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Carbon cycle feedbacks and the worst-case greenhouse gas pathway

Posted on 7 July 2015 by Andy Skuce

The worst-case emissions pathway, RCP8.5, is a scenario that burns a huge amount of fossil fuels, especially coal. The model has sometimes been criticized as implausible because of its huge resource consumption and emissions of ~1700 billion tonnes of carbon (GtC) over the century. Those emissions are based in part on carbon cycle model assumptions, which recent work suggests may be too optimistic. New research shows that future plant growth may be restricted by nutrient availability, turning the land carbon sink into a source. Also, permafrost feedbacks (not considered in IPCC CMIP5 models) may also add significant emissions to the atmosphere under the RCP8.5 pathway. In addition, the latest research on the Amazon Basin reveals that the tropical forest carbon sinks may already be diminishing there. Together, these feedbacks suggest that the greenhouse gas concentrations in the RCP8.5 case could be achieved with ~400 GtC smaller human emissions, making the RCP8.5 worst-case scenario more plausible.

The climate models referred to  in the recent IPCC Fifth Assessment Report (AR5) are founded on one of four Representative Concentration Pathways or RCPs. The key word in RCP is concentration. In the RCPs, the concentration of greenhouse gases is fixed at different times in the future and the climate model (or general circulation model or GCM) uses those atmospheric concentrations to calculate future climate states. Underpinning the concentration pathways are socio-economic and emissions scenarios. There can be more than one underlying emissions scenario capable of producing the concentration pathway.

If you are unfamiliar with RCPs, check out the great guide that Graham Wayne wrote in August 2013 for Skeptical Science.

This way of modelling differs from previous approaches in which the starting point was a story or scenario about economic and social development that led to emissions. These emissions are run through a carbon-cycle model (which may be simple or complex) to produce atmospheric concentrations over time. 

The schematic illustrates the differences in approach. The elements in red boxes are the prescribed inputs into the models, whereas the elements in blue ellipses are outputs. The advantage of the RCP prescribed-concentration approach is that the climate model outputs do not depend to the same degree on carbon-cycle models as they did in the emissions scenario method. The disadvantage is that there is no unique link between concentrations and emissions. The schematic is simplified in that there are feedbacks and loops in the processes that are not illustrated. 

The worst-case scenario among the four Representative Concentration Pathways (RCPs) is known as RCP8.5. The number “8.5” refers to the radiative forcing level measured in W/m2 in the year 2100. RCP8.5, despite it often being called “business-as usual”, has been criticized as an unlikely outcome. While true, that’s more feature than bug, since, as one of the two extreme pathways, it is designed to provide climate modellers with an unlikely, but still just plausible “how bad could it be” scenario.

Let’s look briefly at some of the underlying socio-economic assumptions behind RCP8.5, then we’ll examine how the latest research on the terrestrial carbon cycle makes the GHG concentrations in the RCP8.5 model easier to reach.


The socio-economic model chosen to underpin this pathway is described in Riahi et al. (2011). The model is one in which the following all occur:

  • high population growth;
  • little improvement in energy efficiency;
  • no new greenhouse gas mitigation policy; 
  • heavy reliance on fossil fuels, especially coal.

Although economic growth is assumed to be moderate, the world economy will grow to over $200 trillion (in year 2000 dollars) and the average per-capita income will be about $20,000 per year, roughly equal to current levels in Portugal or the Czech Republic, about double today’s average world income per person.

Population and GDP: The assumption is that population will rise to 12 billion by 2100. This is higher than the UN’s medium estimate of about 11 billion, but within the range of the low and high fertility estimates (7 and 17 billion, respectively). GDP will grow, but modestly compared to some other pathways and is at the low end of the growth ranges used in the AR4 scenarios.

From Van Vuuren et al (2011) as are the following figures. Note that Population and GDP figures are assumptions rather than outcomes of the models. The grey shaded areas are UN population models (left) and assumptions used in AR4 models (right).

Energy intensity and energy use: the assumption is that the energy intensity of the economy (the amount of energy needed to produce one dollar of output will fall, but modestly compared to projections of recent trends. This is one of the most extreme assumptions of the model and is very different from historical trends (this is hardly "business-as-usual"). This leads to a quadrupling of energy use over this century.

Fossil fuel resource use: the assumption is that the energy needs will be met mostly by fossil fuels. The graph below shows historic (since 1950) and projected supplies of primary energy. (The left-hand graph was taken from the Global Energy Assessment (Fig SPM 3) and was squished to be at the same scales as the right-hand graph taken from Riahi et al, 2011, Fig 5.) About half of the energy supply is provided by a gigantic increase in the use of coal: in 2100 coal consumption will be more than five times the usage in 2010. Coal will be used not only for electricity generation but for coal-to-liquids fuel processes to make up for oil production that that will peak in the 2060s at levels that are double the production rate in 2010.


