Help me here: defining dangerous climate change

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Don't you recognize that it's the best time to receive the mortgage loans, which can make your dreams come true.

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Probably the current IPCC impacts chapter is the best source (not that I've read it yet, though).

AFAIK, or can guess, changing precip patterns in areas with large numbers of subsistence farmers will be the highest human cost and body count from AGW, more than intensified hurricanes.

It sounds like you're saying the if endangered species wouldn't be in trouble from AGW alone, then it doesn't matter that they're in trouble from AGW together with all the other human impacts. If that's the argument, I disagree with it. Many species extinctions have multiple causes, not just a single or even a primary cause.

Ultimately this is about drawing a line somewhere in a gray area, and I think it's impossible to be completely scientific about it. That's what policy is about.

I would find a 20 Celsius degree overall warming to be somewhat dangerous because the temperature would then be mostly higher than the body temperature and a lot of air-conditioning would be needed.

Additional information http://www.InteliOrg.com/co2_climate_change.html

There was a conference on "Avoiding Dangerous Climate Change" in Exeter in 2005.
http://www.stabilisation2005.com

DEFRA have published a monstrous book from the proceedings.
http://www.defra.gov.uk/environment/climatechange/research/dangerous-cc…

I've not read it myself, but there's a preface from Tony Blair. So it must be good.

Cheers, Motl. :-)

1. 2 C globally is what in the arctic?
2. ocean acidification (see, for example Science 316 678 2007)
3. sea level rise, already at the high end of the IPCC 2001 models (see Rahmstorf, et al p 709 same issue).

Even the IPCC has doubts about cost/benefit of mitigation at any CO2 level.

From SPM3:
"Limited and early analytical results from integrated analyses of the costs and benefits of mitigation indicate that these are broadly comparable in magnitude, but do not as yet permit an unambiguous determination of an emissions pathway or stabilization level where benefits exceed costs [3.5]."

I derive some entertainment from desperate, pathetic nutters such as Dr Coles and Steve Reynolds. Keep it up guys.

Best,

D

Shishmaref to you oh furry Stoat.

The chapter by Rachel Warren (Chap 11) in Avoiding Dangerous Climate Change (free download from Defra, I think - I'm away from my main machine) is a good overview of degrees of impact versus degrees of warming.

Hansen did a paper (end of last year?) where he discussed definitions of dangerous change.

But Brian has it right. Ultimately the answer is as much about political practicality as it is about scientific certainty. Consider the negotiations about post-Kyoto emissions targets. Do they go for an aggressive 450ppm to give us a good chance of 2-2.4C max rise, or 550 ppm and 2.8C (see WG3 SPM)? There's no "scientific" answer to that question. Science feeds into the answer, but cannot be the sole determinant.

The full "Avoiding Dangerous Climate Change" can be found here:
http://www.defra.gov.uk/environment/climatechange/research/dangerous-cc…

The Hansen paper Gareth mentions was from late 2005, and it's an essay called "A slippery slope: How much global warming constitutes 'dangerous anthropogenic interferenced'?":
http://www.columbia.edu/~jeh1/hansen_slippery.pdf

It makes sense that there is a *desire* to define and frame what dangerous climate change is and what climate change is associated with incurring it. It's after all what the big question is - because in theory if we knew it, it would reveal what we need to do to avoid it. Of course, because of the uncertainties, non-linearities, and risk of large scale singularities associated with climate change, we are generally stuck. The EU, however, still wants to go for a 2C warming limit, even though such a temperature target gives us little or no guidance as to how much abatement we need to undertake in the next few decades. Such a target is politically pleasing to have, I suppose - and as much as I like to, this factor shouldn't be dismissed.

WBGU were among the first (initially back in 1995 I think) to say that anything above 2C warming would be "dangerous". Oppenheimer and Petsonk have an article in Clim Change that outlines the history of defining and setting "dangerous" threshold limits, link here. As far as what impacts are called "dangerous" in the literature, this ranges from the melting of the WAIS, coral bleaching, to permanent loss of X% of income in X% of the population (choose your own X!). Generally speaking, the literature suggests a temperature change of 2-3C above pre-industrial will cause such dangerous impacts.

I don't like hot summers myself, but thats because my environment is poorly adapted to them. My SW facing office windows have no external shading, nor does my house. Plenty of people already live in places far warmer, and appear to lead tolerable lives.

Yeah, but how tolerable would those lives continue to be after a couple of degrees of additional warming? Bearing in mind that temperature is correlated with the success of many insect species, which happen to be vectors for nasty diseases?

