What happens if we overshoot the 2º target for limiting global warming?

Carbon Brief

ImageGen.ashxTwo degrees is the internationally-agreed target for limiting global warming, and has a long history in climate policy circles. Ambition that we can still achieve it is running high as climate negotiators gather in Lima to lay the groundwork for a potential global deal in 2015.

But against this optimistic backdrop, greenhouse gas emissions have continued to rise. With each passing year the scale of the task looms ever larger.  There are very real questions about whether or not the world will be able to stay below the two degree limit.

So what happens if we fail to meet the two degree target? What would it mean to resign ourselves to a post-two degree world? And if not two degrees, then what?

As temperatures rise, so do the risks

Two degrees above pre-industrial temperature has been agreed by countries as anappropriate threshold beyond which climate change risks become unacceptably high.

Global temperature has risen 0.85 degrees Celsius since 1880, according to the latest reportfrom the Intergovernmental Panel on Climate Change (IPCC).

We could be due another couple of tenths on top of that as past emissions take a decade or so to reach their full effect warming. Together with current and expected emissions we’re essentially already committed to about one degree of warming, scientists estimate.

ar5_spmtempsince1850_550x560
Observed global mean temperature from 1850 to 2012, relative to the 1961-1990 average. Coloured lines represent three different datasets. Top panel shows yearly averages, bottom shows decadal averages. Source: IPCC 5th Assessment Report

While the international community uses two degrees as the rule-of-thumb threshold for “dangerous” warming, some climate impacts are already locked-in, particularly for low-lying and island nations. Professor Anders Levermann from the Potsdam Institute for Climate Change Research tells us:

“At one degree we are already experiencing damages. Sea level rise in the long term …  is somewhere in the vicinity of two metres. That puts cities like New York, Calcutta and Shanghai in difficult positions, and they need to protect themselves.”

Rising temperatures have consequences for food and water security, infrastructure, ecosystems, health and the risk of conflict, says the IPCC. And the higher the temperature, the greater the risk those climate change impacts will be serious and damaging.

One of the most direct impacts society feels from climate change is the greater frequency and intensity of extreme weather.In Europe, heatwaves like the 2003 event which killed 70,000 people are already ten times more likely than a decade ago. In the UK, climate change is making extreme wet winters like last year’s about 25 per cent more likely, scientists estimate.

Given that impacts scale with rising temperature, two degrees of warming above pre-industrial levels is thought to represent a climate target that’s both achievable and doesn’t expose us risks that are too difficult to manage.

But suppose we collectively decide the task of keeping to this target is too great, or the price of swift mitigation too high. What are the consequences of exceeding our self-imposed limit?

Three degrees

The IPCC uses four pathways to illustrate how greenhouse gases could evolve this century. The lowest, RCP2.6, is designed to show how warming could be kept below two degrees above pre-industrial levels (blue line in the graph below).

(Note that temperatures in the graph are relative to the 1986 to 2005 baseline, so add 0.61 degrees to get warming above pre-industrial levels.)

If we aim instead to stay in line with IPCC’s second scenario, RCP4.5, that should see global temperature level out at about three degrees above pre-industrial levels (green line).

extendedrcps_575x381.jpg
Extension of the IPCC’s emissions scenarios to 2300. RCP2.6 limits warming to two degrees above pre-industrial levels (blue), RCP4.5 levels out as about three degrees (green). In RCP8.5, temperatures exceed four degrees by 2100 and continue to rise. Meehl et al., (2013)

But even accepting this level of risk would require a strong commitment to mitigation. In the IPCC’s three degrees scenario, global emissions peak around 2040 and start to decline. But the risks at three degrees are already very high, says Levermann:

“Three degrees of warming increases the risk of strong sea level rise from, for example Antarctica, or the collapse of marine ecosystems, such as Arctic sea ice or coral reefs … [It] increases the risk of intensification of extreme events … In short, beyond two degrees of warming we are leaving the world as we know it.”

Four degrees

In the IPCC’s most extreme scenario, RCP8.5, global temperature reaches more than four degrees above pre-industrial levels by 2100. And unless emissions cease altogether, temperatures will continue to rise long past the end of the century.

