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.

Why mature forests matter..more trouble for the Ash tree.

I wrote a few weeks back about the fungus which is infecting Ash trees in Europe (HERE)

At the time I wrote that  “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).”

Well, this week it has been announced that the disease has been discovered in  mature forests in the UK, as this Guardian article discusses (HERE).

My last post about this was a bit technical and clunky, so I will try to keep this easier to read.  I want to try to explain as much of the specifics as I can though, because, when I am finding anything out, I like knowing all the details if possible, rather than just understanding the gist of something.

We are going to take a little detour, as I could just tell you that “Mature forests are important” and let you just take that as read, but I would rather run through the reasons behind my claim.

So, firstly, what is a mature forest?

To most of us, a forest is a forest is a forest, but to biologists, and specifically botanists, there is a difference in the type of forest, and it has to do with a term called succession.

Succession can be thought of as the stages of life of an area, whether that is to do with a wetland, a forest, a field etc.

My ecology textbooks define succession as “Replacement of one community by another, often progressing to a stable terminal community called the climax

To illustrate this I will use the example of how a natural forest progresses, as it is one of the simpler ones to picture, but this also applies to planted forests, although the starting point is different there, as we actually initiate tree growth.

Diagram showing forest succession. Image from the encyclopedia of New Zealand

So, we start out with our bare ground on the left of the diagram, which usually consists of some soil, maybe some grass species (those little guys get everywhere).  This bare ground may be the result of a big event like a glacier retreating, or it could be from the drying up of a lake, or the spread of grasses along a sand dune area.  It can also be derelict land which was previously used as an urban area, things are a little more complex in that case, but the basic order still follows.

If we imagine that we have some rocks on the bare ground, and a few tough grasses to start with.  The rocks provide a nice cosy place for some lichen to start living, whilst the grass roots prevent what little soil or sand there is from blowing away.

The lichen cause damage to the rocks, and small particles of rocks fall off and get trapped on the ground by the grasses, and other particles flying around in the wind also become trapped.  As the lichens and grasses die, or get damaged, they fall to the ground, and begin to form a layer.  Eventually, there is enough of this layer (known as humus) to allow mosses to take hold in the area, and as these die off, they too add to the growing layer of dead stuff on the surface, which allows for bacteria to come in and decompose them, adding nutrients to the new soil.

Over time, the nutrients and soil layer builds up, and small, tough plants can begin to grow there.  These are often other grasses, ferns and very small bushes (Box 2 in the diagram). Microbes, insects, worms etc begin to colonize the young soil.

As the soil quality improves (because the plants there die, and are broken down, and nutrients build up in the soil), and the stability of the soil increases due to the growing number of plants, seeds which are blown in the wind, deposited by animals etc begin to be able to grow, and some of these will be from trees.

Initially, only small, hardy trees can grow, but new species come in and as the conditions continue to improve for plants, taller trees begin to take hold.  These do not grow in the earlier stages as tall trees usually require higher levels of nutrients than bushes, or dwarf trees.

Now we have a young forest, and species of animals and birds begin to colonize the area. Trees begin to grow taller, and form a canopy, this leads to a change in the communities of plants which are on the forest floor, and the ones which die off further improve the soil quality.

Finally, we reach the mature forest stage, where the animal and plant communities are stable, and as a tree dies, a sapling takes its place.  In some ecosystems, trees can stay short, like a new tree, for years until a gap opens up in the canopy, then they all race to be the one to take the place in the sun at the top.  Whilst individual trees may change, the overall structure of the forest stays the same at this point, which provides stability for the animal, bird and insect populations, and leads to the forests which we love to walk in.

Whilst the exact age at which a forest is defined as mature varies (depending on the types of trees which are present), a mature forest is several decades old.  If these forests are able to continue developing, they eventually are classified as Ancient Woodlands, which in the UK means forests which have been there for 400 years or so (the current definition of an ancient forest in the UK is forests which have existed since 1600).

EDIT FOR UPDATE: I have found out that one of the woodlands affected is actually an Ancient Woodland, which is believed to have been in place for around 1000 years. Info about Ashwellthorpe HERE

Ok, so now you know why I feel that mature forests are important.  They provide a stable habitat for wildlife and other plants, and due to the length of time which it takes for them to develop, they are not something which is easily replaced.  They also play a role in preventing soil loss from rain or wind erosion.

Now, you are probably wondering why Ash trees are so important, and why the media have been giving this so much attention.

