A brief introduction to aerosols..

Let’s skip the chit chat and go straight into post number three… Spray-cans will come to mind for many people when they first think of aerosols, but this term has a much more general meaning and plays an important role in climate science. Anything that has a suspension of liquid or solid in a gas is deemed an aerosol. In fact, a spray-can may actually contain hundreds of different individual aerosols.

A simple demonstration that shows the significance of aerosols in the atmosphere is the ‘cloud in a bottle’ trick. A trickle of water is added to an empty bottle and the cap put on. By just shaking and squeezing the bottle (simulating pressure changes that occur every day in the Earth’s atmosphere) no cloud is formed. However, if smoke is added, from a blown out match for example, and then the bottle squeezed, a small vapour cloud becomes visible – try it yourself or see the video of it below!

These tiny particles – aerosols – are necessary for clouds to form anywhere in the world. The photograph below shows how clouds have formed from aerosols released by ships.

Cloud formation using aerosols emitted from ships

Cloud formation using aerosols emitted from ships

Aerosol sources can be both natural and anthropogenic. The photo above is an example of industrial aerosol emissions but other major sources can include deserts (in the form of dust and sand) and from fires / biomass burning, which can be both natural and manmade, even tiny ice crystals can be classed as aerosols. One of the global climate models, Global Earth Observing System Model – Version 5 (GEOS-5) has simulated the emissions and transportation of all major aerosols from September 1st 2006 to March 17th 2007 and the video can be seen below. For a description of the colours see the figure description.

Summary copied directly from you tube link: “Dust (red) blows over the Saharan desert and interacts with two Atlantic tropical cyclones. Sea salt (blue) churned up from the ocean by surface winds is most prevalent along mid-latitude storm tracks and fronts in the Southern Ocean and within tropical cyclones. Organic and black carbon (green) burst from extensive biomass burning in South America and Africa. Sulphate (white) arises from three primary sources: fossil fuel emissions over Asia, Europe, and the United States; a persistently active volcano in Mozambique, Africa; and a large eruption from the Karthala Volcano on Grande Comore Island, Comoros, in January 2007.

Aerosols have both positive and negative radiative forcing effects (for more information on radiative forcing please read my first post! – An Introduction to Global Warming). The diagram below shoes the major contributors to global warming and whether they have a positive effect (a warming – coloured red) or negative effect (a cooling – coloured blue) on the Earth’s climate. The highlighted box shows the contribution from aersols – a very negative effect! However the black lines show how much of this estimation is uncertain. There is a large amount of research currently trying to reduce these uncertainty bars so we can be more sure of how much an overall effect aerosols have on our climate.

Global average radiative forcing estimates and ranges. Oringially produced by the IPCC.

Global average radiative forcing estimates and ranges. Originally produced by the IPCC.

There are two main ways aerosols effect our climate. The first is a ‘direct‘ way. I.e through re-radiating energy (warming the Earth) or reflecting incomming sun’s rays back out to space (cooling the Earth). Again, my first post explains this in more detail. The other way aerosols effect our climate is called the ‘indirect effect‘. As aerosols are needed as ‘seeds’ for clouds to form, an increase in aerosols can cause an increase in cloud number thus increasing the Albedo effect. When aerosols help form clouds they are referred to as ‘cloud condensation nuclei’ (CCN). The more CCN present the whiter the clouds can appear which further increases the albedo effect.

As you read this you may be thinking, the more aerosols present the better in the atmosphere as they have an overall cooling effect of climate however not all effects of aerosols are good. Aerosols can have a huge impact of air quality and atmospheric pollution. The recent smog events in Singapore (shown in the picture below) are thought to have occurred because of aerosols released from illegal biomass burning from Indonesia and travelling up to Singapore where they formed these smog clouds which can have severe health effects.

Comparison of Singapore skyline from Feb 2012 and June 2013. Image found on BBC News website.

