Climate sensitivity wrangles don’t change the big picture on emissions

The sources of data that scientists can use to determine climate sensitivity include ice cores, the cylinders these researchers are holding at the Vostok station in Antarctica. Image credit: Todd Sowers, Columbia University

The sources of data that scientists can use to determine climate sensitivity include ice cores, the cylinders these researchers are holding at the Vostok station in Antarctica. Image credit: Todd Sowers, Columbia University

How much does the world warm up in response to a certain amount of greenhouse gases like CO2 in the atmosphere? It’s a simple question, but its answer depends on whether you mean short-term or long-term warming, and estimates vary according to the methods used. Scientists are currently intensively debating long-term ‘climate sensitivity’, which begs prompts the question: might we be pushing too hard to cut climate CO2 emissions, if this is uncertain?

The answer is no, according to Joeri Rogelj from the Swiss Federal Institute of Technology, ETH Zurich, and his coworkers. They looked at how a range of climate sensitivity values affected their 21st century warming projections in a paper published in Environmental Research Letters last week. “When taking into account all available evidence, the big picture doesn’t change,” Joeri told me. The ‘carbon budget’ of greenhouse gases we could still emit today and in the future is very limited whatever the climate sensitivity, he explained. “Keeping the so-called carbon budget in line with warming below 2°C still requires a decarbonisation of  global society over the first half of this century.”

Climate sensitivity is the measure of how much the world will eventually warm when it reaches equilibrium after a doubling of CO2 in the air. Today, we have upset the normal equilibrium where the Sun’s energy flowing into the atmosphere matches the flow the Earth radiates back out of it. Now more is coming in than leaving, and that’s heating the planet up. Think of the atmosphere as a series of pipes, with energy flowing through them like a liquid. The Earth is a reservoir in the system, filled by an incoming pipe and drained by an outgoing one. CO2 acts like a blockage in the outgoing pipe – it slows the outward energy flow and causes a build-up in the reservoir. When the reservoir gets fuller it can put enough pressure on the blockage for the outward flow through it to again match the incoming flow. Then we’d be at equilibrium, but with a fuller reservoir – a warmer planet. The more CO2 we emit, the worse the blockage gets and the hotter we get before reaching equilibrium.

Not so sensitive

Temperature projections for the RCP3PD (lowest emissions scenario) and RCP8.5 (highest emissions scenario) over the 21st century using the climate sensitivity from the IPCC's 2007 'AR4' statement. Grey areas show the 'likely' range of temperatures that there's a two-in-three chance this scenario will stay within, the thin lines within these ranges show the median average temperature. On the right are temperature ranges for RCP3PD and RCP8.5 for 2091–2100. AR5 is the IPCC's new report, Transient T is the low climate sensitivity scenario, and Climatology is the high climate sensitivity scenario. The thin lines indicate the probability range there's a nine-out-of-ten chance each scenario will stay within. The thick lines are the probability range there's a two-out-of-three chance each scenario will stay within. Diamonds and circles show the median and mean average, respectively. Image copyright IOP Publishing, used via Creative Commons license, see reference below.

Temperature projections for the RCP3PD (lowest emissions scenario) and RCP8.5 (highest emissions scenario) over the 21st century using the climate sensitivity from the IPCC’s 2007 ‘AR4′ statement. Grey areas show the ‘likely’ range of temperatures that there’s a two-in-three chance this scenario will stay within, the thin lines within these ranges show the median average temperature. On the right are temperature ranges for RCP3PD and RCP8.5 for 2091–2100. AR5 is the IPCC’s new report, Transient T is the low climate sensitivity scenario, and Climatology is the high climate sensitivity scenario. The thin lines indicate the probability range there’s a nine-out-of-ten chance each scenario will stay within. The thick lines are the probability range there’s a two-out-of-three chance each scenario will stay within. Diamonds and circles show the median and mean average, respectively. Image copyright IOP Publishing, used via Creative Commons license, see reference below.