Ash is the fourth commonest tree in the UK, and many forest areas have it as the dominant species. (source: Woodland Trust). Birds such as woodpeckers and owls live in Ash trees, as they are easy for them to hollow out, and they provide food and a habitat for a diverse range of animals, insects, mosses and lichens (For more info, see the Royal Forestry Society link HERE)

Ash woodlands are part of the UK Biodiversity Action Plan, and are listed as a priority habitat in the plan. (See page 60 in this PDF), and they say:

“Mixed ashwoods are amongst the richest habitats forwildlife in the uplands, notable for bright displays of flowers such as bluebell.. primrose..wood cranesbill and wild garlic . Many rare woodland flowers occur mainly in upland ashwoods, such as dark red helleborine.., Jacob’s ladder.., autumn crocus.., and whorled solomon’s seal … Some rare native trees are found in these woods, notably largeleaved lime… and various whitebeams…. Upland mixed ashwoods also harbour arich invertebrate fauna, which may include uncommonor declining species. The dense and varied shrub layer found in many examples can in the southern part of the types range provide suitable habitat conditions fordormice… The alkaline bark of old ash (and elm where it still survives) supports an important lichen flora…. ”  (Latin names have been removed, hence the dots)

Aside from the ecological importance, it has a long history in the UK and Northern Europe, it is suggested as  the “world tree” from Norse Mythology (Yggdrasil) and Ash has been used in the UK since early history, as everything from spears to walking sticks, furniture etc.

So, seeing as these woodlands are on the Priority habitat list, you are probably thinking that the government has taken immediate action on this, and had in fact begun to assess this threat as soon as they heard of it.  Well….not quite.  George Monbiot over at the Guardian has pointed out that the government were made aware of the threat to the Ash tree in the UK some time ago, before the imported infected trees were discovered, and that even importers of Ash trees were recommending that action be taken (link HERE)

So, what is the government in the UK doing?  They have announced that a ban is being implemented starting Monday (link HERE).  Bear in mind this fungus was first found in the UK 8 months ago (February)…. Would it have taken so long for action to be taken if we were talking about infected livestock?

I apologise for the very long post, but I feel it is important that I explain exactly why we need to make sure that we do not lose our Ash trees the way we lost our Elms, and I am very angry at the government response.

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.

Corals and Starfish

I thought I would write this post today as it is about some organisms we have already covered (quite extensively, as I got a bit carried away!): in these posts: Here, Here and Here as well as Here.

Browsing the news sites while munching breakfast this morning, I came across this story in the Guardian about the Great Barrier Reef and also this one about Caribbean reefs (Full report link in further reading), and I thought it would be a good idea to write the post I forgot to before, about the threats facing coral reefs today.

The articles mention 3 things which are affecting reefs,storms, predation and bleaching.

Storms are perhaps the most obvious cause of damage, the waves during a storm hit the coral reefs (which are in shallow waters, and so take the full force of the waves as they near land), and this breaks the coral.  This video shows the aftermath of a hurricane on a Mexican reef:

Bleaching occurs when the organisms which live inside the corals are expelled by the corals, leaving the reef looking white:

Bleached coral, image from NOAA Coral Reef Watch

This bleaching has a number of causes, changes in the level of incoming solar radiation can affect particularly sensitive photosynthetic organisms, as too high light levels can be damaging to the cells which they use for photosynthesis,prolonged exposure at low tides and bacterial infections can also impact the organisms.

A number of the causes can be linked to human activity, some of which you will have heard of, and others which are less familiar, so I will briefly cover the main ones here:

Ocean acidification is the result of increasing carbon dioxide levels in the ocean, as shown by this equation:

CO2 (aq) + H2\leftrightarrow H2CO3 \leftrightarrow HCO3 + H+ \leftrightarrow CO32- + 2 H+

What this says is that carbon dioxide and water react to form a product known called carbonic acid (which you may know from sparkling water drinks). This compound forms ions (a molecule or atom which has a charge) of bicarbonate (HCO3) which you may know from bicarbonate of soda used in cooking,and hydrogen ( H+). These then react further to form carbonate (CO32- ) and two hydrogen (2 H+).  The + or – indicates whether there is a positive or negative charge, so carbonate has a negative charge of 2.

Now, the reason this makes the ocean more acid is that, as you know, the scale used for measuring acidity is the pH scale.  This scale is basically about how many hydrogen ions there are in a solution (in this case, sea water). More hydrogen ions in a solution mean that the solution is more acidic.

This is balanced usually by calcification, which is the reaction of calcium with carbonate to form calcium carbonate (chalk or limestone are the best known forms of this), which is used in the shells of many organisms, and is used in coral reef building.  The problem comes when more CO2 is entering the oceans than can be taken out by natural processes, which leads to the oceans increasing in acidity.  One of the problems with this is that, as you probably know, acid and chalk or limestone do not go so well together, and increasing acidity affects the organisms with shells, or corals.  This puts them under stress, and they can expel the microorganisms which live inside the coral tubes.

