Have recent concentrations of carbon dioxide in the atmosphere pushed some parts of the climate system beyond their tipping points? And, if so, how deep in trouble are we? This page is a layman's attempt to distil the answers found in current scientific literature.
SAFE DANGEROUS CATASTROPHIC
Based mainly on Hansen et al. (2008).Preliminary data from Mauna Loa Observatory (NOAA). Further details
Those are average concentrations of atmospheric carbon dioxide measured in ppm units and pending some recalibrations and checks. The first concentration is the latest weekly average, while the other two give the daily averages approximately 1 and 10 years ago. Here, "1 year ago" means exactly 365 days back in the record. Similarly, "10 years ago" equates to 3653 days (3 more days to account for leap years). Notice that, although CO2 seems to build up very slowly year on year, the jump in concentration after 9 years is more than significant: an increase of roughly 4.97 % between the two historical daily averages. Go further back in time and that jump becomes huge.
At Mauna Loa Observatory (MLO), an atmospheric station part of NOAA's Earth System Research Laboratory (ESRL) and located on the Hawaiian volcano of Mauna Loa. The fact that it is up high in the middle of the Pacific Ocean gives MLO a particular advantage when measuring CO2 levels: it is well away from any sources on the ground. This is one of the reasons why these levels have been recorded there since the 1950s.
Parts per million. According to the Wikipedia, 1 ppm
is equivalent to one drop of water diluted into 50 litres. Hence its use to describe small relative concentrations such as that of CO2 in the air. Despite its low concentration though, carbon dioxide has a potent impact on the atmosphere and, by extension, the climate: the so-called greenhouse effect.
Careful study of air bubbles contained in ice cores tell us that in the mid-1700s, before the Industrial Revolution, CO2 concentration stood at about 280 ppm (Etheridge et al., 1996). In fact, CO2 has remained roughly between 260-280 ppm since the end of the last glaciation, approximately 11,000 years ago (Monnin et al., 2004). This period, known as the Holocene, has also been characterised by a relatively stable and warm climate. And it is this climate what may have helped the early discovery of agriculture (Gupta, 2004) and subsequent development of civilisation (Van der Leeuw, 2008). Rockström et al (2009, p.2) believe that:
So, being the upper end of that range, 280 ppm seems a good enough starting point.
[...] we must take the range within which Earth System processes varied in the Holocene as a scientific reference point for a desirable planetary state.
Those are atmospheric CO2 concentrations of historical or environmental significance:
315 ppm: first annual average of direct measurements throughout 1958 on Mauna Loa. This is the starting point of the Keeling curve, the world's longest unbroken record of atmospheric carbon dioxide concentrations.
350 ppm: initial reduction target proposed in Target Atmospheric CO2: Where Should Humanity Aim?, a seminal paper written by former NASA climatologist Dr. James Hansen et al. (2008).
385 ppm: annual average for 2008, when the above paper was published. Last time CO2 was that high was during the middle Pliocene, around 3 million years ago (Ballantyne et al., 2010).
420 ppm: approximate peak concentration suggested by some projections (Meinshausen, 2006) that could possibly prevent a rise of more than 2° C in global average temperature. Nations committed to that 2-degree target at the Copenhagen climate summit in 2009. Since then, it has been shown that even 1° warming still leads to dangerous climate impacts (Smith et al., 2009; Hansen & Sato, 2012), which makes the 2-degree target too high (Anderson & Bows, 2011).
As Prof. Tim Lenton explains in the video below,
it's where a small change makes a big difference. A critical threshold at which small variations can lead to large consequences. Like when you push on a light switch. Beyond the "tipping point", even a very gentle nudge will make the switch flip and the lights turn on. This is potentially dangerous since certain tipping points can cause sensitive parts of the climate system to change abruptly over a few decades, instead of centuries or millennia (Lenton et al., 2008). Humanity is simply not used to that rate of change. On top of that, some changes cannot be "switched off": they may well be regarded as permanent because any possible reversal is unlikely to happen on a reasonable human time scale. Fortunately, even after crossing a tipping point, there is still a window of opportunity to stop the change in question from becoming irreversible. Such window could last up to what is called a point of no return. The problem is understanding the complexities behind all these points.
The colours indicate how likely a given CO2 concentration is of crossing a tipping point. Hansen et al. (2008) suggest that sustained CO2 levels higher than 300 ppm may trigger such a scenario, with potentially catastrophic consequences beyond 350 ppm. The colour code is largely based on that and other studies:
SAFE: less than 300 ppm
No tipping points seem to lie within this range.
DANGEROUS: 300-350 ppmThe CO2 tipping point for several climate components lie within this range. Among these components is the Arctic sea ice, estimated to be at risk of disappearing within this century (Lenton et al., 2008; Overland & Wang, 2013). It has been classified as one of the most concerning abrupt effects of climate change (White et al., 2013). And, although reversible, it can lead to irreversible changes such as the slow release of vast amounts of methane from the sea floor. Now, bear in mind that methane's warming effect lasts shorter but is much stronger than that of CO2. From May & Caron (2009),
it is to carbon dioxide what an espresso shot is to herbal tea. So much so that high levels of methane are considered to have played an important role during the greatest die-off in Earth's history: the Permian-Triassic extinction (Benton & Twitchett, 2003).