Energy sources. Historical 1950-2008 and projected in RCP8.5 2008-2100. See text for references.

These staggering assumptions for fossil-fuel use naturally raise questions of resource adequacy. In terms of the estimates of reserves of fossil fuels, the RCP8.5 model uses (roughly, by my own calculations using the figures given in Table SPM-3 of the GEA report) twice the current coal reserves, two to three times the oil reserves and half of the gas reserves. This is not quite as unreasonable as it seems because resources are constantly converted to reserves through development. In terms of reserves + resources, the RCP model uses, by 2100, about 10% of the current coal resources, nearly all of the oil resources and around one-quarter of the gas resources. It should go without saying that this exploitation will involve aggressive development of the world’s unconventional and low-grade resources and it will have huge financial and environmental costs.

See text for data sources. Author's own calculations.

Dave Rutledge of Caltech has used logistic curve-fitting of production histories to estimate world coal resources and he claims, contrary to the GEA, that coal resources are inadequate by a large factor to meet the demands of RCP8.5. (See, for example this PowerPoint presentation, slides 27-30.) There is a good discussions in GEA Chapter 7, pages 435-437 on the "Peak Debate" mainly focussed on oil and further discussion on coal reserve and resource estimates following page 461.

Policy: The Riahi et al. model factors in no greenhouse gas mitigation policies. Despite this, the model does assume that effective action will be taken to reduce local and regional pollutants such as sulphur dioxide, NOX and black carbon, basically assuming that current practices in rich countries today will be adopted by developing countries as their economies grow.

This perfect storm of high population growth, slow improvements in energy efficiency, ruthless exploitation of fossil fuels and non-existent climate policies leads to the dire climate outcomes of the RCP8.5 scenario. Perfect storms and worst-case scenarios like this are, by definition, unlikely.

So, can we forget about RCP8.5? Not so fast. For one thing, the economic growth forecast is relatively moderate and a higher one could easily make up for, let’s say, actual lower population growth than assumed in the model. Secondly, and more importantly, nature could provide humans with a helping hand to reach those lofty CO2 concentration targets through the combination of natural terrestrial sinks becoming less effective, along with new sources of carbon emissions appearing as a result of rising global temperatures. Let’s briefly look at the latest research on carbon cycle effects and see what difference they will make.

Land carbon storage

All climate models incorporate some form of carbon cycle component. Some are simple, some complex. The most sophisticated are the Earth System General Circulation Models (ES-GCMs). When it comes to modelling terrestrial storage of carbon, the models account for the increasing effect of carbon dioxide fertilization (C), but they do not (except for two models) account for the effects of nitrogen fertilization (N) and none of them, phosphorus (P).

As any suburbanite knows, lush green grass requires not only water, carbon dioxide and sunshine, but also a supply of nutrients, among which nitrogen and phosphorous are the most important.

A recent study in Nature Geoscience by Will Wieder and three colleagues performed modelling to determine what effect limiting N and P supplies would have on plant growth in a RCP8.5 scenario. Robert McSweeney at Carbon Brief has a good summary of the findings.

New inputs of N into the terrestrial ecosystem come from fixation of atmospheric N. New inflows of P are small and come from weathering of mineral soils and rocks. The nutrients can be moved around somewhat by wind and water, but the natural supply is generally limited to what can be found locally. Once local constraints on N and P supply are factored in, the rate at which plants can grow is limited to about one-third of the rate that has been predicted in the CMIP5 models reported in the AR5 IPCC report. This is shown in “a” in the graph below as the difference between the nutrient-unconstrained growth in black and the growth limited by N and N+P nutrient supply in pink and blue.

From Wieder et al (2015). 

There’s also a big difference in the cumulative amount of carbon stored in the terrestrial system, once N and P are limited. As shown in part “b” in Wieder et al.’s figure, the terrestrial carbon store over the 21st Century changes from a net sink of 125 GtC to a net source of 156 GtC once nutrient constraints are imposed. In other words, there’s a difference of about 280 GtC between what the AR5 models calculate will be locked up in the terrestrial biosphere compared to what might be the case if the supply of key nutrients is limited.

It is worthwhile pointing out that the uncertainties on all carbon-cycle models are very large. There are some cases where nutrient-constrained models still produce a net carbon sink and cases where the unconstrained AR5 models predict a carbon source. There is nothing hard-and fast about any of these results and a great deal more research is needed. For further reading on the research on the role of forests as carbon sinks and emerging research, I recommend The hunt for the world’s missing carbon by Gabriel Popkin in Nature News. Nature 523, 20–22 (02 July 2015) doi:10.1038/523020a

That 280 Gt of carbon has to go somewhere and will end up being divided between the atmosphere and oceans. An alternative way of looking at this would be, for a concentration-defined pathway, we could instead subtract that 280 GtC from the human inputs of carbon over the 21st Century to produce the same GHG forcing. The mean fossil-fuel emissions for RCP8.5 in AR5 models are 1685 GtC, so those could be reduced to about 1400 GtC in an N + P nutrient-limited scenario. That would significantly reduce the amount of coal we would need to mine to reach the RCP8.5 GHG forcing, making the pathway more easily achievable. That’s not good news, but it gets even worse once we incorporate permafrost feedbacks.


