2011-02-09 18:47:26The rationale for 350 ppm
James Wight

jameswight@southernphone.com...
112.213.148.195

EDIT: This is the advanced version. See below for the intermediate and basic versions.

Yes, I know this is super-long (4,000 words to be precise). However, I intend to also write a shorter, more basic version to complement it. I know we don't usually do different information levels for blog posts but I think it would be a good idea in this case. This one has all the details for those who are interested, while the short version would hammer the key points so they don't get buried.

I don't intend to publish this one until I have finished the basic version. When both versions are complete we can compare them and decide which one to publish first.

I have had some difficulty getting my head around this subject, so if you think I've misunderstood any aspects, please don't hesitate to set me straight.


In 1992, 154 nations signed the United Nations Framework Convention on Climate Change, with the objective of achieving “stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” This raised the question: what exactly would constitute dangerous anthropogenic interference? In 2008, a team of climatologists led by James Hansen set out to answer that question, and came to the startling conclusion that we are already over the limit: the current level of atmospheric carbon dioxide is already in the danger zone.

The amount of CO2 in the atmosphere has increased from about 280 ppm preindustrially to 390 ppm today, and continues to rise by 2 ppm/year as we continue to burn fossil fuels. In their paper, “Target Atmospheric CO2”, Hansen et al argue we should aim to reduce it to 350 ppm in order to stabilize the Earth’s climate. And we must hurry, because that task will soon be an impossible one. Their reasoning is complicated, but worth taking some time to understand given that it concerns the future of the world.

The 350 ppm target is based not on climate modeling, but on how the climate has responded to past greenhouse gas changes in the real world. Estimating a CO2 target from paleoclimate is fraught with uncertainties, but the assumptions made by Hansen et al are not unreasonable ones. Likewise, their value judgements on what is “dangerous” are, in my opinion, no-brainers. Their paper covers a broad array of topics, but at its centre is the question: how sensitive is the Earth’s climate when you include “slow feedbacks”?

Climate sensitivity and slow feedbacks

Climate sensitivity is the amount of global warming you get from doubling CO2 (or an equivalent forcing, which is about 4 watts per square metre or W/m2), and determining its value is the key problem in modeling future climates. Usually we define climate sensitivity as including only “fast feedbacks” such as water vapor, sea ice, clouds, and dust (ice is a feedback because it affects the reflectivity or “albedo” of the surface). Because this definition comes from a landmark 1979 report by the National Academy of Sciences, whose lead author was Jule Charney, it is often called “Charney sensitivity”. For clarity I will call it “fast-feedback sensitivity”.

But in the long run (and as we shall see, the long run has current policy implications), what will be important is the climate sensitivity when you include not only fast feedbacks, but also “slow feedbacks” such as ice sheets. Greenhouse gases can also be a slow feedback, but Hansen et al do not count it as one because they want to know the long-term sensitivity to an unamplified greenhouse gas forcing.

Fast-feedback sensitivity is 3°C

There is a broad consensus that fast-feedback sensitivity is 3°C for a doubling of CO2. Model estimates come with large error bars that have proven difficult to reduce as climate models have become more realistic over the decades, because modeling all the positive and negative feedbacks is so complicated. However, studying past climate changes, which obviously include all existing feedbacks, allows us to circumvent that problem, and paleoclimate-based estimates converge on the same number, 3°C.

Hansen et al reconfirm this with ice core data, by comparing the Holocene (the relatively stable interglacial climate of the last 10,000 years) to the Last Glacial Maximum (LGM) 20,000 years ago. Most of the warming between those two intervals was caused by ice sheets and greenhouse gases, themselves slow feedbacks on tiny orbital forcings sustained over long periods. But for the purpose of finding fast-feedback sensitivity, those slow feedbacks are considered to be forcings (confusing, I know). It is then straightforward to compare the combined forcing (6.5 W/m2) to the global temperature change (5°C), and derive a fast-feedback sensitivity of 0.75°C per W/m2 or 3°C per CO2 doubling, as predicted.

But here we’re more concerned with slow-feedback sensitivity.

What about slow-feedback sensitivity?

We don’t currently have models that include slow feedbacks (which is why the IPCC hasn’t taken them into account), so paleoclimate is the only available tool to estimate them. Further complicating matters is the fact that slow-feedback sensitivity is not stable over geologic time. The ice sheet feedback will only work if there is ice to melt, thus climate sensitivity is higher when the planet has ice on it. On an ice-free Earth, the albedo feedback approaches zero, and slow-feedback sensitivity is about the same as fast-feedback sensitivity (remember, we’re not counting greenhouse gas feedbacks).