The UN Intergovernmental Panel on Climate Change (IPCC) says that the warming at equilibrium after doubling CO2 would likely – more than a two-in-three chance – be between 1.5 and 4.5°C. It’s extremely likely – more than a nineteen-in-twenty chance – that it would be above 1.5°C. The most likely value seems to be near 3°C, but several recent studies have put it either at the top or bottom of the usual range. And though this may sound like a dry, academic, argument, these numbers are regularly used as ammunition in the political conflicts over climate change. “Each time a new climate sensitivity estimate is published, statements are made about its policy implications,” Joeri underlined.

One of Joeri’s specialities is using climate models to project how our CO2 emissions influence the odds of meeting targets for limiting global warming. He’d previously been in a team who found that to stay under the 2°C threshold governments agreed in Copenhagen in 2009 emissions would have to peak by 2020. Being aware of the ongoing debate, in their main study Joeri and his coworkers therefore fed four climate sensitivity estimates into their existing models. They had previously used estimates from the IPCC’s 2007 report that gave an average figure of 3.0°C, and added its newly-released changes, which remain very similar. The other two estimates they used had average climate sensitivities of 1.9°C and 3.9°C.

Using all these estimates in each of four different scenarios ranging from low to high CO2 emissions, the temperatures Joeri’s team projected by 2100 look surprising at first glance. The greatest sensitivity increased temperatures by around one sixth, and the lowest reduced them by about a quarter. But these more extreme climate sensitivities are one-third higher or lower than the IPCC estimates, respectively. Why aren’t the temperatures changing by a third too? It’s related to the earlier definition of equilibrium. Not enough time has passed for the reservoir-filling to have finished, so the CO2 blockage’s full warming impact hasn’t yet been felt.

Dangerous optimism

That delayed impact on warming feeds through to limit the effects on the targets we need to set. In the team’s lowest emission scenario high climate sensitivity would cut the chances of keeping warming below 2 °C from 81% to 72%. Low sensitivity would improve the chances to a 98% likelihood we’d meet the target in that scenario. However, if our emissions stay on their current path low sensitivity would delay passing the 2 °C threshold by less than a decade. “There is no scientific support to diminish the urgency of emission reductions if warming is to be kept below 1.5 or 2°C, the two temperature limits currently being discussed within the United Nations,” the scientists write.

One reason optimistically avoiding tackling greenhouse gas emissions is dangerous is because continuing to build fossil fuel power stations would commit us to using them. Once the CO2 they emit is in the air it will be even more costly to get back towards staying within temperature limits, Joeri warned. “Only betting on the results from studies with lower estimates would neglect the information we have from other studies that end up at the higher end,” he added. “At this point, we are not able to say which of these methods will turn out to be superior in predicting the actual climate sensitivity value.”

This graph shows the greenhouse gas emissions in 2020 (left) and 2050 (right) that would keep us within various temperature thresholds. In it, higher climate sensitivities lead to a lower 'carbon budget' emission allowance within which the world stays below each theshold. Lower sensitivities allow bigger carbon budgets for each target. The thresholds are a 1.5°C (blue), 2°C (green), 2.5°C (yellow) and 3°C (orange) rise relative to preindustrial temperatures during the 21st century, and the emission ranges shown give at least a two-out-of-three chance of limiting warming below each level . The fat bars are for the climate sensitivity from the IPCC's 2007 report (AR4). Light shaded areas represent the minimum–maximum ranges; the dark shaded areas represent more likely pathways. The thick black horizontal lines show the median average values. The other climate sensitivity figures are simply represented by narrow vertical lines - AR5 is the IPCC's new report, Transient T is the low climate sensitivity scenario, and Climatology is the high climate sensitivity scenario. Horizontal solid and dashed purple lines are 1990 and 2010 emission levels, respectively. Image copyright IOP Publishing, used via Creative Commons license, see reference below.