Pointing out that you can stand a couple of degrees warming because other people already tolerate worse is somewhat narcissistic - they're going to suffer it too, and they're a lot closer to the limits than you are.

However, I agree in broad terms about the difficulty of assessing what constitutes "dangerous climate change". It is, at best, a SWAG - scientific wild-assed guess. The rigourous approach would be to define any climate change as dangerous, since we know so little about the eventual outcomes.

[I knew that bit would get misunderstood. If it gets (year round?) 2 oC warmer than here, that puts me... somewhere in France, climatologically? Is that worse? Similarly, that place in France is now somewhat further south, and so on. Who is to say which of these is "worse"? -W]

The rigourous approach would be to define any climate change as dangerous, since we know so little about the eventual outcomes.

I would disagree. This would be a rigorous precautionary principle approach - but rather simplistic, because there are places (Canada, Russia, Scandinavia) that would likely see some benefits. This is not to say we can cancel out Bangladesh's woes with Manitoba's gains, of course. But we can't say any change is dangerous, given that there is always some natural variability.

A problem I have with these targets is that they divert our horizons to the long term, rather than to actually doing some - any - mitigation NOW. We have so much inertia built into our socio-economic systems that we have to start now doing at least a little bit - otherwise we're just making it harder for ourselves down the line. (I co-authored paper in the Avoiding Dangerous Climate Change book about this - apologies for the plug, ha.)

[Oh yes, I forgot the winners-and-losers bit, but its important: why should, say, Siberia struggle to limit its GHG emissions to avoid climate change that would be beneficial to it? -W]

How about using the word risky, as in somewhat risky, risky, very risky and insanely risky.

Steve Schneider has been going around giving talks arguing that "dangerous climate change" cannot be defined in an unambiguous and objective way. The costs (or benefits) of climate change depend on the stakeholder, depend on which metric you choose (dollars, lives, species, quality of life, inequity ...), and depend on unquantifiable considerations such as ethics.

Much as I like precise and objective definitions, there is a lot of truth in that point of view.

Politically, I think that some artificial definition will have to be made for the sake of having something to agree on, but I don't know what that ultimately will be.

I am reminded of the quote, "Since it is generally impossible to measure what is important, bureaucrats instead turn their energies toward making important what is measurable" (J. M. W. Slack, Egg and Ego).

why should, say, Siberia struggle to limit its GHG emissions to avoid climate change that would be beneficial to it? -W

From an economist's point of view, the answer is simple: they're overusing a public good (the atmosphere), and there are consequent negative externalities affecting others. Hence, it must be limited/stopped. Whether they agree is another question...

What's dangerous? Nothing, looking backward as a survivor or descendant of survivors.
The past is dead, the costs are sunk. "If only" isn't part of the accounting process.

Look at what we're beginning to learn about the cascade of changes when ecologies are disturbed:

"... the decimation of various tribes throughout North and South America through epidemics of European illnesses. The massive reduction in human population had serious ramifications for other native species, in ways that are still emerging today.

"The essential emptying of two continents created a major imbalance in the food web and altered the very landscape. Many plant and animal species that were traditionally consumed by humans ballooned to abnormally high levels, known as "outbreak populations." Among these species were several types of oak, bison and of special interest and importance, the passenger pigeon. ...

The outbreak species were the ones as European hominids followed European smallpox across the continent --- smallpox found what now appears to have been a stable ecology under human management; once smallpox removed most of the native humans, the species they had kept limited like bison and passenger pigeons exploded. The 'natural bounty' of the Americas was the short term outbreak of such species. There were so many bison and passenger pigeons, no one could imagine a limit to them.

http://www.mongabay.com/images/external/2005/Bison_skull_pile-1870.jpg

What's dangerous?

Blind ignorant chanage without a baseline study. Disturbance. House of cards.

Tha'ts what's happening. Half the songbirds are gone since 1950. Half.

http://www.bayjournal.com/article.cfm?article=337
"the Breeding Bird Survey, conducted since 1966. The Breeding Bird Survey documents the number of each species noted along specific survey routes. By analyzing the data, researchers can tell whether a species is declining or not. Though some annual changes may seem small, the effect over time can be devastating. For example, a bird species declining at a rate of 2 percent a year translates into a decline of more than 50 percent over 25 years.