The planet is currently in an ice age, with a hundred-millennium cycle from brief “interglacial” periods like the Holocene, when ice sheets are confined to Antarctica and Greenland; to long “glacial” periods like 20,000 years ago, when global temperature plunged by 5°C, ice sheets covered much of Canada and Europe, and sea level fell by over 100 metres. The Northern Hemisphere has been in an ice age for the duration of the Quaternary period of glacial-interglacial cycles, which began 3 million years ago. Antarctica has been in an ice age for no less than 34 million years, or the second half of the 65-million-year Cenozoic era.

Hansen et al use the ice core record of the late Quaternary (the last few glacial-interglacial cycles) to estimate the recent slow-feedback sensitivity to a specified greenhouse gas forcing. As before, about half of the global temperature change in each cycle was from ice sheet feedbacks and half from greenhouse gas feedbacks (though they in turn were ultimately caused by tiny variations in the Earth’s orbit). Since here we’re defining greenhouse gases as a forcing and ice sheets as a feedback, the result is a slow-feedback sensitivity that is double the fast-feedback sensitivity, or 6°C.

However, all of this is ignoring greenhouse gas feedbacks, which we know exist in the real world. For the moment, the carbon cycle is acting as a negative feedback, as oceans and vegetation are removing some of our CO2 emissions (and we still stand a chance of getting back to a safe level). But as global warming continues, those carbon sinks are expected to fill up and start emitting CO2, as they have done during the glacial-interglacial cycles. If we warm the planet too much, we could trigger a release of methane (CH4) trapped on the ocean floor, with catastrophic effects. Eventually, excess CO2 is removed from the atmosphere by a negative weathering feedback, but this takes hundreds of millennia.

You’ll find some discussion of greenhouse gas feedbacks in a recent book review by Andy S, but for the moment I think it worth noting that the most important thing Hansen et al 2008 ignores is likely to make things even worse.

So, during the late Cenozoic the total climate sensitivity to greenhouse gases has been 6°C. Half of that is from fast feedbacks, and the other half from slow feedbacks. In the early Cenozoic when there was no ice on the planet, or in a possible future in which we’ve melted all the ice, there is no ice-albedo feedback and the climate sensitivity is 3°C. If you counted greenhouse gas feedbacks as feedbacks and not forcings, you’d get an even higher slow-feedback sensitivity.

Are slow feedbacks still as strong?

But will there be an equally large ice-albedo feedback on global warming today, now only the ice sheets of Greenland and Antarctica remain? To answer that question, Hansen et al extend their paleoclimate survey back to before the advent of ice in Antarctica, zooming out to look at the entire 65 million years of the Cenozoic. On this timescale the orbital cycles that caused the glacial-interglacial flips are mere noise on top of a long-term cooling trend. And as it turns out, that long-term climate change can only be explained by CO2.

Hansen et al take sediment core data and make one simple adjustment to derive global deep ocean temperature. (Specifically, the oxygen isotope ratios which are used as a proxy for temperature are also affected by ice volume, so they assume only half of the change during the late Cenozoic ice age is due to temperature.) The resulting record tells us the deep ocean temperature difference between the peak warmth 50 million years ago and the recent glacial periods was a whopping 14°C.

That breaks down into 8°C cooling until 35 million years ago, a 3°C difference between then and today, and another 3°C between today and glacial periods. The latter is noticeably less than the 5°C observed in ice cores, and we know why: we would expect deep ocean temperature to have changed less than global temperature in the icy late Cenozoic as it approached the freezing point. Thus Hansen et al assume the 3°C difference between 35 million years ago and today also translates to about 5°C globally. The relationship is less clear for the ice-free early Cenozoic, so for the 8°C they allow a conservative range of ±50%.

Using the values of fast-feedback and slow-feedback climate sensitivity derived from the Quaternary glacial-interglacial cycles, Hansen et al calculate the total change in climate forcing required over the 50 million years of cooling. The ice-albedo feedback accounts for about half of the 10°C difference during the late Cenozoic, confirming their slow-feedback sensitivity estimate of 6°C, so only about 7 W/m2 of original forcing are required over that period. Assuming the 3°C fast-feedback sensitivity for the ice-free period, the forcing that caused the earlier 8°C cooling was 11 W/m2, give or take a few W/m2.

