Climate change is making the planet warmer, though there has been debate recently about just how quickly. The global balance of radiation (heat and light) has started to change as a result of increased greenhouse gases. Due to the urban heat island effect (good definition here), this temperature increase will be stronger and more noticeable in urban areas.

Sim City example City
Created using Sim City 4 (Electronic Arts, 2003)

There are two strands to addressing a problem such as future climate change; adaptation and mitigation. Adaptation means that we change our behaviours or manipulate our environment in order to be able to cope with the changes that occur. Mitigation means that we try and prevent the changes from happening, or we try to slow them down. Of course, for the best outcome we need to achieve both mitigation and adaptation; we must work hard to adapt to the new conditions that the changing climate brings but by doing so we should not worsen the initial problem. Instead we must try to bring adaptation measures in line with measures that will help to lessen the impact of the problem.

In the case of hot cities, it might seem simplest to undertake a ‘business as usual’ approach and focus on regulating the indoor temperature of buildings using air conditioning in the summer whilst enjoying the possible reduced heating demand during the winter months. Air conditioning can be expensive, both in terms of energy demand and monetary cost for energy provision. Reliance on air conditioning alone also does not address the underlying issue: it may be a valid adaptation technique but if the extra energy demand is not met through renewable energy sources, the carbon burden of the business increases. Furthermore, reliance on air conditioning does not address heat pressure on vulnerable communities such as the poor, elderly or infirm. A recent study in Vienna (described here) showed that elderly residents outside of care facilities tend to stay inside during heatwave periods, unaware that their indoor environment may be warmer than it is outdoors. This highlights the importance of adaptative behavioural measures and filtering of knowledge and instruction to those communities that need it.

Adaptive behavioural measures to cope in hot urban areas range from closing curtains/shutters during the hottest time of day to altering activity and working patterns to undertake the greatest activity during the cooler morning and evening hours. These changes to the working day would need to start at the highest levels of business and be rolled out in conjunction with an established heatwave early warning system in order to avoid loss of income from reduced workforce efficiency/availability. Information and social care for heat-stress vulnerable city residents must be made available in affected areas.

Though we can adapt to hot cities, the cost of insufficient or tardy adaptation measures could be high. This study into death risk for elderly city residents showed that where urban areas do not cool down sufficiently at night, elderly residents are twice as likely to die during heat waves than those living in the suburbs. One of the distinguishing features of the urban heat island effect is that temperature increase relative to the rural surroundings is stronger overnight than during the day. This means that whilst in rural areas there may be some reprieve from the heat overnight, in the city a continued heatwave situation is more likely. In order to reduce this risk, we need to focus on mitigation at the same time as adaptation. The direction for mitigation is twofold: lessen the urban heat island by cooling the city and reduce the fossil fuel carbon emitted by the city as a unit.

If we want to cool down the city, we need to address what it is about the city that makes it hotter than the countryside in the first place. Previously on Ground to Sky, I wrote about the difference in moisture and heat balances between vegetated and urban surfaces. Urban surfaces are often darker than vegetated/soil surface and therefore they absorb more sunlight. Not only this but they are capable of storing this heat for a much longer period of time (accounting for increased heat release overnight relative to the rural land surface). Vegetation also cools the surface by allowing the evaporation of water through its leaves (transpiration) and by shading the ground. Greening of the city is widely accepted to improve the heat balance of urban areas and may also benefit air quality.  These areas also contribute to improved quality of life for nearby inhabitants and raise capital for the area (as evidenced quite well in this TED talk by Marjora Carter, which also provides a strong case for addressing the inequity in urban planning policy between rich and poor city areas). Urban planning is a city-wide discipline but is in most cases conducted on a neighbourhood by neighbourhood basis. Additionally, building design is something conducted on a building by building basis but individual buildings affect the surrounding neighbourhood. Mitigation of the urban heat island and efforts to decarbonise and decentralise energy (bring energy production closer to the centre of demand) is something that requires careful consideration at the city scale.

Adaptation and mitigation of urban heat is a current area of research and a driver for engineering and technological development. Sustainable city planning, neighbourhood and building design is higher and higher on the political agenda in the West so we are moving in the right direction. However, in the developing world, urbanisation is occurring at an alarming rate (see this article discussing sustainable urbanisation in India) and monitoring of urban meteorology and air quality is limited or completely lacking in some developing cities. Research focused on methods to accomodate the growing city population in a way that will mitigate urban heat issues for the future is an urgent requirement in developing countries.