"Looking at the data collected in the past 29 years, researchers have documented the changes in bird populations in the Chesapeake Bay watershed. The yellow-billed cuckoo, gray catbird, least flycatcher and Baltimore oriole are declining at less than 1 percent per year. The wood thrush, whip-poor-will, Eastern wood peewee, chipping sparrow, barn swallow and scarlet tanager are declining between 1 percent and 3 percent a year.

"More startling is the decline in warblers. Many of these small, colorful birds, favorites of bird watchers, breed in forests and swamps. The golden-winged warbler is declining at an average rate of 7 percent a year; the black-and-white warbler between 4 percent and 5 percent; the Cerulean warbler by more than 4 percent; the black-throated blue warbler by more than 3 percent; and the Prothonotary warbler between 2 percent and 3 percent. "

Per year.

That's good news for the pesticide companies, since they never found a way to sell songbird services.

Oh, too bad about the honeybees. Fruit and vegetable pollinators? Oh, we can't stop spraying; too many bugs around. Maybe we better save that corn from the gasahol folks, it's wind pollinated.

Or, if you like people :^j

http://earthobservatory.nasa.gov/Library/GlobalWarmingUpdate/Images/sea…

William: most of the papers on the subject start with the problem of what 'dangerous' even means. The Hadley study does a reasonable job of this.

Hansen's latest is the ACP article from last week, which I comment on at the 'cave'. He seems to define dangerous as 'likely to cause runaway feedbacks', by implication, at least.

If by 'dangerous' you mean 'increasing the likelihood of vulnerability to injury, damage or loss', and if the picture is holistic in scope, the answer to the question 'how much warming is dangerous?' must be 'very little indeed.'

But in this case, the answer is provided by the definition, so it isn't very helpful. Perhaps a risk specialist can give actual numbers for the percentage by which a perceived risk must rise for the level of risk to become 'dangerous' as opposed to, say, merely 'risky'.

As has already been pointed out, it also depends on whether you are going to fix on human risk or environmental risk (not that they are truly separable), on direct or indirect risk, on economic loss, or loss of life or livelihood.

The only useful definition I can offer at the moment for 'dangerous climate change', or here, dangerous warming, is 'any level of warming at which point, or beyond, the lives or livelihoods of a group of people, or the survival of a key component of an ecosystem, is placed at imminent peril, beond their immediate ability to respond or adapt.'

However, this may be extended, a la Hansen, to 'a level at or beyond which a chain of feedbacks might be set in progress, the unavoidable consequence of which...etc.'

The answer to the first might well be 2C (above pre-industrial, or present? - you don't specify), the answer to the second is unknowable, but might be less, or more. The use of palaeo exemplars gives rise to the assumption that 1.5-2C is comfortably within this definition.

"why should, say, Siberia struggle to limit its GHG emissions to avoid climate change that would be beneficial to it?"

Would it? Rhetorical?

I spend my life trying to remember to use words such as variously qualified 'likelys' to quantify anthro climate change doubt and here we have a 'would be'? This is the kind of certainty we get from contrarians :( like 'it's the Sun', 'it's UHI' - but we don't know that. Surely we don't know it 'would be' benificial? Melting permafrost? More mozzies? Changes to forests? And perhaps drying bogs? All 'would be' benificial?

Perhaps my views are going out of fashion, but I, if I could, would leave this little planet as I found it or better. Ok, I know it's not going to happen, but it's what I'd try to do if it were possible. Radically changing Siberia is an experiment, just like our big one with the atmosphere, it's damn silly imo.

[OK, I didn't mean to state that with certainty. But its one place often mentioned as likely to gain from warmth. The idea remains -W]

If you want a British-sourced answer to your question, try my post which links to scientists' documents on what constitutes 'dangerous' in terms of climate change (as understood in early 2005), with the way the language was used by politicians to influence G8 discussions back then (Mike Hulme's comments) and the overuse of overly dramatic adjectives in the media since then, and how the impacts of climate change are being reported and explained, degree-by-degree Celsius, in Mark Lynas' book today (which sounds like the answer to your question).

Dear William,

maybe You should pay some attention to this document (written by Rik Leemans, dutch prof. of climatology):

http://assets.panda.org/downloads/extremeweatherreportdoesnaturekeepup…

From the summary:

"This study indicates that it is likely that the IPCC 'reason for concern' level of 2°C is actually too high. Even with small changes, there will be disproportionately large changes in the frequency and magnitude of extreme events and consequently unpredictable but devastating impacts on species and ecosystems with even a moderate climate change increase of 1 to 2°C."

