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Why it's urgent we act now on climate change

What the science says...

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A large amount of warming is delayed, and if we don’t act now we could pass tipping points.

Climate Myth...

It's not urgent

"There are many urgent priorities that need the attention of Congress, and it is not for me as an invited guest in your country to say what they are. Yet I can say this much: on any view, “global warming” is not one of them." (Christopher Monckton in testimony to the US Congress)

Global warming is an increasingly urgent problem. The urgency isn’t obvious because a large amount of warming is being delayed. But some of the latest research says if we want to keep the Earth’s climate within the range humans have experienced, we must leave nearly all the remaining fossil fuels in the ground. If we do not act now we could push the climate beyond tipping points, where the situation spirals out of our control. How do we know this? And what should we do about it? Read on.

James Hansen, NASA’s top climatologist and one of the first to warn greenhouse warming had been detected, set out to define dangerous human interference with climate. In 2008, his team came to the startling conclusion that the current level of atmospheric carbon dioxide (CO2) is already in the danger zone.

Since the Industrial Revolution, atmospheric CO2 has increased from 280 to 390 parts per million (ppm). Don’t be fooled by the small number – 390 ppm is higher than CO2 has been in millions of years. CO2 is rising by 2 ppm per year as we continue to burn fossil fuels. To stabilise the Earth’s climate, we must reduce CO2 to the relatively safe level of 350 ppm. And we must hurry, because the task will soon be an impossible one.

The 350 target is based not on climate modeling, but on past climate change (“paleoclimate”). Hansen looked at the highly accurate ice core record of the last few hundred thousand years, sediment core data going back 65 million years, and the changes currently unfolding. He discovered that, in the long term, climate is twice as sensitive in the real world as it is in the models used by the IPCC.

The key question in climate modeling is how much global warming you get from doubling CO2, once all climate feedbacks are taken into account. A feedback is something that amplifies or cancels out the initial effect (eg. interest is a feedback on a loan). The models include “fast feedbacks” like water vapor, clouds, and sea ice, but exclude longer-term “slow feedbacks” like melting ice sheets (an icy surface reflects more heat than a dark surface).

Both models and paleoclimate studies agree the warming after fast feedbacks is around 3°C per doubling of CO2. Slow feedbacks have received far less attention. Paleoclimate is the only available tool to estimate them. To cut a long story short, Hansen found the slow ice sheet feedback doubles the warming predicted by climate models (ie. 6°C per CO2 doubling).

Long-Term Climate Sensitivity

The global climate has warmed only 0.7°C, but has not yet fully responded to our past emissions. We know this because the Earth is still gaining more heat than it is losing. There is further warming in the pipeline, and Hansen’s results imply there’s a lot more than in the models. If CO2 remains at 390 ppm long enough for the ice sheet feedback to kick in, the delayed warming would eventually reach 2°C. That would result in an Earth unlike the one on which humans evolved and a sea level rise of not one metre, not two metres, but 25 metres. Imagine waves crashing over an eight-storey building.

It’s hard to dispute this would be “dangerous” climate change. But how quickly could it happen? In the past, ice sheets took millennia to respond, though once they got moving sea level rose several metres per century. But maybe ice sheets can melt faster if CO2 rises faster, as it is now doing. The IPCC predicted they would grow by 2100, but instead they are starting to shrink “100 years ahead of schedule”. Once an ice sheet begins to collapse there is no way to stop it sliding into the ocean. We would suffer centuries of encroaching shorelines. The climate change we started would proceed out of our control.

If ice sheets can melt significantly this century, then Hansen’s long-term warming has near-term policy implications. The tragedy we have set in motion can still be prevented, if we get the Earth to stop accumulating heat before slow feedbacks can kick in. To do so we must target the greatest, fastest-growing, and longest-lived climate driver: CO2.

Under business as usual, we are heading for up to 1,000 ppm by 2100, or nearly two doublings (and that’s not including possible carbon feedbacks). This would surely be an unimaginable catastrophe on any timescale. Even the mitigation scenarios governments are quarreling over are based on IPCC assessments now several years out of date. The lowest CO2 target being considered is 450 ppm, which Hansen concluded would eventually melt all ice on the planet, raising sea level by 75 metres. The Earth has not been ice-free since around the time our distant ancestors split off from monkeys.