In my next article on Ground to Sky, I will discuss the surface energy balance (of radiation and heat) in more detail, hopefully providing an insight into the physics that gives rise to issues such as the greenhouse effect and the urban heat island.

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No matter where you are in your research from starting out and seeking the gap in the knowledge, to rounding off that last few touches to a journal article, there will come a time when your desk looks like this.

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Perhaps your desk doesn’t look like this, and you are tidy in the way you read papers, perhaps reading them all on digital devices (honourable undertaking that I haven’t yet mastered – I dislike sitting down to read at length if the thing I’m reading is not printed in black on white paper). But no matter how you read, or what your desk looks like, we all reach the point where the mountain accumulates and threatens to bury you under an endless web of “author publications,” “paper citations” and “cited by.” Here follows a few pointers from my experience on literature searching, reading journal articles and citation management.  It’s really advice for beginning environmental science PhD students, but it might be valuable to others interested in research methods.

Literature searching

When embarking on a new topic, the easiest way to find information is through a keyword search. You can do this simply on abstract database sites like Web of Science. If you’re using one of these it’s worth restricting the search to the last 5 years for an initial investigation. The latest papers will tell you the state of the research environment for your chosen topic right now. They can help you identify the current knowledge gaps and by going through a few of them you can usually identify a few common papers.  By looking at the papers they all cite, you will eventually find your way to the seminal works in your field and possibly review articles.  Seminal papers (highly cited) can identify the key authors in this specialism. By looking at this author’s other work and following the “cited by” web from the seminal papers, it is easy to gain a very quick impression of the literature in your chosen field.  Review articles can be very helpful but bear in mind unless you are interested in first principles, generally the more recent a review article, the more useful it will be to new research.

When literature searching in an expansive topic, the question will soon arise “how far back in time is too far back?” and/or “how remotely related to my research is too remotely related?” The days of limited access and hidden texts in vaults is gone. Today scientists have easy access to reams of information dating back to the turn of the 20th century and beyond.  For an initial literature search, I would advise not casting the net too far back.  In the early stages of a project, the most important thing you can do is find out what people are doing right now, and how your proposed project can fit in to the current body of literature.  When you are deeper into your project, perhaps devising methodologies for experimentation and statistical analysis this is when older/less related studies can be helpful.  At these stages, keyword searching for specific techniques can be very useful, and don’t discount the use of textbooks and manuals.  During my PhD viva, I was accused of re-inventing the wheel somewhat when I failed to cite papers from the late 1980s and early 1990s that had used a similar technique.  This was not out of disrespect, it was due to not looking far enough back down the chain of literature to identify these early papers.  This is how sometimes a reliance on the most recent papers can catch you out – if the papers do not cite these older methodologies and results then how will you find out about them?  I have learnt to cast the net wider and further back when considering experimental design by other researchers in a given field.  In the latter stages of a research project, seeking out comparable results and items to discuss can easily turn into yet another expansive search. In my PhD, I ended up citing papers from atmospheric chemistry and microbiology sources, despite my main focus being soil physics.  It’s easy to forget in your specialism that environmental science is a systems science subject and as such you may need to draw in discussion from a wider range of source material than you might have initially thought.

Finally DON’T restrict your literature search to the peer-reviewed journals. There is a wealth of information to be found online, using Google (consultancy reports, governmental reports, white papers…) and offline in books and magazine publications. Wikipedia, even though you’ll never cite it, can give you the lowdown on a new topic very quickly and provide invaluable keywords for database searches. Though you will streamline and academise your sources later on (particularly when writing up!) you shouldn’t restrict sources of information at the early stages.

