Month: Jun 2019

Lesson 6: water vapour absorption

CO2_H2O_absorption
Photo found on web, attributed to Robert Rohde’s “Global Warming Art” which I can’t find a live link to.

The image above shows the “absorption spectra” of H2O (water – in blue) and CO2 (carbon dioxide – in pink). The absorption is because light (electromagnetic radiation) at each wavelength causes the water or carbon dioxide molecules to change their vibration from one way of vibrating to another. Because water has so many different ways of vibrating, a very large number of wavelengths are absorbed. You can see that the edges are “jagged” – actually, if you zoom in on any one part of the spectrum, you can see that it’s actually made up of lots and lots of lines.

trans10
(Image from: http://www.gemini.edu/sciops/telescopes-and-sites/observing-condition-constraints/ir-transmission-spectra, showing the absorption spectrum of the atmosphere above a mountain for two levels of water vapour in the atmosphere – almost none in black, and a bit in blue-green). Note the picture at the top is about absorption whereas, this graph is for transmission – so where this is high, there is low absorption and vice versa.

The top image has a wavelength scale in microns (micrometres – 1000 times bigger than the nanometres I’ve used so far). The sun’s spectrum is in the wavelength range from 0.3 microns to around 3 microns. The Earth’s thermal infrared emission spectrum is from 4 microns to 40 microns.

The dominant greenhouse gas is water vapour. Water vapour absorbs more infrared radiation than any other gas because of the many, many different ways the molecule can vibrate. And because that radiation (light) is absorbed, it doesn’t get released into space and the Earth has to heat up to maintain a thermal balance between the incoming solar radiation and the outgoing thermal infrared radiation.

So, what would happen if all 7.5 billion of us boiled a kettle and released water vapour into the atmosphere simultaneously? Well, the simple answer is – it would rain. The atmosphere can only hold so much water (the exact amount depends on temperature and pressure) and when it’s exceeded, the water condenses into clouds and, eventually, rain. The water cycle is a very complex, but also very rapid feedback loop. The exception would be if we boiled those kettles in the upper atmosphere. There it is harder to make clouds, and the extra water vapour creates significant increased warming. This is one of the problems with aeroplanes – they are not only creating carbon dioxide, but also releasing water vapour into the high levels of the atmosphere.

The aeroplane effect is more complex still – aeroplanes burn fuel and the by-products are carbon dioxide, water vapour and nitrous oxides – all greenhouse gases. Emitting water at high altitude creates increased warming – but this, too, eventually falls as rain. The carbon dioxide has a much longer lifetime. If we all stopped flying, the water vapour would disperse quickly – the carbon dioxide would stay around for decades. Note that if we powered our planes with hydrogen, they would still emit water. (See also: this Guardian article from 2010). Of course, planes also make contrails – which means more clouds (see comments below about clouds).

However, hotter air can hold more water than colder air. So if the air temperature increases (for whatever reason), there is more water vapour in the atmosphere, which in turn leads to more “greenhouse effect” heating. This is known as a “positive feedback” – positive in the sense that it makes the effect bigger, rather than that it’s a good thing!

We don’t have a lot of global records of water vapour levels, so it’s hard to put precise numbers on the amount of water vapour in the atmosphere and how that’s changing with time. But there are indications from spot measurements (where it’s been measured in one place for a long time) that water vapour is increasing as the atmospheric temperature increases.

Of course, increased water vapour also leads to more clouds and we’re still not completely sure what more clouds means for the climate. On the one hand, more clouds means more sunlight is reflected – reducing the incoming energy and therefore cooling the Earth. On the other hand, more clouds means that more heat is held in at night (we all know clear nights are the coldest), therefore heating the Earth. For clouds at high altitudes we know that the keeping heat in is the bigger effect (positive feedback), for clouds at low altitudes, reflecting sunlight is a bigger effect (negative feedback). Overall, there’s a lot of uncertainty in what we understand about both the positive and the negative feedback mechanisms. The latest IPCC report concluded that the positive feedback was likely to be more significant than the negative feedback – but it’s still not clear by how much. “Cloud feedback” is the biggest uncertainty in climate models (and better satellite data is needed to improve our understanding of it). The climate modellers have been predicting that doubling the carbon dioxide will change the Earth’s temperature by “something between 1.5 ºC and 4 ºC”. The reason they give a range is almost entirely due to our lack of  – almost all of that range is caused by our lack of understanding of cloud feedback. As we improve our understanding of clouds, we’ll reduce that range (and recent studies suggest the lower end of that range was too optimistic).

 

Lesson 5b: More on atmospheric absorption

800px-Atmospheric_Transmission

This image comes from the Wikipedia article on the greenhouse effect.