- I think it is worth of diggiung through,

good reading, best

Alexander

Or something is also here:

http://www.oecd.org/dataoecd/6/29/2483789.pdf

"...ecosystem model comparison (Cramer et al., 1999), where several models were used to simulate the impact of a standardised climate change scenario. This scenario
simulated a global mean temperature increase over 4 oC. Somewhere between 2 oC and 3 oC the analysis shows that soil respiration starts to dominate over photosynthesis, which leads to a rapid decline in NEP."

- so I think, to limit the warming up to 2C is reasonable ;-)

It seems to me the ocean pH is going to hit hard before any of the warming happens. In fact regardless of whether warming happens; even faster if we go with the smoke-and=mirrors (that's the orbital sunshade and sulfate-cloud) engineering route. Are you including this in what you're considering? I don't hear much from the biologists, all told. I don't understand why they're not better represented in scenario-building yet.

I would disagree. This would be a rigorous precautionary principle approach - but rather simplistic, because there are places (Canada, Russia, Scandinavia) that would likely see some benefits.

They may see some benefits, they may see some disbenefits, but if you don't know what is going to happen, then there is always a degree of danger. Any situation where you cannot predict the outcome of your actions is dangerous.

But we can't say any change is dangerous, given that there is always some natural variability.

Are you seriously arguing that natural variations cannot be considered dangerous? So if a perfectly natural flood happens to drown a few million Bangladeshis, they're not really dead?

I think precedent is quite an important underpinning idea informing people's opinion as to what is dangerous climate change. Basically, if we are out of the past ranges, and do not properly understand the system, something unpredictable might happen. We may not know how likely it is, it might be zero percent or 90% likely, we may not know what it'll actually be, but we do know that if it didn't happen for millions of years, while conditions were stable, we ought to have a fair shot at continued stable conditions, if we don't stray outside the range of past experience.

I think this type of argument is interesting and valuable, but does not exactly lend itself to scientific quantification.

Now, if we knew that with 3C of climate change, all bees might die with 20% probability and therefore we humans were to follow 4 years later with 90% probability ;-) we'd have something pretty hard. However, we don't and so far none of the consequences of climate change we can predict is serious enough to make economic growth impossible, let alone cause the end of humanity.

The eco-system stuff is interesting, and for people who worry about species or eco-systems for their own sake that may be enough reason to say make do without a car, but the link with human welfare seems to me, largely, pretty speculative.

-----

And as you are mentioning polar bears, the Economist has an interesting story on the subject of species inflation.

They inform us that "polar bears are just brown bears that happen to be white."

http://www.economist.co.uk/opinion/displaystory.cfm?story_id=9191545

And after a bit of digging, I find, they are right. Polar bears and brown bears can interbreed and produce fertile offspring, which by the classical definition of species means that polar and brown bears are the same species, just like black and white dogs or cats (or ... humans).

Heiko, I trust you, if not the Economist, must realize how outdated that "Noah's Ark" definition of species is? Look up population, population structure, population dynamics, biodiversity, variation.

"Could we rule out the risk of economic collapse if we restricted out CO2 emissions (this is, I think, from JA)."

Absolutely. Here's an easy way of getting an upper bound on the costs. Suppose we made a collective decision to replace all fossil fuels with solar (the most expensive of the renewables, but essentially unlimited in supply) by 2050, and had no technological progress, improvements in energy efficiency etc. Allowing for costs of storage and so on, solar might cost four times as much as the fossil fuel alternative. Since energy is about 5 per cent of output value, this would reduce income by 20 per cent.

If the average growth rate to 2050 under BAU, this would reduce it to 1 per cent; a substantial slowdown, but still growth rather than decline, let alone collapse.

Of course, this upper bound is way in excess of a realistic estimate. With a quadrupling of price, demand would certainly decline by at least 50 per cent, bringing the cost down to 10 per cent of income. Then there would be induced innovation and so on.

And that's for complete elimination of fossil fuel consumption, which no-one is proposing. A cut by 50 per cent relative to BAU would be less than half as expensive by the low-hanging fruit principle. So, even with highly pessimistic assumptions, it's almost impossible to get a cost estimate higher than 5 per cent of income.

Googling on species and the terms given by Hank Roberts didn't provide much useful material.

What's attractive about the old definition is that it is clear. If interbreeding giving fertile offspring isn't the criterion, what prevents us from declaring that there are several species of humans?

"If interbreeding giving fertile offspring isn't the criterion,"

That's just a necessary condition.