What was the forcing? The ice-albedo feedback contributed to the late Cenozoic cooling, but something caused it. The continents were close enough to their current positions 50 million years ago that their effect on albedo was negligible. The Sun’s brightness increased by 0.4%, a forcing of just 1 W/m2 and in the wrong direction. However, CO2 levels fell from over 1,000 ppm in the early Cenozoic to merely 170 ppm in Quaternary glacial periods, approximately a factor of eight, or 12 W/m2 — the only forcing which even comes close to explaining the observed cooling.

As an aside, the reason CO2 varied so greatly was that continental drift affected the geologic carbon cycle: the imbalance of emissions from volcanoes versus absorptions from weathering and fossil fuel formation. I say geologic carbon cycle because these processes are far slower than the cycle between atmosphere, ocean, and vegetation that is important on human timescales. CO2 increased from 65 to 50 million years ago as India’s relatively rapid motion reduced sedimentation in what is now the Indian Ocean, but subsequently decreased as the rise of the Himalayas exposed new rock to the air. This natural CO2 cycle is of mainly academic interest, because we are now emitting CO2 thousands of times faster than volcanoes can.

Proxy records of CO2 are uncertain (the error bars are small for the recent past when CO2 was low, but very large at its peak in the early Cenozoic), but nevertheless the broad sweep of CO2 must have been mainly responsible for Cenozoic climate change, with perhaps some contribution from other greenhouse gases. So Hansen et al calculate the CO2 history that best explains the temperature history. In their chosen scenario (which matches the glacial-interglacial cycles and predicts a peak of 1,000-2,000 ppm 50 million years ago, within the broad range of proxy-based estimates), CO2 was about 450 ppm just before Antarctica became glaciated. 35 million years ago 450 ppm was the freezing point, but if we pass it in the opposite direction it will be the melting point.

The greenhouse gas forcing and global temperature in the current interglacial is about halfway between the Quaternary glacial periods and the formation of the Antarctic ice sheet 35 million years ago. That means the slow albedo feedback is still very much in play. It means we can look forward to much more warming in the pipeline than previously thought. And it means 450 ppm, if sustained long enough for slow feedbacks to take effect, would eventually return the Earth to an ice-free state, raising the global sea level by 75 metres.

How much warming is in the pipeline?

The forcing associated with the dramatic human-caused CO2 spike since 1750 is about 1.8 W/m2 (and rising by 0.2-0.3 W/m2 per decade). However, as yet the climate has responded to only part of this forcing. We know this because the Earth is still gaining more heat than it is losing. This global energy imbalance tells us there is still warming in the pipeline.

The delay is caused by two sources of inertia in the climate system: the oceans and the ice sheets. Only the former is included in the climate models which IPCC projections are based on. The oceans warm quickly at first, reaching the first third of their response within a few years and the second third within a century, but take over a millennium to fully respond. The oceans are thus “hiding” about 0.6°C of future global warming. However, the long-term sensitivity of 6°C implies that the slow ice-albedo feedback will contribute another 1.4°C, making a total of 2°C.

To put this in perspective, 2°C of further warming is enough to take us back to the Pliocene several million years ago, when sea level was 25 metres higher. Such a climate has not existed since before the evolution of humans.

How slow are slow feedbacks?

One of the scariest parts is that “slow feedbacks” may not be as slow as everyone used to think. Although in the past ice sheet collapses have taken millennia, perhaps that was only because orbital forcing changed very slowly. Perhaps ice sheets could melt faster if the climate changed faster. You only have to look at the glacial-interglacial cycles to see that ice sheets can melt faster than they build up. And though it takes a lot of energy to get ice sheets moving, once they are in motion they can collapse rapidly.

In the past, sea level changes of metres per century were not uncommon; instead it is the stability of the Holocene that is unusual. In a particularly dramatic example 14,000 years ago, the sea level rose 20 metres in just four centuries. Even during the last interglacial 125,000 years ago sea level was not as stable as once thought, apparently varying by several metres. In the present, we observe the ice sheets shrinking “100 years ahead of schedule” — the IPCC expected them to grow during this century! The fact that ice sheet models do not predict these events seen in the real world suggests they are missing important positive feedbacks.