This graph shows the greenhouse gas emissions in 2020 (left) and 2050 (right) that would keep us within various temperature thresholds. In it, higher climate sensitivities lead to a lower ‘carbon budget’ emission allowance within which the world stays below each theshold. Lower sensitivities allow bigger carbon budgets for each target. The thresholds are 1.5°C (blue), 2°C (green), 2.5°C (yellow) and 3°C (orange) rises relative to preindustrial temperatures during the 21st century, and the emission ranges shown give at least a two-out-of-three chance of limiting warming below each level . The fat bars are for the climate sensitivity from the IPCC’s 2007 report (AR4). Light coloured areas represent the minimum–maximum ranges; the dark coloured areas represent more likely pathways. The thick black horizontal lines show the median average values. The other climate sensitivity figures are simply represented by narrow vertical lines – AR5 is the IPCC’s new report, Transient T is the low climate sensitivity scenario, and Climatology is the high climate sensitivity scenario. Horizontal solid and dashed purple lines are 1990 and 2010 emission levels, respectively. Image copyright IOP Publishing, used via Creative Commons license, see reference below.

Journal reference:
Rogelj, J., Meinshausen, M., Sedláček, J., & Knutti, R. (2014). Implications of potentially lower climate sensitivity on climate projections and policy Environmental Research Letters, 9 (3) DOI: 10.1088/1748-9326/9/3/031003

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29 Responses to “Climate sensitivity wrangles don’t change the big picture on emissions”

  1. climatehawk1 Says:

    Minor wording choice note: to “beg the question” is to fail to answer it. “Prompt the question” means what I think you intend here.

  2. Jim in IA Says:

    The reservoir analogy is good for me. We have a large flood control reservoir on a river near us. Most of the time, the incoming flow from various rainfalls is matched by the outflow below the dam. The outflow can be left unchanged most of the year. In the spring with rainfall and snowmelt, the outflow gates need to be opened up or else the reservoir fills and tops the spillway.

    We have no outflow gates to adjust readily on Earth. Our CO2 gates are big and cumbersome. We don’t even know how quickly we can adjust them. Politically, we have a hard time deciding who will be in control and how much. Some even say don’t worry about it. Nature will take care of on her own.

  3. Richard Mallett Says:

    How do we know that a warming of x degrees Celsius will be harmful ? Plants grow better in warmer climates, and they grow better with more CO2.

    • andyextance Says:

      There are multiple lines of evidence that suggest the benefits and harms we can expect at given temperatures. If you’re focussing on plants, there’s a narrow window, up to 2C, where wheat yields here in the UK could increase, if farmers slap on enough fertiliser. I covered that in this blog entry:
      http://simpleclimate.wordpress.com/2014/03/08/the-climate-challenges-that-my-morning-toast-poses/
      However the scenario is different in different places. You may have seen this infographic from the latest instalment of the IPCC report:
      Crop yields after climate change
      This paper shows maize yields fell 20% in France during the 2003 heatwave:
      http://onlinelibrary.wiley.com/doi/10.1111/gcb.12069/abstract
      And you may be interested in this recent story on how warming is threatening coffee production:
      http://www.theguardian.com/environment/2014/mar/28/climate-change-bad-expensive-coffee-ipcc

      Farming is critical to our survival, but it’s arguable the greatest threat to it doesn’t come directly from temperature. Civilisation has developed over the course of the past 11,000 years when climate has been stable and relatively warm. That has helped kept sea level stable, making grain production possible in estuary and floodplain ecosystems. But the Arctic and Greenland ice sheets are now losing more than 100 cubic km of ice per year. Arctic sea ice is declining faster than predicted, and that plays a key role in protecting the Greenland ice sheet. The IPCC is conservative on sea-level rise, but we are certainly at risk of entering conditions unseen during the existence of human civilisation. (If you want a citation, for most of this paragraph I’m quoting from ‘Storms of my Grandchilden’ by Jim Hansen, former director of the NASA Goddard Institute of Space Studies).

      Thanks for reading and asking the question. I truly welcome it.

      • Richard Mallett Says:

        Are there any historical records of the effect on crops when, according to the Central England Temperature record, annual average temperatures rose from 7.25 C in 1695 to 10.47 C in 1733 (compared to 9.56 C last year) ?

      • andyextance Says:

        That’s not something I know off the top of my head. You’re just as capable of searching Google as me. I’m surprised by and interested in the temperatures you cite though. Can you point me to where they can be found?