Reading scientific papers

When confronted with 25 pages of tightly arranged font, equations and figures, it is excusable to feel daunted (especially if there are 20 more to follow).  The first thing to ask is: how much of this do I need to read to get the messages I need at my current research stage?  There will always be the papers that you’ll read from end to end; seminal papers, review articles (relevant sections) and the most recent work in your field.  You’ll end up coming back to many of these papers time and again, to look at different sections. So there’s really no need to go through each one with a fine tooth comb. Instead: (the following assumes you’ll always read the full abstract first – recommended in any case)

  • If you are beginning a project, focus your reading on the introduction, discussion and conclusions.  Find out the justifications for their research, the aims of their research, what they found out and what they suggest doing next.  Delving to deep into the methods and results can waste time at this stage, there’ll be a lot to get through and you can keep reference to the most useful papers to read in depth later.
  • When developing your methodologies focus on the methods sections (funnily enough). Follow method citations even if the paper itself is outside of your field. You might not be interested in the results of the cited paper at all, just the method they used to get those results. Whatever you do, don’t look at the paper and go “oh, ‘Journal of Cattle Management’, that’s not relevant to my research” when the authors have used an experimental or statistical method that could be.
  • When you have results you’ll probably want to compare them to others in the literature. You can identify results in abstracts much of the time (numerical abstracts are quite ‘in’ nowadays if you compare to abstracts from 20/30 years ago). Looking at results sections of relevant papers can also give you a great deal of information on how best to visualise your own data and the types of statistical analyses that are most useful.
  • Writing up and discussion stages.  Joy, this is the fun part. Sit yourself down and conduct another thorough literature search on your own observations. Has anybody found this before? How did they explain it? Be prepared – you might end up outside of your primary subject area again. Prepare to let this stage hoover up your time. Think of yourself as the Sherlock Holmes of the literature. Have you exhausted all leads? All possible explanations from the papers you know of?  Keep an eye on introductions, results and discussion sections.
  • Conclusions, finalising writing up, preparing papers. Well done. Now be prepared to do another literature search. Years have probably passed by now (sorry but it’s true!) since you searched your main keywords right at the beginning. You don’t want your last cited paper to be from 2 years ago. If you’ve kept abreast of the literature throughout, well done! If not, the 2013 papers in your subject area are calling you.

Citation management

I think it’s nearly enough about this. Writing about literature searching is almost as wearing as doing it. The last thing I want to do is highly recommend to all new researchers that you use a citation management system.  I let my university’s inability to provide the software for free to a student’s personal computer to put me off use of this software. As a result I wrote a thesis and managed all 300+ references manually. That is, by hand. That is, checking the reference list and the text match up and no mistakes have been made, slowly, tediously. I am still doing so in the papers relating to my PhD topic as I have no reference manager set up. Copy-and-paste is no way to go about this while sensible options are available.

Now I am staff, I have a staff computer and so I can have citation management software installed. My platform of choice is EndNote, which also has a web-based version to cite on the move!  There are also free web-based programs (i.e. Mendeley) which I haven’t used but seems pretty good to me (but anything would seem pretty good to someone who processed their PhD citations manually. If there is one thing you take away from this blog article it is not to do that).

Good luck in all your literature searching and reading endeavours!

In my next ‘life in environmental science’ article, I’ll talk about the topic I introduced earlier: environmental science as a system science discipline.  Using my research career thus far and its progress from soil to atmospheric science as an example, I’ll show how the many strands of earth and environmental science are highly interrelated. The next article on Ground to Sky will return to the science, and I will be discussing current thinking on how we can cool down our hot cities.

Greenhouse gases are top of the worldwide green agenda and all around the world industrious groups of people are seeking to reduce emissions. Whether by legislating against irresponsible fossil fuel use, encouraging energy efficiency or creating traffic-free zones (and many, many other methods besides), the only way we’ll know whether such schemes are having an effect is by knowing the rate of emission of gases into the atmosphere.  We need to be able to do this to a fine enough level of detail to be able to tell if our reduction strategies are successful or not.

This is no mean feat. The atmosphere is notoriously complicated, as is the land surface with sinks (take up gases) and sources (emitters of gases) springing up all over the place, often as quick as you can count and varying wildly according to environmental conditions.  In spite of these difficulties, scientists, industry and policy have created methodologies and taken steps towards putting some figures on greenhouse gas emission.  There are three main methods, which I will introduce in turn.

1. Emissions inventories.

I like to call this the “let’s just tot up what we know” method.  It relies on accurate figures for the extent of a certain activity (i.e. how many cars there are, how many power stations are running and for how long) and an accurate figure for how much greenhouse gas is emitted by each of these activities. These are called activity data and the emissions factor.