With emissions accelerating faster than they are now for the next few decades, global temperature rise in RCP8.5 reaches five degrees by about 2120 and six degrees by 2150. This is a worst-case scenario, says Levermann, but that doesn’t mean it’s not a possibility.

Professor Richard Betts from the UK’s Met Office is the coordinator of a new international project called Helix, which looks at the impact of very high levels of warming. He tells us:

“[I]t’s very difficult indeed to know what a two degree world will look like, let alone four degrees or even six.”

But as the latest IPCC report notes, it’s clear the risk of triggering very large, abrupt or irreversible changes in the climate system increases the higher temperatures get:

“With increasing warming, some physical systems or ecosystems may be at risk of abrupt and irreversible changes … Risks increase disproportionately as temperature increases between one to two degrees Celsius of additional warming and become high above three degrees Celsius”

Since temperatures have risen almost one degree already, three degrees “additional warming” here means about four degrees above pre-industrial levels in total.

Continued warming will at some stage trigger the Greenland ice sheet to gradually collapse, although scientists can’t say precisely at what temperature this would occur, the IPCC report says:

“For sustained warming greater than some threshold, near-complete loss of the Greenland ice sheet would occur over a millennium or more, contributing up to seven metres of global mean sea level rise.”

Natural ecosystems are also set to suffer under higher temperatures, Betts tells us:

“Most life on Earth tends to be adapted to its current conditions, so if conditions change then species either need to move … or adapt to new conditions, or die out. [The] resilience of the natural world seems to be being reduced as a direct consequence of other human actions, through land use and habitat loss, so it’s a double-whammy for ecosystems.”

Understanding how rising global temperature translates to risks for society and natural ecosystems is critical to prepare for, and strive to reduce, the scale of impacts. But predicting consequences for different regions is difficult because while global temperature is a good indicator of global change, local impacts can be much more pronounced, Levermann says.

rsz_ar5globalimpacts_575x810
A comparison of climate impacts across the world by the end of the century under RCP2.6 and RCP8.5. (a) Average surface temperature, (b) average annual rainfall, (c) Arctic sea ice extent and (d) change in ocean pH. Source: IPCC 5th Assessment Report

A continuum, not a precipice

For international climate policy purposes, it makes sense to think in terms of the climate damages expected at different degrees of warming.

Two degrees is an appropriate middle ground between what we can no longer avoid and the level of further risk we’re willing to accept, Levermann suggests.

“We can’t really keep to one degree target anymore … At three degrees warming, Greenland is going to vanish and corals are going to be largely extinct… I would personally argue three degrees is too much and one degree is no longer achievable so two degrees is a reasonable target, but that is for society to decide.”

But setting two degrees as a boundary into “dangerous” climate change only works as a political target if its understood as a point along a continuum, not as a climate precipice, Levermann warns.

In other words, failing on the two degree target doesn’t mean we should all give up and go home. But admitting defeat means accepting a greater level of risk – and at that point preventing temperatures straying too far above two degrees should be paramount.

As to what counts as unacceptably high risk, that comes down to a judgement call, Betts concludes:

“How much these changes ‘matter’ or not is largely a matter of personal values and ethics … [W]e have to judge whether we think the benefits to ourselves are worth the risks to other species or future generations of our own.”

Only a few years on since countries agreed on two degrees as an appropriate level of climate ambition, it’s important to remember the science that underpins the agreement. As Professor Rowan Sutton told a Royal Society meeting this week, decision-making on climate comes down to our appetite for risk. Any decision to expose ourselves to higher climate risk should at least be a conscious and deliberate one, if not necessarily a prudent one.

 

Source: Carbon Brief. Reproduced with permission.

Comments

29 responses to “What happens if we overshoot the 2º target for limiting global warming?”

  1. Pedro Avatar
    Pedro

    Does anybody know what temperature increase has on human population growth? Some studies have shown that increased frequency of heatwaves shows a spike in heat related deaths. I can imagine that death from starvation/disease and severe storms/ bushfires could reduce world population especially in poorer countries.

