Climate Change Future

Grim scenarios on a 2 to 6 degrees celsius hotter Earth

NASA Earth Observatory, based on IPCC Fourth Assessment Report (2007)   




Temperature projections to the year 2100, based on a range of emission scenarios and global climate models. Scenarios that assume the highest growth in greenhouse gas emissions provide the estimates in the top end of the temperature range. The orange line (“constant CO2”) projects global temperatures with greenhouse gas concentrations stabilized at year 2000 levels. Source: NASA Earth Observatory, based on IPCC Fourth Assessment Report (2007)

A stark appraisal of the synthesis report of the IPCC 4th assessment (AR4) reveals some disturbing realities.

First, stabilisation scenarios indicate that to have a reasonable chance of avoiding 2 to 2.4°C warming, we will need to achieve global emissions reductions of 50 to 85 percent by 2050, relative to year 2000 output and a levelling off by no later than 2015. On a globally equitable basis, the burden on developed nations will be higher (80 to >95% by 2050) because of their disproportionately high per capita emissions. Second, there is no mitigation scenario proffered for <2°C, yet the model scenarios do take us into the intemperate realm of 4.9 to 6.1°C of planetary heating.

The IPCC, it seems, is not at all confident that global society will be able to implement the sort of wholesale socio-economic restructuring that is likely to be required to stave off substantial global warming this century. It seems, therefore, quite necessary to assess the likely implications of scenarios for a 2 to 6°C warmer Earth.  I did this recently in a speech to the Manning Clark House conference ‘Imagining the Real’, and followed this up at the 2008 JD Stewart Address I delivered to the University of Sydney. You can download the slides and listen to the audio here. My Climate Change Q&A seminar #4 on 19 September will cover similar ground, with a slightly different slant.

In brief, analogue climates from deep time may help in this “imagining the real” – when the real is something never before witnessed by humanity, or indeed, at its most extreme, by most species now occupying the planet.

The average lifespan of a species is 1 to 10 million years, and yet to approach conditions 4 to 6°C hotter than today’s climate, we must look back to the world of the Eocene, some 35 to 50 million years ago. The world was then a very different place – there was no permanent ice cap shrouding Antarctica, sea levels were considerably higher than today, and deserts were more widespread. The tundra and boreal forests were limited or non-existent.

The cooling descent into the icehouse conditions of the Quaternary, 1.6 million years ago through to the present, was a slow progression that took place over tens of millions of years – an unimaginably vast stretch of time. Although this slide from a Cretaceous greenhouse world towards a “modern climate” was punctuated by occasional rapid climatic reversals, the globally hot conditions of the deeper past were never again re-visited. Now, through the actions of modern civilisation, we risk returning to the Eocene (or earlier) within the geological wink of an eye – a matter of a mere century or two. How will Earth, and its diaphanous clothing of life, cope?

Simply put, even under the most stringent mitigation scenario proposed by the IPCC AR4 (scenario I, in which CO2 equivalent is limited to 445 – 490 ppm), and more recently by the Garnaut Review, there is a high confidence that a slew of what can only be described as catastrophic impacts (30 percent species loss, major coastal flooding, most corals bleached, significant global water stress), will unfold! Indeed, beyond the mitigation scenarios, the fossil-fuel intensive business-as-usual – right to the bitter end – runs off the chart of possible impacts, with a disturbingly plausible risk of up to 6.8 – 8.6°C heating. This would truly be “game over” for humanity and most other life on this planet.

In this context, I focussed in the JD Stewart Address on describing a range of potential future impact scenarios and tipping elements, under scenarios of 2 to 6°C warming. These include the instability of the large polar ice sheets, which threatens major sea level rise, possible collapse of major carbon sinks in the ocean and on land – exacerbating committed climate change, increased intensification of the hydrological cycle and the related problems of desertification and water stress, severe impacts on biodiversity and ecosystem services – interacting with ongoing stressors such as habitat loss, and erosion of the natural, agricultural and infrastructure capital that is required to support modern civilisation. I also presented an overview of recent scientific research in this area of extreme outcomes, and speculated on some of the more worrying “low risk, high consequence” tipping points that we risk breaching, beyond 1 to 2°C of warming.