Flux = sum of all (activity data x emissions factor).

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I call this the “tot up what we know” method because all that goes into an inventory is the emissions sources we know about.  They also usually only include human activities or have a subjective or modelled term for natural sources and sinks.  We have several uncertainties in the inventory method and they require extensive checking:

  • is the activity data right? i.e.  is our traffic count up to date?, are the power companies telling the truth about what’s coming out of that chimney?, are we up to date with all of the new houses?
  • are the emissions factors right? i.e. are we simply estimating over a large range of possible values?
  • are we summing up all of the sources we know about? what about the sources that we don’t know about? could we be overestimating the total emissions by ignoring the effect of vegetation uptake?

Because of questions like these, inventories work best in a coarse resolution (over a large area, like a country).  They aren’t checked using measurements and are best for making broad decisions (i.e. cutting back on fossil fuel energy in a certain country) than narrow ones (i.e. implementing a neighbourhood road closure and cycling scheme).

2. Direct measurement.

Measurement of greenhouse gas fluxes can happen at all sorts of scales. From a chamber placed over a square metre of soil right up to an instrument mounted on a tall tower above a city.  For the purpose of this discussion I will focus on these tall tower instruments.  The instrument on the new tower in the drawing below is designed to capture fluxes from a wide area.  So all of those terms that we put in our emissions inventory (plus or minus the influence of the sources/sinks that we ‘forgot’) are contributing to the flux measurement.

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This particular system measures the difference in concentration of greenhouse gas between two vertical air packets (I will come back to how this works some other time).  This tells us the overall flux.

To be able to interpret this, we need to know where the fluxes measured by the instrument are coming from.  That is, we need to know the area of the ground that is sending gas up to be captured by the instrument.  We don’t want to consider sources that aren’t being included in the measurement and likewise we don’t want to miss any.  To do this, we need a ‘source area’ or ‘footprint’ model.  This can calculate the area of ground we can expect the fluxes to be coming from and then we can investigate how they might be influencing this measurement.

Hm, you might think this is a little vague.  You’re right, when it comes to making decisions about changing the number of cars in your city or increasing the amount of green space, it makes sense to be able to tell more exactly where the sources really are and whether or not the action has made a difference to their extent.  But if you want to monitor what the overall flux is over the city, what it really is, through observations rather than approximations, a direct measurement scheme is what you need.

How about a middle ground between the two?  A method that includes all the knowledge we have about what is out there generating greenhouse gases (inventories) plus real greenhouse gas measurements to tell us if we’re going wrong?  This brings me to the final method, which is newest to science and very much still in development.  This is one of the things that is keeping me busy during the working day right now.

3. Measurements + Model (the “inversion” method)

Inversion methodology is where we start to get really clever.  I will explain this step by step.

a. Take the inventories and make it as accurate as you possibly can for your chosen area.

b. Place an instrument on a tower that measures concentrations of greenhouse gases.  (Note: this is concentration not flux.  The instrument tells us what the concentration of gas is, not the change in that concentration).

c. Use a meteorological forecast model to tell you what the winds are doing throughout the surface km of atmosphere.

d. Place the inventories and the wind data (plus some other information such as the height of the boundary layer and the surface roughness) into a chemistry-transport model.  This model can give you an idea of what the concentration of the greenhouse gas in question is EXPECTED to be.

e. (And here’s the clever bit) Use the real data from the atmosphere, plus the expected concentrations and then run the model BACKWARDS.  This “inversion” of the method uses everything we know to ‘go back to source’ and tell us where we should be checking our inventories a little more closely.

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This last method has a lot of potential. But right now it is in its pilot stages.  New science is consistently being published in developing this method and industry is becoming aware of the economic opportunity in providing governments with such a service.

The method that an international group/national govenment/local authority will choose depends on what they wish to get out of the investigation. Benchmark figures? Real measurements? In-depth analysis? Participating in new development of new scientific technology? The future is bright for estimation methodology, so don’t let “how will we even know if it works?” put you off taking your bike to work instead of your car.

In the next article at Ground to Sky, I will be discussing the ups and downs of wading through huge piles of scientific literature and providing some tips from my experience.  Following this I will return to the technical science and will be discussing some of the methods that we can use to cool down our hot cities during the summer months, without compromising winter warmth.