    1. nakedChimp Avatar
      nakedChimp

      Before people starve or die from disease they will try everything to survive.. are you sure you want to be witness to that?

      1. Pedro Avatar
        Pedro

        I definitely don’t want to witness mass disease epidemics, starvation and war and wouldn’t wish it on anybody. Sadly I don’t think you or I have much choice. Unless the worlds politicians all suddenly find a heart, a good deal of common sense and a will to cooperate to do something to dramatically to reduce GHG pollution, it will be too little too late.

        With the current state of world politics it is easy for me to imagine human population being reduced significantly with all the negative disruption that that entails. The only positive I can see is that less people equals less consumption. It would be good to have a global population goal of 2 billion achieved peacefully over the next 200 years. A good deal on enlightened thinking has to happen first. Women in 3rd world or repressive countries overall have to have much more reproductive, and economic control.

        1. Alan Baird Avatar
          Alan Baird

          The chances of politicians allowing a DEPOPULATION of any country is ZERO. All politicians LOVE growth, especially of population. If one asked a polly whether he (usually) wanted to lead a country of 20 million or 50 million and you’d get the Kev Rudd answer: the latter. Too many is never enough. Peter Costello would give strenuous support. China has been criticised ad nauseam for trying to restrict population. The locals then all did what they usually have done and go 100% for males and get rid of daughters due to cultural attitudes. I applauded this decision ‘cos all the boys would be wandering around wifeless in a generation’s time. The hoi polloi were not disposed to thinking ahead. You’ve just got to be cynical about the usual human race. Fick to the deaf-knock.

          1. Pedro Avatar
            Pedro

            Many 1st world countries if you exclude migration would be close to zero or negative population growth. So it seems that wealth and being confident that you have access to reasonable care in old age is good for reducing family size.

            Bit a worry with countries like china where that are far more male children. An oversupply of men with little chance of finding a partner will lead to some serious social problems. In a worse case scenario civil war to get rid of excess men.

            Population growth is the invisible elephant in the room. I agree no politician is going to touch this one especially with the current ‘growth economic model’

    2. Bart_R Avatar
      Bart_R

      The direct correlation between temperature and population growth would be so complex as to defy a cause-and-effect explanation.

      For example, while heat-related deaths are significant, seasonal contagion-related deaths are far larger annually, and those tend to happen during the cooler months, though they spike when there are unseasonal warm spells in those cold months, and are far worse in warmer lattitudes than colder. Germs love damp surfaces and can survive in torpor due mild cold so long as they have moisture.

      Populations tend to have developed strategies to deal with the usual temperatures in their seasons, and when those temperatures are unseasonal, population behaviours tend to be counterproductive. This applies as much to birds — with avian influenza — as people, for example.

  2. iflyjetzzz Avatar
    iflyjetzzz

    “Global temperature has risen 0.85 degrees Celsius since 1880, according to the latest reportfrom the Intergovernmental Panel on Climate Change (IPCC).”

    So, you’re measuring global temperatures where the starting point is the end of the little ice age and during the period where temperatures rose, solar flare activity was so high that the time period is known as the Modern Maximum. Strange; who would ever conclude that the Sun heats the earth? That’s crazy talk: it’s got to be something done by man; it can’t be nature.
    Yes, let’s ignore Occam’s Razor.

    1. Icarus62 Avatar
      Icarus62

      The net effect of natural climate forcings since the end of the ‘little ice age’ is approximately zero, so the warming since then is anthropogenic.

      1. iflyjetzzz Avatar
        iflyjetzzz

        LOL! So you’re going to ignore the Modern Maximum? This is precisely why you people are viewed as cultists. You are unable to comprehend that the SUN’s solar energy output is not constant.

          1. iflyjetzzz Avatar
            iflyjetzzz

            Awesome report. I assume you didn’t bother reading the comments by the 5 physicists. 4 of them stated that fluctuations in solar activity have a large influence on the earth’s temperature. A couple of them flat out stated that the IPCC is incorrect in its assumption that CO2 is primarily responsible for post-industrial global warming.