  4. Richard Mallett Says:

    Go to http://www.metoffice.gov.uk/hadobs/hadcet/ and “Download data”. Then click on (under “Mean HadCET data”) “Monthly HadCET mean.txt 1659 to date (32 KB)”. This gives the monthly averages, followed by the yearly average, for each year from 1659 (see Format). This shows many periods of quite dramatic warming and cooling in past centuries. The Hadley Centre usually updates it very quickly after the end of each month.

    • andyextance Says:

      Thanks! Taking a quick look at the dates you mention, even if we did get agricultural data for that period in the 17th and 18th century it’s not quite an apples-to-apples comparison temperature-wise. In the 36-year period from 1695-1793, the annual CET only exceeds 10°C twice. If we look after 1793, there’s no indication of an ongoing warming trend. The CET only exceeds 10°C three times from 1716-1751. But in the period from 1978-2013, it exceeds 10°C 17 times. Likewise the period from 1695-1733 only exceeds the average CET from 1961-1990 nine times, 1716-1751 exceeds it 15 times, 1978-2013 exceeds it 27 times. Current warming in the CET is pretty exceptional. I expect it will be possible to learn valuable lessons if there were historical crop yield records for this period, but they might be harder to earn than just a direct comparison.

      I’m going to see if I can post a graph here to illustrate that point. Hold on.

    • andyextance Says:

      Here’s the graph:
      Central European Temperature comparisons

      The vertical axis is the difference from the average CET from 1961-1990 for each year’s average CET, and the horizontal axis is the number of years through the period indicated through the legend. You’ll note 1733 (right hand end of red line, middle of green line) is unusually warm compared to its period, while 2013 (right hand end of blue line) is unusually cold. You’ll also note how much of the blue line is above zero – much more than the other lines. That’s what I was talking about in the previous reply.

      Hope that helps!

  5. Richard Mallett Says:

    Yes, the question is whether crops are more affected by absolute temperatures, or by changes in temperature. From about 1900-1980, there was a ‘slow but steady’ increase; whereas prior to that, there were more dramatic swings up and down (albeit about a lower mean, but the swings were so dramatic that the temperature in 1733 was higher than today)

  6. Another Week of Global Warming News, March 30, 2014 – A Few Things Ill Considered Says:

    […] 2014/03/29: SimpleC: Climate sensitivity wrangles don’t change the big picture on emissions […]

  7. Richard Mallett Says:

    Here is a comment from Tony Brown of http://www.climatereason.com regarding agriculture in the 17th. and 18th.century :-

    “In the late 17th century it was still very much an agrarian based society and to some extent a subsistence society (although much less than in Medieval times) So what happened with the weather from season to season really mattered. Poor springs meant crops could not be planted at the right time or that poor growth might mean they would need to be replanted. Heavy rains would mean flooding which might affect planting, yield and harvest. Cooler summers would mean poorer yields and that crops might not ripen fully. Hard winters physically destroyed apple trees by frost penetration and oaks cracked and were killed by the cold thereby reducing the acorns needed for pigs. Food in short supply would invariably send prices soaring and much more time was spent trying to keep warm. Also there were physical impediments-such as snow and flooding-to travel.

    So whilst the wealthy living in towns could overcome cold weather the vast majority living hand to mouth in the countryside would be badly affected. The church was active in providing relief to the poor and whilst many died from the cold the effects were nowhere near as bad as during other cold periods in earlier centuries when tens of thousands perished from cold and famine.

    So, yes warmth was very important to society’s well being and a parallel can be drawn to today that a very cold winter would impact greatly on peoples fuel bills and a cool wet year would still impact on farming but of course we are able to overcome these problems with our modern infrastructure. However, I think if there was a protracted cold period we would find that society would suffer greatly. For that reason I wish we had a Plan B to cover cooling as well as a Plan A to cover warming.”

    My own belief is that this is what we should be doing in case the pause is followed by an upturn or a downturn.