The carbon release from thawing of the Arctic permafrost are not included in the AR5 models. I wrote about the latest research in a Skeptical Science article in April, 2015 Permafrost feedback update 2015: is it good or bad news? The review by Ted Schuur and colleagues estimated that on the RCP 8.5 scenario, some 145 ±15 GtC will be released over the rest of this century.

Some of this accelerated soil decomposition could add some N fertilization and alleviate some of the N constraints in Arctic plant growth. So, there is risk of some double counting if we were to simply add the 145 GtC from the Arctic to Wieder et al.’s 280 GtC from fertilization constraints. This would have to be calculated using a properly integrated model, but let’s assume for now that the combined effect of permafrost thaw and N and P constrained plant growth would be about 400 GtC.

That would reduce the amount of fossil fuel emissions required to produce the RCP8.5 GHG forcing to about 1300 GtC from the AR5 figure of 1685 GtC. For the sake of comparison, 400 GtC is approximately the total amount of carbon produced historically from fossil fuels and cement from 1750-2013.

To put it more plainly, if we follow the RCP8.5 "business-as-usual" pathway, nature may add to our emissions (relative to current IPCC expectations) as much additional carbon as we have emitted from fossil fuels since the Industrial Revolution began. To repeat, that carbon feedback is not factored in to the latest IPCC assessment.

Alas, it doesn’t end there.

Tropical forest die-back

A recent paper by Roel Brienen and 90 or so co-authors examined the effectiveness of the Amazon rain forest as a carbon sink over the past 25 years. Again, Robert McSweeney has a good summary at Carbon Brief.

The key findings were:

• The Amazon is still acting as a net carbon sink, but its effectiveness has been diminishing over the past 25 years. Simple linear extrapolation of the rate of biomass change would predict that it could change to a net source of carbon over coming decades.
• The forest productivity measured on a per-Hectare basis increased in the 1990s but levelled off in 2000-2010.
• Trees have been dying off more quickly over the 1990-2010 period.

The reasons cited (see also the accompanying Nature article by Lars Hedin) for the tree mortality are: a) the faster the trees grow, the quicker they die; b) drought periods, as we saw in 2010; and c) possible limitations of N and P nutrients.

What this amounts to is an additional divergence from prevailing assumptions that the biosphere will continue to provide a strong net sink of carbon throughout the century. This effect is largely, but perhaps not completely, independent from the processes described by Wieder and will provide an additional boost to carbon-cycle feedbacks to those from nutrient limitations and permafrost thaw.

The size of this tropical forest effect over the 21st Century is unclear. The Amazon Basin contains about 150-200 GtC in living plants and soils. It is not known how much of this carbon will be lost over coming decades. Nor is it known how much carbon will be lost from other, less well-studied tropical rainforests in Africa and SE Asia. Brienen et al. estimate that an increase of 3.8 GtC in necromass (dead wood) produced since 1983 has yet to reach the atmosphere. That's equivalent to more than two years of current US carbon emissions.


Compared to the existing IPCC models, terrestrial carbon cycle processes could provide an additional net feedback of 400 GtC or more over this century following the RCP8.5 scenario. This is a quantity roughly equivalent to historical human fossil fuel emissions to date. The implication is that the gigantic fossil fuel consumption envisioned in the RCP8.5 socio-economic model could be reduced by 25% or so and we would still achieve the 8.5 W/m2 greenhouse gas forcing required in the model. This makes this worst-case scenario much more likely to be achievable. Nevertheless, humans would still have to demonstrate ingenuity and determination in exploiting even that reduced amount of fossil fuels, while at the same time remaining oblivious to the climate consequences.

We ought, of course, to be able to do much better than that, but our recent history shows that we are perfectly capable of demonstrating short-term, resource-exploitation ingenuity at the same time as being heedless when it comes to longer-term environmental consequences. The huge and unpredicted increase in the production of unconventional oil and gas resources in North America in just a few years has shown what we can do to exploit resources once we are motivated. We certainly should not console ourselves that RCP8.5 is beyond our reach just because the present estimate of fossil fuel resources appears insufficient. The recently quantified carbon cycle feedbacks that may occur if we follow the path of no mitigation make the achievement of the RCP8.5 greenhouse gas forcing level all too plausible.


Brienen, R. J. W., Phillips, O. L., Feldpausch, T. R., Gloor, E., Baker, T. R., Lloyd, J., ... & Marimon, B. S. (2015). Long-term decline of the Amazon carbon sink. Nature519(7543), 344-348.