If the ice sheets can begin to respond on the timescale of a century or so, then the “slow” warming in the pipeline has near-term implications. Human civilization developed with the relatively stable sea level of the last seven millennia. More than a billion people currently live within 25 metres of sea level. Yet once an ice sheet begins to collapse there is no way to stop it from sliding into the ocean. We would be subjected to centuries of encroaching shorelines. But this tragedy we have set in motion can still be prevented, if we reduce CO2 before it is too late.

So where does the 350 target come from?

Humanity has become the driver of the Earth’s climate — human forcings are now far greater than natural ones — but that doesn’t mean we can control it. Unfortunately the climate system contains tipping points, beyond which the climate change we started would spiral out of our control.

The good news is that the inertia in the climate system means that even if CO2 has passed the “tipping level” (say, 350 ppm) for a given tipping point (say, an ice-free Arctic), we may not yet have passed the “point of no return”. The bad news is that nobody knows exactly where the point of no return is, and we probably won’t know until we’ve already passed it. Hypothetically at least, we might still be able to prevent a tipping point by bringing the global climate back into energy balance before it has time to fully respond.

As well as the paleoclimate-based estimate of warming in the pipeline, many of the changes currently unfolding confirm the conclusion that we have already exceeded the safe level of atmospheric CO2. Hansen et al estimate that restoring energy balance is necessary to save the Arctic sea ice (if it’s not already too late); to stop the expansion of the subtropics which will cause desertification in places like Australia; to prevent glacier loss which will cause water shortages; to relieve coral reefs from the twin stresses of global warming and ocean acidification; and of course to stabilize the ice sheets. All these problems are already beginning to occur, many faster than predicted.

How do we get the planet back in energy balance? The problem of setting a target is complicated by the existence of many other human effects on climate besides CO2, but CO2 is clearly the dominant one. It is the largest and fastest-growing forcing. The non-CO2 forcings roughly cancel out anyway: the warming effect of other greenhouse gases is offset by the temporary dimming effect of reflective particle pollutants (though the latter is not known with satisfactory precision). In the long run, CO2 is most important for the warming in the pipeline from slow feedbacks, because it has the longest lifetime in the atmosphere. Whichever way you look at it, CO2 is the main event.

So now we finally arrive at the central conclusion: a long-term target for atmospheric CO2. To restore the planet’s energy balance, we need to reduce CO2 to less than 350 ppm. The 350 number refers to CO2, not CO2-equivalent, for the reasons explained above. This is not to say other forcings should be ignored, but controlling them would not make much difference to the long-term CO2 target. The recommendation may be revised as we obtain better measurements of the total forcing and resulting energy imbalance, but 350 ppm provides a useful benchmark for the scale of action that is needed.

Can we get back to 350?

If Hansen is correct and ice sheets can respond faster than has been assumed, then his long-term CO2 target has near-term policy implications. We need to get CO2 back to 350 ppm as soon as possible. We still have a window of opportunity to get back to 350 ppm, but that window is rapidly slamming shut. Stabilizing the CO2 level will require rapidly reducing global emissions until carbon sinks can absorb carbon faster than we emit it. Hansen et al argue the only realistic way to reduce emissions fast enough is to phase out coal.

Why target coal? Because CO2 has such a long atmospheric lifetime, we must leave most of the remaining fossil fuels in the ground if we are to have any hope of achieving the 350 goal. Of the three conventional fossil fuels (coal, oil, and gas), coal has by far the largest reserves. The phaseout of coal needs to include any conversion of coal to oil or gas — using up coal reserves at a slower rate would make little difference, because the carbon would still build up in the atmosphere and much of it would stay there for a very long time. Remember, carbon sinks have limits. The fundamental problem is with the coal being burned at all.

Hansen et al calculate that if we phase out coal by 2030, CO2 could peak at around 425 ppm in 2050. Their scenario demands that we also not burn unconventional fossil fuels like tar sands and oil shale, whose reserves are as yet untapped but thought to contain even more carbon than coal. What about conventional oil and gas? There is dispute among energy experts over exactly how much oil and gas is left. Some think we’ve already burned about half of the available reserves and thus production must peak soon, while others argue there is more oil and gas if we want to go to the effort of extracting it. If the former is correct, or if the latter is correct but we leave the least accessible oil and gas in the ground, CO2 could peak at just 400 ppm as early as 2025.

