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All IPCC definitions taken from Climate Change 2007: The Physical Science Basis. Working Group I Contribution to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change, Annex I, Glossary, pp. 941-954. Cambridge University Press.

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Why results from the next generation of climate models matter

Posted on 4 April 2019 by Guest Author

This is a re-post from Carbon Brief.  Prof Stephen Belcher is chief scientist at the UK Met OfficeDr Olivier Boucher is head of the Institut Pierre Simon Laplace (IPSL) Climate Modelling Centre; and Prof Rowan Sutton is director of climate research at the UK National Centre for Atmospheric Science (NCAS), University of Reading.

The first results from a new generation of global climate models, which are valuable tools for understanding climate change, are now becoming available from climate research centres around the world.

These new climate models make maximum use of advances in technology – such as increased supercomputing power – and feature many improvements in their treatment of Earth’s climate system. These include better representation of the weather systems that bring us wind and rain, the clouds within those weather systems, and aerosols – the myriad of small particles in the atmosphere that come from natural sources and human activities.

An unprecedented amount of information is available from the new models about the changing character of weather processes in a changing climate, which is important for understanding our exposure to climate hazards and how to make society more resilient to climate change.

Many of the new models from centres around the world have been recently finalised, with others due to be completed over the coming weeks. They will be included in the next international comparison of climate models, known as the sixth “Coupled Model Intercomparison Project” (CMIP6). This will provide the foundation of climate model information for the Intergovernmental Panel on Climate Change’s (IPCC) sixth assessment report (AR6) – which is due to be published in 2021.

From an international policy perspective, an important function of climate models is to provide evidence for estimates of the permissible global greenhouse gas emissions available to stay within a given level of global warming. This is known as a global “carbon budget” and varies in size according to the temperature goal in question and the defined likelihood of staying below these thresholds.

The climate agreement signed by governments in Paris in 2015 aims to keep global temperature rise this century to “well below” 2C above pre-industrial levels and to pursue efforts to limit the temperature increase even further to 1.5C.

A key factor in determining carbon budgets is how sensitive the Earth’s climate system is to increases in CO2. One measure of the long-term response of the climate over hundreds of years is known as the “equilibrium climate sensitivity” (ECS), which is defined as the temperature increase when CO2 has doubled and the climate system has come into equilibrium. The higher the ECS is, the smaller the remaining carbon budget has to be to meet a particular climate target.

Early results suggest ECS values from some of the new CMIP6 climate models are higher than previous estimates, with early numbers being reported between 2.8C (pdf) and 5.8C. This compares with the previous coupled model intercomparison project (CMIP5), which reported values between 2.1C to 4.7C. The IPCC’s fifth assessment report (AR5) assessed ECS to be “likely” in the range 1.5C to 4.5C and “very unlikely” greater than 6C. (These terms are defined using the IPCC methodology.)

The chart below shows how the early estimates from the CMIP6 models (red bar) compare with the CMIP5 models (yellow) and the assessment of ECS range from AR5 (blue). It should be noted that the CMIP6 range is preliminary and could change as more modelling centres publish their results.

Assessment range for ECS from IPCC AR5 (blue bar; thick bar denotes likely range, thin bar extending from it shows values below which ECS is “extremely unlikely” and above which ECS is “very unlikely”), range from CMIP5 (orange bar) and preliminary estimates of ECS values from new global climate models (red bar).

The IPCC estimates its assessed range of ECS through multiple lines of evidence, including the following:

  • Global climate models, which are powerful tools for understanding the effect of greenhouse gases on climate;
  • Simple models constrained by observed changes in the instrumental record (since 1850);
  • Using estimates of past climate going back thousands of years, which are inferred from proxy measures, such as ice cores and tree rings, combined with simple models;
  • New techniques to study and quantify climate processes, such as the interactions between clouds and radiation

For AR5, simple models constrained by observed changes in the instrumental record tended to give values of ECS generally in the lower part of the likely range of 1.5 to 4.5C, whereas global climate models tended to give ECS in the upper part of the likely range.