Hedin, L. O. (2015). Biogeochemistry: Signs of saturation in the tropical carbon sink. Nature519(7543), 295-296.

Johansson, T. B., & Naki?enovi?, N. (Eds.). (2012). Global Energy Assessment: Toward a Sustainable Future. Cambridge University Press. PDF

MacDougall, A. H., Avis, C. A., & Weaver, A. J. (2012). Significant contribution to climate warming from the permafrost carbon feedback. Nature Geoscience,5(10), 719-721.

Popkin, G. (2015) The hunt for the world's missing carbon. Nature 523, 20–22 (02 July 2015). 

Riahi, K., Rao, S., Krey, V., Cho, C., Chirkov, V., Fischer, G., ... & Rafaj, P. (2011). RCP 8.5—A scenario of comparatively high greenhouse gas emissions.Climatic Change109(1-2), 33-57. PDF

Schuur, E. A. G., McGuire, A. D., Schädel, C., Grosse, G., Harden, J. W., Hayes, D. J., ... & Vonk, J. E. (2015). Climate change and the permafrost carbon feedback. Nature520(7546), 171-179.

Van Vuuren, D. P., Edmonds, J., Kainuma, M., Riahi, K., Thomson, A., Hibbard, K., ... & Rose, S. K. (2011). The representative concentration pathways: an overview. Climatic change109, 5-31. PDF

Wieder, W. R., Cleveland, C. C., Smith, W. K., & Todd-Brown, K. (2015). Future productivity and carbon storage limited by terrestrial nutrient availability. Nature Geoscience8(6), 441-444.


Appendix: a note on the low emission scenario

The carbon cycle papers discussed above look only at feedbacks in the high emission scenario or, in the case of the Amazon paper, report recent trends. To try to estimate the effect that these phenomena will have under a lower human emissions scenario requires guesswork. What follows is just that, so reader beware.

According to MacDougall et al (2012) permafrost feedbacks up to 2100 for the RCP2.6 scenario would be about 40% of those for the RCP8.5 case. Applying that percentage to Schuur's permafrost emissions for RCP8.5 gives 60 GtC of emissions from permafrost by 2100 under low human emissions.

The nutrition effect noted by Wieder will be small under a low emissions scenario and I will assume that it is zero for these purposes.

There is already 3.8 GtC of dead wood in the Amazon that has accumulated since 1983, according to Bienen et al. That carbon has not yet reached the atmosphere. Assuming this trend continues for several more decades, let's assume a 10 GtC feedback from the Amazon by 2100, noting also that similar processes may be playing out in other tropical forests.

The IPCC AR5 Summary for Policy Makers contains the following paragraph (my highlighting).

Using those numbers, updating them with actual emissions from 2012-2014 and subtracting the carbon cycle feedbacks gives this table:

On this estimation, the carbon cycle feedbacks from permafrost and reduced tropical forest sinks could reduce our "safe" emissions by 20-30% (incidentally, about the same percentage as for the high-emissions scenario). The time period we have left at current emissions rates would be reduced by 6 years, to as little as 16 years, if we give ourselves a two-thirds chance of staying below two degrees, once we factor in carbon cycle feedbacks.

I should stress again that these are just my estimates and they are intended for discussion purposes only. Properly integrated studies that incorporate all of the latest carbon-cycle feedback studies under different emissions scenarios have yet to be done.


Thanks to Will Wieder for patiently answering my questions.

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Comments 1 to 19:

  1. This is what happens when there's little upside and large downside in the uncertainties - the more things are refined the greater chance that they're worse than previously thought. I can't recall any time there was a new discovery or refinement that made the situation look a bit more optimistic than pessimistic.

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  2. tmbtx @1, you do not consider the proof that the Earth cannot undergo a runaway greenhouse effect good news?  Or the recent indications that climate sensitivity may not have a long tail?  The later does not eliminate the risk of severe impacts from climate change, but it does largely eliminate the risk of extreme impacts, which is surely good news.  If you are not finding any evidence of optimistic developments, you are relying to much on biased sources.  There is good news out there, particularly in the technical development of low carbon technologies (renewable energy, electric transport).

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  3. Tom, I would agree that Earth not being susceptible to a runaway greenhouse effect is good news. However, it would not be necessary for Earth to have temperatures hot enough to burn us to ashes or pressures that would crush us to jelly and yet be entirely unsatisfactory for the continued existence of our species. My personal level of optimism would have to be described as "cautiously hopeful." Humans are a stubborn lot...perhaps they can shift that mule-headedness toward a more constructive purpose. 