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. Natural carbon sinks would absorb about 25 ppm by the end of the century. Forestry and soil policies (for example, net reforestation by 2015) might be able to wipe off another 25 ppm.

It won’t be easy but it appears to be still possible to get back to 350 ppm by century’s end. On the other hand, if unlimited coal-burning continues for even one more decade, CO2 can be expected to remain in the danger zone for a very long time.

 

Conclusion

Global warming is an increasingly urgent problem. It doesn’t appear that way because of the inertia of the climate system and the slowness of slow feedbacks. But we must act now before we push the climate beyond a tipping point where the situation spirals out of our control. As climate blogger Joe Romm likes to say, the time to act is yesterday.

Fast-feedback climate sensitivity is 3°C, but slow-feedback sensitivity is as high as 6°C when there are ice sheets on the planet, as there are today. Even worse, those slow feedbacks may not be nearly as slow as we used to think. This means there is a large amount of warming already “in the pipeline”, though it is not yet too late to prevent it. To do so we cannot avoid targeting the largest, fastest-growing, and longest-lived forcing; a greenhouse gas which has been a major cause of climate change over geologic time: CO2.

A CO2 level of 450 ppm (a target which Hansen himself had previously proposed) would eventually melt all the ice on the planet. Both observations of the climate change currently underway, and the paleoclimate-based estimate of slow-feedback sensitivity, suggest even the current level of 390 ppm is too high. If CO2 is at or above its current level for too long, it will eventually result in a planet unlike the one on which humans evolved: a planet 2°C warmer and with sea level 25 metres higher. Imagine waves crashing over an eight-storey building. It is hard to dispute that this would be “dangerous” climate change.

To stabilize the climate, we must return the Earth to energy balance. And in order to do that, we need to reduce CO2 to 350 ppm, as soon as possible. To meet this target we must leave most of the remaining fossil fuels in the ground. We need to 1) rapidly phase out coal (including coal-to-liquid-fuels), 2) not burn the tar sands and oil shale, 3) not burn the last drops of oil and gas, and 4) turn deforestation into reforestation. And we must hurry: one more decade of business as usual would make this goal practically impossible. If we fail, we face an uncertain future.

I’ll leave the final word to Hansen et al, whose concluding statements are 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.

2011-02-10 03:44:20excellent
dana1981
Dana Nuccitelli
dana1981@yahoo...
38.223.231.252

Great post James, very interesting.  It would be nice to create both basic and intermediate versions, and perhaps publish the intermediate as a blog post (so that you don't have to leave out too much, but it's a more reasonable length).

I'd recommend being a little more precise about the doubled forcing (3.7, rather than 4 W/m2). 

When you say "Climate sensitivity is the amount of global warming you get from doubling CO2 (or an equivalent forcing..." it may also be worth noting that different forcings have different efficacies, so the sensitivity isn't just dependent upon the forcing, but the efficacies aren't terribly different.  You can just reference the advanced climate sensitivity is low rebuttal, where I discussed the issue.

2011-02-10 05:03:14Very thorough
Daniel Bailey
Daniel Bailey
yooper49855@hotmail...
97.83.150.102

If you're looking for a powerful graphic to depict the "sweet spot" of climate stability allowing humans to develop civilization, I've previously used the following graphic from Climate Progress with great effect:

 

 

2011-02-10 08:58:29
Riccardo

riccardoreitano@tiscali...
93.147.82.126

Good idea and good job.

As a general comment, you should mre clearly underline that this is not a forecast and not even a projection; it's just a possibility, a risk we're facing. The same apply for the timing. The examples of past collapses of ice sheets mostly come from glacial times with much more ice over the continents at lower latitudes than now. We don't know if Greenland and Antarctic ice sheet will show the same rapid collapse; again, it's just a possibility.

I think you could make it shorter, even this advanced version.

2011-02-11 19:06:50How we should roll this out
John Cook

john@skepticalscience...
124.186.217.214
James, this is an Important post so we should do it right when rolling this out. Can I suggest the following:

Make this a rebuttal to an argument. So someone find an example of a skeptic article that this is a response to. Something arguing that we don't need to restrict CO2 limits perhaps? Maybe there's an argument already in our database? By making this a rebuttal, we can do multi-levels.

next, write a Basic version of this. This can be the intermediate version (although it's borderline advanced). And I think the Basic version should be the blog post. Plus you should write it with the goal of getting this in the Guardian.

