Hot, cold and dry!

Last time, we covered the extremely humid and wet areas of the planet, the rainforests.  Today we go to the other extreme, the desert, and semi-desert climates.

These are the climates classified in Group B in the Koppen Classification, and are distributed as shown in these images below:

Hot desert regions, BWh in the Koppen Climate Classification. Image from Wikipedia

Cold desert regions BWk classification. Image from Wikipedia

Semi-arid regions BSh and BSk classification. Image from Wikipedia

All these regions are extremely dry, and most are extremely hot too, as you can see, these regions include the Sahara, the Gobi desert, Arizona, Nevada, Central Australia, Iran and large chunks of the Middle East.  Whether an area is classified as arid or not depends on the evapotranspiration that I mentioned last time, which is the sum of evaporation and transpiration from plants.  This image illustrates evapotranspiration:

Evapotranspiration. Image from Wikipedia

All the arid, and semi arid regions have more evapotranspiration than precipitation (fancy word for rain and snow etc, stuff that falls out of the sky).  This means these areas are not able to support much plant life, and consequently, the number of animals which can live in the region are limited.   There are of course, plants which survive, and indeed thrive in these conditions, and they have some amazing adaptations to allow them to do so, the same for the animals which live in these conditions.

It is often assumed that deserts have no rain at all, but, some have over 25cm a year.  The evapotranspiration however, is greater than this.

So, what makes these regions so dry, and mostly hot?

You remember last time I mentioned the ITCZ?  Well, arid climates fall around 30°S and 30°N, which is between the first green lines above and below the equator in this image:

Arid regions are located around the bands with an H to the north and south of the Equator (marked by the ITCZ on this image). Image from Wikipedia

As I mentioned in my first post on climate (HERE), the air circulations from the equator to the edge of the tropics are known as Hadley cells.  To recap, the warm air rises at the equator, and moves north or south (depending on the hemisphere), as it cools, it begins to descend.  This descent occurs around 30°S and 30°N.  This air has deposited much of the moisture it picked up at the equator already (mostly over the rainforest regions), so it is much drier when it reaches these latitudes.

For interested geeky parties like me, these latitudes are also known as the Horse Latitudes (Allegedly due to some old naval tradition for running around on a deck of a ship with a horse…).  They are characterised by high pressure, which leads to clear, cloudless skies (due to the lack of water vapour in the atmosphere) and very high temperatures.  Up here in the North, when we have high pressure it is associated with those lovely (Or not, depending on your perspective) hot summer days, and those crispy cold awesome winter days (Guess which I prefer!).  The lack of clouds in these regions means that whilst the days can be scorchingly hot, the nights can be extremely chilly. The highest recorded temperatures are from desert regions, and they can reach over 40°C in summer on a regular basis.

The deserts which are not hot all year round tend to be the ones at a higher altitude, and these occasionally receive snow in the winter, but their summers are still very hot, and they have very little rainfall.

The semi-arid regions are found bordering desert regions, and the main difference between these, and the arid regions is that they receive a little more rainfall, and so are able to maintain a wider variety of plants and animals.

Now, I should mention that whilst these areas are deserts, Antarctica is also one huge desert, but is not on the list of arid regions.  This is because Antarctica has a polar climate, which we will get to in a later post..

Next time, we will move onto the climate I am most familiar with, the wet, windy temperate climates, where we will bump into the Jet Stream, the Gulf Stream, and probably a few other streams too!  I will try not to make it too long, promise!

Classifying climate

So, as promised last post, today we will look at the different climate zones on the planet, and some of the main features of each.

As mentioned last post, climate refers to the long term patterns in a region, and these are usually classified using the Köppen classification system, which divides the Earth into regions with similar annual variations in climate.  This is how the planet looks under this system:

Köppen climate classification map. Image from Wikipedia

Don’t be put off by all the strange abbreviations at the bottom, the basic thing is that there are 5 main groups, and then lots of sub-groups within each one.