Instead of stepping on or easing off the accelerator, we need to be slamming on the brakes. We must not only slow the rise of CO2 in the atmosphere, but reverse it. We must reduce CO2 from 390 to 350 ppm as soon as possible. That should stop the planet’s accumulation of heat. Stabilizing the CO2 level will require rapidly reducing CO2 emissions until nature can absorb carbon faster than we emit it – in practical terms, cutting emissions to near zero.

The only realistic way of getting back to 350 ppm is leaving most of the remaining fossil fuels in the ground. We must:

1) phase out coal by 2030. It is not enough to slow down coal-burning by converting it to liquid fuels, because CO2 stays in the atmosphere for a very long time. The fundamental problem is with the coal being burned at all.

2) not burn tar sands or oil shale. Their reserves are virtually untapped but thought to contain even more carbon than coal. Canada cannot keep burning them.

3) not burn the last drops of oil and gas if their reserves are on the high side. If it turns out we have already used about half, then we can safely burn the rest.

4) turn deforestation into reforestation. We’d still be left with the gargantuan task of removing CO2 from the atmosphere. Nature can absorb some carbon, but it has limits.

CO2 Emissions and Atmospheric Concentration with Coal Phaseout by 2030

It won’t be easy, but with these actions CO2 could peak around 400 ppm as early as 2025 and return to 350 ppm by century’s end. I believe we can achieve this; it’s primarily a question of political will. But our window of opportunity is rapidly slamming shut. Even one more decade of business as usual, and CO2 can be expected to remain in the danger zone for a very long time.

I should point out estimating a CO2 target from paleoclimate is fraught with uncertainties. I’ve had to simplify for this short article. I explain in more detail on Skeptical Science, or you can read Hansen’s paper free here. If there is one lesson recent climate research should teach us, it is that it’s a mistake to call uncertainty our friend. Arguably the most important aspect Hansen ignores, carbon feedbacks, is likely to make things even worse. There is more than enough reason to heed Hansen’s warning.

Right now we stand at an intersection. What we do in this decade is crucial. If we choose one path, by the end of the decade the world could be well on its way to phasing out coal. If we choose the other, we face an uncertain future in which the only certainty is a continually shifting climate. I’ll leave the final word to Hansen et al, whose concluding statements were pretty strongly worded coming from a dense, technical, peer-reviewed paper:

Present policies, with continued construction of coal-fired power plants without CO2 capture, suggest that decision-makers do not appreciate the gravity of the situation. We must begin to move now toward the era beyond fossil fuels. […] The most difficult task, phase-out over the next 20-25 years of coal use that does not capture CO2, is Herculean, yet feasible when compared with the efforts that went into World War II. The stakes, for all life on the planet, surpass those of any previous crisis. The greatest danger is continued ignorance and denial, which could make tragic consequences unavoidable.

Basic rebuttal written by James Wight

Update August 2015:

Here is a related lecture-video from Denial101x - Making Sense of Climate Science Denial


Last updated on 5 August 2015 by MichaelK. View Archives

Printable Version  |  Offline PDF Version  |  Link to this page

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Comments 1 to 25 out of 48:

  1. This is the only refutation I could find in the list by taxonomy that I thought might explain why "geo-engineering" doesn't solve the problem. After all, if geo-engineering would work, then that would reduce the urgency. So this is the perfect place (at least under the existing taxonomy) for addressing it. There is, after all, a a claim circulating the rumor mill now that sulfuric acid high altitude aerosols will solve the problem. I do not think we can explain the popularity of this belief solely in a one-sided reading of the Wikipedia article
  2. I'd like to see the Basic article expanded in various ways.  This article doesn't mention a time frame as to when we might see costly effects.  Sounds like we have at least hundreds of years if not thousands before Miami sinks beneath the waves.

    The article doesn't discuss any problems other than sea level rise.  Will climate change leave agriculture unphased and as productive or more productive than it currently is?  If production drops, how log before we might see climate based drops in production?

    Over time frames like hundreds of years geo-engineering becomes more feasible.  And over longer time frames, smaller yearly investments can be made.

  3. Perhaps I'm on the wrong page, but...  The climate myth the way I've heard it is:  "Nothing is going to happen for a long time, so we don't need to do anything now."  This is mostly in the context of melting ice sheets in Greenland and the Antarctic.