Increasing temperatures in the oceans also places corals under stress. We all have a temperature range which we can survive within, and some organisms have smaller ranges than others, especially those which live in zones with fairly constant temperatures.

Apart from concern about coral reefs and other marine organisms, one of the problems with a warming ocean is that warmer liquids can hold less gas (Like when a beer goes flat as it gets warm, this is because the carbon dioxide in the beer is released as it warms up). The image below shows the solubility of carbon dioxide in water as temperature increases.  One of the implications of a warming ocean is that it will be less able to store carbon dioxide, and so will release more to the atmosphere.

Solubility of CO2 in water. Image from Wikipedia

At present, there is both increasing acidity in the oceans, and increasing temperatures, because the temperature increase is not enough to reduce the carbon dioxide entering the ocean, because it is not saturated, which means it is still able to take dissolve gases.

This movement of carbon dioxide into the oceans is a very important part of the carbon cycle.  The diagram below illustrates this cycle, and as you can see, the largest store of carbon is within the oceans. The numbers represent gigatons of carbon (A gigaton is 1 followed by 9 zeros tons, or 1000000000 tons)

Carbon Cycle, the bold black text is the amount of carbon stored in each place, and the purple text is the flux, or amount moving between each area. Image from Nasa.

Finally, and apologies for boring you with the chemistry above, the most abstract of the relationships for today: Crown of Thorns starfish.

Very brightly coloured Crown of Thorns starfish. Image from Wikipedia

More usual colouring for a Crown of Thorns: Image from Wikipedia

This is definitely a very pretty starfish, but it is bad news for our friends the corals.

It preys on corals, and does so by climbing on top of the coral and turning its stomach inside out (extruding) to dissolve the corals tissue with digestive acids.  Whilst predation in itself is not a problem, and is part of a normal ecosystem, there are occasionally explosions in the population of these starfish, and there is a discussion about the causes of this.  At present, the likely cause appears to be an increase in nutrients in the ocean.

This can be a bit abstract, so I will try to explain it.  I wrote in THIS POST about how the high levels of phytoplankton in the Antarctic are due to the nutrients from the continents being carried to Antarctica by ocean currents.  This ties in to today’s post because there has been observed to be a link to periods of increased rainfall or floods, and an increase in the population of this starfish a few years later. (Link to article in Further Reading)

The mechanism appears to be as follows: During periods of heavy rainfall on land, a lot of nutrients are washed off with the rainwater (runoff).  These nutrients primarily come from fertilizers used for agricultural purposes.

These are carried downstream to the oceans, where they accumulate, and are carried on currents.  These added nutrients allow for an increase in the population of phytoplankton, and these are food for starfish larvae.  This increase in food allows for more of the larvae to survive to mature into starfish, which then feed on the reef.

This video shows a survey being done of the Crown of Thorns population on the Great Barrier Reef

Finally, overfishing, both for food, and for aquarium fish can affect the balance of the ecosystem, as this video briefly discusses.

What can we do about this?

Well, reduction in the amount of fertilizers used in agriculture would reduce the run-off, but this has an impact on agricultural yield for the communities in the regions affected by the monsoon flooding.  In Australia and other places, population control measures for the starfish have been implemented with varying levels of success.

The one thing we can do something about is the overfishing of reefs and other areas, by buying sustainable fish, and not having exotic fish in home aquariums.

The good news is that reefs do recover from bleaching effects, given enough time, and provided that the surrounding ecosystem is not too damaged, and that the effect is a “pulse”, that is, a one off event which causes for example a sudden surge in ocean temperature (like El Nino).  Sustained increases in temperature, predation or acidity may be harder to recover from.

Further Reading and links

Australian Institute of Marine Science page on Crown of Thorns:

Birkeland,C: Terrestrial runoff as a cause of outbreaks of Acanthaster planci (1982)

Brodie et al: Are increased nutrient inputs responsible for more outbreaks of crown-of-thorns starfish: An appraisal of the evidence (2004)

Cox et al: Acceleration of global warming due to carbon cycle feedbacks in a coupled climate model (2000)

CRC Reef Research Centre: Controlling Crown of Thorns

Graham et al: Coral reef recovery dynamics in a changing world (2001)

IPCC page on Ocean Acidification:

IUCN workshop on Caribbean reefs report:

NASA page on Ocean Carbon:

Orr et al: Anthropogenic ocean acidification over the twenty-first century and its impact on calcifying organisms (2005)