The red bit is the sunlight coming down. The drawn line is roughly what’s at the top of the atmosphere (there is some loss because of Fraunhofer lines, but basically it’s a perfect blackbody) and the coloured in red bit is what reaches the Earth’s surface. The missing wavelengths are absorbed – and you can see below why:

  • UV and blue are absorbed by ozone in the upper atmosphere and “Rayleigh Scattering” (the thing that makes the sky look blue in the daytime and red at sunset).
  • The near infrared (sunlight with wavelengths too long for us to see) is absorbed mostly by water vapour (and a few wavelengths by carbon dioxide) (more to come)

The blue and purple lines are what the Earth would emit at different Earth temperatures (the one most to the left is for a blackbody at 310 K – or about 35 ºC – the one furthest right for temperatures around 210 K – or about -63 ºC). The solid blue bit is the only bit that gets through the atmosphere. All other wavelengths are absorbed by water molecules (“water vapor” plot) or by carbon dioxide or other greenhouse gases.

The diagram is drawn for a normalised blackbody curve – so you can see the thermal infrared one and the solar one on the same picture. In reality the solar one would be much “higher” as well – and the blue ones would not only shift left with higher temperatures, but also get taller.

Because so much of the output spectrum is absorbed, the Earth will heat up until it’s output is equal to its input: it needs to be at a hotter temperature for the energy in the coloured in blue bit to be equal to the area under the whole curve at a lower temperature.

This is known as the ‘greenhouse effect’ – but that’s actually a poor name. Yes, there is some real “greenhouse effect” in a greenhouse: the sunlight gets through the glass, but the thermal radiative energy of the surfaces in the greenhouse, emitting thermal infrared radiation, can’t get back out again … but actually the main reason real greenhouses warm up is that hot air can’t escape… ah well!

Lesson 5: Atmospheric absorption

So in Lesson 4, we learnt that if the Earth had no atmosphere, but still reflected about the same amount of sunlight as it does now, it would be at about -15 °C to -20 °C on average to be in “thermal equilibrium” where the energy coming in from the sun matched the energy coming out through the Earth’s own, thermal infrared, blackbody radiation.

Of course, we all know from our personal experience that the average temperature of the Earth (averaged over the whole Earth, whole day, whole year) is a lot hotter than that. So what is it that the atmosphere does?

To think about that, let’s start with a revision of Lesson 3 about light being absorbed and emitted by atoms. First, the “electromagnetic spectrum” is what I drew in lesson 1: it is the “rainbow” in the visible, and extends that to other wavelengths of electromagnetic radiation. If you look at the visible spectrum (the rainbow) of the sun, you see black lines in the spectrum. These are known as Fraunhofer Lines after the scientist who first described them (see lesson 3b).

Light coming from inside the sun “excites” an atom in the outer parts of the sun, which means that an electron goes to a higher orbital. Then, when the atom returns to its lower state, it releases light with the same wavelength: but it does so in a random direction. So the amount of light heading towards us decreases at that wavelength and we see a black line in the solar spectrum.


In the Earth’s atmosphere the same thing happens – both on the way down and on the way up. Every atom has its own set of lines where it absorbs. But additionally, molecules can absorb lines too. In the atom case, the absorbed energy from the light is used to move a very light-weight electron up to another orbital inside the atom. With molecules, the absorbed energy from the light makes the molecules vibrate in new ways. Since in molecules the things moving are much heavier atoms (rather than very light electrons), all this happens with a lower frequency – and molecular absorptions are in the thermal infrared.

Incoming light from the Sun reaching the top of the atmosphere is in the UV, visible and near infrared spectral region. The UV is absorbed by atoms (and some molecules like ozone) this light gets re-emitted but in all directions, including out of the atmosphere, and is lost. That’s how our ozone layer protects us from harmful UV. Other visible wavelengths are absorbed by the atmosphere too – some Fraunhofer lines are due to atoms in the Sun, others are due to atoms in our atmosphere. This means that some wavelengths do not make it down to Earth.  But this absorption is only a few lines, and it doesn’t affect the overall amount of energy reaching the surface very much.

The Earth’s emitted radiation is in the thermal infrared. This longer wavelength (lower energy) light gets absorbed by molecules to make them vibrate in lots of ways.

Wikipedia has some great images of water molecules vibrating:

The yellow ball is the oxygen. The blue balls (which should really be much smaller than the yellow ball) are the hydrogen atoms (H2O!). Imagine you were holding a model of this with springs for the bonds and balls for the atoms. You can imagine that there are lots of ways for the molecule to vibrate and rotate. Each transition from one way of vibrating to any other way of vibrating requires just the right amount of energy supplied through light at “just the right wavelength”. So you can imagine there are lots and lots of thermal infrared wavelengths that get absorbed by all the water molecules in the atmosphere. And, while that light can also be re-emitted, that will be in any direction – including straight back down to Earth and into the path of another molecule.