Imagine you’re out walking in your local big city on a bright and very hot day. Before you set out from your suburban home, you noticed a light breeze on the air but here, upon exiting the underground station, the air is heavy and still. You have a look around you. Where did that nice breeze go? A nearby tall building captures your attention, you look up and there on the roof is a pair of small wind turbines, furiously turning in the wind that has been deflected above your head.

You frown, why is the wind all the way up there, it’s down here on street level that you need to feel its benefit. Because it is a nice day, you decide to walk to the big city park. A short distance later, you turn off of the wide road and into a side street. Suddenly, the breeze hits you but it is much stronger than you remembered, whipping around your clothes and your hair. Looking around, you realise that this street is much more narrow than the road with the train station on it. Because it is facing a different way, the wind is channeled directly into it and is being buffeted by the tall buildings on either side.

You’re thankful when you reach the end of this street. You turn again into a street running parallel to the first. It isn’t quite as wide as the first street. Once you turn the corner, the wind has gone again and you walk a little while before you have to cross over. When you do, you can feel the wind again, pulling at your clothes and the stack of papers you were taking to the park to read. Better not lose them.

A few turns later, through narrow streets with hardly any wind at all and a hot, humid feel to them, you reach the park. It feels cooler here and the light breeze has returned. You settle yourself in the shade of a tree and begin to read.

The route of this hypothetical journey through the city is shown in the below drawing. Take a moment to look at the airflow that is depicted on each stage of the trip.

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The street with the train station on (1) is wide and runs perpendicular to the direction of the wind. The wind hits the tall buildings and is deflected upwards, into the flow of the air turbine which is positioned to capture the most consistent wind direction. The narrow street which is parallel to wind direction (2) funnels any wind that has not been deflected by tall buildings, decreasing flow at the edges but increasing it down the centre of the road. The next road perpendicular to wind direction (3) is not quite as wide as the first and the buildings are not quite so tall. The walker was first in the ‘wake’ of the building (the area in which the building has interrupted the air flow) but when they crossed over, they exited the wake and were once more affected by the air stream. The narrow streets that are perpendicular to wind direction (4) do not receive much replenishment of air on this day. The air feels warm here and a bit difficult to breathe thanks to the traffic. If the air is not getting replaced due to the flow simply ‘skipping’ over the top of this street – what might be the consequence for air quality? It’s a relief for the walker to reach the park (5), where the trees are having a cooling effect on the warm air and that breeze has returned as a result of the wide open space without building interference.

The structure of the city (i.e. where are all the tall buildings and how are they oriented) has a large influence on the city’s microclimate. The effect of the city buildings on the meteorology of the region has lots of important conseqences for city planning, building design, placement of energy services such as wind turbines and structures such as tall masts and for air quality and greenhouse gas dispersion. The climate of the city and the breathability of the air is vastly important for the quality of life of city residents and workers and the experience of visitors.

In my next blog article, I will take a step outside of the city and look at things on a much larger scale, talking about the many methods in which a country can estimate its greenhouse gas emissions and the current science on which of these methods is closest to giving us a good answer.

The atmospheric boundary layer (ABL) is usually defined as the portion of the atmosphere that is directly influenced by the land surface. It ranges in height from around 50 to 4000m but is usually in the range of 1 to 2 km in depth. Due to a combination of turbulence and static stability (in simple terms: the way in which air naturally arranges itself when subjected to different temperatures and pressures) the air tends to arrange itself in layers (stratification). In the case of the boundary layer there is a region of very stable air which ‘caps’ the air below it that is interacting with the earth’s surface.

You can observe the boundary layer using equipment that can detect aerosols. The aerosols will be restricted by the cap at the top of the boundary layer, so the instrument will see high concentrations of the aerosols inside the boundary layer then a ‘pause’ followed by a well mixed zone in the free atmosphere above.

Speaking generally, the boundary layer is unstable during the day, when the warm surface is heating the air and creating the turbulence that is mixing the air well within the boundary layer itself. It is stable at night, as the ground is cooler than the air and mixing is reduced. At night the boundary layer is much shallower than during the day. The height of the boundary layer will begin to increase as the ground heats up and decrease as the ground cools down. All of this is driven quite strongly by two things: the weather conditions (windy and cloudy = a neutral boundary layer) and the land surface. At the time of writing, only one of these things we can control.