Climate scientists will need to assess how new understanding of ECS from the various lines of evidence compares. They will all be considered by the IPCC for AR6 due in 2021.

The chart below shows an assessment of climate sensitivity estimates published since the year 2000. Each dot shows the best estimate of ECS from an individual study, while the bars show the range of possible values found by that study. The colour indicates the type of study. The black bar on the right-hand end shows how the early CMIP6 estimates compare.

Updated compilation of climate sensitivity studies featured in the Carbon Brief climate sensitivity explainer, adapted from Knutti et al 2017. Bar on the far right shows the range of preliminary estimates of ECS values from the new global climate models.

The next step is for climate scientists to understand in detail why some of the new models are showing this shift in ECS – and how this fits with other lines of evidence. This includes looking at other measures of sensitivity, including “transient climate response” (TCR), which measures the rate of warming.

TCR is defined as the temperature increase at the instant that atmospheric CO2 has doubled, following an increase of 1% each year. This measure is arguably more useful for looking at changes we might expect over the current century, as it deals with shorter timescales than ECS.

The international community of scientists working on this new generation of climate models meets together for the first time next week from 25-28 March in Barcelona at the CMIP6 model analysis workshop. This has been organised under the auspices of the United Nations World Climate Research Programme (WCRP). It is an exciting opportunity for modelling centres to compare notes about the performance of their models and for the community to start thinking about the implications of this new rich seam of information for climate policy, including causes of past climate change and projections of likely future rates of change.

If it turns out that there is enough evidence to corroborate the higher ECS values from new-generation climate models then there would be important implications for carbon budgets. A higher ECS means a greater likelihood of reaching higher levels of global warming – even with deeper emissions cuts. Higher warming would allow less time to adapt and mean a greater likelihood of passing climate “tipping points” – such as thawing of permafrost, which would further accelerate warming.

The exact implications will only become clear once more analysis work is done using the latest generation climate models. In the meantime, the IPCC’s special report on 1.5C, published last year, remains the most up-to-date and robust assessment of the carbon budgets needed to meet the Paris goals.

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Comments 1 to 7:

  1. Improved awareness and understanding like this is essential and always helpful.

    And the parallel development of effective mitigation actions like CO2 removal (See BBC article here) is important and can also be helpful.

    But their helpfulness can be challenged by morally corrupted leaders. Stephen Gardiner presented improved understanding of how and why such moral corruption occurs in his 2011 book "A Perfect Moral Storm: The ethical tragedy of climate change".

    Such morally correpted leaders can be expected to continue to try to excuse doing less to reduce the use of fossil fuels. They may:

    • claim that they like the new science, but, because of the diversity of model results, claim that more certainty needs to be established before they will penalize the current generation just because of 'hopes' that doing so will limit the 'potential harm done to future generatons'.
    • claim that because of the technological potential to remove CO2 they can do less to reduce the use of fossil fuels, rather than adding the CO2 removal as an action to help do less harm to future generations.
    • abuse the claim of the potential for future generations to develop more and 'better' technology for CO2 removal as justification for choosing to be more harmful now.
    • continue to unjustiably claim that, based on past evidence and economic models (a history of development based on unsustainable and harmful pursuits and models that are far less reliable than even the older climate models), the economy only ever increases in value and the richer future generations will easily be able to enjoy their future while also brilliantly solving every problem (correcting every harm) created by the previous generations who chose not to correct how they lived.
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  2. Regarding the article. The evidence does indeed point at a climate sensitivity of 3 degrees and its good the new models are helping refine this. The problem is this spread of climate sensitivity numbers is still quite large and 3 degrees is still not hugely certain. Politicians will look at this spread of numbers and be bewlidered and uncertain: and they hate that. It's really important that climate senstivity is pinned down so you can say you are 95% sure it's a certain number.

    Regarding Direct air capture. I agree with OPOF. I think this probably has a place and enough potential to draw down some limited atmospheric CO2, but its going to have big problems doing more than this. These machines are still expensive and use a lot of specialist materials that are not in infinite supply.You will also have to find places to store the carbon, which will also require a lot of transport, and you will need unprecedented global cooperation.