One can only consider it a damning indictment on our collective vacillation, inaction and deliberate stalling to date, in facing up to this problem (Australia and the US being two historically prominent curmudgeons), that we are now facing the stark choice between a bad situation, a catastrophic situation, or a civilisation-terminating situation.

The European Union somewhat arbitrarily defined “dangerous anthropogenic interference with the climate system” (what we were supposed to avoid, according to the 1992 UN Framework Convention on Climate Change) as being anything over 2°C warming – and many claim this is already too much for comfort, given the changes we are witnessing already, at 0.8°C warming.

Given what is at stake, there is some dark humour to be had in contemplating that only 6 of 177 AR4 mitigation scenarios actually allow for the possibility of avoiding 2.5°C!

There is good news, however, if policy makers will just take heed. The costs involved in moving fast to address the emissions problem and avoid the catastrophic global scenarios that threaten to “awaken the Balrog” of 2 to 6 degrees of planetary heating, are incredibly small, or perhaps even beneficial overall, and that’s before we count the social and environmental cost of not taking action.

By Barry Brook

Barry Brook is an ARC Laureate Fellow and Chair of Environmental Sustainability at the University of Tasmania. He researches global change, ecology and energy.

25 replies on “Grim scenarios on a 2 to 6 degrees celsius hotter Earth”

Barry and others, one thing that always confounds me is the perception that “On a globally equitable basis, the burden on developed nations will be higher (80 to >95% by 2050) because of their disproportionately high per capita emissions.”

I totally agree with the concept that higher cuts will have to be made in countries with high emissions per capita at the moment, and that “permits” will need to be allocated in an equitable manner. But is it not true that the tradable nature of the rights to emit carbon means that a company in say the USA that still has a considerable investment in some infrastructure with significant operational life ahead may not find it sensible to purchase the right to emit carbon, with a nation looking to develop an industry selling its carbon rights and choosing to invest in a low-carbon technology?

This is especially true in say Australia with a low population but lots of resources that are in high demand. Our mining industry probably makes more $$$ per unit carbon than most, so they will be able to afford carbon permits on the carbon market.

So to me, national targets are also something that become quite redundant under a global ETS… with the initial allocations of permits being the real issue.

One can imagine less reputable dictators jsut flogging their nation’s carbon rights to buy a few more rolls-royces while the population replains in poverty…


Barry, could you please elaborate on why 6C above the pre-industrial level will result in ‘near-worldwide deserts’, even though equivalent temperatures in the Eocene didn’t? In fact, if anything the opposite; I suspect there’s a good reason why it was called the ‘Eocene Optimum’ rather than ‘Eocene Maximum’.


Mark Duffett @2, I’m in transit and so can only answer briefly – I will detail this tomorrow. In brief, it is due to the rate of change – a shift in the planetary climate of 6C over 10 or even 1 million years, in the absence of other perturbations, allows plenty of scope for adaptation. So a planet 4-6C warmer than at present might resemble a Cretaceous or Eocene world at equilibrium. But we are going to return to these conditions in 1-2 centuries, 4 or 5 orders of magnitude faster than most past transitions and perhaps faster than the great mass extinction events of the past, which were driven by similarly abrupt changes.

So, if you don’t have a sufficient luxury of time (or space, due to humans impinging on all natural systems) to allow for shifting ecosystems, evolutionary adaptation, or building of new belts of tropical forests and high latitude tropical ecosystems, you get savanna and desert instead. If you trigger feedbacks (tipping points) in the carbon cycle, then you have transient conditions that may be geologically ‘instantaneous’, but on human time scales, take “forever” (i.e. millennia to hundreds of millennia to re-equilibriate). So for the planet we, as a civilisation, know, it’s hell and high water.


