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FAQ on climate models: Part II

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This is a continuation of a previous post including interesting questions from the comments.

More Questions

  • What are parameterisations?

    Some physics in the real world, that is necessary for a climate model to work, is only known empirically. Or perhaps the theory only really applies at scales much smaller than the model grid size. This physics needs to be ‘parameterised’ i.e. a formulation is used that captures the phenomenology of the process and its sensitivity to change but without going into all of the very small scale details. These parameterisations are approximations to the phenomena that we wish to model, but which work at the scales the models actually resolve. A simple example is the radiation code – instead of using a line-by-line code which would resolve the absorption at over 10,000 individual wavelengths, a GCM generally uses a broad-band approximation (with 30 to 50 bands) which gives very close to the same results as a full calculation. Another example is the formula for the evaporation from the ocean as a function of the large-scale humidity, temperature and wind-speed. This is really a highly turbulent phenomena, but there are good approximations that give the net evaporation as a function of the large scale (‘bulk’) conditions. In some parameterisations, the functional form is reasonably well known, but the values of specific coefficients might not be. In these cases, the parameterisations are ‘tuned’ to reproduce the observed processes as much as possible.

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FAQ on climate models Vanliga frågor om klimatmodeller

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We discuss climate models a lot, and from the comments here and in other forums it’s clear that there remains a great deal of confusion about what climate models do and how their results should be interpreted. This post is designed to be a FAQ for climate model questions – of which a few are already given. If you have comments or other questions, ask them as concisely as possible in the comment section and if they are of enough interest, we’ll add them to the post so that we can have a resource for future discussions. (We would ask that you please focus on real questions that have real answers and, as always, avoid rhetorical excesses).

Part II is here.
Det finns en svensk översättning tillgänglig här.
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Start here

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[Note this is page is updated regularly. Please notify us of any dead links. Last update: 10 May 2022.]

We’re often asked to provide a one stop link for resources that people can use to get up to speed on the issue of climate change, and so here is a selection. Unlike our other postings, we’ll amend this as we discover or are pointed to new resources. Different people have different needs and so we will group resources according to the level people start at.

For complete beginners:

NCAR: Weather and climate basics
Center for Climate and Energy Solutions: Climate basics
Wikipedia: Global Warming
NASA: Global Warming
National Academy of Science: America’s Climate Choices (2011)
Encyclopedia of Earth: Climate Change
Global Warming: Man or Myth? (Scott Mandia, SUNY Suffolk)
Open Learn: The Basics of Climate Prediction

There is a booklet on Climate Literacy from multiple agencies (NOAA, NSF, AAAS) available here (pdf).

The UK Govt. had a good site on The Science of Climate Change (archived).

The portal for climate and climate change of the ZAMG (Zentralaanstalt für Meteorologie und Geodynamik, Vienna, Austria). (In German) (added Jan 2011).

Those with some knowledge:

The IPCC Frequently Asked Questions are an excellent start (AR4 version here , updates were provided in the 5th Assessment report (pdf) and again for AR6).

The UK Royal Society and US National Academies of Science produced a joint Q&A on climate change in 2014, and an update in 2017.

RealClimate: Start with our index.

Informed, but in need of more detail:

Science: You can’t do better than the IPCC reports themselves (AR6 2021, AR5 2013, AR4 2007, TAR 2001). Also the Climate Science Special Report for the US National Climate Assessment.

History: Spencer Weart’s “Discovery of Global Warming” (AIP)

Informed, but seeking serious discussion of common contrarian talking points:

All of the below links have indexed debunks of most of the common points of confusion:

Please feel free to suggest other suitable resources, particularly in different languages, and we’ll try to keep this list up to date.

A Slovak translation is available here

Tłumaczenie na polski dostępne jest tutaj
A Bulgarian translation is available here (via Ivan Boreev).

Does a Global Temperature Exist?Existe uma temperature global?