    While something is obviously possible, sucking all the carbon from the air obviously looks like it would require tens of thousands of machines, perhaps millions, and would strain the planets resources and economy beyond the limit. I think the use of massive levels of direct air capture to suck all the excess CO2 out of the air is lord of the rings fantasy stuff. So using this technology as an excuse to go on burning fossil fuels looks delusional. There is a difference between well reasoned evidence based technological optimism and pure wishful thinking.

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  3. Interesting article! ... The ECS from my simple, 1-box "Temp vs CO2" modeling is 3.24 C with a 1st-order time constant of 14.6 yrs. Data from 1958 to Feb-2019. I don't know how to do the statistical math to determine the various uncertainty ranges. Could someone give me a link that will teach me how to do that for a model correlation like this? Thanks!

    The math I'm using is: Temp(@yr n) = a/Ln(2) x Ln(Cn/Cb) x [1-exp(-1/k)] + Temp(@yr n-1) x exp(-1/k) ... where, a=ECS; Cn=CO2 for yr n; Cb=CO2 for the baseline yr; k=time constant; Temp(@yr n-1) is temp at previous year of Temp(@yr n). ... Note: I am using a macro to find the best-fit a & k constants. It does this by incrementing one of the constants up & down (in tighter and tighter incremental steps), and then, at every incremental step, it uses solver to find the least squares best fit for the other constant, until the incremental steps get very small. I get the same results either way, no matter which constant increments, and which constant is found by solver.

    This Excel model file is located HERE [file name = TempRise vs CO2-Rise_Rev2(2019-02).xlsm]

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  4. Something related. First successful model simulation of the past 3 million years of climate change discussed here. "Our results imply a strong sensitivity of the Earth system to relatively small variations in atmospheric CO2....The climate sensitivity of the model is around 3°C global warming for a doubling of CO2 concentration, which is at the center of the range of current best estimates of climate sensitivity that range between 1.5 and 4.5°C."

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  5. I wonder if the new models take aerosols into consideration.  Have we done enough research for them to be realistically included.

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  6. @william - To find out would be as easy as an 8th grade science assignment...  LINK.  

    Within 30 seconds, I discovered that there are ongoing detailed discussions of aerosols and constraints on how to to incorporate them in CMIP6.  (Searchng fo aerosols and CMIP5 also yields impressive results on the massive amount of research and ongoing discussion about the role of aerosols vs AGW/CC in the current report.)

    Many people "wonder" about and ask questions about science at public internet forums and news sites, which is not the best approach.  Robust and comprehensive free scientific information is readily availble on the internet. 

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  7. I follow CMIP and IPCC developments with great interest, and my daily Skeptical Science emails like the one that linked to this article are greatly appreciated. 

    Science and science reporting websites and web pages are targeted at varying levels of reader technical expertise. Good, accurate information is always right at the tip of your fingers via Google and other internet search engines, although clever folks in AGW/CC denialist and anti-scientist websites can overwhelm and bury actual peer-reviewed science and legitimate science reporting under mountains of clever and persuasive b.s.

    Websites like this John Cook project - Skeptical Science - have for me replaced Science News Magazine, my favorite back in the 1980's and 1990's. Back then, I worked in sales and tech support for a company that developed and sold early examples of scientific software programmed to work on MS-DOS PC's. Science News was our most effective advertising venue, because at the time, it was the most read multidisciplinary scientific news magazine.

    Science News and Skeptical Science both publish short informational articles that allow one to keep up with current scientific research and news - small bits of knowledge and accurate summaries to keep us informed and provide incentive for further reading.

    It is best to look at climate science as a continuous endeavor that produces periodic reports and summaries - and no endpoint. Each report here at Skeptical Science is just another milestone as the work continues.  Scientific papers and reports - in this case, about climate science - are simply more pieces of a never-finished puzzle that can be seen as an evolving and clearly recognizable threat to human civilization.

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