      2. Scott Sinnock Avatar
        Scott Sinnock

        We don’t understand “natural climate forcings” any better and even worse than we do “anthropogenic climate forcings” Where, thermodynamically, does 1.5 watts per meter squared per doubling of CO2 come from as the Anthropogenic forcing factor anyway. My understanding it is a “consensus’ figure with no thermodynamic reasoning or empirical evidence, but a “feel right” number (with a range).

    2. Bart_R Avatar
      Bart_R

      The Little Ice Age?

      http://www.woodfortrees.org/plot/hadcrut4nh/from:1850.25/mean:6/every:12/detrend:-0.17/mean:11/plot/hadcrut4nh/from:1850.75/mean:6/every:12/detrend:-0.17/mean:11/plot/hadcrut4tr/from:1850.25/mean:6/every:12/detrend:-0.17/mean:11/plot/hadcrut4tr/from:1850.75/mean:6/every:12/detrend:-0.17/mean:11/plot/hadcrut4sh/from:1850.25/mean:6/every:12/detrend:-0.17/mean:11/plot/hadcrut4sh/from:1850.75/mean:6/every:12/detrend:-0.17/mean:11/plot/esrl-co2/normalise/mean:31/offset:0.4

      Occam’s Razor tells us to be as parsimonious in exceptions as possible. It is often confused with Newton’s Corollary, simplify assumptions so much as possible (but no simpler — h/t A. Einstein). Collectively with the principle of universality of cause and effect of like properties of like bodies, these form the foundation of Newton’s Philosophy of Science, which goes on to state that the strongest inference upon these principles from all observations is considered accurate or most nearly true until new contradictory observations require that inference be amended.

      What caused the LIA? When exactly did it start and end? Observation and inference tell us it was a combination of solar activity — which accounts for perhaps one tenth of a degree — and a long chain of volcanic eruptions high enough to cause stratospheric transport of sulfate aerosols, with the last of these in 1815, plus albedo feedbacks.

      Following the well-expressed Medieval Climate Anomaly (approximately A.D. 910–1257), which comprised the warmest decades of the last millennium, our tree-ring-based temperature reconstruction displays an abrupt regime shift toward lower average summer temperatures precisely coinciding with a series of 13th century eruptions centered around the 1257 Samalas event and closely preceding ice-cap expansion in Arctic Canada. Furthermore, the successive 1809 (unknown volcano) and 1815 (Tambora) eruptions triggered a subsequent shift to the coldest 40-y period of the last 1100 y.

      http://www.pnas.org/content/111/28/10077.short

      By 1865, the LIA was done. Incidentally, since 1990, there have been an unprecedented string of volcanic eruptions, and yet even when we ought have seen LIA-like temperatures due this, plus low solar activity, plus the low phase of ENSO, we’ve instead seen the dozen hottest years on record, including probably the five hottest years of the Holocene.

      Occam’s Razor tells us it’s us, it’s our fossil waste dumping, that does this.

      1. iflyjetzzz Avatar
        iflyjetzzz

        If there were any predictive AGW models that did not diverge from real world data, you might have a valid argument. However, all CAGW models use short cherry picked timeframes from which to predict future temperature increases. They are all littered with type I and type 2 statistical errors.

        Fortunately, the public is getting weary of CAGW forecasts that are continually proven to be incorrect. CAGW cultists have cried wolf too often.

        1. Bart_R Avatar
          Bart_R

          Which requires us ask, what GCMs do not diverge from real world data?

          The first candidate ought of course be Hansen 1988 Scenario “B”, the one most like what happened in human events with the success of the Montreal Protocol, effects of the fall of the former Soviet economy, Great Recession of 2008, switch from oil to natural gas (and coal to natural gas) of electrical utilities, and so forth. Hansen skipped ENSO and volcanoes in his model, so we have to either add them into the GCM, or preferrably subtract them from the real data.

          Oh look. Hansen “B” is an EXACT match for 2014, and it’s only been a quarter century, not even halfway through the model run.

          You claim Type I & Type II statistical errors. Which ones, exactly? Please, be specific.

