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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Why it's urgent we act now on climate change

What the science says...

Select a level... Basic Intermediate Advanced

Global warming is an increasingly urgent problem. The urgency isn’t obvious because some warming is being delayed, and even more warming is yet to kick in. Some of the latest research says if we want to keep the Earth’s climate within its natural range during the time that humans have existed, we must leave nearly all of 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.

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)

In 1992, 154 nations signed the UN Framework Convention on Climate Change, with the objective of preventing “dangerous anthropogenic interference with the climate system.” This raised the question: what exactly would constitute dangerous human interference? James Hansen, NASA’s top climatologist and one of the first to warn that greenhouse warming had been detected, was the lead author of a masterful 2008 paper which set out to answer that question. 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 – in many countries, 390 ppm is over the legal limit for blood alcohol content while driving. More pertinently, it’s higher than CO2 has been in millions of years. CO2 is rising by 2 ppm per year as we continue to burn fossil fuels. Hansen and colleagues argue that 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 how the climate has responded to past greenhouse gas changes in the real world. From a comprehensive look at past climate change (“paleoclimate”), Hansen concluded that, in the long term, climate is twice as sensitive in the real world as it is in the models used by the IPCC. This is the most complicated part, so bear with me for a few paragraphs.

The key problem in climate modeling is determining the value of “climate sensitivity”, or the amount of 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 ice sheets (ice is a feedback because an icy surface reflects more heat than a dark surface). There is a broad consensus that the fast-feedback climate sensitivity is 3°C. Though model estimates come with large error bars, paleoclimate-based estimates converge on the same number, 3°C.

Slow-feedback sensitivity has received far less attention. Paleoclimate is the only available tool to estimate it; Hansen used the highly accurate ice core record of the last few hundred thousand years. Further complicating matters, slow-feedback sensitivity is not stable over geologic time. The ice sheet feedback will only work if there is ice to melt, thus slow-feedback sensitivity is much higher when the planet has ice on it. To cut a long story short, it turns out the total climate sensitivity is as high as 6°C when there are ice sheets on the planet, as there are today. That is, the slow ice sheet feedback doubles the warming predicted by climate models.

Long-Term Climate Sensitivity

So why the urgency? The global temperature has risen only about 0.7°C. The answer is the climate 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 climate sensitivity implies there’s a lot more than in the models. If CO2 remains at or above 390 ppm long enough for the ice sheet feedback to kick in, the delayed warming would eventually reach 2°C (ie. 2.7°C above preindustrial times). 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 could rise 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”. If ice sheets can melt significantly this century, then Hansen’s long-term warming has near-term policy implications. Once an ice sheet begins to collapse there is no way to stop it sliding into the ocean. We would be subjected to centuries of encroaching shorelines. The climate change we started would proceed out of our control.

Hansen confirmed his results using sediment core data, which allow a longer-term view, tens of millions of years. The Earth has been in an ice age since Antarctica froze 34 million years ago, around the time our distant ancestors split off from monkeys. The major cause of the descent into the current ice age was a natural decline in CO2 (related to continental drift and thousands of times slower than the current rise). According to Hansen’s calculations, the freezing point came when CO2 fell to 450 ppm. If we pass 450 ppm in the opposite direction, it will be the melting point. A return to an ice-free Earth would mean a sea level rise of 75 metres.

Finally, Hansen looked at the changes already unfolding. He concluded the current CO2 level also puts us at high risk of an ice-free Arctic Ocean in summer, desertification in Australia and equivalent latitudes, water shortages for hundreds of millions from glacier loss, and the devastation of coral reefs. But 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 cannot avoid targeting 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 even including possible greenhouse gas 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. Unfortunately this is the same level which Hansen concluded would eventually melt all the ice on the planet.

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 is to leave most of the remaining fossil fuels in the ground. Because CO2 stays in the atmosphere for a very long time, burning them at a slower rate makes little difference. Of the three conventional fossil fuels (coal, oil, and gas), coal has by far the largest reserves. It is not enough to slow down coal-burning by converting it to liquid fuels. The fundamental problem is with the coal being burned at all.

We also must not replace coal with other fossil fuels. We really don’t want to burn the tar sands and oil shale, whose reserves are virtually untapped but thought to contain even more carbon than coal. Unfortunately, the Canadian government is actively encouraging their burning; this cannot go on. What about conventional oil and gas? Energy experts disagree on how much is left. If it turns out we have already used about half of their reserves, then we can safely burn the rest (assuming we’re not burning any other fossil fuels). If the reserves are on the high side, we shouldn’t go to the effort of extracting the least accessible drops.

Supposing that we succeed in halting the rise of CO2, we will still be left with the gargantuan task of removing it from the atmosphere. Nature can absorb some carbon, but it has limits. We would also need to change our forestry and soil practices to absorb carbon rather than emit it.

It won’t be easy but it appears to be still possible to get back to 350 ppm, if we

1) phase out coal by 2030.

2) not burn the tar sands and oil shale.

3) not burn the last drops of oil and gas.

4) turn deforestation into reforestation.

CO2 Emissions and Atmospheric Concentration with Coal Phaseout by 2030

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.

For fear of being called alarmist, I should point out that estimating a CO2 target from paleoclimate is fraught with uncertainties. I’ve had to simplify somewhat for this short article. If you want to learn more, I explain Hansen’s methods and results in detail on Skeptical Science, or you can read the full paper for free here. But 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, greenhouse gas feedbacks, is likely to make things even worse. There is more than enough reason for concern 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, scientific 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. Continued growth of greenhouse gas emissions, for just another decade, practically eliminates the possibility of near-term return of atmospheric composition beneath the tipping level for catastrophic effects.

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.

Intermediate 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

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Comments 1 to 9:

  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!

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