CATASTROPHIC: beyond 350 ppm
There was a time when the planet was so warm that it was essentially free of ice. Then, during the Late Eocene, world temperatures went down and, some 35 million years ago, they were low enough for permanent ice sheets to form over Antarctica. Well, it turns out that a decrease in CO2 was behind that cooling (Pagani et al., 2011). If you think about it the other way, it means that a rise of CO2 beyond a certain threshold (somewhere between 350-550 ppm) can ultimately make the planet ice-free. And that may bring more than just severe droughts and flooded cities: it could put us on a path towards a mini-version of the so-called runaway greenhouse effect (Hansen et al., 2013). If that weren't enough, evidence gathered by Kutterolf et al. (2012) and Nyland et al. (2013) point to a possible link between ice retreat and volcanic activity. Some scientists like Prof. Bill McGuire go even further, claiming that dramatic climate change could set off more earthquakes too. Others remain cautious. The debate is on the table.
This is the subject of much research. The decline of summer Arctic sea ice is probably an early effect of crossing the tipping point for Arctic sea ice in general (Livina & Lenton, 2013). Snape and Forster (2014) suggest that
ice-free conditions could occur as early as 2032. On the other hand, although the loss of that ice can have serious global consequences, these at least do not seem to be abrupt: the release of methane that it may cause is unlikely to be sudden (White et al., 2013)... As far as we know. Because, you see, the climate system is so complex that it might hide other processes linked to that sea-ice loss that could indeed bring about rapid change. Prof. Stephen Schneider, one of the pioneers of modern climatology, explained it eloquently during a TV interview in 1979:
[00:05:35] So, what we're really doing is we're insulting our global environment at a faster rate than we're understanding it. And the best we can do in all honesty is say:Look out! There's a chance of potentially irreversible change at a global scale based upon the benefits of use of energy. And it's very tough for us to know whether those benefits of energy today are worth the potential risks of environmental change for our children.
Way beyond it. As explained above, the science is clear: taking into account all uncertainties, concentrations above 300 ppm can ultimately trigger uncontrollable and even abrupt climate shifts. And, remember, the current concentration is 400.35 ppm! Granted the specifics are not so clear: exactly when, how or at what level those shifts happen. All the more reason to be cautious and act sooner rather than later. Especially knowing that sudden changes have already happened in the past. The Younger Dryas, for instance, was a very rapid shift to colder temperatures that occurred in the Northern Hemisphere over 12,000 years ago, towards the end of the last ice age. Although its causes are not yet completely understood, one possible explanation is the shutdown of the North Atlantic thermohaline circulation. Simulations conducted by Jahn and Holland (2013) show that the melting of Arctic sea ice can affect such circulation. On The Climate Wars, a 2008 BBC documentary series, Prof. Iain Stewart asked rapid climate change expert Prof. James White if he was genuinely scared about events like the Younger Dryas. His answer was quite sobering:
[00:37:05] Yes, I don't think anybody could not get scared. If you really thought... If you understood just how fast that was and how big this change was... Just how fundamental it was... If something like that happened to us... And it's important to recognise we've not seen anything like it... If something like that happened to us today, we would probably not be able to grow enough food, we would not have enough freshwater... It would challenge even the most industrialised society to adapt. And THAT is scary.
Courtesy of our policymakers' systematic procrastination, the quickest most effective solution we have been left with is radical change in consumer behaviour. Either that or overhaul the entire economic system, a slow process we have to set in motion anyway. But we still need to start somewhere soon. So, in short: consume less now. Buy less, drive less, fly less... Make frugality fashionable again. Just like our parents and grandparents did not so long ago! In an interview at the 2013 Warsaw Climate Change Conference, Tyndall Centre's Dr. Alice Bows-Larkin and Prof. Kevin Anderson were very clear about the urgency to slash consumption:
KEVIN ANDERSON: [00:05:02] In the short term, the only way we can get our emissions down is to actually reduce the level of energy we consume. Now, we can also put low-carbon energy supply in place [...][00:05:26] but it's going to take us 20, 30 years to do that. [...][00:05:43] So we'll have to consume less. And there's absolutely no way out of that. The maths are absolutely clear. [...][00:07:09] over the last 10 or 15 years we’ve moved from what were quite high carbon lifestyles to these completely profligate, extraordinarily high carbon lifestyles, and we’ve made them normal, that, actually, what we did 10 or 15 years ago, if we did that now, we’d think we were strange.
ALICE BOWS-LARKIN: [...][00:08:16] the earlier you can actually cut those emissions, the easier it will be to achieve change overall and the more likely it is that we’ll be able to achieve these grand climate goals. But that points to doing things that you can do in the short term. So you can address consumption and behaviour and demand-side measures. That’s not saying it’s easy. It’s going to be incredibly challenging. But we don’t have much debate about consumption and about demand. When I go to energy meetings, we talk primarily about energy supply, particularly electricity supply. But to change that takes a long time.
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Ballantyne, A., Greenwood, D., Damsté, J., Csank, A., Eberle, J. and Rybczynski, N. (2010). Significantly warmer Arctic surface temperatures during the Pliocene indicated by multiple independent proxies. Geology, 38(7), 603-606. DOI: 10.1130/G30815.1
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I use a dead simple REST web service I wrote some time ago. All it does is extract the values from the entry in this web feed corresponding to the latest weekly average. The XML file of the feed is fetched only once a day to guarantee that the bandwidth party can go on for the folks at ESRL/NOAA. Not that it is a huge file but just in case.
The service's base URL is http://www.hqcasanova.com/co2. The available endpoints are:
http://www.hqcasanova.com/co2/all Human-readable summary of all three averages
http://www.hqcasanova.com/co2/delta Estimated percentage change between historical daily averages (9 years)
http://www.hqcasanova.com/co2/help Brief help page
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