    The arguments I would expect to see are:

    1)  There are short term problems.  For example ocean acidification and coral bleaching.  Heat waves.  Reductions in agricultural production.

    2)  Changes, especially cheap ones, take a long time to take effect.  For example, the U.S. car fleet turns over in 20 years.  If we stopped selling gasoline and diesel powered cars today and only allowed the sale of electric cars, it would take 20 years to get all the gasoline and diesel powered cars off the road.

    3)  There is huge inertia.  If we stopped emitting new CO2 today, and held existing concentrations constant, we would see the earth continue to warm up in quite some time.  

    With business as usual, we will see accelerating CO2 emissions causing deteriorating climate, and we will also see deteriorating climate as the Earth tries to reach equilibrium with the CO2 already emitted.  Both of these will mean that Miami will be flooded sooner (50 years) rather than later (100 years), plus we will have less time to react.


  4. Dr David Mills was on youtube years ago saying it is now impossible not to go over 440ppm...! He also said it was being debated whether it was possible to go over it and then come back down under it but seeing as that was years ago I'm sure someone has information on where that specific debate is now.

    (Dr David Mills was the candian guy who wanted to do Solar Thermal in Australia after having trained and invented processes in Australia but no dice so went to America and no dice, so, well... I suppose he gave in the end!)

  5. You said here that carbon concentrations will peak at 400 ppm in 2025 under the ideal situation, but it's only 2015 and we're already at 400 ppm, and I see no signs of global emission reductions happening soon. Is this evidence that we will pass the point of no return?

  6. anticorncob6...  The writer said concentrations "could" peak around 400ppm, but clearly we're screaming past that level right now. That was written four years ago, and I'd have to say was a very optimistic outlook.

  7. Oh, and regarding "point of no return..."  "Point of no return" would likely not be a term anyone would use since it leaves too many loose ends.

    We are currently at about 0.8C over preindustrial global temperature, and with thermal inertia we've banked about 1.2C of temperature rise no matter what we do. 

    That, in and of itself, means there are going to be aspects of climate change that we can't stop and will have to adapt to. After we pass 2C over preindustrial temps we risk passing tipping points where we don't know how much additional warming will result. Researchers are urging us not to pass that 2C limit. At around 0.2C/decade... meh, we have a little bit of time, but we desperately need to be enacting policies now that can keep us below that 2C limit.

  8. @6, There was a famous interview/commerical-I-suppose on Youtube a number of years ago(not sure how many) where Dr David Mills was saying there is now no way we can stay under the information being floated around does seem a little contradictory and the denial brigade can almost be forgiven for driving mack trucks through what seem like gaping holes of information. Yes, it all depends where you go for information of course.

    My point was that I'm guessing a lot more than 440ppm is locked in- I'm not sure how This David Mills character came up with his numbers so I am of course merely guessing/being potentially paranoid... the IPCC reports are known to be conservative by nature for instance and there was an article just recently on Sterling Engines being tested in South Africa over the last 4 years... combining this with the new phenomenons of formula-e racing and possibly Virgin competing with Tesla for the electric car market and Tesla itself saying it may just go into producing batteries the world does seem to be displaying something of a turn !

     (I could find that video but it may take some searching... the only relevant information was that he though 440ppm was locked in and they didn't know if it was possible to go over the limit and then come back down under at that time but that it would be impossible to not break it in the initial sense!)

  9. @7, another indicator worthy of attention in my mind is that Bjorn Lomborgs political message was that 3C was a more sensible target... what this indicates I don't like!

  10. Hi, I just read the book No One Is Too Small to Make a Difference by Greta Thunberg. Wow, what an emotional rollercoaster. I've played the Cranky Uncle game for hours, taken the edx101 course, surfed the skeptical science website, and argued with deniers.

    Book no one is too small Greta Thunberg

    Yet, only in 2022 have I heard about net zero emissions. Even then, I thought it was by the year 2050. Greta Thunberg makes the case that global climate change is an existential urgent crisis. That we need net zero by 2030. Is this really true?

    As a millennial I feel a lot of the same emotions that the older generations are out of touch when I say I cannot get a job or having trouble with the basics like a roof over my head, running water, heat in winter. I find I get scolded by the older generations and they offer out of touch simplistic solutions blaming the victim or even calling me a liar.