Starting at the beginning of the alphabet:

Group A is the Tropical climate regions, this is broadly the band around the equator, where we find tropical rain forests (group Af), monsoons (Am), and Savannah regions (Aw).

These regions have some of the most productive places on the planet, covering the Amazon Rainforest, the Indonesian rainforests, and the rainforests of central Africa. They also encompass the great plains of Africa, the Serengeti, and places in the US such as Hawaii (Aw) and Florida (Am) and the northern coast of Australia (Aw).

So, what does it mean to have a tropical climate? Well, it means the mean (average) temperature is 18 degrees centigrade (Which is a nice warm day for me here in Denmark!), it also means that the day length is roughly equal throughout the year, and the temperature does not have the wide range of seasonal, or daily variations you find in the mid-latitudes.

Looking a little deeper into each: Af, the Rainforest zones, are the zones closest to the equator, these have minor variations in weather throughout the year, being situated in the Intertropical Convergence Zone that we covered last post.  This image shows the location of the ITCZ in January, and July each year, and you can see that it lines up with the rainforest, and other tropical zones.

Intertropical Convergence Zone location. Image from wikipedia

The rainforest zones have a lot of rain, each month, the average rainfall is around 60mm, which means they get 720mm of rain, on average, every year. The actual rainforests themselves have far more than this, with an average 200cm of rain per year.The reason for this difference is that not all locations within the rainforest zone are actually rainforests, there are cities, and arable lands, and grassland within these areas.

These regions, as well as being very warm, are very humid (As warm air can contain more water than cold air, and there is less wind than in other regions to disperse the moisture heavy air). Much of this humidity is from evaporation from the oceans, but large rainforests such as the Amazon have high levels of evapotranspiration (Evaporation from plants, and transpiration, which is the loss of water from plants when they have their stomata open) that much of the rain which falls over these rainforests actually comes from within it, meaning that they have their own mini water-cycle going on!  This is also why rainforests have a higher rainfall than the zone as a whole.

The tropical monsoon zones (Am) are characterized by, as the name implies, a monsoon, and have a dry month, and little annual variation in temperature.

We usually associate the term monsoon with extremely heavy downpours, but, the term monsoon does not refer to the rain specifically, but to the change in wind direction, which brings this rain.  The change in wind direction has different causes on different continents, but, is (usually) predictable, and arrives at the same time every year, meaning many of these regions have agriculture which relies on the arrival of the monsoon each year at a certain time for the crops, and any delay or shift in timing can have a large impact on the crops, and the food supply.

The monsoon we are most familiar with is the Indian monsoon, so I will briefly explain the mechanism behind this.

In India, in the summer, the air is very warm, the further you go into the continent, the warmer the air becomes, and at the northwest end of the sub-continent is a desert, the Thar desert, which has extremely warm air during the summer months.

This warm air rises, and because the air which has risen needs to be replaced, cooler air moves in, and this air moves from the ocean northwards over the continent.  This air contains a lot of moisture, from evaporation over the ocean.

So, now we have this very wet air moving in to replace warm air which has risen, which would result in normal summer rains if it was not for the rather large blockade at the northern end of India, which we know as the Himalayas.   The warm, moisture filled air cannot move further north into Asia due to the mountains, and it is forced to rise.  As it rises, it cools, and cool air cannot hold as much water as warm air, so, like the bottom of a water balloon bursting, the moisture is deposited out of the atmosphere back to the ground.

Whilst this is vital for agriculture and the plants on the subcontinent, there is also a lot of damage caused every year by flooding and landslides, with houses and roads washed away.

Chittagong in Bangladesh is a city within the tropical monsoon zone, and these next images show the average temperature, and the rainfall for each month.  Note the low variation in maximum temperature, and the very obvious spike in rainfall during the monsoon season.