    • andyextance Says:

      That’s an odd argument, which the CET record you cited earlier shows. Rolling 10-year averages (ie the average for the decade leading up to each particular year) in the CET have been over 10C every year since 1998. No other decade in the entire record has been over 10C on average. The coldest decade was 1691-1700, at 8.07C. Today we’re already 2C warmer than that. We’ve got a lot of cooling to do until we get back to the late 17th century. Our crops, even our modern technology grew out of what the world had then. From the temperature we are now, it would be a lot easier to adapt to being 2C cooler – a temperature people have experienced during modern record-taking times – than to being 2C warmer – a temperature unheard of in the CET, and there’s already evidence to show the world hasn’t seen temperatures this warm in 44,000 years.

      Can I ask what makes you think we should be looking into a cooling plan B? Personally, I’d welcome a 0.5C cooling. Do you think it would be a bad thing?

      Thanks again for commenting – I like that you make points you believe in without getting angry about it.

      Here’s a graph of the averages I was mentioning earlier.
      10-year CET rolling averag

  8. Richard Mallett Says:

    Yes, I agree that we are experiencing higher temperatures now than in the 17th.-18th. centuries, of course. As I said, the question is whether it’s absolute temperatures that cause problems, or rapid changes in temperature that cause problems. Michael Mann says it’s the rate of change that challenges our adaptive capacity.

    The claim that this is the warmest in 44,000 years would contradict the belief that the Holocene Optimum of 8,000 -6,000 and 4,000 years ago, and the Roman Optimum of 2,000 years ago, were warmer than today; so one piece of evidence would not be enough to prove the point.

    An 0.5 C cooling would not be a bad thing – we have gone down from 10.70 C to 9.56 C annual average (CET) in the last two years. A return to a Dalton minimum (which could happen if solar activity returns to the low levels seen then) would be a problem.

    I don’t get angry – I have been attacked by the warmists and the sceptics alike :-)

    • andyextance Says:

      Any chance I can convince you to look longer than two years in terms of whether temperatures are going up or down? And don’t just go straight for 1998 either! Chaos and cycles like El Nino play a big role in temperatures – I think we can agree on that – and that’s why averages of at least a decade are way more meaningful than using individual years or what’s happened over the last two years as a basis for comparison. I’d be very grateful if you tried to think along those lines!

      And even though there has been a slowdown in warming, there are lots of reasons why a cooling is unlikely. Before we get into it, please take note of this report by the UK’s Royal Society – its most esteemed scientific institution, founded by the likes of Newton and Hooke – and the US National Academy of Sciences – set up specifically with the support of Abe Lincoln to give the US the best advice on science. It specifically and unequivocally say that ‘the world is warming and will warm by a further 2.6-4.8C by the end of the century’. Fancy pointing out what part of their argument is flawed?

      Given as we’ve been talking about the CET, it’s interesting to note that neither the minimum temperature or minimum decadal average temperature in that record fall during the Dalton minimum (if you take the Dalton minimum as being 1790-1830). Jim Hansen is his book “Storms of my Grandchildren” estimates the negative climate forcing effect at a standard solar minimum as about 1/8 the current positive forcing from CO2. How much more negative might it be at a Dalton minimum? Eight times as much? Averaging across the Dalton minimum period temperatures in the CET were 0.43C below the average temperature from 1961-1991. The relatively cool 2013 was still 0.09C warmer than the 1961-1991 average, and the decadal average leading up to 2013 is 0.63C warmer than the 1961-1991 average. The effect seen during the Dalton minimum would be kind of handy at this point.

      Hansen also says the following that you might find interesting:

      “A few geologists continue to speak as if they expect Earth to proceed into the next glacial cycle, just as it would have if humans were not around. That glacial period would begin with an ice sheet developing and growing in northern Canada. But why would we allow such an ice sheet to grow, and flow, and eventually crush major cities, when we could prevent it with the greenhouse gases from a single chlorofluorocarbon factory? Humans are now ‘in charge’ of future climate. It is a trivial task to avoid the net negative climate forcing that would push the planet into an ice age. But it is not an easy task to find a way to stop the growth of atmospheric greenhouse gases, most notably carbon dioxide.”