In a previous blog article, I wrote a little about how different land surfaces change the fluxes of heat as a result of the differences in how they absorb or reflect radiation and how they behave around water (affecting the amount of heat remaining after water is evaporated). One of the most important things about this difference in the heat balance is the effect that it has on the boundary layer, which has been observed to behave differently in the city to in the countryside. This difference is largely related to how the boundary layer develops. For example: if the city is warmer than the country, the unstable boundary layer is likely to persist later into the day. The city not only affects the heat balance but also the wind, a fact that anybody who has had to walk down a road between tall buildings on a windy day can attest to. Particularly if they were carrying an umbrella at the time. This increased turbulence as a result of the extra ‘surface roughness’ (I’ll come back to this another time) in the city, might also affect the dynamics of the forming or deforming boundary layer.

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One thing that all this certainly does affect is the distribution and concentration of pollutants in the city. A stable boundary layer (like overnight) is shallow and doesn’t mix very well with the atmosphere above. The result is increased pollutants close to the surface even without taking into account the actual emission of the pollutants because:

a) they cannot mix easily with the atmosphere above

and

b) the amount of space in the atmosphere that they can mix in has become limited.

The result is high pollutant concentrations in the air. In some cities under certain conditions ‘urban plumes’ of pollutants develop, spreading downstream of the city. In very hot cities there is the added complication of high turbulence inside the city creating enhanced updrafts that can rise above the rural boundary layer. When combined with weak downdrafts in the surrounding countryside, the net effect could be continual recirculation of pollutants into the city and the associated negative impact on air quality.

We can make adjustments to the heat balance of the city using effective city planning and energy management techniques. This is a field of science that is new and the importance of many of the individual contributors to the overall picture are not yet quantified (for example: the net cooling effect of trees, the net heat loss from buildings at the city scale).

In my next blog article I will discuss this topic in a little more detail, focusing more on the effect that the city has on the wind and the resulting influence on pollution.

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It was a cool but bright day in October, and I took a few steps back from my equipment to take a photograph. You can see from the picture here the variation in grass quality from the near to middle distance. The equipment had been fenced off during the cattle grazing period of the farm calendar. Ironic really, because if you look close at the photograph you can see that the grass inside the markers on the closest side is dark green. The grass here is enriched with cattle urine, that myself and my PhD supervisor had sprinkled down on the grass inside the markers with watering cans whilst bemused cattle looked on from over the electric fence.

The white boxes allowed me to measure the release of greenhouse gases from the field surface. Next to the boxes, some porous tubes were buried in the soil. By measuring the greenhouse gas concentrations inside those tubes I was able to determine some of the key biological production and physical transport processes and see how these changed when the soil had been subject to addition of the cattle urine. Along with measurements of soil chemistry, moisture and temperature, an overall picture could be developed of what heppened in that soil to create the observed greenhouse gases at the surface and how these things changed over time.

Controlled field experiments like these are crucial if we are to understand the effects that changing land use can have on the natural environment. Today, computers become ever more powerful. Processors and memory that once filled whole rooms can now fit into the palm of your hand. And scientists have capitalised on this new technology; something that was once a dream has become a feasible reality: modelling the Earth’s complex environmental system and making inferrences about how this system might change into the future. Now we move ever closer to being able to enter into a computer how many cattle we put on our field and being told the amount of fertilisation that will occur, a value for how much nitrogen might make it into nearby waterways and the effect that the cattle will have on the greenhouse gas balance of your piece of land.

This might sound highly ambitious, and it is. Yet these models continue to appear and are in continuous development and improvement; researchers validate these models using measurements from the field and look at improving understanding of processes and process interaction so that the amount of uncertainty in our estimates may be reduced (JULES land surface model, ECOSSE carbon model). Models are known by scientists to be a tool in development, a simplification of complex systems to allow us to search for missing information in the knowledge base and give us clues as to how a known process might change in the future as human activity or natural variation alters land use, biodiversity or climate. Issues arise when model data is misreported or uncertainty in the models is not adequately described. It’s important to always remember that models (particularly the largest and most complex) can have missing or inadequately described processes (Stephens and Bony, Science, 2013 (paytoview) ; Carbon Brief, 2013 (free commentary)). In order to ensure that models are telling us the right things, we need field studies (validation studies) and in order to know what to put in to the models in the first place, we need field studies (parameterisation studies). Models only tell us what they know about, which is what we input into them. A complex computer model can give us far more detail on complex processes than we would ever be able to get without them, but without field measurements to provide references and comparisons, we will never know if we’re going wrong.