Average Temperature Chittagong. Image from

Rainfall in Chittagong, Bangladesh. Image from

The cool thing for me about the Indian monsoon is that it only exists because of the Himalaya mountain range blocking the passage of the air further into Asia.  Why do I think this is super cool?   Because, the Himalayas are a relatively new mountain range, geologically speaking, and are only there because India went zooming across the ocean faster than continents usually move (one theory suggests this is due to the plate being thinner than most other continental plates, and so lighter, and able to move faster.  It may also have got a slight speed boost by passing over a hotspot in the Earths mantle) and smacked into Eurasia around 50 million years ago.  To put this in a time perspective, this was after the era of the dinosaurs.  The Himalayas are currently growing at around a centimeter every year, due to the Indian plate moving north-east faster than the Eurasian plate is moving north.

Finally, onto the Savannah zones (Aw).  These regions have very obvious wet and dry seasons.  While these areas can have a lot of rain during their wet months, it is not enough rain to qualify them as monsoons.  This change in weather is due to the movement of the ICTZ, and the rainbelts which lie around it.

This image illustrates the dry season in Brasilia, which is as marked a difference as the monsoon in India:

Rainfall in Brasilia, image from

Eeep….this post got a little longer than I expected, so, next time, we will briefly (I promise!) cover the B and C zones!

*Tiptoes back in*

So…it has been a while.  You know how it goes, you get caught up in something (in my case my final semester of my bachelor) and next thing you know, it is 8 months later, and then you feel awkward about returning after such a long break.

Anyway, I was asked if I would write up some posts on the IPCC report, so I thought it was as good a reason as any to get back into blogging!

I thought I should start out with an introduction to climate, as for me, the systems themselves are fascinating, and for a lot of people, climate is some abstract concept only heard in relation to carbon dioxide and climate change, when in reality, it is so much more.  I hope I can show you at least a little window into our awesome planet in this post.

Climate is defined as “The pattern of variation in temperature, wind, humidity, atmospheric pressure and other variables over a long period in a given region”

This is different from weather, which is the temperature, wind, humidity, atmospheric pressure etc at a given time in a specific location.

So, climate is “It is cold in Sweden”, weather is “It snowed today in Stockholm”.

The main factor affecting climate on Earth is the incoming solar radiation, which is unequally distributed across the surface of the Earth, being more concentrated at the equator, and more diffuse at the poles.
Solar Radiation Distribution. Image from

This means the atmosphere is heated more at the equator, and less at the poles.  As hot air rises, and cold air sinks, this unequal heating causes movement of air.  The air heated at the equator rises and moves towards the poles, whilst the cooler air moves towards the equator to replace the rising air.

This results in loops of air circulation, which are known as Hadley cells in the tropical regions (near the equator), Ferrel cells in the mid latitudes, and polar cells at the poles.

Global Atmospheric Circulation Cells. Image from wikipedia

In the areas where the air in the Hadley cells is descending, we find desert regions, this is due to the air being cooler, and containing less water than the warm, rising air at the equator.

These cells of circulating air play an important role in many of the large scale phenomena such as the monsoon circulation in Asia, formation of hurricanes, and the jet streams and trade winds, which are caused by a combination of the circulating air cells, and the rotation of the Earth.

The rotation of the earth means that air currents do not move in a straight line, but appear to rotate to the left, or right, depending on the hemisphere.  These two videos demonstrate the effect.  This first one is a simple experiment showing the phenomena:

This second video shows this effect in action, and is an animation of annual air circulation (I actually had an image of this as my desktop background for quite some time, as I really love the visualization).   You can see the deflection of the air currents, and the band across the equatorial areas known as the Intertropical Convergence Zone (ITCZ), which is where the northern and southern hemisphere air meets.

Next post in this series will cover the major climate zones on the planet, and some of the driving forces and phenomena in each of them.


So, after an extended break, normal service is resuming, although the post today will be slightly off the usual topic of the evolutionary journey.

As I mentioned yesterday, I have been writing my bachelor project, and I thought I would share with you a little of the topic I have written it on.

