Angus Ferraro

A tiny soapbox for a climate researcher.


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Transformational Climate Science – the future of climate research

On 15-16 May a diverse group of climate researchers gathered at the University of Exeter to discuss the state of climate change following the publication of the IPCC Fifth Assessment Report and the future of the field. In a previous post I discussed some of the key themes. Here I’m going to summarise some of what went on at the conference in terms of how we should proceed with climate research in the future. It will be biased towards physical science, since that’s my personal area of interest.

What are the outstanding challenges in climate research? What are the areas that need further investigation? Should the IPCC process function as a driver for new research efforts?

Science & policy panell (left to right): Thomas Stocker, Saffron O’Neill, Georgina Mace, Andrea Tilche, Asuncion St Clair, Chris Field. Credit: University of Exeter via Flickr.

I think the final question there is an especially interesting one. The role of the IPCC is to bring together diverse research findings and assess our state of knowledge. And yet, sometimes it is seen as an end in itself. One of the speakers at the conference noted he sometimes sees research justified as ‘important for the IPCC assessment’, and that this is a big turn-off. If that’s the best thing the researcher can say about their work it’s probably not going to be that interesting. Of course, it might be that the research is fascinating and yields new insight into some of the big challenges of contemporary climate science. In that case the authors should say so. The challenges of contemporary climate science are not challenges because the IPCC says so; they are challenges because there are scientific and policy questions that need answering. Thomas Stocker, in his remarks, noted that one of the most important things to do in future climate research is to continue with ‘curiosity-driven research’. There are many examples of pure research that did not have any obvious application spawning major advances, often with great commercial success.

I’m no science policy scholar, so I won’t discuss where the balance should lie between ‘pure’ and ‘applied’ research, but this conference provided some food for thought. Some speakers emphasised both equally, generating a tension which isn’t easily resolved. Indeed, the majority of the ‘challenges’ identified at the meeting fell on the ‘applied’ side in the sense that they were suggestions to make climate research more policy-relevant. Perhaps that is unsurprising at a meeting structured around the IPCC, with its strong emphasis on policy-relevance.

One of the main challenges identified during the meeting was moving from the robust aspects of climate theory to those phenomena which actually matter to people on the ground. Robust aspects of climate theory are largely thermodynamically driven, argued Stephen Belcher. We understand that the accumulating energy balance of the Earth will lead to warming, and that the land will warm faster than the ocean. We understand that surface warming leads to greater evaporation and consequently, on average, greater precipitation. But the things we really care about are rather smaller in scale. We experience climate through weather events, and these are influenced as much by dynamic as thermodynamic factors. Unfortunately, we have much less confidence in our understanding of these dynamical processes. They have smaller spatial scales and shorter temporal scales, and so they are much more computationally demanding to model. They involve processes which are not well understood. Ted Shepherd has spoken similarly about the need to focus on the climate dynamics of global warming. It certainly seems like a fertile area for future research, though also a very challenging one.

On the subject of things that people actually care about, Mat Collins and David Stephenson both discussed moving from simplistic averages to the broader statistics of climate. We experience climate through weather, and we care about it most of all when it’s extreme. It’s the ‘tails’ of the probability distribution of weather events that we care about. Unfortunately, said Mat Collins, we don’t really have a good idea about how to assess this. Our current batch of climate model simulations are a statistically questionable sample – they have known deficiencies, biases and interdependencies. We need to address this or develop techniques to deal with it.

On the theme of translating our physical understanding into more relevant information, there was also some discussion of modelling of the politico-economic systems. Integrated Assessment Models attempt to do this, but there is no coordinated intercomparison of these models like there is for climate models. Some at the meeting objected, saying we don’t have good enough theory to be able to credibly model economics. Perhaps that’s true, but just because something is complicated and uncertain doesn’t mean we shouldn’t try to model it; in fact, perhaps it means we should! An intercomparison would at least help us know where we stand.

A final note: this continued emphasis on relevance seems to me to require a greater role of values in presenting stories about what humans care about. Simon Caney spoke about the major breakthrough of including ethicists and philosophers in WG3. More broadly, I think a move to greater policy-relevance would need everyone involved to be crystal clear about what is factual and what it normative (value-based). People were mostly good at that in this meeting. A productive discussion on climate change needs good-quality factual basis and a wide range of normative viewpoints. There was even some discussion about how it might required new forms of collaborative decision-making.

Regardless, the very necessary shift towards policy relevance will mean the potential for even greater controversies. Sam Fankhauser spoke about the need to develop very clear channels for communication to help get around this: ‘whatever we say will be used in that very emotional debate’. It’s difficult and sometimes downright unpleasant, but I think ultimately we have to embrace that.