      Does Mann talk about rates of change or ‘transient response’? It would be nice to get a link to that comment. One of the things my plumbing analogy above doesn’t really capture is that we’re adding CO2 – I suppose we would be pouring in more fatty gunk to block up the drain in my analogy. That obviously means the slowly filling tank would get even fuller – we’d get even hotter – before outward flow gets back to normal. There’s a delay before equilibrium is regained, so by emitting CO2 now we’re putting off the point when the the tank stops getting fuller – temperature levels out – ever further into the future.

      These are some of the reasons why I don’t think the Dalton minimum conditions are likely to be that much of a risk, and why thinking it might be a threat were they to recur and preparing accordingly is likely to be a waste of time and effort. There are many more – I expect you’re familiar with most of them.

      • Richard Mallett Says:

        Another relevant piece of information referred to the CET at
        http://wattsupwiththat.com/2014/04/13/earth-to-lovejoy-0-9-c-in-a-century-is-not-huge/

        Professor Lovejoy of McGill University has said that we have had a ‘huge’ fluctuation since 1880 of 0.9 degrees Celsius.

        Christopher Monckton of Brenchley (after some preliminary discussion of the ‘hockey stick’ graph) takes 1894-2013 inclusive (two 60 year PDO cycles) in the CET = 0.90 C warming, compared to 0.89 C warming in the GISS / HadCRUT4 / NCDC records. This is to demonstrate that CET is a good proxy for global temperature changes over long time scales. (This was the part that I was most interested in)

        Monckton then takes the 100 years 1663-1762 inclusive = 0.90 C warming in the CET.which is a higher rate of change than Lovejoy’s ‘huge’ 0.9 C in 1880-2013.

      • andyextance Says:

        OK, so we’ve clearly moved on from the idea that we need to establish a plan in case we get cooling. That’s fine by me. However your argument that the rate of change is more important than the absolute temperature is based on a reference you can’t produce. I don’t really find that satisfactory.

        The rapid warming from 1695-1733 you highlight is interesting, however. Although you’re also using individual years when I asked you not to, it is still clear when you use decadal averages that there is a more rapid warming – in central England, at least – than today, just stopping well before the period reached modern temperatures. You can see both aspects of that in the CET 10 year rolling average graph above. But that lower temperature is important because absolute temperature does play a vital role when it comes to problems like ice sheets melting, which is the largest threat from climate change in my opinion.

        The Monckton comment also relies on individual years, which are affected by the chaotic and cyclic factors I mentioned before. If you take difference in the decadal average leading up to 1669 (the record only starts in 1659 so I can’t do a decadal average leading up to 1663) and the decadal average leading up to 1762, you get a figure of just 0.121C. If you want a full century, the difference 1669-1769 is -0.074C.

        On the point of whether it really is the warmest in 44,000 years, admittedly the Holocene optimum is the key hurdle to overcome there. As well as the Gifford Miller paper, Shaun Marcott’s also backs up the idea it’s warmer now than it was during the Holocene optimum – you’re probably familiar with it. And I’m not sure the argument that CO2 isn’t having a significant effect because it was warmer in the past without high CO2 levels stands up to a lot of scrutiny. You call it the ‘usual argument’. Is it one you subscribe to personally? Because I’d have to point out that even if and when other effects take the lead in historical warming, that doesn’t rule out CO2 having a driving role at other times. And actually, when you look at the evidence the graphs for the world’s temperature and its CO2 levels, such as in figure one of this open-access Jim Hansen paper, they look remarkably similar.

        Do you really give credence to these arguments that you quote? Or are you just playing devil’s advocate? Do you take things you learn here and use them in discussions at WUWT, for example? I really hope that you do.

      • Richard Mallett Says:

        It looks like I have to reply to my own comment, rather than to yours. Since Mann’s quote was just something in a blog post (which is why I couldn’t find it again a week later, as I follow a lot of blogs) it wasn’t of great importance anyway.

        I used the warming of 1695-1733 – in what sense is that using individual years ?