I could write a post which would keep my good e-friend Argyle Sock very happy, with lots of statistical stuff in it, but, honestly, if I see the terms “not normally distributed” one more time, my head may go pop!

Also, the actual topic is much cooler than the stats (even though they are very cool too of course!).

I would like to introduce to a group of pigments called anthocyanins.  These are the reason for the colour of blueberries, blackberries, some grape species, olives and many other fruits, and purple or red colourings in flowers, like these pansies:

Violet pansies: The purple colour is caused by anthocyanins in the petals. Image from Wikipedia

The amazing bright red colours you see in autumn leaves is also down to anthocyanins.

So..why have I spent this last semester writing about pretty red colours? is where it gets a little more interesting (not that pretty purple flowers are not interesting of course!)

Anthocyanins are also found in leaves which are not about to fall off the tree.  Some plants have red leaves when they are young, and some plants can turn on and off the red colouring under certain conditions

Sciencey term for the day: This is called phenotypic plasticity….a phenotype is the appearance something has due to certain genes being expressed. If a phenotype is plastic, it means it can change under certain environmental conditions.

So, I have been looking at a particular plant, which is usually green and grows in shallow lakes.  These lakes dry out a bit in the summer (Yes, we have summer in Denmark, sometimes!), and so some of these plants may end up out of the water.  If this happens, 90% of these plants turn red, until the water covers them again.

The reason I have been looking at this plant (with a bunch of other people who are much better at scary maths than me!) is because it is not entirely clear exactly what the red stuff in the leaves does.  In some plants it seems to act a bit like a sunscreen, in others it seems to stop the plant being eaten by insects.

The amazingly awesome Leaf-cutter ants (Several posts on them will come along a bit later in our evolution journey) will not harvest leaves which are red. This may be because insects do not see red the way we do, they do not have the parts in their eyes which can collect red light.

Leaf Cutter Ants carrying leaves off to their underground farm Image from wikipedia

It has also been suggested that anthocyanins can help plants survive during cold or drought conditions, so, there seems to be a whole lot of stuff that this pigment helps with.

The health food industry has even been getting in on it, and you can buy anthocyanin supplements…this is because in plants, they work as anti-oxidants, and we are always hearing about how having free radicals is bad for us, and so we should eat blueberries, or whatever the cool food to eat this week is (its usually the most expensive one!).  I am not entirely sold on this idea, as, last time I checked, I am not a plant.

So anyway, I have been working with an awesome group of my fellow students, and we have been trying to make these poor plants very stressed to see what happens. We grew them under some very bright lights, and then did some tests where we zapped them with..well…an even brighter light, to see what they did.

Personally, I find the plant we have been studying more interesting than the actual pigment we have been looking at, although, that has been fascinating to learn about.  The plant itself has no stomata, which are the holes which plants use for taking in carbon dioxide and giving out oxygen and look like this in extreme close-up:

Stomata on a tomato leaf Image from wikipedia

Because this plant has no stomata, it has to breathe through its roots, which is extremely cool!  This means that it can only live in lakes which do not have a lot of nutrients in them, because, if you increase the amount of nutrients which end up at the bottom of a lake, you decrease the amount of carbon dioxide which is produced by the bacteria eating all the dead stuff at the bottom.

As more dead stuff ends up at the bottom of the lake, the bacteria down there start using up more oxygen than is available, and so the only ones that can live there produce methane (CH4) instead of carbon dioxide (CO2). This is what is found in my favourite land-types, the wetlands, bogs and moors.

Oh, yes, I forgot to show you a picture of the plant!  This is the little thing I have spent 6 weeks growing, and then several weeks cutting up and zapping with bright lights!

Lobelia Dortmanna. Image from wikimedia

So, I hope I did not bore you too much with the slightly off-topic post, and next time we will pick up on the evolutionary trail again, with some weird and wonderful creatures. If you prefer plants, hang in there, we will get to them in a few million years or so!