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  4. Tom - I specifically am referring to the behavior of the climate system. Even more specifically I'm referring to how gaps in the understanding seem to turn out a bit worse than best guesses. There is nothing strange about this as scientists are usually cautious in their conclusions and making guesses without sufficient study. I suppose finding that we won't likely turn into Venus is a good thing, but the geologic record seemed to make that pretty clear to begin with. Certainly there are good things in our response with regards to renewable energy, etc. I'm not saying we're all dead and that there's not hope (well not most days) but that the size of the problem seems to grow a bit with each incremental advance. Concluding that our supposed worse case scenario is a lot easier to reach than previously believed is consistent with that.

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  5. Tom Curtis @2. Paleoclimate indicates that at geologically short timescales the climate can in fact do the equivalent of a runaway greenhouse - for example the end-Permian, end-Triassic, Capitanian etc. When CO2 rates overwhelm the terrestial and surface ocean sinks the Earth can undergo rapid, lethal warming. For example see this post and this post.

    The rock weathering thermostat does indeed prevent a runaway greenhouse over timescales of millions of years - but that doesn't prevent centuries-multi-millennia-length abrupt warming events which have happened several times in Earth's past.

    On the long tail, again paleoclimate does indicate a very long tail of 10s to 100s of thousands of years for anthropogenic climate change. For an article on that topic see this post.

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  6. I hate to add to the pessimism but there is evidence that the oceans, which have been "scrubbing" about 28% of our CO2 for us, will decline in their capacity to do this. In a recent study, Randerson et al showed that the ocean feedback will overtake the terrestrial feedback by the end of the century. This is because as the ocean warms it resists uptake of CO2, and a warming ocean reduces circulation, which diminishes the ability to refresh the saturated surface ocean with deeper water. A slowed biologic "pump" from sluggish, hot, acidifying oceans may also be a factor.

    In other words, the oceans will effectively amplify our emissions in the coming decades, over and above all the worst case scenarios mentioned in this article.

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  7. howardlee @5, a runaway greenhouse effect is when the Climate Feedback Paramater, α, approaches zero.  The Climate Feedback Parameter is defined by the IPCC:

    "Formally, the Climate Feedback Parameter (α; units:
    W m–2 °C–1) is defined as: α = (ΔQ – ΔF)/ΔT, where Q is the global mean radiative forcing, T is the global mean air surface temperature, F is the heat flux into the ocean and Δ represents a change with respect to an unperturbed climate."

    Here is one of the diagrams from the post to which I previously linked:

    In this diagram, the difference between the green line (representing the OLR prior to any forcing) and the red line (representing the OLR after a forcing is applied) is the change in OLR required to reach a temperature equilibrium.  As you will note, the y axis is in W/m^2, and the x axis in degrees K, meaning the instantanious slope of the blue line is the Climate Feedback Parameter for a given GMST.  As the slope approaches zero, you require a larger and larger temperature response to achieve equilibrium until, eventually, the temperature response to achieve equilibrium is greater than that consistent with liquid water on the Earth's surface.

    In simpler terms, the Cimate Feedback Parameter is the inverse of the Climate Sensitivity Parameter, λ, which is defined by the IPCC:

    "The climate sensitivity parameter (units: °C (W m–2)–1) refers to the equilibrium change in the annual global mean surface temperature following a unit change in radiative forcing."

    Fairly simply, as α approaches zero, λ approaches infinity.  So, a runaway greenhouse effect is that condition in which, until the oceans boil dry, climate sensitivity is for all practical purposes infinite.  Neither of the two posts you link to suggest evidence of this state, nor even use the term "runaway greenhouse effect".

    With regard to the "long tail", I clearly discussed the long tail of climate sensitivity, ie, the extended tail of the Probability Density Function of climate sensitivity as estimated using some statistical methods.  It has nothing to do with the long duration until equilibrium climate sensitivity is reached.

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  8. howardlee @6, the consensus view on ocean uptake is represented schematically by David Archer thus:

    From memory, this is for a 1000 Petagramme pulse of CO2.  As you can see, the oceans keep on absorbing CO2 strongly for several hundred years after the pulse, eventually absorbing around 80% of CO2, before chemical weathering of different sorts eventually, and very slowly removes the rest (over hundreds of thousands of years).

    The models showing this pattern have successfully retrodicted the decline in CO2 concentrations following the PETM, so it is highly unlikely that they will fail for the CO2 increase expected over the next few hundred years with BaU.

    Yes, some scientists think differently, but they are a distinct minority.  It makes no more sense to work on the assumption that they are right than it does to work on the assumption that that other distinct minority much loved by Anthony Watts are right.

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  9. tom Curtis @ 5 - OK I'll grant you nobody is talking of Venus-style, ocean-boiling runaway greenhouse. My point was that the paleo record shows several abrupt, strong global warming episodes.

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  10. IPCC scenarios do not convey the full gravity of the situation. They are usually limited to 2100 and people tend to think that 4.9 ºC (projection for RCP8.5) is the worst that can happen, bad as it is.