Comparison of Singapore skyline from Feb 2012 and June 2013. Image originally from BBC News website.

Any questions or comments please write them below, or perhaps even a request for future posts?? Next time I plan to give a very brief overview of the history of the Earth’s temperature has changed over 65 millions years. They’ll be videos in that one as well!

Thanks for reading.

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The Milankovitch Cycles

Hello everyone, welcome to my second blog post. I wanted to start off with a diagram that I should have included in my last post. It shows an overview of the world’s energy budget, which includes several terms I described before, including the albedo effect and the greenhouse effect!

Original image found here: http://www.nasa.gov/audience/foreducators/topnav/materials/listbytype/Earths_Energy_Budget.html

Original image found on the NASA website here.

In this post I want to explain how the Earth’s temperature is affected over longer timescales as a result of changes in its orbit. These changes can have a significant impact on the Earth’s climate and temperature. The variations in the Earth’s orbit were first discovered by a mathematician named Milankovitch in the early 20th century, whose theories were confirmed in the 1970s. However, to understand why these cycles have such an effect on temperature I should first review the Earth’s annual cycle and why it has seasonality.

The Sun emits energy which travels to the Earth as rays. The amount of heat generated in the atmosphere depends on the angle these rays hit the Earth (their angle of incidence). The figure below shows an example of how the Sun’s rays reach the Earth at two areas. The rays around the equator have a much higher angle of incidence (~90⁰ in relation to the Earth’s surface) and so they cover a relatively small area. In energy terms there is a higher Watts per metre squared value (Wm-2). The rays that reach much higher latitudes have a lower incidence angle and therefore diffuse themselves over a larger area (lower Wm-2). This combined with the Earth being tilted on an axis explains our seasons (NB. Not represented in the figure). In northern hemisphere summer time (NHST) the North Pole is tilted towards the Sun and so the northern hemisphere experiences incoming rays at a high angle of incidence. The opposite occurs northern hemisphere winter time (NHWT).

Sourced from here.

Sourced from here.

As I stated before, the Earth’s orbit around the Sun is not fixed. Milankovitch discovered that it varies in three main ways, of which all have an impact on its climate. The three Milankovitch cycles are:

Eccentricity

The Earth’s orbit around the Sun is not spherical but in fact elliptical. When the distance between the Earth and Sun is greatest this is called ‘aphelion’ (see figure below) and when it’s at its shortest, ‘perihelion’. The elliptical orbit can vary greatly. The two extremes of the Earth’s eccentricity can be seen on the diagram summarising the Milakovitch cycles below. The variation between summer and winter temperatures is at its most extreme when the difference between aphelion and perihelion is largest. The time it takes for the Earth’s orbit to change from its most elliptical shape to its most spherical and back again is ~95,000-100,000 years. This period of time is known as its cycle.

Aphelion & Perihelion Sourced from here.

Aphelion & Perihelion
Sourced from here.

Obliquity / Axis Tilt

The Earth’s axis is currently on a tilt of 23.45⁰ however this can vary from 22.1-24.3⁰ over a ~41,000 year cycle. When the angle is decreased the difference between summer and winter climates is less extreme.

Precession

As well as the Earth’s axis being tilted it also wobbles about this axis. Think of a spinning top just before is comes to a rest. Currently the North Pole is pointing towards the North Star, however in ~12,000 years’ time it will be pointing in a completely different direction which will make NHWT in June and summer time will be in December! A full cycle takes 19,000-24,000 years to complete. As the precession is effectively swapping our seasons this has a large impact on the temperature of the Earth, as well as its climate.

Original image from the UCAR website found here.

Original image from the UCAR website found here.

