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.

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)