          Keeping in mind that less than 2% of the lines of evidence for AGW involve GCMs, and so what you say is only one fiftieth as important as you make it sound, with your spurious “C” and all, I’m not exactly counting on a very illuminating reply, so don’t worry about disappointing. The bar you’ve set yourself is extremely low.

          1. iflyjetzzz Avatar
            iflyjetzzz

            Bart, The Hanson model you cite is an atmospheric CO2 model, not a temperature model. We are debating temperature forecasts, not CO2 levels. I concede that CO2 levels have risen in the last 70 years, and that world food production has risen in lock step with increased CO2 levels. Look, higher CO2 levels are a GOOD thing! Unless you think that increased world food production is a bad thing.

            Based on your question with respect to type I and type II statistical errors, it’s obvious that you do not understand anything about statistics because if you did, you wouldn’t ask the question.
            Type I is a false positive correlation.
            Type II is a false negative correlation.

            For Type I, AGW cultists have falsely over correlated the link between CO2 levels and the earth’s temperature.
            For Type II, AGW cultists have intentionally falsely under correlated natural causes of the earth warming.

            I see some merit in the greenhouse gas theory (it IS theory), but I am very troubled by the extreme overemphasis on its importance in the earth’s natural temperature variability. There are much clearer connections with naturally occurring forcings. I am extremely troubled with the IPCC dismissing ALL natural causes of global warming over the last 70 years, as should all rationally thinking people.

          2. iflyjetzzz Avatar
            iflyjetzzz

            By the way, my statement that world food production has risen in lock step with CO2 levels is true, but it is not statistically significant because there isn’t a large enough sample size (not enough annual crop reports); it may be a Type I error.

          3. Bart_R Avatar
            Bart_R

            World food production has statistically risen with world use of artificial fertilizers and industrial farming methods, genetically modified and specialty bred seed, and advances in crop disease and pest control, as well as intensive irrigation.

            Factoring out those influences, your statement is meaningless.

            That would be an example of Type 0 Error: not even understanding the question.

            Plant mass increases, and plant senescence is induced, by raising CO2 to the levels of an ethylene & gibberrellin plant hormone antagonist. The parallel is force-feeding steroids to every infant on the planet. Of course you get longer limbs and bigger muscles, but at the cost of lower overall health and shrunken and deformed glands.

          4. klem Avatar
            klem

            Ah so that explains why growers pump up the CO2 levels in their greenhouses, because they prefer old plants with lower health and shrunken, deformed glands. Makes sense.

            Learn sumptin new every day.

          5. Bart_R Avatar
            Bart_R

            We generally pump up the CO2 levels during the day only, and work to drop CO2 levels at night as much as possible. Of course, when I was a kid, that was easier, since there was so much less CO2 in the air.

            When we do pump up the CO2 during photosynthesis, it’s with enormous levels of fertilizer and water, and cooling sprays. It’s rare for greenhouse growers to care about the reproductive glands of hothouse plants for sale: the point of such ornamentals isn’t to breed future generations or furnish nutritious food but to be showy on the shelf.

            If you do learn ‘sumptin’ new every day, try to allocate a day to goodwill reading. It’s well worth learnin’.

          6. Bart_R Avatar
            Bart_R

            My bad.

            I’d concluded that because you spoke the jargon, you understood at least some of what you were saying.

            I see now I was mistaken.

            Sorry to have troubled you.

          7. klem Avatar
            klem

            No need to apologize Bart, we thought same thing about you.

            Cheers

          8. Bart_R Avatar
            Bart_R

            To help you out, http://pubs.giss.nasa.gov/abs/ha02700w.html is the Hansen (not ‘Hanson’) study of horizontal heat transport (aka ‘temperature forecasts’ over long timespans) in a GCM under three proposed scenarios (A, B, C) from high to low of CO2 emission from fossil sources.

            And no one is still debating these points. That ship has long ago sailed. The reason over 17,000 climate studies published in the past two decades simply accept as fact the truth of AGW is that over 4,000 of those studies independently confirm AGW using the scientific method propounded three centuries ago by Isaac Newton in his Philosophy of Science.

            You have begged the question (an invalid form of argument) on your Type I “example” (propaganda).