This would be a rigorous precautionary principle approach - but rather simplistic, because there are places (Canada, Russia, Scandinavia) that would likely see some benefits. This is not to say we can cancel out Bangladesh's woes with Manitoba's gains, of course.

The gains in Manitoba are likely to be minimal. Already communities in the north are facing difficulties. Lengthy roads (some more than 500 km) built across frozen lakes and bog are used to bring in heavy supplies like fuel and construction material. In recent years warmer conditions have delayed the construction of these roads and there has been problems in bringing in sufficient supplies. The only alternative is to fly everything in (they use aircraft from the 1940s). All-weather roads are being considered for some places but would be expensive because of the rocky terrain and the numerous bridges required.

Although agriculture may shift, with warming in the north and drying in the south, over most of the province the soils are poor and thin. The topography is uneven, very unsuited to modern agriculture. There is also the problem of the infrastructure being in the wrong place.

Heiko, use Google Scholar.

This may help, just to pick one anthology:
http://books.google.com/books?id=uanzAYFCeokC&dq=mayr+%22population+spe…
http://books.google.com/books?id=uanzAYFCeokC&pg=PA93&dq=mayr+%22popula…

Anecdotally, remembered from my college biology decades ago:

The first collection of butterflies made from Atlantic to Pacific across the new USA identified quite a few distinctly different species in one genus (it may have been the Checkerspot, memory fails).

Sometime in the mid-20th Century a young biologist named Ehrlich made the same transect, collecting the same butterfly --- but in between each of the points collected on the original transect.

The butterflies would also have been identified as distinct species, using the old criteria. But described taxonomically (this well before DNA analysis was easy) each collection also fell in between those made a century or more ago on either side.

What in fact was being described was a transect across local populations.

Have a look at the links above. Selection doesn't produce "species" -- selection changes the frequency of genes in populations.

The current great extinction --- our impact is already comparable to that of an asteroid strike -- is losing genetic diversity.

His butterfly work led Ehrlich to think furiously on this question -- what can we afford to lose before ecologies fall apart?

http://scholar.google.com/scholar?num=100&hl=en&newwindow=1&safe=off&q=…

...The classic rivet-popper hypothesis (Ehrlich and Ehrlich, 1981) pictures a madman randomly removing airplane rivets. Some rivets may not be crucial to the functioning of the airplane, but, at some point, removal of a rivet will cause the plane to crash. ....

You can find quite a bit on this if you read the ecological restoration literature.

Simple assertions of belief that we can continue losing biodiversity and trust that the ecology will still work have no support in the research. That's ideological wishful thinking you're hearing.

Stoat asks [Oh yes, I forgot the winners-and-losers bit, but its important: why should, say, Siberia struggle to limit its GHG emissions to avoid climate change that would be beneficial to it?]

Rabett replies: permafrost becoming a sometimes thing is an entirely different and nastier type of muck.

Astonishing how the world is perfect right now. So perfect that even a decrease in permafrost is regarded as a step downhill by the true believers. In the face of such insanity the only rational position is scepticism.

Ahhh, if only it were that simple, Mugwump. Can you tell me what conditions will bring about a decrease in permafrost?

>what prevents us from declaring that there are several species of humans?
Nothing stops you from "declaring" your opinion, of course. It's been done. Not good science though.

Nor is blog-flogging the "white bears are brown bears so who cares" talking point. It's PR, bad PR.

Species was a convenient taxonomic tool, it's a useful one as a snapshot in time.
Reproductive isolation is another term to look up. This is a fascinating area of science.

If you don't think natural selection and diversity is beautiful and worth understanding, well, I don't know how much biology you have, but some would help. Try E.O. Wilson's work, if you aren't familiar with his writing.

Here, from thirty years ago, a page from a sample of how to approach a complicated question that's still fascinating people today:
http://www.jstor.org/jstor/gifcvtdir/di000208/00129658/di960230/96p0014…

This might help:
http://www.molbio.wisc.edu/carroll/Endless_Forms.html

The Wikipedia article on species is quite helpful:

http://en.wikipedia.org/wiki/Species

Until I read the article in the Economist and did a bit of digging afterwards I hadn't realised how soft and subject to interpretation the concept actually is.

Part of what you're facing in this issue is politics. When the first serious attempt was made at the federal level in the USA to deal with environmental problems, protecting ecologies was anathema to many of them. The outcome was the protection of "endangered species" --- the cute ones, usually the obvious ones. The ecologists merely hoped that protecting "charismatic megafauna" would have the outcome of protecting the environment in which the cute fuzzy things lived.