Collated information on Ash dieback

I have been asked by a few people what they can do about the Ash dieback problem in the UK, and whilst I said yesterday that there was no official guidance for the general public yet, there are some informative documents prepared by the forestry commission.

Most of these are in the original post I made (HERE), but as they are at the end of a long post, I will put them up on here again, with the new information.

The Forestry Commission has published a visual guide to symptoms: Ash Dieback Disease

The Forestry Commission has also updated its guidance, with information that this is now a quarantine pest, and gives contact details if you think you have come across a site of infection. It also outlines how it is thought that the ancient woodlands have become infected. HERE

I used this Forestry Commission PDF in my original post, it is fairly easy to read and gives an overview: Chalara Fraxinea

The Woodland Trust has updated their guidance since I last read it, with the advice that any suspected infections should be reported at once: HERE

The Government has said that 50,000 trees have already been destroyed: (BBC source HERE)

EDIT for update: There is a website which will be launching Monday at ASHTAG and there will be a smartphone app launching Monday too.  I will post details of that when it arrives.

I mentioned on my fellow blogger Argylesocks post (HERE) that my concern is that with the onset of autumn, we will not know until next year what the extent of the infection is, as the fungus is now in the leaf litter over the winter.

I hope that these are isolated cases in woodlands, but I do not understand why it took from February til late September for this to get attention, or be publicised.

For my non-UK readers, I apologise for the UK-centric posting the last few days.  I am currently writing a post relating to a US issue, so will put that up over the soon.  I will be returning to my evolution based posting next week, but will keep putting up posts of interest to current environmental issues, to go a little more in depth into those.  If you have a story from anywhere relating to environmental or ecological issues which you would like to read a bit more in depth on, you can chuck me an email at squirrellyskeptic at gmail dot com.

Leaves, keys and fungi

So, there was this story yesterday in the Guardian about how ash trees are at risk from a fungus:

This topic has been in the media a fair bit lately, but very few of the stories have gone into the mechanisms and details, so I thought I would write briefly about those, as they are fascinating, and can help with understanding the problem better.

Most of the stories in the media have just said that it affects leaves, which is a very vague description.

So, first of all, to make sure we all know the tree we are talking about, this is an Ash tree, otherwise known as Fraxinus excelsior:

Fraxinus excelsior, the common Ash. Image from Wikipedia

And here is it’s close-up (It doesnt get red-eye like I do, and is always photogenic!)

Close up of the leaves and “keys” (fruit) of the common Ash. Image from Wikipedia

So, now we have met the victim, lets meet the perpetrator (Sorry, I am catching up on CSI episodes at the moment, so excuse me if I go a bit Horatio Caine).

This is where it can appear a bit confusing, because this fungus actually has two names:  The one most mentioned in the media is Chalara Fraxinea and it looks like this when it is grown in a lab:

Chalara fraxinea, politely posing in a petri dish. Image from Federal Institute of Technology Zurich

This is the fungus mentioned in the Forestry Commission factsheet about this problem (See further reading for link).

This is what is known as an anamorph, which means it is the asexual reproductive phase of this fungus.

I think the reproduction of plants, fungi and small micro-organisms is really cool, so I am going to explain it a bit here as it can seem a bit confusing (I remember getting tied in knots trying to revise this for functional biology!)

The asexual reproduction of fungi such as this species involves producing spores (from the greek spora, which means seeding, or sowing), which you might know from the puffball mushroom, when you kick it, it gives off a load of dust-like stuff, which is actually the spores for the next generation of the fungus, which looks like this:

Puffball mushroom releasing its spores. Image from wikipedia

Each of those spores is a potential new fungus, provided it lands in a suitable environment for growth.  This method of dispersal is very haphazard, and this is why these organisms produce so many spores.  It is a bit like closing your eyes and throwing a handful of seeds randomly out on a bit of ground and hoping for the best.

They are formed by mitosis, which is also how our cells in our body are replaced and is in itself a really really cool process (especially when you see slides of it), and which I will cover in depth in a later post.