In the next blog article, I’ll return to the technical articles and write a little more about city environments and their influence on the lower atmosphere. In the next insight article I will write about the importance of literature in a scientist’s life and the perils that are associated with under and over exposure to the enormous piles of reading that accumulate in the scientist’s in-tray.

Atop most of the world’s land surface, there is a layer of soil varying in depth from a mere few centimetres through to over ten metres in well established peat bogs. The majority of the world’s cities were founded in the most sensible places for human inhabitation – fertile lands close to a source of water. As time advanced, so did the extent and population density of many of these cities and towns and gradually bare and vegetated soils came to represent the minority land use within city borders.

Today, in many cities, parkland and other greenfield sites are considered premium resources. Townhouses bordering parks fetch the highest house prices and nearby green space is prerequisite for the majority of buildings of state and homes of the rich. Urban vegetation takes the form of trees planted by the roadsides, parks large and small, turf playing fields, urban farms and forestry and, importantly, residential gardens.

Across the world, city planners are coming to value the importance of greenery and soil within the city bounds and seek to maximise it. Why have they come to this conclusion? Not only to drive up the house prices and provide a pleasant environment to live in for city citizens but also because of the key influence that soils and vegetation have on natural cycles of heat, water and pollutants in the surface to atmosphere interface. These ‘services’ that the soils provide are termed “ecosystem services” and they are as important in the city as they are in natural environments. Ecosystem services might be cultural (house prices, space for recreational activity, aesthetic value) or provide habitats for animals and plants. They might be productive (city farms and allotments) or they might regulate the environment (take up carbon, capture pollutants such as heavy metals, cool the city down).

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In the bottom panel of the above graphic I have drawn three different land surfaces and how they impact three important things; heat, water and carbon dioxide. The size of the arrow is a rough indicator of the magnitude of the movement of that commodity. Outbound heat is much greater from the tarmac surface than from the vegetated or soil surfaces for several reasons. Firstly, tarmac is a dark surface and it absorbs a larger amount of incoming solar radiation. But soils can be dark too – so why no large heat loss from soil surfaces? Because soils are often wet as well as dark, in fact quite often a darker soil is a wetter one. And heat is required to evaporate this water, therefore less heat is absorbed and the warming effect is reduced. On vegetated soils the plants mediate this evaporation through their water-upake systems, a process known as transpiration. Back to the tarmac. The second reason it is a heating surface is that it does not absorb water, it pools on the surface. So instead of a consistent and steady evaporation flux you have either rapid evaporation from a pool on the surface, or if there is drainage, little cooling effect from water. In fact the tarmac is a barrier to downward water flow (infiltration) which is why urbanisation in areas that are naturally floodplains can cause serious flooding issues if nowhere is provided for displaced water to go. Finally, another important ecosystem service is the regulation of carbon dioxide by vegetation. This effect in some city neighbourhoods could be substantial enough in the summer to balance out the carbon dioxide from human sources (UC Santa Barbara, 2012) though it would take a lot of vegetation to do this at the city scale – to the extent that such an environment might no longer qualify as an urban space (University of Helsinki, 2012).

For further information on this fascinating and complex subject, I recommend this detailed review by Pataki et al (2011) which is available open-access here. This article provides analysis of key ecosystem services provided by urban ecosystems and their related costs (here termed ‘ecosystem disservices’).  It also provides a useful summary in Table 1. of the magnitude of impact of each of the discussed ecosystem services and the level of uncertainty associated with current knowledge.

In the next technical article on Ground to Sky, I will describe in more detail the influence of land surface type on heat, including the startling impact that hot cities have on the lower atmosphere. Before this, I will write the first of what I hope will be many insight articles into my life in science, focusing on taking field measurements of soil properties and greenhouse gases and why measurements are critically important now, as environmental science moves towards an entirely modelled world.