75CFDA33-4183-4D54-9393-81C6E28FAAD9 Created with sketchtool. 400 ppm Carbon Dioxide Milestone

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  Last updated April 5, 2017 at 4:33 pm

This article was originally published on 22 May 2013 on riaus.org.au.

Last week the world’s atmosphere took a giant leap backward. For the first time in more than 3 million years, the atmospheric concentration of carbon dioxide (CO2) reached 400 parts per million (ppm). This was recorded at the National Oceanic & Atmospheric Administration’s Earth System Research Laboratory located on Mauna Loa in Hawaii right out in the middle of the Pacific Ocean, a long way from major sources of CO2  in Asia and North America. What this means for the environment is uncertain but it’s likely that, if no measures are taken to reduce this level of greenhouse gas, we could be headed for 3-40C warming globally by the end of this century.

Already a lot of ink has been spilt about the many possible and disastrous outcomes from the continuing rise of greenhouse gasses and, whatever the scenario, the future does not look good. But, being a palaeontologist, I thought it might be instructive to take a more detailed look back at the last time the world had these concentrations of CO2 and see what things were like. How did the world look 3 million years ago?

Well firstly, there were no humans. Even the most primitive members of our genus Homo had yet to evolve. Instead, the genes that built you and me could be found in a more distant ancestor, a species of Australopithecus roaming large areas of Africa. We were very much part of the fauna, regularly falling prey to leopards and other predators and spending most of our time walking around on open grassy plains (although we were still capable climbers if trees were around). Culture and industry were as simple as you can get –stone hammers used almost unmodified from the condition they were found and there was no use of fire, agriculture or mobile phones. Our total number was probably less than one million and we had no more impact on the environment than any other species of mammal weighing in at 40-60 kg and thinly distributed across our habitat.

The animals that we shared the planet with then would be completely unfamiliar to us today. Even mammoths, Diprotodon and other beasts of the Megafauna had yet to evolve. That more recent megafauna appears to have evolved as a response to the Ice Ages that have occurred over the last 2 million years. Yes, Ice Ages (plural) because there have been a dozen or so interspersed with warmer interglacial periods over the last couple of million years. We are still in the midst of this series of glaciation cycles, currently we are in a warmer interglacial event. But since the world last bathed in 400 ppm CO2, vast sheets of ice up to a couple of kilometres thick have marched across the northern continents and retreated at least 12 times.

Instead of this periodic global freezer, 3 million years ago the world was a much warmer place. On average, global temperatures were around 3-40C warmer than they are today with polar temperatures as much as 100C warmer. There was still some glaciation in Antarctica, in fact some parts of that southern continent have had ice cover for the last 70 million years, back when the last dinosaurs roamed the Earth, but Antarctica was not gripped in a continent-wide ice sheet as it is today. The northern polar regions were ice free with a continuous seaway covering the North Pole. The huge quantities of water that are now locked up in the polar regions was free to mix with the oceans of the world 3 million years ago. That would have pushed up sea levels by as much as 40 metres. A rise in sea levels of that magnitude today would inundate all of Australia’s capital cities except Canberra.

The point I’m trying to make here is that the world was a radically different place 3 million years ago and a lot has happened in the intervening period. I’m not saying that we are going to see a reversion to these conditions anytime soon just because CO2 levels have gone back to that ancient level – the world’s climate is far too complex for that to occur. Even if global temperatures do track the CO2 concentrations and warm by 3-40C, it would take a long time, probably several centuries, for this to melt all the ice at the poles and drive up sea levels.

But these are all linked systems and while those may not be the outcomes we will see in our lifetimes, that is the direction that we’re pushing the system and it is where things are likely to go over the longer term. And ‘long term’ is what concerns those of us who study Earth history. We can look back and predict the future based on our observations of the past. If you want to know what a world looks like with 400 ppm CO2 we can tell you with some alarming accuracy.

But ‘long-term’ as we see it, is usually irrelevant to other people. No one is interested in what the world will be like in a million years or 100,000 years. Even a century is barely within the grasp of most people. But what about a couple of decades? That’s within our lifetimes but outside the political cycles of 3-4 years. We can perhaps grapple with outcomes on those scales even if our political masters don’t appear to be able to. Suddenly changes looming in the world become personally relevant if expressed in timeframes of decades. So what will the world be like 20 years from now if we continue to generate greenhouse gasses at the rate we have been for the last century?

Again, this is a complex situation and modelling it is equally complicated. However, it appears likely that within 20 years we will be living in a world that is another degree warmer than it already is. That’s enough to substantially rearrange weather patterns around the world leading to a diversity of outcomes dependant on where you are. Rain and flooding events will increase in some areas while other places will experience more drought and less water. The habitat for most species of animals and plants will shrink, driving some to extinction. One estimate posits that half of all plants and a third of all animals will have half the area to live in by 2080. Changes in weather patterns will result in less arable areas available for agriculture and thus, less food to go around. Coral reefs will struggle in warmer waters and we will see the continuing loss of the Great Barrier Reef.

Of all the likely outcomes over the next few decades of continuing to increase CO2 concentrations in the atmosphere, it’s hard to come up with anything positive. And, of course, the next few decades will only be the beginning of an ongoing trend. Our grandchildren will inherit a world in a much poorer state than was received from our grandparents. We really can’t afford to take a world with more than 7 billion people on it back to a time when Australopithecus timidly strode the plains of Africa.

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About the Author

Paul Willis
Paul is a respected leader in the science community with an impressive career in science. He has a background in vertebrate palaeontology, studying the fossils of crocodiles and other reptiles. He also has a long history as a science communicator, with a career spanning as Director of The Royal Institution of Australia, presenter and host for Australia’s Science Channel, working for the ABC on TV programs such as Catalyst and Quantum as well as radio and online. He’s written books and articles on dinosaurs, fossils and rocks and is finding new ways to engage the people of Australia with the science that underpins their world. Follow him on Twitter @fossilcrox.


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