As I mentioned earlier in this post, this fungus has two names, the asexual form C.fraxinea and the sexual form Hymenoscyphus pseudoalbidus. Now, maybe it is just me, but I found it a little confusing initially to understand how one organism can have two names, or even two life cycles when I first started reading about this.

This image shows the life cycle of an Ascomycete, which is the group of fungi which this particular one belongs to.  The asexual cycle is the loop off to the left of the diagram.

General life cycle of an ascomycete. Image from Penn State University

From what I gather from reading several journal articles on this species, it seems that the asexual form is on the leaf litter, and dead wood on the forest floor, and this is not infectious (or pathenogenic to use the sciencey word).

It all goes a bit nasty for our Ash trees when it is in the sexual form, H.pseudoalbidus .  It is called “pseudoalbidus” because there is another species called H.albidus which is not responsible for this problem in Ash trees, but appears physically similar.

This is what the fungus looks like:

H.pseudoalbidus on a branch. Image from Institute of Technology, Zurich

This confusion with two different names for the sexual and asexual form of fungi will be less confusing soon, as in 2013 they are changing the naming structure, so that there is one name for a species of fungi, regardless of which stage of the life cycle it is in.

As you can see from the diagram, the asexual form of the fungus only refers to the spores,  everything else within its lifecycle is classified as H.pseudoalbidus. Calling this C.fraxinea in the media is quite confusing, but understandable, as many journals refer to this fungus as C.fraxinea.

The cycle of infection appears to be, that the spores remain in the litter, or on dead branches over the winter, and then, in the summer, it germinates, and becomes the white mushroom thingies.  These release spores, which are spread by the wind, and some end up on the leaves of Ash trees, and on the branches.  These form structures known as mycelium which are basically a mass of threads, and it is these which are responsible for the damage to leaves and branches, if they get into a gap in the bark, they form lesions like on this branch:

Necrotic lesions on a branch. Image from EPPO (European Plant Protection Organisation)

These are also known as cankers, and result from the death of the tissues.

The fungus also damages the leaves, as shown in this image:

Leaf dieback as a result of fungal infection. Image from EOL

The dead branches and leaves then fall to the floor, and the cycle begins again.

This is a relatively new infection in Ash trees, first being noticed in the mid 1990s.

There are ongoing discussions as to why this has arisen, as this fungus has been known since the late 1800s, but as the non-infectious H.albidus.  There is discussion about whether climatic stress has weakened the trees resistance to infection, or whether the infectious version of this fungus is better suited to the milder climate conditions over recent years, or whether this new infectious form is a mutuation which has arisen recently.

Whatever the cause, the result is devastating. Denmark has lost around 90% of its Ash trees since the infection arrived, and other European nations are reporting large scale losses of Ash trees.  The infection appears to have arrived in the UK (Which is usually protected from these types of infection because of its island status) by importing of young trees which were carrying the fungus.

So far, it seems that the fungus has not managed to infect “wild” trees in the UK, and the government has begun a consultation, which will end on the 26th of October, which could lead to a ban on imports of Ash (and given the severity of the threat, I would hope that a ban is imposed).

Further Reading: (Most are very easy to read, with the exception of the journal article at the end, they are mostly from the Forestry Commission, and similar bodies) (Rapid Risk Assessment)$FILE/pest-alert-ash-dieback-2012.pdf

Krautler & Kirisits: The ash dieback pathogen Hymenoscyphus pseudoalbidus is associated with leaf symptoms on Ash species (2012)



Just one more…

Final post about the ocean for now, honest!

I linked a clip from this in my last post, but have just watched the full episode, and thought it was one for sharing with you all.

BBC: Blue Planet, Deep Trouble covers some of the coral reef things I mentioned today, as well as other issues my fellow blogger and awesome sciencey person Argylesock has been covering:

Tomorrow is back onto the journey through evolution, with more on the cowry.