent paper that supports the statement in this article that CO2 levels during the Pliocene were comparable? If so, please provide a link. Thanks very much.

  3. Beware that the rate of CO2 rise today (2.2ppm/Year) is about 140x the 4MY steepest rate of rise (0.016ppm/Year) and temperature rise today (+1.78C/Decade) is about 11x the rate at which the planet exited the last ice-age (+0.16C/Decade). Only sea-level rise exiting the last ice-age (4.8mm/Year) is greater than today (3.2mm/Year) but then, there were super-glaciers over Nth America and Europe melting at a prodigious rate whereas only Greenland and Antarctica are the largest one’s left today.

    But my main concern is for species that can’t adapt or migrate fast enough. Coral Bleaching has already destroyed significant portions of the Great Barrier Reef which places the total reef’s demise (95% gone) at around 2050. Many other life-forms will be impacted leading to the next greatest extinction event since the KT event 65MYA.

    We had it in our power to change the Earth today so we have it in our power to change it back!

  4. There are humans and their is the surrounding reality. What humans think about reality, how they operate and co-operate within it, will ultimately determine their fates as an intelligent race, as a singular complex global system.

    Then there is Reality. She is merciless but fair, complex but beautiful. There is a reward to trying to understand her as she truly is, abstractly and concretely. There are consequences to misunderstanding her, as well. Physical reality, she is really something.

    Ultimately, our fate as a race will be determined by how accurately we understand her collectively, as a global conscience.

    1. There’s a lag effect. It takes time for the oceans to expand due to the thermal pressures put upon them. The predicted rise in sea levels that are touted quite often are mainly due to heat effects that we’ve ALREADY induced due to the CO2 increase that’s occurred. If we keep pumping more of the gas into the atmosphere, the heating will not only increase but the rate of change will too. The reason why the sea level now isn’t 5 metres higher is because the oceans haven’t expanded to that extent…yet. If we keep going on with our CO2 emissions, they will. Probably within a few hundred years or maybe earlier, depending on how much higher the temps end up going.

      1. Carl

        That is speculation. The rational interpretation of current CO2 levels and temperature compared with the past is that the they aren’t nearly as closely related as computer models suggest. One can speculate endlessly like a taxi driver pretending he is lost as the meter continues to revolve.

        It is as likely that CO2 lags temperature than the reverse due to the release of CO2 from warmer oceans.

  5. The idea that the last time CO2 levels were > 400 ppm was in the Pliocene has been repeated many times in the media and internet. To estimate atmospheric CO2 for over a million years ago we need so-called proxy methods, all of which require careful assumptions. Here I point out that the claim that CO2 exceeded 400 ppm in the Pliocene is based on a few extreme estimates, mostly from sites (locations) that systematically overestimate more recent CO2 levels, while the majority of published Pliocene CO2 values are in the 250-400 ppm range. The last time we have consistent evidence for pCO2 over 400 ppm is in the Early Oligocene epoch more than 26 million years ago. To read more, and comments on the various proxy methods, and a summary of ten recent proxy studies, see http://descentintotheicehouse.org.uk/when-was-co2-last-at-400-ppm-and-what-was-the-climate-like/

    Paul Pearson

  6. Pingback: Anonymous
  7. Atmospheric carbon dioxide at 400 parts per million, represents only 0.04% of the total composition of the gases in our atmosphere. The main gases are Nitrogen (78%), Oxygen (21%) and argon (0.5%). Thus an increase of carbon dioxide from 350 ppm (0.035%) to 400 ppm (0.04%) is not a significant change in the atmosphere (and not enough to have an effect on climate) Actually, carbon dioxide enters the air every day from volcanoes, forest fires, burning of fossil fuels, respiration of living creatures, etc. It is recycled and is taken up by all green plants, grasses, shrubs, as well as trees. By way of the complex reactions of photosynthesis, carbon dioxide is turned into carbohydrate and becomes the basis of our entire food supply. Without caarbon dioxide, life on this planet would not be the same.
    All of the above can be verified in standard chemical reference books.

    1. “Thus an increase of carbon dioxide from 350 ppm (0.035%) to 400 ppm (0.04%) is not a significant change in the atmosphere (and not enough to have an effect on climate)”

      I take it you’re not aware that the difference between an ice age and the current interglacial is around 100 ppm of CO2? The ice core data from several sites show that during an ice age the CO2 ppm figure is around 190 and during interglacials about 290. Fossil Fuel burning has now pushed the figure up to around 400 ppm.

      1. That is because CO2 has no relationship with climate in the ppm that we see then and now. That is the point he was trying to tell you. I believe in a way you made his point as well. The amount is a trace amount.

        C02 did not cause the ice age nor did it end the ice age.

        That does not mean we should not study the climate. It is all very interesting without a chicken little scenario.

  8. I believe the sun determines the cloud cover by allowing cosmic rays to form clouds. I also think Fern Gulley sucked while being forced to watch it in elementry school. I think ipcc people should watch A Land Before Time and The Little Mermaid because life adapts to climate for billions of years and shapeshifting octopi can coerce entire kingdoms.

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