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Does a global temperature exist? This is the question asked in a recently published article in Journal of Non-Equilibrium Thermodynamics by Christopher Essex, Ross McKitrick, and Bjarne Andresen. The paper argues that the global mean temperature is not physical, and that there may be many other ways of computing a mean which will give different trends.

The common arithmetic mean is just an estimate that provides a measure of the centre value of a batch of measurements (centre of a cloud of data points, and can be written more formally as the integral of x f(x) dx. The whole paper is irrelevant in the context of a climate change because it missed a very central point. CO2 affects all surface temperatures on Earth, and in order to improve the signal-to-noise ratio, an ordinary arithmetic mean will enhance the common signal in all the measurements and suppress the internal variations which are spatially incoherent (e.g. not caused by CO2 or other external forcings). Thus the choice may not need a physical justification, but is part of a scientific test which enables us to get a clearer ‘yes’ or ‘no’. One could choose to look at the global mean sea level instead, which does have a physical meaning because it represents an estimate for the volume of the water in the oceans, but the choice is not crucial as long as the indicator used really responds to the conditions under investigation. And the global mean temperature is indeed a function of the temperature over the whole planetary surface.

[Read more…] about Does a Global Temperature Exist?Existe uma temperature global?

Has Pacific Northwest snowpack declined? Yes.

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There has been a bit of a flap here at the University of Washington over the state of the snowpack in United States Pacific Northwest region. The Seattle city mayor, Greg Nickels (a well known advocate for city-based CO2 reduction initiatives) wrote in an Op-Ed piece in the Seattle Times that

The average snowpack in the Cascades has declined 50 percent since 1950 and will be cut in half again in 30 years if we don’t start addressing the problems of climate change now. That snow not only provides our drinking water, it powers the hydroelectric dams that keep our lights on.
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Aerosols: The Last Frontier? Aerossóis: A Última Fronteira?

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Guest commentary from Juliane Fry, UC Berkeley

The recently released IPCC 2007 Fourth Assessment Report Summary for Policymakers reminds us that aerosols remain the least understood component of the climate system. Aerosols are solid or liquid particles suspended in the atmosphere, consisting of (in rough order of abundance): sea salt, mineral dust, inorganic salts such as ammonium sulfate (which has natural as well as anthropogenic sources from e.g. coal burning), and carbonaceous aerosol such as soot, plant emissions, and incompletely combusted fossil fuel. As should be apparent from this list, there are many natural sources of aerosol, but changes have been observed in particular, in the atmospheric loading of carbonaceous aerosol and sulphates, which originate in part from fossil fuel burning. While a relatively minor part of the overall aerosol mass, changes in the anthropogenic portion of aerosols since 1750 have resulted in a globally averaged net radiative forcing of roughly -1.2 W/m2, in comparison to the overall average CO2 forcing of +1.66 W/m2.
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What triggers ice ages? O que Dispara as Eras Glaciais? Buzul Çağlarını Tetikleyen Nedir?Qu’est ce qui déclenche les glaciations?

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by Rasmus Benestad, with contributions from Caspar & Eric

In a recent article in Climatic Change, D.G. Martinson and W.C. Pitman III discuss a new hypothesis explaining how the climate could change abruptly between ice ages and inter-glacial (warm) periods. They argue that the changes in Earth’s orbit around the Sun in isolation is not sufficient to explain the estimated high rate of change, and that there must be an amplifying feedback process kicking in. The necessity for a feedback is not new, as the Swedish Nobel Prize winner (Chemistry), Svante Arrhenius, suggested already in 1896 that CO2 could act as an amplification mechanism. In addition, there is the albedo feedback, where the amount of solar radiation that is reflected back into space, scales with the area of the ice- and snow-cover. And are clouds as well as other aspects playing a role.