If all goes to plan, will be a nice thing to casually mention to your lecturers :-)

2011-02-12 05:00:12rebuttal
dana1981
Dana Nuccitelli
dana1981@yahoo...
38.223.231.252

It's tough to pinpoint exactly which argument this would be a rebuttal for, but maybe "CO2 limits will make little difference"?  Except that argument tends to be more specific - like Monckton telling Australians that they only produce 1.5% of global emissions, so their CO2 limits would make little difference.

It's tough because this is a long-term issue, and "skeptics" tend to ignore the long-term.  I think we'll have to come up with a new skeptic argument for this one, like "CO2 is nothing to worry about" or something along those lines.

2011-02-12 16:36:27Which rebuttal?
James Wight

jameswight@southernphone.com...
112.213.148.195

Maybe “It’s not urgent”?

My only reservation about using this for a rebuttal is that “dangerous” is to some extent subjective, so hypothetically you could choose not to accept the implicit value judgements made by the authors of the Target CO2 paper. Eg. it will take far longer than a century for the full impact of slow feedbacks to be felt so you might say the timescale is not worth worrying about; the risk of significant ice sheet feedback this century is unknown so you might dismiss that possibility as too unlikely to worry about; etcetera.

Dana, I don’t think it’s really necessary to go into efficacy of forcings and so on – the article is already long enough and it doesn’t make a difference anyway.

Daniel, I’m not sure about the “sweet spot” graphic because the “future” part doesn’t exactly correspond to what I’m talking about in this post.

Riccardo, I will think about your points.

I am working on the basic version and will get back to you soon.

2011-02-13 07:18:56not urgent
dana1981
Dana Nuccitelli
dana1981@yahoo...
69.230.107.233
Not urgent might work because if we're going to get down to 350 ppm, we need to start reducing emissions immediately.  You'd have to make that point in the rebuttal though, but that would be a simple addition.  The other good thing is that nobody has tackled 'not urgent' yet, so you could do at least 2 versions for it without stepping on any toes.
2011-02-19 01:48:16Intermediate version
James Wight

jameswight@southernphone.com...
112.213.164.22

Okay, here is the basic intermediate version. It's 1,900 words long, which is probably a disadvantage if we try to get it into the Guardian, but I couldn't bring myself to cut it down further. I've thrown in a couple of images though presumably they wouldn't be included in a Guardian version. Let me know what you think.


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. But 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. How do we know this? And what should we do about it? Read on.

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 non-icy one). 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 [add link to advanced version] 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, slow feedbacks double the warming predicted by climate models.

 

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 slow 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 largest, fastest-growing, and longest-lived climate driver: CO2.

Under business as usual, we are heading for up to 1,000 ppm by 2100 (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.

If we do all these things CO2 could peak around 400 ppm as early as 2025 and return to 350 ppm by century’s end. Personally 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 [add link to advanced version], or you can read the full paper for free here. But I think 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.

2011-02-19 21:48:32Comments on shorter version
John Cook

john@skepticalscience...
144.131.205.143
James, the shorter version is great but it's such an important topic, I think it still needs to be shorter. I would make your original version the advanced rebuttal of "it's not urgent", this 'shorter' version could be the intermediate version and and even shorter version would be the basic version and also the blog post that we try to get into the guardian.

I'm not sure of appropriate length. Maybe a max of 1000 words? Anyone have a clue of typical word lengths of Guardian articles?

I would suggest that the treatment of climate sensitivity needs to be simplified. I would just talk about the physical processes in an intuitive manner. Perhaps even get through that part without even using the term climate sensitivity. You can communicate the same message just by saying if we double co2, fast feedbacks like water vapor & sea ice will warm us up to 3C, slow feedbacks will amplify the warming another 3C.

2011-02-19 23:29:081,000 words could be tricky
James Wight

jameswight@southernphone.com...
112.213.164.22

Now I understand how Peter Hogarth must feel! Brevity is definitely not one of my talents.

My original intention was to shorten it from 4,000 to 1,000 words, but somehow it ended up being nearly 2,000 words. I managed to shorten the most technical parts a lot, but I’ve also tried to make other parts more accessible, which can sometimes lead to adding more words for context. I’ve also tried to emphasise key points, but it can be hard to find a balance between simplifying until it sounds alarmist, and droning on without getting the message across.