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Paper review: On the misinterpretation of the North Atlantic Oscillation in CMIP5 models

Citation: Davini, P. & Cagnazzo, C. (2013),  On the misinterpretation of the North Atlantic Oscillation in CMIP5 models, Clim. Dyn. doi:10.1007/s00382-013-1970-y.

The North Atlantic Oscillation (NAO) is a big hitter in the world of atmospheric dynamics. It’s everywhere, and it (or its close relations the Arctic Oscillation and Northern Annular Mode) invoked in pretty much every explanation of why midlatitude countries get the weather they do. But what is it?

Positive phase of the NAO (image from Columbia University)

In the simplest terms, the North Atlantic Oscillation is an expression of the location of the jet stream, a band of strong winds which circles the globe at a latitude of about 45 degrees. Weather systems form on the jet stream and are steered along with it. The jet stream wobbles north and south (like most fluids, it forms eddies), and depending on its latitude it might send a storm into the Mediterranean or to the United Kingdom.

The NAO is often used as a neat way to summarise the behaviour of the jet stream. In the real atmosphere it describes a specific pattern of variation in its position, but as this paper (and others) shows, this pattern isn’t exactly the same in climate models.

An aside before we begin. It’s not really correct to say the NAO explains why the jet stream and weather systems are where they are. The NAO must be a certain way if the jet stream is where it is. It’s just a description, like saying, ‘we got a storm because we are at the latitude where the storm was’. The reasons for the way the jet shifts north and south are complex and really not obvious, and we’re learning more about it all the time.

So that’s what the NAO is in general terms. When we analyse atmospheric data we give it a mathematical form. This involves a tricky but really rather clever analysis technical called Empirical Orthogonal Function analysis (or Principal Components analysis). I won’t go into it here but it’s basically a way to pull out a pattern according to which something varies. If you think of the pattern of variation of a swinging pendulum, it’s main variation pattern is a back and forth swing. For the jet stream, the main variation is a north and south wobble – the NAO.

Negative phase of the NAO (image from Columbia University)

The convention is, when the NAO is in a negative phase the jet is shifted southward, and when it is in a positive phase it is shifted northward. Generally, it’s also wavier in its negative phase than its positive phase, and more waviness means more ‘blocking’, which is when we get big meanders, producing high pressure systems which hang around for a week or so and give stable, calm weather.

But some climate models don’t have an NAO that behaves like this. For some of them, the main variability includes too much northward wobbling and not enough southward wobbling. Another group has variability which is more like a pulsing of the speed, going from fast to slow and back again, without any wobbling. For some the north-south wobbling is simply too weak.

This paper shows the weakness of the mathematical definition of the NAO – it’s not the same between different models! The examples above represent different physical processes so it’s meaningless to lump them together like this. But, importantly, all the models do have something like the real-world NAO – that is, the north-south wobbling. It’s just that the standard mathematical definition doesn’t always pick that out. This is the peril of using a mathematical construction with no physical basis to define variations in the jet latitude which are the result of specific physical processes.

Obviously it’s better to think of things, if we can, in terms of the physical processes involved rather than this rather obscure mathematical idea. Davini and Cagnazzo end their paper with this recommendation:

…since climate models represent a slightly different world with respect to the real one, special caution must be applied…when the NAO is studied. We conclude suggesting that instead of using the NAO to study the North Atlantic variability it would be better to adopt diagnostics based on jet stream position and strength


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Rainfall and climate – a dynamic problem

This was originally posted on the EGU blog, GeoLog.

Rain is grace; rain is the sky descending to the earth; without rain, there would be no life. – John Updike

Rain quenches the thirst of soils and vegetation, fuelling ecosystems and much of the world’s agriculture. Whether it ruins a day on the beach or destroys a season’s harvest, it makes humans deeply aware of their vulnerability to the vagaries of the atmosphere. It’s important to understand how rainfall changes in a changing climate. Here, I will describe the issues in understanding precipitation changes and how two recent papers help to solve the puzzle.

Predicting rainfall is difficult. It is a small-scale phenomenon, especially in the towers of convective cloud in the Tropics. Weather forecasting models are just beginning to capture them properly at scales of a kilometre or so, but climate models, which have to be run for decades rather than days, calculate atmospheric conditions on scales of hundreds of kilometres. Rainfall has to be simplified in these models, since we cannot calculate the physical properties of individual clouds. These simplified representations are called parameterisations. A precipitation parameterisation relates the average rainfall over a large area to the average amount of water in the air. Different models do this in different ways and, because it’s a simplification, there is no definitive ‘right’ way. This means there is some disagreement among climate models about how rainfall will change in the future, especially in the Tropics (areas on the figure which are not stippled).

Climate model projections of precipitation change in a future with high greenhouse gas emissions. Left: current generation of models, Right: previous generation of models (around 2005). Top: December-February, Bottom: June-August. Stippling shows areas where models largely agree. White areas show complete disagreement among models (source: Knutti & Sedlacek, 2013).