    It is plausible to assume that we're not leaving any fossil fuel underground. We are going to sell it and burn it to the last dollar, if we just continue our business-as-usual behavior. Try to talk seriously with any politician or executive of the fossil fuel industry about giving up available oil or coal.

    If we burn it all, and considering long-term climate sensitivity, we'll eventually pour enough CO2 in the atmosphere to reach about 20 ºC over land (reference below). This pathway we're in is tantamount to destroying the planet we live in. That's not something people gather from reading the mild-mannered IPCC report.

    Hansen 2013: Climate Sensitivity, Sea Level, and Atmospheric CO2

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  11. Sorry, on my post above I meant a warming of 20 ºC over land.

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  12. Tom Curtis @ 8 - I was referring to the work of Zeebe & Zachos who were looking at the PETM compared to today. Most people estimate the rates of carbon release in the PETM were much slower than modern rates. But the recovery curve is broadly similar to Archer's figure you post. Here's a relevant figure:

    Comparison of the effects of anthropogenic business-as-usual emissions (total of 5000 Pg C over 500 years) and PETM carbon release (3000 Pg C over 6 kyr) on the surface ocean saturation state of calcite.

    Regarding strong ocean absorbtion continuing for centuries, I'm simply highlighting research that may not apply to us. In addition to the Randerson et al paper there are others reporting the same thing such as this, and this.

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  13. Howardlee and Tom C,

    The ocean will take up CO2 from pulse but over a long time though, the graph is misleading, look at the X axis on those graphs and that is in a very hypothetical situation were CO2 releases just entirely stop into the atmosphere.

    If we stopped all fossil fuel emissions tomorrow CO2 levels would still rise from land use, permafrost melting, forest fires (also not in IPCC) and the land sinks becoming acute sources as vegitation shifts and we have at least 0.5C warming to come, and in general warming tends to cause the releass CO2 in all the last Ice ages, most likely due to Southern Ocean linked mechanisms.

    Then everyone talks about grand children when the climate right now is quite literally becoming very extreme everywhere, so the impacts are now not tomorrow or in several decades.

    Then the question is how long does it take to get to equlibrium temperature?

    Hansen and others say 80% in 100 years, and ~99% in 1000 years.

    CO2 now ~400ppm.

    Last CO2 400ppm was the early Pliocene and Miocene, 3-5C to 4-6C warmer respectively.

    And people somehow claim we have a carbon budget; isn't this a debt situation?

    And if this is a debt situation, isn't replacing all the power supply with renewables and cars with electricity merely going tp add to that debt.

    That is if there is no CO2 budget, but a debt, then anything that requires CO2 emissions to make it is merely adding to the debt, even if it does slow the rate of debt accumulation in the long terms as fossil fuel usage is reduced. And at present all renewables carry a CO2 cost, and a significant biodiversity cost as well (mining rare earth metals, toxic wastes, mining steel, concrete, aluminium, solvents, etc,etc,etc), and soberingly we are in a rapid mass extinction and climate change is only just taking hold.

    Hope, is there hope?

    Unless we get CO2 below 350ppm by 2100, 2C is inevitable and considering the extremes we are already having how can that be at all safe?


    350ppm by 2100 given only 60% warming materialised gives 1.8C still.

    A 2C rise is a 5-SD shift in the world's mean temperature (using natural variation for the last 2000 years), and that is like increasing the average height of men by 15inches, extremely extreme.

    And it is becoming more and more apparent that in terms of weather there are other influences that are making things even more extreme that just a shift in temperature might be expected to produce, like Arctic rapid warming, the front ending of deluge events resulting in greater peaks of flow to exacerbate flash flooding and so on and on.

    Therefore as the climate dramatically shifts before our eyes we pretend there is room to move.

    And we locked into at least another 10 years of higher than 1990 emissions?

    And there are things like, New York is unsustainably vulnerable to sea level rise and storm surge increase, yet is anyone saying lets move?





    Anyway lets hope the ocean do perform a miracle as Tom suggest for it is hard to see any other way out of this as even letting the mobile phon eis too great a sacrifice to anyone I've asked.

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  14. Ranyl - I still have some hope. Maybe that's just the part of my brain rationalizing the recent purchase of a home in Holland... 

    I'm not following though how switching to renewables will add to a carbon debt. Seems like it would be a net positive change. 

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  15. tmbtx,

    Presuming there is a carbon debt, then any additional carbon emitted to atmosphere adds to that debt, (i.e. any additional carbon emissions), therefore the carbon needed to manufacture renewables or anything else for that matter is an addition to the debt.

    All renewables are carbon emissions dependent to manufacture, as they are made of materials like concrete, aluminium, steel, metallic grade silicon, rare earth metals, etc, all of which require large scale mining, large amounts of energy and create significant amounts of general and toxic waste.