    Did I get distracted by the pandemic, George Floyd's murder, and possible nuclear war between Russia and Ukraine? With all my climate activist since 2016 did I really miss that we only have a 50% chance to avoid a climate catastrophe of runaway greenhouse effect if we go for a 2 degree Celsius increase by 2050 or whatever Thungberg said in her book?

    How urgent is climate change? Thank you in advance. :)

  11. "PollutionMonster at 14:13 PM on 8 March 2023"
    "How urgent is it?" is a value question. Another value question is "How much threat to diversity does humanity's anthropocentrism" cause to the long-term survival of species-populations in the wild (10,000 years)? And I agree, how do we quantify "How much threat"?

    In an anthropocentric (human-centered) context, judging from what I'm witnessing, we were out of time long ago.

    I need only point to the proliferation of nuclear warheads and greenhouse gases to bolster my case. I don't see much genuine effort by governments and corporations to do the real work, and make long-term decisions for the benefit of humanity and biodiversity's long-term existence; we continue to pass the buck to future generations.

  12. PollutionMonster @10,

    You ask about the timing for reaching net zero carbon. That single timing doesn't properly capture the task in hand.

    The science strongly suggests that increasing global temperatures by more than +1.5ºC risks potential dramatic climate change. To prevent such rise, there is just one scenario presented within the IPCC AR6 that fits the bill - SSP1-1.9.

    AR6 SPM Fig8.a

    This SSP1-1.9 scenario does include a timing of 2050 for net zero carbon but it also requires a halving of global net carbon emissions by 2030 and large net negative carbon emissions post-2050. These net negative emissions amount to roughly extracting atmospheric CO2 equal to all the emissions post-2007 and storing them away safely. (There are many saline aquifers around the world which this CO2 could be desolved into after its extraction from the atmosphere. These extractions from the atmosphere are additional to the natural draw-down of CO2 into the oceans.)

    But it seems it is only the 'net zero' message that is being heard by politicians. So calling for an earlier 'net zero' is probably a useful message.

  13. "These extractions from the atmosphere are additional to the natural draw-down of CO2 into the oceans.)"

    And we have no idea if, at all, the global ocean will continue to act as a viable carbon sink, not to mention methane. Then there's the political will and economic resources to make the abrupt ideological and technological changes needed, assuming that critical tipping points were not breached long ago. I'm assuming that we don't know everything to know about the neew climate change and our test-tube earth mentality.


  14. EddieEvans @13,
    The net carbon sink into the oceans is far more predictable than the carbon interchange in/out of the biosphere. There is still some uncertainty and re-assessment (eg Watson et al 2020) in the matter but generally the only big variable is the ocean surface temperatures. So as long as we prevent massive SST rises, I would think it is safe to say "the global ocean will continue to act as a viable carbon sink." The actual size of that sink over the coming millennium will thus depend on how well we do preventing AGW but otherwise it's size is fairly predictable. What is far less predictable under AGW is the biosphere as a source/sink.

    You also raise the threat of methane, this usually focusing on natural feedbacks and the melting permafrost. In the past I was rather worried by the poor coverage of this subject in the scientific literature but having dug into the subject I now feel more comfortable about it. Additionally the absence of significant methane fluxes resulting from the significant permafrost melt in recent decades is a reassuring sign.

  15. According to the following article, we are on the precipe of multiple climate tipping points. As they say, "Hold onto your hat, we're in for a wild ride."

    Risky feedback loops are accelerating climate change, scientists warn by Emma Newburger, Climate, CNBC, Mar 6, 2023  

  16. MA Rogers has correctly clarified that the total harmful warming impact is what matters. Limiting the impact to 1.5 C needs to continue to be the focus. And the reality that the peak impact will almost certainly exceed 1.5 C needs to be understood to mean that wealthy people today need to be paying for safe/harmless technological extraction of CO2 from the atmosphere. That extraction will be expensive and never be profitable. And the spending of tax money on it rather than other things will never be "most" popular.

    That is the challenge. Leadership has to do something unpopular and unprofitable to benefit future generations. The diversity of developed socioeconomic-political systems is tragically lacking in the development of that type of leadership. And it is now undeniable that humanity only has a future if it develops governing of all significant human activity in ways that understandably limit and correct harm done.