The opponents to the idea of protecting the natural world focused then --- ever since, and very narrowly -- on arguing that any proposed "species" wasn't really a species, there were plenty more of them. There might be only a handful of green terwilligersn known or discovered when looking at the proposed new development property by the seaside, but there were zillions of orange terwilligers across the next mountain, over in the desert --- and of course they were all the same species so there weren't any endangered ones.

So we've had judges deciding that fish farm salmon are the same species as any and all of the many runs of wild salmon in rivers ---- so none of the wild salmon need protection. That made the company that owns the fish farms happy, and the developers happy, and the logging companies happy.

Not good for the environment at all.

Bringing us back to our charismatic fuzzy host's question ---- what needs protection, really?

And that's "White bears are brown bears so who cares." In a nutshell.

Nathan Rive implies that Manitoba would gain from climate change. Unfortunately, so far the evidence does not support this.

Communities in the north are heavily dependent on lengthy roads (some over 500 km) built across frozen lakes and bogs for bringing in heavy supplies such as fuel oil and non-perishable foods. In recent years freeze-up has been delayed and it has not always been possible to bring in everything by truck. The only alternative has been to fly in the supplies at great expense (air freight rates are considerably higher than they are between major centres).

Consideration is being given to building some all-weather roads but the rocky topography and numerous lakes and rivers would make this a very costly operation.

Agriculture is likely to shift, with the south becoming drier, negating the effect of the longer growing season, and the north warming. However, in the north the soil is generally thin or absent and the uneven terrain does not lend itself to modern agriculture. In addition, the infrastructure will be in the wrong place.

I already submitted a post with these points but for some reason it did not appear. I hope this has better luck.

So many Flanders and Swann moments at Stoat. As to why one should not wish for the permafrost to melt, among other things

Mud, mud, glorious mud
Nothing quite like it for cooling the blood
So follow me follow, down to the hollow
And there let me wallow in glorious mud
The fair hippopotama he aimed to entice
From her seat on that hilltop above
As she hadn't got a ma to give her advice
Came tiptoeing down to her love
Like thunder the forest re-echoed the sound
Of the song that they sang when they met
His inamorata adjusted her garter
And lifted her voice in duet

The permafrost will not bloom, it will turn to muck.

Lo and behold, snipped from the comments at Pharyngula, forewarning:
http://scienceblogs.com/pharyngula/2007/05/spare_biology_from_the_opini…
-------------
"... before 2008. One of the biggest things they can do is neuter the ESA.

So get ready to see some rather alarming decisions coming from the USFWS, but please keep in mind, they're not actually coming from the Service. The administration just writes what it wants and makes the Service put its name on it.

I'll give you a hint: The phrase "throughout its range" is a phrase you're going to hear a lot, and the number of species on the list is about to go down.
---------------

I had an interesting thought experiment a few days ago regarding dangerous climate change. Let's just for a minute assume (it's an experiment, remember!) that we would be about to plunge into an ice age within the next, say, two hundred years. We know the conditions on earth during an ice age rather well, so we would somehow know what to expect. Furthermore, we would have a lot of well-working climate models at our service to tell us the things we don't yet know. Global average temperatures were about to drop by 5 degrees Celsius until 2200. Now, would you consider this a "dangerous" change? My idea is that one can get a sort of crude comparison for what we are about to face in our warming world. If you think an ice age is not actually "dangerous", well, I suppose you would not expect global warming to lead to "dangerous" interference with human life. But if you think a thick layer of permanent ice above your head, given that you live somewhere in Northern Europe or Canada *would* be a dangerous thing, and mainly so for the social effects such a change has (not to forget William missing his English wine) - then your answer towards what constitutes "dangerous" global warming will surely be different.

Try this experiment out for yourself, I'd be keen to read your answers!

[It is potentially an interesting thought experiment, one thing perhaps that it tells you is that cooling is more obviously dangerous than warming :-) Disregarding the sea level rise of course -W]

I tend to think that something that might 'trip' a condition that would lead to a change of climate on the scale that that led to the Permian mass extinction - which seems to have been a climate change event, would rather 'dangerous'.

95% of species on Earth were wiped out - and I understand it took at 5C trigger to lead to the release of methane from the continental shelf which gave a total increase of 10C..

Or so one popular theory goes..

I suppose if your life is already compromised by climate change - as could be the case in Darfur or New Orleans - then you might consider we've already passed the threshold??