        Perhaps we should concentrate on why an absolute temperature of (say) 2-3 C above our current level of 9.56 C in the CET record would be catastrophic. (See below)

        Regarding ice sheets, currently the Arctic ice seems to be retreating, while the Antarctic ice seems to be expanding, so (once again) we can’t draw any global conclusions there.

        It looks as though Marcott’s graphs combine ice cores and tree rings, which is the same criticism that has been levelled at Mann. There is an interesting series of ice core graphs at http://www.nature.com/nature/journal/v489/n7414/full/nature11391.html?WT.ec_id=NATURE-20120906

        Of course, the Antarctic is a huge place. At http://www.antarcticglaciers.org/glaciers-and-climate/ice-cores/ice-cores-antarctic-peninsula/ are some ice core graphs from different parts of the Antarctic peninsula

        Regarding CO2 and temperature, I was mainly referring to the 1695-1733 warming – if that wasn’t due to CO2 levels, it would be interesting (and important) to know what did cause a rise of 0.065 C per year, compared to our expected rise this century of 0.030-0.055 C per year.

        I do believe what the CET tells me. I sometimes present what WUWT tells me to ‘warmists’ but they hardly ever respond. Perhaps you can help me here.

        What year (would you say) did Anthropogenic Global Warming begin ?
        What has been the rise in CO2 in ppm since then ? (C)
        What has been the rise in temperature from the CET record since then ? (T)
        What was the mechanism by which a rise of (C) caused a rise of (T) ? If we don’t know the mechanism, how do we know that one caused the other, if we don’t know what caused the 1695-1733 warming ?
        What part of (T) was caused by other factors such as ocean and solar effects ?
        What level of temperature in the CET record would (in your opinion) be ‘a bad thing’ and why ? 2-3 C above the current annual average 9.56 C is 11.56-12.56 C annual average – is that where you would ring your alarm bell ?

      • Richard Mallett Says:

        An interesting comment on the current state of play at http://judithcurry.com/2014/04/17/climate-change-what-we-dont-know/

      • Richard Mallett Says:

        An interesting assessment of the IPCC projections at http://wattsupwiththat.com/2014/04/16/a-clear-example-of-ipcc-ideology-trumping-fact/

  9. Richard Mallett Says:

    I only mentioned two years because you asked if an 0.5 C cooling would be a bad thing. More later.

  10. Richard Mallett Says:

    Regarding predictions that temperatures will rise by 2.6 C to 4.8 C by the end of the century (87 years), that would compare with (let’s take the CET period from 1963-2011 as the most extreme in recent years) 10.70-8.47=2.23 C in 48 years, so for 87 years that would be 4.04 C. So if the 1963-2011 trend continues, then that’s entirely possible, but the question is whether 0.030-0.055 C per year warming (if it happens) will be ‘a bad thing’ – in 1695 to 1733 it increased from 7.25 C to 10.47 C = 3.22 C in 38 years, or 0.085 C per year, and people obviously coped, and probably coped better than in the cooling that we mentioned last time.

    Regarding the Hansen quote about carbon dioxide, the claim is that (as I mentioned last time) temperatures were comparable, or even warmer, hundreds and thousands of years in the past, when there were much lower amounts of carbon dioxide. This is the usual argument against CO2 having a significant effect.

    Mann definitely mentioned rates of change, though I can’t find the quote now on a quick search, sorry.

  11. Richard Mallett Says:

    Since we were talking about ice, there are some interesting graphs of global and hemispheric sea ice anomalies at http://wattsupwiththat.com/reference-pages/sea-ice-page/ showing that sea ice is currently increasing overall, despite declining in the Northern hemisphere.

  12. Richard Mallett Says:

    Another interesting discussion about the 2.0 C threshold and the IPCC AR5 WG2 0.1 C per decade threshold (much less than the 1695-1733 rate of 0.85 C per decade, or the RS/NAS predicted rate of 0.30-0.55 C per decade) at http://www.climate-lab-book.ac.uk/2014/when-will-we-reach-2c/

  13. Richard Mallett Says:

    More on ice core records from the Late Holocene :-
    http://www.nipccreport.org/articles/2011/nov/16nov2011a2.html


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