por Rasmus Benestad, com contribuições de Caspar & Eric

Em um artigo recente da Climatic Change, D.G. Martinson e W.C. Pitman III discutem uma nova hipótese que explica como o clima pode mudar abruptamente entre eras glaciais e períodos interglaciais (quentes). Eles argumentam que as mudanças na órbita da Terra ao redor do Sol em isolado não são suficientes para explicar as altas taxas de mudanças estimadas, e que deve necessariamente haver a ação de um mecanismo de feedback (ou retro-alimentação) amplificando o processo. A necessidade de um feedback não é nova, pois o sueco ganhador do Prêmio Nobel (Química), Svante Arrhenius, já havia sugerido em 1896 que o CO2 deveria agir como um mecanismo de amplificação. Além do mais, existe o feedback do albedo, pelo qual a quantidade de radiação solar que é refletida de volta ao espaço é escalonável com a área de cobertura de gelo e neve. E existem nuvens bem como outros aspectos envolvidos.

Orbital forcing A hipótese de Martinson & Pitman III formula que a entrada de água doce funciona em consonância com o ciclo de Milankovitch e o feedback de albedo. Eles concluem que os ‘maiores’ términos podem somente acontecer após um acúmulo de gelo grande o suficiente para isolar o Artico, inibindo o fluxo de entrada de água doce até um ponto em que o aumento da salinidade na camada superficial, através de um vagaroso e contínuo crescimento do gelo marinho, causa uma inversão das águas marinhas do Ártico (pelo efeito na circulação atmosférica e nas correntes oceânicas). A inversão vertical traz água quente de baixo para cima, promovendo condições mais favoráveis ao degelo. A salinidade também tem um papel, mas a hipótese não menciona variações de gases de efeito estufa (GEE). Algumas questões: Martinson e Pitman III esqueceram disso? Ou os GEE representam somente uma pequena contribuição? E, não poderiam as mudanças nos GEE explicar boa parte da variabilidade? Por outro lado, parece plausível que mudanças na salinidade e na entrada de água doce poderiam afetar a formação de gelo marinho e a convecção profunda. Contudo, até o presente, a hipótese proposta por Martinson and Pitman III é meramente uma especulação, e estamos aguardando para ver se a hipótese pode ser testada através de experimentos de modelos numéricos (o que pode requerer modelos oceânicos e de gelo marinho com maior resolução que os atualmente usados em modelos climáticos globais). Seria interessante conduzir experimentos para avaliar a significância individual da água doce, dos GEE e o efeito combinado.

Uma reação ao trabalho de Martison e Pittman é: Onde está o cálculo de energia? Gases de efeito estufa contribuem somente com alguns W/m2, em contraste com uma forçante >40 do ciclo sazonal de Milankovich. Para esta nova idéia ter mérito, teria sido melhor ter no mínimo fluxos de calor em paralelo com a forçante radioativa do CO2. Estudos de modelagem anteriores encontraram que GEE produzem aproximadamente 50% de todo Último Máximo Glacial (inglês, LGM) para a resposta da temperatura atual (veja por exemplo Broccoli & Manabe), a outra parte sendo o albedo, etc., que respondem ao ciclo sazonal de irradiância. É muito difícil isolar completamente as causas individuais pois as mudanças nos GEE podem produzir alterações na distribuição de nuvens e gelo marinho. Mas a grosso modo, se você rodar um LGM e somente somente reduzir o nível do mar, introduzir as calotas de gelo, mudar a vegetação, adicionar alguma poeria (embora esta ainda seja grosseira), então você alcançaria ao redor de 50% do caminho que você quer ir. Mude a concentração de GEE e você chegaria mais próximo. Isso é mais ou menos o que Manabe e Stouffer mostraram há quinze anos atrás. A questão é se realmente precisamos de algo mais, e se esse ‘algo mais’ tem força suficiente.

traduzido por Ivan B. T. Lima e Fernando M. Ramos.


