By Guest Author 13/05/2020 13


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Earth had several periods of high carbon dioxide levels in the atmosphere and high temperatures over the last several million years. Can you explain what caused these periods, given that there was no burning of fossil fuels or other sources of human created carbon dioxide release during those times?

James Renwick, Te Herenga Waka — Victoria University of Wellington

Burning fossil fuels or vegetation is one way to put carbon dioxide into the air – and it is something we have become very good at. Humans are generating nearly 40 billion tons of carbon dioxide every year, mostly by burning fossil fuels.

Carbon dioxide stays in the air for centuries to millennia and it builds up over time. Since we began the systematic use of coal and oil for fuel, around 300 years ago, the amount of carbon dioxide in the air has gone up by almost half.

NOAA

Apart from the emissions we add, carbon dioxide concentrations in the air go up and down as part of the natural carbon cycle, driven by exchanges between the air, the oceans and the biosphere (life on earth), and ultimately by geological processes.

Natural changes in carbon dioxide

Every year, carbon dioxide concentrations rise and fall a little as plants grow in spring and summer and die off in the autumn and winter. The timing of this seasonal rise and fall is tied to northern hemisphere seasons, as most of the land surface on Earth is there.

The oceans also play an active role in the carbon cycle, contributing to variations over a few months to slow shifts over centuries. Ocean water takes up carbon dioxide directly in an exchange between the air and seawater. Tiny marine plants use carbon dioxide for photosynthesis and many microscopic marine organisms use carbon compounds to make shells. When these marine micro-organisms die and sink to the seafloor, they take the carbon with them.

Collectively, the biosphere (ecosystems on land and in soils) and the oceans are absorbing about half of all human-emitted carbon dioxide, and this slows the rate of climate change. But as the climate continues to change and the oceans warm up further, it is not clear whether the biosphere and oceans will continue absorbing such a large fraction of our emissions. As water warms, it is less able to absorb carbon dioxide, and as the climate changes, many ecosystems become stressed and are less able to photosynthesise carbon dioxide.

Earth’s deep climate history

On time scales of hundreds of thousands to millions of years, carbon dioxide concentrations in the air have varied hugely, and so has global climate.

This long-term carbon cycle involves the formation and decay of the Earth’s surface itself: tectonic plate activity, the build-up and weathering of mountain chains, prolonged volcanic activity, and the emergence of new seafloor at active mid-ocean faults.

Most of the carbon stored in the Earth’s crust is in the form of limestone, created from the carbon-based shells of marine organisms that sank to the ocean floor millions of year ago.

Carbon dioxide is added to the air when volcanoes erupt, and it is taken out of the air as rocks and mountain ranges weather and wear down. These processes typically take millions of years to add or subtract carbon dioxide from the atmosphere.

In the present day, volcanoes add only a little carbon dioxide to the air, around 1% of what human activity is currently contributing. But there have been times in the past where volcanic activity has been vastly greater and has spewed large amounts of carbon dioxide into the air.

An example is around 250 million years ago, when prolonged volcanic activity raised atmospheric carbon dioxide levels dramatically. These were volcanic eruptions on a vast scale – lasting for around two million years and causing a mass extinction.

In the more recent geological past, the past 50 million years, carbon dioxide levels have been gradually dropping overall and the climate has been cooling, with some ups and downs. Once carbon dioxide concentrations became low enough (around 300 parts per million) between two and three million years ago, the current ice age cycle began, but the warming our emissions are causing is larger than the natural cooling trend.

While Earth’s climate has changed significantly in the past, it happened on geological time scales. The carbon in the oil and coal we burn represents carbon dioxide taken up by vegetation hundreds of millions of years ago and then deposited through geological processes over millennia. We have burned a significant proportion within a few centuries.

If human emissions of carbon dioxide continue to increase through this century, we could reach levels not seen for tens of millions of years, when Earth had a much warmer climate with much higher sea levels and no ice sheets.The Conversation

James Renwick, Professor, Physical Geography (climate science), Te Herenga Waka — Victoria University of Wellington

This article is republished from The Conversation under a Creative Commons license. Read the original article.


13 Responses to “What caused major climate change in the past?”

  • Over the last 3- 4 million years we have evidence that there has been repeated cycles of warm and cold periods.
    https://www.sciencelearn.org.nz/resources/1480-whanganui-rocks-and-climate-cycles

    Our scientists tell us that the temperature difference between the coldest and warmest periods at that time around NZ are up to 12DegC

    https://www.tandfonline.com/doi/pdf/10.1080/03014223.2005.9517776

    The interaction of oceanic, atmospheric and solar cycles continues to be the key driver of Earth’s climate.
    It doesn’t seem that CO2 has played much of role (if at all) in our recent climate history.

  • Hi Maggy

    Here’s the relevant bit from your first link:

    “Until a million years ago, each climate cycle lasted about 41,000 years. Since then, the cycles have lasted about 100,000 years. The cycles are related to the Earth’s rotation and its orbit around the Sun, but reasons for the change a million years ago are not known.”

    So, 12 degrees change over a period of 59,000 to 100,000 years, or around 0.00012 degrees change per year.