I’m not convinced that 6 or 8 degrees of warming would be “game over” for mankind.

I’m speculating and don’t have any solid evidence, so solid evidence of bigger disasters would certainly trump these speculations. But I’d bet we could be surprisingly adaptable. Such a warming could turn much of our tropics into a barely habitable wasteland, and require mass migration to the Arctic/Antarctic regions. Maybe our food supply would shrink by half or three quarters. Mass starvation. Major wars, collapse of society with a return to the middle ages. But not game over. Maybe the next 50 years of technology advance will help. But then not far from 50 years ago we thought we’d see hover cars and space colonies on Mars by now.

Compared to the alternative of paying the equivelant of a few cups of coffee a week or something extra on our electricity bills.


Or if climate is not as sensitive to CO2 as claimed, then 6 degrees is just a cataclysmic, catastrophic and calamitous fantasy.

Or if it’s higher than claimed, then 6 degrees is just an overly sanguine fantasy.

And, of course, taking Energy and Environment as a serious science venue is simply … fantastically ignorant.


“And, of course, taking Energy and Environment as a serious science venue is simply … fantastically ignorant.”


Now your objections to the content of the paper are?


Michael Huber @7: Agreed – it is impossible to know if it would spell extinction for humans – we are an incredibly resilient species, but then again, have never been faced by a challenge like this. But as to the Civilisation we all know in this modern world, a 6C temperature rise all but guarantees its demise.

PeterW@10: If verified, the Douglass & Christy 2008 paper would be a spectacular overturning of all that we know about climate science from thousands of peer reviewed papers. Such paradigm-shifting papers are a rare event indeed, and are commonly published in Nature, Science, PNAS etc. This is a 12 page ms published in the non-ISI listed Energy & Environment, after presumably being culled by all other standard technical journals. The analysis is a time-series based correlation of temperature, ignoring any lag effects, and predicated on the UAH record being correct and all other temperature records (including RSS) being wrong. The results contain 3 laughably simplistic linear regressions with unverified assumptions about the linearity of the relationship between delta-T and ENSO and some hand-waving about black carbon forcing. It is difficult to critique further because there is nothing of substance to hack it. In short, it is neither convincing nor robust.


Apropos the prospect of “major coastal flooding”, this recent paper and associated discussion at RealClimate are apposite. Bottom line is 0.8m global sea level rise by 2100 is most likely, certainly no more than 2m. This is more than the most recent IPCC range estimate, but considerably less than some of the worst case scenarios that have been bandied about.

Of course this does not imply that sea level rise will stop in 2100, but the rate constraints do set the parameters for plausible adaptation.


Mark @12, 2m seems to be an upper bound for a Greenland melt contribution by 2100, I agree. However, the estimates on discharge from the West Antarctic Ice Sheet (WAIS) are based on some more heroic assumptions, and open to far greater uncertainty. As such, I still wouldn’t rule out 3-5m in a century if the WAIS starts to collapse along the grounded pinning points that are currently held secure by the huge Ross and Filchner-Ronne ice shelves — which Pfeffer et al. consider secure this century. Others, including Hansen, are less optimistic, and given the breakup of the Wilkins shelf this winter due to high SST, who knows.


Such paradigm-shifting papers are a rare event indeed, and are commonly published in Nature, Science, PNAS etc. This is a 12 page ms published in the non-ISI listed Energy & Environment, after presumably being culled by all other standard technical journals

PeterW, just to clarify, no scientist who believes his work overthrows a huge body of existing science is going to diminish that work by publishing it in the pseudo-science equivalent of World Wide Weekly by choice, where it is guaranteed to be ignored unless someone tacks it to the wall over the department urinal. Or some denialist posts a link to it on a blog. They’ll only publish there because they know that the paper isn’t going to hold up to the standards of a prestigous journal like Nature or PNAS.


re: #12 Mark Duffett

Do you live anywhere near a coast, and have you attended long-term planning meetings for local governments in dealing even with +1m SLR by 2100?