    A related point is that it is harmful to cause increased CO2 to be absorbed in the oceans. The fact that CO2 will continue to be absorbed in the oceans is not a positive.

    Also, a lack of significant methane release from massive thawing of permafrost (a miss named item) is not a helpful positive.

    It is essential to remain focused on the need to end harmful activity regardless of its developed popularity or profitability abnd related popular 'perceived to be positive' misunderstandings (and that applies to authoritarian as well as democratic governing).

  17. I read all the responses, and I want to thank all of you. :) Climate justice is a major part of climate change. That rich nations including the United States, France, and United Kingdom need to reach zero emissions by 2030 so that poorer nations have time to develop and have some emissions until 2050.

    Thunberg in her book referenced some specific page of the IPCC page 100 or so stating that there is a 50% chance of runaway greenhouse effects beyond human control at 2C and only 34% chance at 1.5C. Is this true? That there really is that high a chance that climate change will be the end of everything? Did I misread? I haven't read the source material, navigating the IPCC report is difficult.

  18. PM @17... Would you have the precise quote from Thunberg's book related to "50% chance of runaway greenhouse effects beyond human control at 2°C"? 

    My suspicion is that's not an entirely correct assessment, though I'm confident Thunberg's book went through a thorough review by researchers prior to publication. My understanding is, past 2°C we move into a realm of much greater uncertainties. Also, even at 2°C significant feedbacks (say, from methane releases) remain long tail uncertainties. But I could be wrong.

  19. Most of the quote is here in this NPR article. Risk of setting off irreversible chain reactions NPR.

    I would have to reread the book to get the exact quotes, I read in local bookstore.

    ""The popular idea of cutting our emissions in half in 10 years only gives us a 50% chance of staying below 1.5 degrees [Celsius], and the risk of setting off irreversible chain reactions beyond human control." Thunberg


  20. Hm... Yeah, those are quotes from 2019 and I think she's probably conflating two issues. One being the likelihood of staying below 1.5°C or 2°C, and the other being the likelihood of setting off irreversible feedbacks. To my understanding, they're two different issues with very different confidence levels. 

  21. I"m with Rob. The writing of that specific sentence could be clearer. The "50% chance" part is definitely associated with the "staying below 1.5 degrees", but the comma that follows that separates the "50% chance" probability from the "risk of setting off irreversible chain reactions".

    Two possible re-writes that would make the writer's intentions clearer:

    1. "The popular idea of cutting our emissions in half in 10 years only gives us a 50% chance of staying below 1.5 degrees [Celsius]." It also brings a "risk of setting off irreversible chain reactions beyond human control."
    2. "The popular idea of cutting our emissions in half in 10 years only gives us a 50% chance of" avoiding two outcomes: "staying below 1.5 degrees [Celsius], and the risk of setting off irreversible chain reactions beyond human control."

    Expect "The Usual Suspects" to insist that the only possible interpretation is the one that fits their preconceived notions of Greta Thunberg.

  22. Bob Loblaw @21

    I am unsure who "The Usual Suspects" are. I am pretty good at debunking  obvert denial, but I still have a knowledge gap when it comes to the subtler aspects of climate change.

  23. PM (may I call you PM?):

    There is a classic line at the end of  the movie Casablanca, where the police captain (Renault) says to Rick (the main character played by Humphrey Bogart), "round up the usual suspects". The connotation is that the police have a list of people they know are usually associated with many crimes, and they'll take the blame.

    I'm basically pointing out that there are certain players in the climate change "debates" who will most certainly take the least charitable interpretation of that quote. We've seen them do similar, many times before.

  24. MARoger@

    "The net carbon sink into the oceans is far more predictable than the carbon interchange in/out of the biosphere."

    Using the global ocean as a carbon sink has consequences for biodiversity, increasing acidification. There's no free lunch, and no eternal waste disposal for the Anthropocene, I gather. I'm not up to date on the latest research; I left the ocean as a sink with Roger Revelle. I will update my understanding for sure.  There are no positives in any of these GHG matters.

  25. Eddie:

    SkS did a series on the ocean acidification issue a number of years back.

    Part 0 provides an index to the series.

    After it was complete, it was turned into a downloadable booklet.

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