If we think about precipitation in general theoretical terms, we can find laws which must be followed and use them to make predictions, as Issac Held & Brian Soden did in their study of how the hydrological cycle responds to global warming. Rain is caused by the upward transport of water vapour from the surface into the atmosphere, where it condenses, forms clouds and rains out. The amount of moisture going up must, of course, balance the amount coming back down as rain.

As the climate warms, the amount of water vapour a fixed mass of air can hold increases. This means that, as long as the circulations transporting water upwards remain the same, the total amount of water vapour going upwards must increase – which means the amount of rain coming down must also increase. This is called the ‘rich get richer’ mechanism, because it increases rainfall in regions where there is already a lot of rain driven by upward moisture transport. It’s a fundamental mechanism driven by thermodynamic laws…but that doesn’t mean it’s the only thing going on.

Convective raincloud in tropical Africa (photo credit: Jeff Attaway).

If climate model projections followed the ‘rich get richer’ mechanism, precipitation would increase most in the regions with the most precipitation currently. In fact it is more complicated than that. Robin Chadwick and his colleagues explored the effect of weaker vertical motions in a warmer climate. We can understand this by thinking about what carbon dioxide does to the vertical temperature profile. It warms the mid-troposphere (about 5 km up) more than the surface. To get convective upward motion, the air at the surface must be less dense (i.e. warmer) than the air above. Warming the air aloft suppresses this motion. The Chadwick decomposition calculates the part of the precipitation changes caused by changes in moisture (which goes at about 7% per K) and the part caused by the reduction in upward transport. They find the two tend to roughly cancel each other out, which means the spatial shifts in precipitation are determined by changing patterns of surface temperature (since warm surfaces produce upward motion).

Sandrine Bony and her team decompose precipitation changes into two main components rather than three: one is the ‘dynamical’ component, associated with changing upward motions, and the other is the ‘thermodynamical’ component, including changes in atmospheric moisture content. Unlike the Chadwick method, the thermodynamical component is not designed solely to represent the ‘rich get richer’ mechanism. This means the thermodynamical component isn’t just a 7% per K increase; it includes things like the spatial changes in surface temperature. The dynamical component isolates the change in precipitation caused by changes in upward motion.

Monsoon raincloud over a lake in the Tibetan Plateau (photo credit: Janneke Ijmker).

The ‘rich get richer’ rule of thumb becomes increasingly irrelevant at smaller scales. This is frustrating, because these are the scales we really care about! It’s not particularly useful knowing what will happen in a general sense over the whole Tropical region. Farmers want to know what will happen to the seasonal rains on their small piece of land.

Bony also points out that geoengineering schemes which aim to reduce incoming solar radiation to cool the planet’s surface would leave the dynamical component of precipitation change untouched. This is because the dynamical component is caused by the warming of the mid-troposphere by carbon dioxide, and this remains even if we cool the surface. It is an example of the inexact nature of the cancellation between carbon dioxide increases and geoengineering schemes to decrease the amount of carbon dioxide in the atmosphere, and demonstrates that the only way to stop carbon dioxide-driven climate change properly is to stop emitting carbon dioxide.

Bony and Chadwick’s decompositions show how one can glean a lot more information from climate model projections than one would expect from first glance. We have established some general facts about climate change related to the Earth’s energy budget. In that sense we understand quite well what will happen in a warming climate. However, there is still a lot of diversity between model projections, most of which comes from differences in the dynamical response. Local changes in rainfall are related to changes in circulation, and this is the area in which a lot more work needs to be done.

References:

Bony, Sandrine, Gilles Bellon, Daniel Klocke, Steven Sherwood, Solange Fermepin & Sébastien Denvil, 2013: Robust direct effect of carbon dioxide on tropical circulation and regional precipitation, Nat. Geosci., doi:10.1038/ngeo1799

Chadwick, Robin, Ian Boutle & Gill Martin, 2013: Spatial Patterns of Precipitation Change in CMIP5: Why the Rich don’t get Richer in the Tropics. J. Climate, doi: 10.1175/JCLI-D-12-00543.1

Held, Isaac M., Brian J. Soden, 2006: Robust Responses of the Hydrological Cycle to Global Warming. J. Climate, 19, 5686–5699. doi: 10.1175/JCLI3990.1

Knutti, Reto & Jan Sedláček, 2013: Robustness and uncertainties in the new CMIP5 climate model projections, Nat. Clim. Change, doi: 10.1038/nclimate1716


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Paper review: Stratospheric versus tropospheric control of the strength and structure of the Brewer-Dobson circulation

Citation: Gerber, Edwin P., 2012: Stratospheric versus Tropospheric Control of the Strength and Structure of the Brewer–Dobson Circulation. J. Atmos. Sci., 69, 2857–2877.
[pdf] from Edwin Gerber’s web page

The stratosphere lies above the troposphere (which is where most ‘weather’ happens) between about 10-50 km altitude. Its circulation is in many ways counter-intuitive, thanks mainly to the influence of waves.