For those of you who read my first blog post you will recognise the next figure but hopefully now the cycles in the Earth’s temperature will make more sense to you. You can definitely see the impact of the Milankovitch cycles within the plot. These sorts of temperature plots have sometimes been used by climate sceptics to show the degree of natural variability in the Earth’s Climate and how current climate change is minimal compared to the past. This is extremely difficult to justify, especially when comparing two extremely different timescales. As you can see from the plot below we are actually in one of the Earth’s warmer periods on this timescale, so if anything we should be expecting Earth to be moving into a cooler period in the future. However the rate of global warming we have experienced in the last 150 years has not occurred for several million years. As I said in my last blog post I will do a future post about the history of the Earth’s temperature in more detail, however next time it’ll be more chemistry with a brief introduction into aerosols!

Record of the Earth's temperature and Carbon Dioxide concentrations taken from ice core data in Antarctica.

Record of the Earth’s temperature and Carbon Dioxide concentrations taken from ice core data in Antarctica.

A brief Introduction to Global Warming

Hello and welcome to my first blog post! I can’t really start without an explanation of how global warming occurs, as this is one of the most fundamental reasons why climate science exists! So here we go…

The Earth’s atmosphere allows life as we know it to exist. Without it, global temperatures would be around 33-35 oC colder. The gases that make up our atmosphere can absorb some of the sun’s energy leading to an increase in temperature. Energy from the Sun reaches the Earth in the form of light and heat. The Earth has the ability to absorb all of this radiation but some is reflected back into space either by the Earth’s surface or by its atmosphere. This process is called the Albedo Effect and without it the Earth would be much warmer. The Earth re-radiates energy at a slightly longer wavelength that can be absorbed by some atmospheric gases. These gases then re-radiate this as thermal energy.

A measure of how strong an effect this re-radiated energy has on the Earth’s temperature is called a gas’ radiative forcing. Radiative forcing is the difference between radiant energy received by the earth and energy radiated back into space, and its units are in watts per meter squared (Wm-2). The gases that contribute most to the greenhouse effect, in terms of their radiative forcing, their lifetimes and also their abundance in the atmosphere include carbon dioxide (CO2), water vapour (H2O) and methane (Ch4). The relationship between temperature and concentrations of these gases is strikingly positively correlated, as the graph below shows. In another blog I plan to explain more about how the Earth’s temperature has changed throughout geological history.

Record of the Earth's temperature and Carbon Dioxide concentrations taken from ice core data in Antarctica.

Record of the Earth’s temperature and carbon dioxide concentrations taken from ice core data in Antarctica.

Almost all atmospheric gases have both natural and anthropogenic (man-made) sources. In fact, for most of the major greenhouse gases natural sources outweigh man-made. Since the Industrial Revolution, however, anthropogenic emissions have been rapidly increasing. The plots below, published by the Intergovernmental Panel on Climate Change (IPCC), show just how high the atmospheric concentrations have risen. It is this increase, or offset from the pre-industrial balance between sources and sinks, that is causing temperatures to rise. For more information on the IPCC and its soon to be released 5th assessment report click here.  Or if you want a more detailed summary of all the aspects of climate change this ‘summary for policy makers’ is really interesting.

Timeseries of Methane (CH4), Carbon Dioxide (CO2) and Nitrous Oxide (N2O) emissionsfor the last 10,000 years.

Time series of Methane (CH4), Carbon Dioxide (CO2) and Nitrous Oxide (N2O) emissions for the last 10,000 years.

Of course natural variability also affects the earth’s climate and can contribute to global warming. Examples of natural variability include variations in the Earth’s orbits (i.e. Milankovitch cycles – more to come in another blog), solar radiation variability, lunar tides and interactions between the oceans and the atmosphere. The world’s scientific community agree that the Earth is currently warming however there is a degree of discussion about how much human impact is contributing to global warming. The IPCC has stated that ‘the observed increase in global average temperatures since the mid-20th century is very likely due to the observed increase in anthropogenic greenhouse gas concentrations’. Very likely being classed as a >90% probability.

Thanks for reading what I hope is the first of many blog posts.  Next up I will discuss changes to climate that occur on longer times scales, i.e changes to the Earth’s orbit about that sun.