If you haven’t, you should. You might be less sanguine about “plausible adaptation”. Even really rich areas (like the San Francisco Bay Area) are worried about what it’s going to cost us. Please look at website for such a meeting that I attended earlier this year. The attendees mostly worked for local governments, and were impressive to me because they are *trying* to think about 50+ years, they’ve already stopped most sea-level development, they’ve already tried to preserve marshland buffers. We were using a +1m 2100AD estimate, and it was already expensive.

You should attend a scenario planning exercise, in which a team is handed a description of an imaginary bay-side town, and asked for a strategy for 50-100 years. The politics will get *really* ugly, especially in deciding where to put dikes or not, and how high you’re willing to build before they’re abandoned. If your property is outside the dike, do you get compensated? Told “tough”? Fight hard to get included?

This is in a place that is environmentally conscious, well-educated, rich, has generally-competent local governments, is used to dealing cooperatively with disasters, is used to dealing with water issues, is used to protecting coastlines … and even a +1m rise will be expensive. Some areas will have to be abandoned.

Speaking of dikes, note that most of our (worldwide) coastal infrastructure was built by with help of US$30 oil over the last century. By 2100, there will still be a little oil, but it won’t be $30. In many places, if people want dikes, it will probably be pick and shovel, maybe with some electric bulldozers, maybe with some biofuels, but not with cheap petroleum.

You might be aware that living behind dikes tends to need electricity for pumps. That’s relevant also.

In a lot of cases, huge chunks of coastal infrastructure will have to be given up, because people won’t be able to afford building dikes and running the pumps.


Re. Barry Brooke, #11:

This is a 12 page ms published in the non-ISI listed Energy & Environment, after presumably being culled by all other standard technical journals.

I doubt Christie submitted it anywhere else, as I think he’s smart enough and knowledgeable enough to be aware of its flaws and of the fact that it would fail peer review. If true that means he must have published it solely in the hope of giving spurious ammunition to so-called “skeptics”, which is dishonest and an attack on science (and if false he’s a lot less smart than he seems – so either way it reflects badly on him).



Re. Barry Brook, #13, another thing that keeps being ignored in such discussions is the difference between transient and equilibrium SLR. Even if the transient SLR is “only” 80cm in 2100, far larger SLR would be inevitable before equilibrium is reached, even if GHG emissions had been reduced to zero by then, due to feedbacks and the GHG already in the atmosphere. And although adaptation to (say) a 5m equilibrium SLR would admittedly be more manageable if it took several centuries rather than one to reach that level, it would still mean the end of many very large coastal cities and some entire countries.



Also, with respect to ice melt, the general focus on the effect on SLR ignores the (IMO) equally serious issue of the permanent droughts that will result in some large regions from inland glacial melt. As I understand it, all of the countries that currently rely on the Andes summer melts for their water supply, for instance, are likely to be mostly uninhabitable by 2100 on current trends, as are several other large regions such as the “Stans” (Pakistan, Uzbekistan etc.) and parts of China. Surely that is at least as serious an issue as SLR?



Dave raises a point at #17 that I often find myself reminding complacent folk of, and I think it is important to emphasise the fact that figures such as ‘xcm in a century’, or ‘ycm increase by 2050’, or ‘zcm by 2100’ do not account for the equilibrium values of sea level rise. This is a serious deficiency in policy determination, as response to sea level rise should be designed with final values in mind, rather than arbitrary intermediate values that provide convenience but distracting reference points.

Another point that I made several years ago, to engineers at one of the local government coastal planning meetings that John Mashey mentioned at #15, is that appears to me to be much less constructive to speak of things such as the height/contour profile of ‘one in a hundred years’ flood levels, when changing climate regimes will alter the frequency and severity of such. Flood levels would be more informatively defined in terms of various specific heights/contour profiles above a local datum, with regularly updated estimates for the frequency at which such inundations would occur for these various heights above the datum/contour, given the values of relevant parameters as they exist. This would provide an easier frame of reference with which to make planning decisions.