Wave motions are ever-present in the atmosphere on a huge variety of scales – from turbulent eddies to thousand-kilometre planetary waves. They can basically be understood as motions which dump momentum (which changes the wind speed) when they break. Think of waves surging onshore at the beach. Out at sea, the wave doesn’t actually move water horizontally. It just moves water up and down. The wave moves but the water doesn’t. As the shore gets closer and the sea shallower, the wave breaks and water rushes forward. Edwin Gerber’s paper describes the behaviour of the wave-driven Brewer-Dobson circulation in the stratosphere.

The Brewer-Dobson circulation (BDC) transports air poleward from the tropics. This happens because waves from the troposphere are produced by mountains (think about putting a rock in a stream and imagine the swirls this produces), move up into the stratosphere and break. This breaking decreases the wind speed and air drifts towards the pole and then sinks. This is because of the principle of conservation of angular momentum. Basically, if you are spinning in a computer chair really fast, you will find it difficult to pull your arms in. If you slow down (imagine this is the effect of some waves) you will be able to pull your arms towards your body (your ‘pole’). The BDC is driven by the breaking of very large planetary waves (thousands of kilometres long)

The Brewer-Dobson circulation (schematic from University of Frankfurt)

Gerber uses a fairly simple atmospheric model to look at the behaviour of the BDC. The model isn’t meant to be like the real world. Instead it’s meant to simulate the fluid dynamics of the atmosphere in a way which looks something like the real world. It follows the same physical laws as the real world. This means the physics we learn about using the model can be applied to the real world.

This paper picks out two ways in which the BDC can change.

  1. Tropospheric control. Increase the wave activity and the BDC will become stronger.
  2. Stratospheric control. Increase the strength of the polar vortex (westerly winds in the winter hemisphere of the stratosphere) and the upward movement of waves will change.

The effects of the control mechanisms on the BDC can be easily seen by looking at the ‘age‘ of the air. This is the time since the air was at a given level in the atmosphere (in this paper, 100 hPa, or around 20 km). The picture below shows the age for different model setups. Red colours show older air, indicating a slower circulation.

Fig. 1 from Gerber (2012). Age of air. Left: (top) weak wave activity, (bottom) strong wave activity. Right: (top) weak winds in the stratosphere, (bottom) strong winds in the stratosphere.

Tropospheric control is the easier one to understand. If the BDC is driven by waves, increasing the wave activity will make it stronger. What does ‘increasing the wave activity’ mean? Well, the amplitude (‘size’) of the waves can be increased by changing the height of the mountains producing them. This is the what the study does. It sounds silly, because for the foreseeable future the mountains on this planet are going to stay much the same. But this is just the simple approach Gerber took in the paper. In fact, the amplitude of the planetary waves can be changed by large-scale changes in the air pressure in the lower atmosphere (such as the ‘Arctic Oscillation’, which is an expression of the latitude of the jet stream).

The stratospheric control mechanism is a little more subtle than the tropospheric one. Changing the winds in the stratospheric polar vortex changes the way waves behave. If the vortex is very weak, waves are trapped at the base of the stratosphere (under 20 km). This means the bottom part of the BDC speeds up, but the top part slows down. If the vortex is strong, waves can move all the way up into the stratosphere, so the top part speeds up.

Gerber’s experiments agree with some previous work indicating the BDC has two branches: an upper and a lower (you can see this in the first picture: the lower part is around 20 km while the upper part goes up to around 50 km). They seem to affected in different ways. The lower branch is most susceptible to changes in the waves coming from below; the upper branch is modified by changes in the way waves behave once they are in the stratosphere.

Why is this important? The BDC transports ozone to the poles, and ozone is important for protecting living things from harmful ultra-violet sunlight. Changes in the upward motion at the bottom of the stratosphere also change the level of the tropopause (thick blue line in the first picture). This is important because the level of the tropopause governs how much water vapour gets into the stratosphere. In the troposphere, extra water just rains out, but in the stratosphere extra water vapour exerts a warming influence on the Earth’s surface. If the tropopause gets lower it moves to a warmer region of the atmosphere, which allows more water vapour to get into the stratosphere. So it’s important we understand how and why the BDC does what it does.

It is unusual to find a single-author paper of such detail nowadays. It is also unusual to find such a readable paper on a highly technical subject. As I have said I find stratospheric dynamics quite tricky, but this paper really nicely illustrated the important physical principles without giving me a headache. There is a lot more in the paper than in this summary, but I hope it has at least given you a flavour of the clear, focused experimental design and the excellent presentation of its arguments.