            You have begged the question again for your Type II propaganda “example”.

            Your argument from personal incredulity, also, is simply invalid. No one cares that you’re troubled, except your mother.

            No one denies natural forcings, however when examined closely all identified natural forcings scale far below the power of the GHE due AGW on periods exceeding two decades. The Hale Cycle is two orders of magnitude less influential than CO2 with feedbacks, and has been swamped from the global temperature signal for the past half century. La Nina generally accompanies the coldest years in a century, but the past twenty years’ La Nina events have included some of the twelve hottest years ever on either the instrumental or paleo record.

            You’re very bad at this, and I say this with as much kindness as I can muster.

  3. Bob Bingham Avatar

    Sea level rise alone will displace millions of people who will then be looking for a new home. Put this together with massive crop failure and I would predict war on a grand scale.its not going to be easy what ever happens. http://www.climateoutcome.kiwi.nz/climate-threats.html

  4. Bart_R Avatar
    Bart_R

    Something people are missing is that AGW is a seasonal activity. The warming leads in winter, as extreme cold is more susceptible to warming, but is more intense in summer, when there’s more sunlight converted to IR for Green House Gases to trap in the troposphere, and it leads in the Northern Hemisphere, which means we’ll hit the pain points soonest in the more developed nations.

    http://www.woodfortrees.org/plot/hadcrut4nh/from:1850.25/mean:6/every:12/detrend:-0.17/mean:11/from:1910/plot/hadcrut4nh/from:1850.75/mean:6/every:12/detrend:-0.17/mean:11/from:1910/plot/hadcrut4tr/from:1850.25/mean:6/every:12/detrend:-0.17/mean:11/from:1910/plot/hadcrut4tr/from:1850.75/mean:6/every:12/detrend:-0.17/mean:11/from:1910/plot/hadcrut4sh/from:1850.25/mean:6/every:12/detrend:-0.17/mean:11/from:1910/plot/hadcrut4sh/from:1850.75/mean:6/every:12/detrend:-0.17/mean:11/from:1910/plot/esrl-co2/normalise/mean:31/offset:0.6

    We are hoist on our own petard. China, the USA, Russia, Germany, Japan, the UK, South Korea and Canada will experience all of the worst effects of the warming consequences of fossil wasted dumping before Brazil and India of the ten worst polluters. Those effects will include rise in extreme weather such as winter storms and cold, summer heat waves, intense winds, drought, flood and cyclone.

    The non-heat consequences to aquatic stoichiometry, plant hormone imbalance and soil microbe disruption will continue to be global, and collectively more costly to food production than the warming effects.

  5. Asteroid Miner Avatar
    Asteroid Miner

    At 6 degrees C [11 degrees F], humans go EXTINCT. We are going to have a population crash from 7 or 8 billion to what? Seventy thousand or zero? That is the remaining question. GW is already slowing food production compared to what it would be without GW. Famine is the most popular kill mechanism. This planet cannot support more than 3 billion humans permanently. Without GW, our population still must drop as aquifers run dry.

  6. Bart_R Avatar
    Bart_R

    Continuum, or precipice?

    It’s comforting to think in terms of the Fallacy of Equipartition, that things will always be gradual, that trends will be smoothly and regularly predictable. That way, we can get used to things little-by-little, like a frog being boiled.

    But that’s not how Nature works. The principle curve of natural trends is the Sigmoid, the S-shaped slow-to-start, suddenly takes off shape of population statistics and mortality rates. The curve of precipices, and tipping points, black swans and unpredictability.

    1. Scott Sinnock Avatar
      Scott Sinnock

      Nature is exponentially decaying processes back toward thermodynamic equilibria in response to disturbances of the equilibria. A sigmoidal curve is also a cyclical normal gausian curve

      1. Bart_R Avatar
        Bart_R

        Again, you’re falsely assuming equipartition.

        We know from Vostok that the two-sided sigmoid of CO2 is anything but Gausian normal, and we know fossil waste dumping is anything but Nature.

        Debris remnants in sediments from iceberg rafts show us to expect decadal upward pulses of up to seven meters as the ice fields of Antarctica and Greenland break up.

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