Not sure I want to drop the term “climate sensitivity” – then I wouldn’t be able to use my diagram which I was quite pleased with. Okay, I admit that’s a rather selfish motivation.

Anyway, I’ll see what I can do...

2011-02-20 07:33:10Pic
John Cook

john@skepticalscience...
144.131.205.143
BTW, I love that feedback pic too! :-) But you could still use that without 'climate sensitivity' - in fact, that pic is an example of what I was talking about - you're explaining the physical processes without getting bogged down by terminology.
2011-02-22 17:26:23Draft basic version
James Wight

jameswight@southernphone.com...
112.213.164.22

Cutting words is proving to be nearly as hard as cutting carbon. Anyway, here’s a 1,382 1,366-word version for the forum to pick over:


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 [add link to advanced version] short, Hansen found the slow ice sheet feedback doubles the warming predicted by climate models (ie. 6°C per CO2 doubling).

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.

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 [add link to intermediate/advanced version], 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.

2011-02-23 03:53:59comments
dana1981
Dana Nuccitelli
dana1981@yahoo...
38.223.231.252

A few suggestions:

I think you could cut out this sentence: "Don’t be fooled by the small number – in many countries, 390 ppm is over the legal BAC while driving."  If you don't, you should define the acronym "BAC".  But if you just want to express that this is a large amount, you could add that it's a 40% increase.

"but exclude longer-term “slow feedbacks” like ice sheets. (Ice is a feedback because an icy surface reflects more heat than a non-icy one.)" => "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 approximately 3°C"

"The global climate has warmed only 0.7°C," => The planet's surface has warmed only 0.8°C,"

Link for warming in the pipeline: http://www.skepticalscience.com/monckton-myth-10-warming-in-the-pipeline.html

Link for "dangerous" climate change: http://www.skepticalscience.com/monckton-myth-5-dangerous-warming.html

2011-02-23 14:16:51
James Wight

jameswight@southernphone.com...
112.213.164.22
Okay, have made some of the suggested changes.
2011-02-23 19:24:45
Chris Colose

colose@wisc...
69.71.240.56

This is a good post.  A couple of things:

 

-- It's not at all obvious that the ESS is the same as the fast-feedback response with no ice, since ESS incorporates a number of other slow-feedback processes such as vegetation/hydrology that can impact sensitivity. The number of feedbacks relevant on these long timescales and the precise way they interact with the surface and top of the atmospheric energy budget is still a topic that deserves more research. Slower carbon-cycle feedbacks (methane releases for example) aren't often included in the definition but they are relevant as well on these timescales

 -- In discussing the 350 ppm or 450 ppm threshold, it's worth emphasizing that this is rather speculative and subjective, and the idea itself is based largely on the Miocene initiation of Antarctic glaciation.  It's quite possible that you'd have to keep concentrations that high for thousands of years for some major slow-feedbacks to occur, so you can surpass these limits for a century or two without inevitably hitting a "tipping point" (whatever that means).  It's also not generally true that the CO2 threshold to glaciate Antarctica is the same CO2 threshold to deglaciate Antarctica (see for example the bifurcation discussion in my post on the no-CO2 atmosphere I just put on here for review).  From a policy standpoint these type of threshold numbers might be meaningful, and you can talk about them in a scientific context, but they are very loose numbers, and it's just worth driving that point home.  It's not like CO2 levels leveling at 449 ppm for 1,000 years is just fine but 450 is "dangerous."

2011-03-01 23:35:57Hi, Chris
James Wight

jameswight@southernphone.com...
112.213.166.156

You make some good points, but unfortunately there’s only so much information I can cram into one post.

2011-03-07 14:56:01Can't think of how to trim it further
John Cook

john@skepticalscience...
124.186.229.6

You've done well, I would suggest populating the basic/intermediate/advanced rebuttals then turn this into a blog post. Good work, James, a helluva lot of work went into this!

2011-03-09 15:44:20Heard from Guardian
John Cook

john@skepticalscience...
124.186.229.6

Sorry, James, they passed on this one, said it had some overlap with their FAQ section.

Dang shame, I was hoping you could say to your climate science lecturers, "I've published a climate science article in the world's second largest newspaper - have you?". Of course, they'd probably deduct marks for being a wise guy but still would've been cool. But there'll be other opportunities.

How about I email this one to Treehugger then?

2011-03-09 15:50:19Okay
James Wight

jameswight@southernphone.com...
112.213.166.156

Okay.