[Read more…] about What triggers ice ages? O que Dispara as Eras Glaciais? Buzul Çağlarını Tetikleyen Nedir?Qu’est ce qui déclenche les glaciations?

A Linkage Between the LIA and Gulf Stream?

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Michael Mann & Gavin Schmidt

The precise factors underlying the so-called “Little Ice Age” (LIA) have been intensely debated within the scientific community. One key metric in this debate is the spatial pattern of cooling which may provide a ‘fingerprint’ of the underlying climate change, whether that was externally forced (from solar or volcanic activity) or was part of an intrinsic mode of variability.

Surface temperatures in parts of Europe appear to have have averaged nearly 1°C below the 20th century mean during multidecadal intervals of the late 16th and late 17th century (and with even more extreme coolness for individual years), though most reconstructions indicate less than 0.5°C cooling relative to 20th century mean conditions for the Northern Hemisphere as a whole. There is much less data during these time intervals for the Southern Hemisphere, and that severely limits what conclusions can be drawn there. Just what combination of factors could explain this pattern of observations has remained somewhat enigmatic. A new ingredient in this debate comes with a recent paper in Nature by Lund et al.
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Short and simple arguments for why climate can be predicted

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Sometimes, I encounter arguments suggesting that since we cannot predict the weather beyond a couple of weeks, then it must be impossible to predict the climate in 100 years. Such statements tend to present themselves as a kind of revelation, often in social settings and parties after I have revealed for some of the guests that I’m a climatologist (if I say I work for the Meteorological Institute, I almost always get the question “so, what’s the weather going to be like tomorrow?”). Such occasions also tend to be times when I’m not too inclined to indulge in deep scientific or technical explanations. Or when talking to a journalist who wants an easy answer. In those cases I try to provide a short and simple, but convincing, explanation that is easy for most people to understand why climate can be predicted despite the chaotic nature of the weather (a more theoretical discussion is provided in the earlier post Chaos and Climate). One approach is to try to relate the topic to something with which they are familiar, such as to point to empirical observations which most accept (I suppose with hindsight it could be similar to the researchers in the early 20th century trying to convince that nuclear reactions were possible – just look at the Sun, and there is the proof! Or before that, the debate about whether atoms were real or not – just look at the blue sky, and you look at the proof…). I like to emphasised the words ‘weather‘ and ‘climate‘ above, because they mean different things.

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Decrease in Atlantic circulation? Ralentissement de la circulation Atlantique?

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by Gavin Schmidt and Michael Mann

In a sure-to-be widely publicized paper in the Dec. 1 Nature, Bryden et al. present results from oceanographic cruises at 25°N across the Atlantic showing a ~30% decline in the ocean overturning circulation. These cruises have been repeated every few years since 1957, and the last two cruises (in 1998 and 2004) show notable changes in the structure of the deep return circulation. In particular, the very deepest part of the return flow (at around 3000 to 5000 m) has reduced and moved up in the water column compared to previous decades. How solid is this result and what might it imply for climate?par Gavin Schmidt et Michael Mann (traduit de l’anglais par T. de Garidel)

Dans un article largement commenté dans la presse (voir par exemple ici et la) dans le numéro du 1er déc. de Nature, Bryden et al. présentent des résultats de croisières océanographiques à 25°N à travers l’Océan Atlantique qui montrent un déclin d’environ 30% de la circulation océanique “générale”–dite circulation thermohaline-. Ces croisières ont été répétées régulièrement depuis 1957, et les deux dernières croisières (en 1998 et 2004) montrent des changements notables de la structure de la circulation de retour profonde. En particulier, le flux dans la partie la plus profonde du courant de retour (entre environ 3000 et 5000 m) a diminué et est remonté dans la colonne de l’eau par rapport aux décennies précédentes. Quelle est la robustesse de ces résultats et quelles en sont les implications potentielles pour le climat ?
(suite…)

[Read more…] about Decrease in Atlantic circulation? Ralentissement de la circulation Atlantique?