At least, I thought so, and apparently the engineers at the meeting thought so too, as there was much nodding and leaning over and discussion between them. I’m sure that some government bodies calculate policy this way already, but ours at least hadn’t considered this possibility.


The predicament of cotemporary human civilization is that, whereas vested interests and leaders representing vested interests are prepared to pour $trillions into war, bombing hapeless populations in their own homelands, or $trillions into rescuing failed financial speculators – attempting to delay economic meltdown, the powers to be appear agnostic with regard to the acceleration of the polar ice meltdown …

Indeed some even welcome the opening of the Arctic Sea for further oil drilling …

Such is the adaptability of the Genus Homo, having survived the most abrupt climate changes during glacial/integlacial atmospheric upheavals (e.g. 4 degrees in a few decades or even few years, many tens of metre sea level rise over time frames of centuries), chances are there will always be human survivors, mainly in sub-polar regions (as suggested by James Lovelock).

Pre-historic humans were nomadic and could escape to more promising regions, whereas civilization is adapted to climate conditions allowing production of excess food and anchored to energy and food supply routes.

The large mammals, including humans, are physiologically adapted to life in glacial/integalcial conditions of the Pliocene and Pleistocene. Civilization developed under favourable climate conditions along rivers of the great valleys of the Middle East and China fed by snow-melt water. Excess food and energy allowed growth of the species to 6 billion.

Abrupt breakdown of these conditions can only result in return to medieval dark ages or even tribal conditions, where small bands of humans fight for remaining resources …

As Barry indicates, natural habitats take periods of thousands of years and longer to develop, and can not “jump” from one state to another within time scales corresponding to current climate change.

Whether such tagedy can be averted, for example by channelling a few percent of GNP into alternative non-polluting energy in the nearest future, remains to be seen …


If and when mean global temperature increases by more than two or three degrees C (or even if the only increase is by this amount), there will be ecologically/agriculturally profound changes to climatic envelopes around the world. What a lot of denialists seem to miss is that whilst a few extra degrees might provide a pleasant, balmy local climate, the changes will not be consistent nor will they be gentle across the globe, and much of the planet’s current productive lands will be severely strained. Or worse.

So whilst the weather might be great for a soiree on the deck where many Westerners live, the capacity to provide the coffee, cigars, durham wheat, maize, rice, seafood and many, many other grown commodities for satifying basic hungers, or for providing tropical timbers to make the garden furniture for parking our fat Western arses, will be rather less enhanced.

If any denialist or sceptic disagrees, I would dearly love to hear how they think ecological systems will move if temperature does increase.

And the thing is, as others have indicated above, the issue is not whether humanity will become exinct so much as it is whether our societies would effectively cope. Considering how absolutely reliant Western society is on intensive energy use and intensive resource extraction from every corner of the planet, our capacity to deal with any significant shift in access to any such resources will be greatly compromised. One only has to consider what happens when a Middle-eastern war breaks out, or what happens when there’s a drought in a significant cereal-producing country, or indeed what happens when there’s a transport strike and our groceries are held up in transit for a few days. The knock-on from the growth of biofuels is a handy analogy for many countries in this regard.

It takes very little to set our complex societal system to wobbling, and technology will simply not be able to compensate for the fundamental climate shifts that would accompany anything over a couple of degrees of warming. And ironically it will be the ‘advanced’ societies that will crash the hardest – the much-derided peasant communities in many parts of the world are infinitely more able to maintain self-sufficiency even if they suffer changed climate in their local regions, and if significant warming does come to pass it will mostly be the techologically ‘meek’ that will inherit what is left of the earth.

And, somewhat ironically, probably a few survivalists…

Without immediate strategies for modifying our patterns of living, the urban crowds from Sydney, London, Paris, Tokyo and New York would be caught short and thus find the going quite a lot harder.


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