    Meanwhile:

    “According to the NOAA 2019 Global Climate Summary, the combined land and ocean temperature has increased at an average rate of 0.07°C (0.13°F) per decade since 1880; however, the average rate of increase since 1981 (0.18°C / 0.32°F) is more than twice as great.” – https://www.climate.gov/news-features/understanding-climate/climate-change-global-temperature

    So, .07 degrees C per decade is about 0.007 degrees C a year, or .007/.00012=58 times faster than the geological average. The rate in the last thirty years is over 100% faster again, so over 100 times faster than the geological average rate of change.

    Something odd is happening. Something that impacts on and/or amplifies the oceanic, atmospheric and solar cycles. Something that really only started in the mid to late 1800’s. Hmmmmmmmn. I wonder, WHAT could that be?

  • Your heading is your correspondent’s question: “What caused major climate change in the past?” but you don’t appear to have answered this question.

    You say CO2 stays in the air, but this cannot be true. Your graphs clearly show its levels oscillating, which means CO2 levels are regularly declining (as well as rising), and we know ocean organisms in their trillions are born and die every day, removing the carbon from the atmosphere and sequestering it in the ocean or its bottom.

    You say that carbon dioxide stays in the air for centuries to millennia, so what is the evidence for this?

    You say, “The biosphere (ecosystems on land and in soils) and the oceans are absorbing about half of all human-emitted carbon dioxide, and this slows the rate of climate change.” However, it doesn’t affect climate change unless CO2 causes discernible (as distinct from theoretical) warming and it continues for at least 30 years. Since the IPCC claims man-made warming since 1950 was “probably” about 0.5°C from 1951 to 2010, how much would you say climate has changed in that time, and was it due to slightly increased temperature?

    You say, “As water warms, it is less able to absorb carbon dioxide,” and that’s true, but it would be much more honest to reveal that it actually emits carbon dioxide. Have you factored this in to the amount of human emissions and if so, how much was involved?

    You say, “On time scales of hundreds of thousands to millions of years, carbon dioxide concentrations in the air have varied hugely, and so has global climate.” You strongly imply that they’re related. What’s the evidence for that?

    Carbon dioxide is added to the air by volcanic activity, and is removed as rock weathers. The process take millions of years to add or subtract carbon dioxide from the atmosphere. Clearly, this occurs every day, so would it be more correct to say: “These processes take millions of years to add or subtract SUBSTANTIAL QUANTITIES of carbon dioxide from the atmosphere.”

    In calculating the human contribution to airborne CO2, what allowance have you made for CO2 emissions from volcanic activity since 1750?

  • @Ashton Dempsey

    Goodness! I’ve never seen the suggestion that the temperature rose/fell at a constant rate throughout a Milankovitch Cycle before.
    Everything I’ve ever read about our recent past climates has indicated that there were numerous quite rapid fluctuations between cold and warm times (interstadials) during Glacial or Interglacial Periods.
    A review of New Zealand palaeoclimate from the Last Interglacial to the global Last Glacial Maximum
    https://www.sciencedirect.com/science/article/abs/pii/S0277379114005083

  • Hi Maggy – If by “quite rapid” you mean a couple of thousand years for the fastest, yeah.

    12 degrees / 2000 is still 0.006 degrees a year. Since 1981 our rate has been 0.18 degrees a year. Orders of magnitude then.

    Goodness indeed. Possibly with a Gracious Me! to follow.

  • Since 1981 our rate has been 0.18 degrees a year.

    I think you are confusing years and decades.

  • ahhh. True that. So we are currently “only” warming at three times the rate of that significant event.

    In your opinion, if its not CO2 or other man-made cause, what is it that is causing this rapid change?

  • If you bothered to read my first post here, you’d see that I’d mentioned the main factors that must have been operating before Industrial man arrived on scene.

  • Pehaps you could describe the mechanism as it is acting in this period Maggy. No one is denying it happened as you have described in pre-history, but why is it happening now?

    After all, the oceanic and atmospheric are both known vectors for the man-made CO2 mechanism – in shorthand, atmospheric CO2 increases temperature releasing further CO2 from sea water and ice (hugely simplistic but I’m sure you get the drift). If you are asserting that water and air are vectors for a different heating mechanism, what’s the mechanism that aligns with the timeline for this latest increase in temperatures?

  • If you are asserting that water and air are vectors for a different heating mechanism,

    Why would I assert anything like that?

    I will go through this slowly for you:
    Professor Renwick writes: Once carbon dioxide concentrations became low enough (around 300 parts per million) between two and three million years ago, the current ice age cycle began

    I presented evidence from NZ, that over the last 3 million years there have been numerous occasions when the temperature rose and fell …… all while CO2 levels were around 280-300ppm..

    Now as far as I’m concerned, whatever was responsible for climate fluctuations in our very recent Geological past is probably heavily involved in climate regulation nowadays. I don’t think Oceanic and atmospheric cycles and solar cycles suddenly stopped operating around 1860.

  • That conversation with Maggy was gripping, thank you, and well done, Maggy. Let me remind you of the questions I posed above, which are literally statements made by Prof Renwick, none explained and all crucial to the IPCC climate narrative. As you know, scientific statements need justification, so it’s incumbent on the IPCC side to completely justify them to assist public understanding—no one is obliged to refute them. I look forward to your response.

  • To Maggy Wassilieff and Richard Treadgold:
    Am I correct in deducing that you are of the view that any human-made climate changes that might be happening now and which might continue over the next century or more are negligible in comparison with the naturally occurring climate changes which have happened in the past over millenia and million year time frames? So therefore we need not bother trying to limit greenhouse gas emissions?