    There is little doubt that over time producing energy from a wind turbine in comparison to coal, gas or liquid fossil fuels, costs a lot less carbon emissions, causes much less toxic waste, mining disturbance and has a significantly lower adverse impact on general biodiversity. However that doesn't mean wind turbines are clean for they do have associated with carbon emissions and do have adverse affects on biodiversity, they are just far far less dirty than coal.

    Clearly the renewables that have been manufactured should be used until they cease to function, although maybe they should positioned optimally, (e.g. putting the solar panels already manufactured on south facing roofs rather than in fertile fields?)

    However on the premise of stopping all fossil fuels use ASAP, (meaning there is no fossil fuel burning displacement carbon emissions payback for renewables), then they do have a definitive carbon cost and biodiversity impact. Add in the batteries needed and the impacts of manufacturing and disposing of these batteries and the general costs rise, and the end of life of renewables also has to be considered; what is the end of life of a large scale hydro dam?

    Given the gravity of the situation faced and considering the scale carbon debt (Is 350ppm safe? maybe 300ppm (300ppm =1C rise, 6-9m sea level rise), for me the question is how much more carbon emissions and biodiversity impacts can be gambled?

    Further given 1.5C isn't seemingly that safe (considering the weather already?), and 350ppm by 2100 might keep the temperature rise to between 1.5C and 2C, (although 3C-5C more in the longer term and with eventual sea levels rises of ~15-25m, if the early Pliocene is anything to go by), again is 350ppm safe?

    What would getting the atmospheric CO2 levels down to 350ppm take?

    95% power down?

    Not using the power means no carbon emissions and no biodiversity impacts. Further due to the reduced power generation biodiversity should increase and therefore carbon should be sequestered. Therefore reducing down, in terms of energy use saves the most carbon emissions, increases biodiversity and removes carbon dioxide from the atmosphere to some degree as biodiversity enriches.


    Think of rubber plantations, road impacts, road deaths (biodiversity and people), manufacturing impacts, disposal impacts, what do electric cars become at the end of their life, even if recycled a few times?

    And when considering all this also note the degree of adaption that is going to be required, planning for 1:1000 extreme weather isn't easy, and if go up 2C, then 1:1000 event will be mild merely by the shift in mean temperature by 5 standard deviations and this will affect infra structure and crops (e.g. wind, flood, hail, drought).

    Is it possible to go back to sailing rather than flying?

    Would imagine with human ingenuity to ships would be impressive, however who wants to go back to skilled craftsmen building wooden ships as wood in an appropriate amount can be truly sustainable?

    What would it require to actual stop using fossil fuels?

    And what is sustainable?

    Renewables utilise renewable energy sources (e.g. the sun) however they are machines and aren't renewable in themselves (as in they don't renew themselves), as they need to be manufactured and that costs carbon emissions and has biodiversity impacts. Further all renewables also impact biodiversity to some degree in use. Not as much as fossil fuels by any means, however still impacts when biodiversity is falling rapidly, therefore minimising these seems reasonable. And not needing as many renewables by using much less power reduces these impacts the most. That is the less renewables acutely required for functionality the easier it will be to achieve ecosystem regeneration.

    Biomass can potentially be grown sustainably in the long term and without displacing food crops, displacing soil carbon or negatively impacting biodiversity (e.g pesticides, fertilisers), the question is how much? and what is the best done with its limited supply?

    Ambulances or X-Boxes?

    Therefore tmbtx,

    Is there hope?

    I agree overall at the present getting energy sourced from renewables is much better than getting it from fossil fuels, therefore positive to a degree.

    However, again not as positive, as not using the energy.

    And if only it was just about energy production as the debate is so always shifted, when biodiversity losses are just as, if not more worrying, and over fishing in an electric boat is just as impacting as over fishing in a petrol one and cars do kill a lot of birds each year as any wind turbine manufacturer will confirm.

    Personal cars?

    And then there will climate displacement of large numbers of peoples, is current situation that well suited for mass migrations?

    Also if 350ppm is seen as safe, and that is a large carbon debt, the higher we go above that the larger the debt becomes.

    And to get 350ppm given the melting permafrost, forest fires, tree die offs (which will continue to increase in scale as there is warming in the system), basically means becoming massively carbon sequestering as soon as possible. And further the carbon that was taken up into the forest and sea sinks will be re-released as the atmospheric CO2 falls, which means to get 350ppm by 2100 would require removing ~twice amount of carbon dioxide that has been accumulated in the atmosphere plus removing the additions from the permafrost etc.

    At present 1:5 or more (worldwide) don't even feel climate change is happening although I'm not aware of the current perceptions of the biodiversity crisis.

    What scale of change would be needed to achieve that massive amount of true carbon sequestration by 2100, whilst also adapting to the changes to come and repairing the earth's ecosystem?

    Is there hope?

    What are the carbon costs of war and destruction?


    Is there hope?

    For me I suppose theoretically yes, although only if there is a paradigm shift in the perception of the problem, a massive reduction in power use, the cessation of fossil fuels use within 5-10years, and stopping of using non sustainable materials and the transformation of human interactions with nature and each other, in order to facilitate a sustainable society and ecosystem regeneration.

    Is there hope though?

    Not sure, however whilst the focus remains on BAU replacement with renewables then for me it is definitely no.

    Is there hope?

    I wish I could say yes, but as mentioned to most the mobile phone is essential.

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  16. @ranyl #15:

    The Chinese government's inititative smmarized in the following article undermines many of your arguments about why fossil fuel generated energy cannot be replaced by clean enery in a significant amount. (You will undoubetly want to travel to China in 2020 to see first-hand how well their new power distribution system is working. )

    China eyes safe smart-grid system by 2020 to push clean energy, Bloomberg/Japan Times, July 7, 2015

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  17. Dear John.

    I don't make the agrument that in practical terms current energy production can be changed to "cleaner" energy sources, of course it can, if you build enough you'll be able to replace current demandand even build on further to accomadate fot population and affleunce increases.

    There is no doubt there is the technology available to drive huge amounts of energy from availableresources without using fossil fuels. 

    I merely point out that all that additional infra-structure and manufacture of the power generating machines will add a significant amount of carbon to the debt and have biodiversity impacts.

    If China powered down to their current renewable and present nuclear power generation capacity then that addition to the debt would be prevented.

    I do mention it would be sensible to use what we have for the carbon emissions are already in the atmosphere.

    How much extra carbon emissions and environmental degradation can we gamble?

    All depends on whether you like gambling or not and what perception of risk environmental (global warming, ocean acidification, mass extinction, all proceeding at unprecendented rates comparison to earth's geological and recorded history) change induces within your perception.

    Calnifornia's drought is exceptional as is North Koreas, and San Paulo, etc, etc....

    However how you see them is almost already formulated by your general opinion on environmental change (urgent, grandchildren'sproblem), your religious and political beleifs and what people arround you suggest is the way to see it.

    The drought in Syria was severe, and the Arab Spring report suggests this was linked to the climatic changes occuring in the region and that this lay part of the basis for th einitial civil unrest and eventual civil war.

    And then there is nuclear?

    Huge carbon emissions up fronts for making new ones and making lead must have environmental impacts and what is safe waste disposal?

    If you have a carbon budget to spend (gamble with), where would spend it?

    Defences? Building regenerative infra structure (e.g. terracing hills to slow water flow and provide usable growing space), running hospitals? running banks? running governments? until fuel supply truly sustainable and carbon neutral?

    1:5 see no risk at all.

    Anyone suggesting the problem is so severe the transofrmational change is needed immediately as the problem is here, is stigmatised an alarmist or extremist.

    Is there hppe?


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    Moderator Response:

    [RH] Inflammatory comment snipped. Comment reinstated.

  18. Dear JH moderator,

    It was a genral comment of the situation and not intended to inflame anyone.

    Was it the terms used? or do you feel that the general press and others (bloggers etc) don't try to dismiss sincere calls  about the urgency of these matters by portraying the messengers as "crying wolf" without just cause?

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    Moderator Response:

    [RH] I just re-read the comment and my initial interpretation of the comment was that you were saying anyone saying this is urgent is an extremist. I now see you were saying the problem is that people stating urgency get "labeled" an extremist.

    Comment reinstated.

  19. Thank you RH,

    Maybe just delete the conversation about? up to you.

    On "NOT" clean energy, I would suggest people look up large scale hydroelectric methane and carbon emissions on going and the effect of dams on the downstream water ways and river deltas.

    GHG emissions can be greater than fossil fuels emissions especially in the Tropics,

    And they impact biodiversitry once completed, not to mention the large impacts during construction an dhow much concrete do they need?

    And how many dams are CHina building?

    Not to mention funded dams in Laos, how many dams are planned in Lao, and Brazil and Chile say?

    All with large CO2 emissions, concrete, biodiversity impacts etc, all in the name of "clean" energy, when it should really be called, high GHG emitting, highly biodiversity tottally not clean energy, especially in the Tropical regions.

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    Moderator Response:

    [JH] Now you are engaging in one of the favorite games of concern trolls  — throwing factoids against the wall to see which ones might stick. Either cease and desist or have your future posts summarily deleted in their entirity. Enough is enough!

    [RH] It should be noted that there has been a commenter named "ryland" whom JH has been engaging with for the past week or so. That seems to be causing some confusion with comments from "ranyl." (It definitely influenced my previous misinterpreted reading.) We request everyone's patience while we sort it out.

    [JH] I did in fact confuse ranyl with ryland. My sincerest apology to ranyl.

    [RH] Links activated.

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