[Actually, because the water molecules aren’t cold themselves, they are already doing some vibrations of their own – this actually leads to even more wavelengths being “just right” to create transitions between different vibrational modes.]


There are some difficult concepts in here, so I’ll stop and add give space for questions. 

Lesson 4b: Further thoughts on the temperature of the Earth

The ideas developed in Lesson 4 are the basic “radiative balance” that make up the Earth’s “energy budget” (look up radiative balance or Earth’s energy balance on Wikipedia as a starting point). This is all based on a very simple concept – if an object has more energy coming in than going out, it will heat up, until the energy in balances the energy out. Similarly, if an object has more energy going out than coming in, it will cool down until the energy out balances the energy in. All physical systems try to maintain an equilibrium. In the Earth system, the energy “in” is coming from the Sun, the energy “out” is coming from the Earth’s own blackbody radiation – and the hotter it is, the more it is radiating out.

I’ve considerably over-simplified the problem in the example in lesson 4 (and not just by ignoring the greenhouse gas effect – which we’ll come onto next).

First, I’ve ignored any heat generated by the Earth’s core, which does work its way up to the surface (hot springs and geysers). This is ok, though, the heat that comes up from the core is about 0.03 % of the energy that comes from the sun. That is much smaller than the size of the approximations I’m giving above.

Second, to improve this calculation you’d have to properly know the average reflectance in the solar reflective spectral region (from UV to short wave infrared). That’s called the “Earth’s albedo” and will vary from something very high (clouds, snow) to something very low (deep ocean, dark forests). It’s generally assumed that the average albedo is somewhere between 0.2 and 0.4 (search it yourself if you want).

You also need to know the Earth’s emissivity in the thermal infrared. Generally natural surfaces have a high emissivity in the thermal infrared – around 0.8 (low end of shiny snow ice) – 0.96 (deep water), with stone and mud around 0.9. (see: https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA18833). So my calculation should be more like 340 × (1 – 0.3) = 0.9 × σT4. That gives a temperature of 261 K, or -12 ºC.

Third, I’ve ignored the effect of atmospheric and ocean circulations that move the energy around the Earth (though that’s ok with my “no atmosphere” approximation).

But my basic premise holds: without greenhouse gases (next lesson we’ll talk about what they do), the Earth would be really, really cold – with average temperatures in negative teens.  

Before we leave this simplification, it’s worth thinking about what you’d do to do this simple calculation better. You’d probably split the Earth up into little boxes. In each box you’d work out what the average energy (over a day, over a year) coming in from the Sun would be (higher at the equator than at the poles). And you’d work out how much was reflected (more over ice and sand than over ocean or forest) and what the thermal infrared “emissivity” is (i.e. how well that type of surface emits thermal infrared radiation). Then you’d do the energy balance equation in each box and then add that all up for the whole Earth. That would be the beginning of a climate model (more later!)

 

Lesson 4: the temperature of the Earth

temperature of the earth

In this lesson, we’re going to do what physicists like to do – we’re going to over-simplify the Earth and do a “thought experiment”.

So, we’re going to imagine that the Earth doesn’t have an atmosphere and we’ll work out what temperature it “should be”. This builds on the lesson on blackbodies.

First, the Sun is sending light towards the Earth. The Sun is very hot and emitting light in all directions, but the amount of energy coming directly towards the Earth (the solar irradiance) is 1360 W / m2 (ish – we’ll come back to how we measure this later). But that’s the light coming towards the Earth and, of course, half the Earth doesn’t get hit at any one moment (it’s the night time) and towards the poles, that 1360 W gets spread over a much bigger area of Earth.

To understand that consider 1 square metre rings in a row in front of the Earth (top picture): over the equator the light going through those rings forms a circle on the Earth; but over the poles, it would be spread over a much bigger ellipse. So – the average power falling on a square metre of Earth’s surface at any one time (averaged over the whole Earth) is about 1360 / 4 = 340 W / m2 (watts per square metre). That’s like having 4-old fashioned lightbulbs on every square metre of the Earth.

Now, the Earth can’t get hotter and hotter and hotter! It will reach an “equilibrium” where the heat in equals the heat out. (Equilibria are very common in physics). The way it releases energy into space is via its own blackbody radiation. You may remember from our lesson on blackbodies that everything that is hotter than absolute zero radiates energy with a blackbody curve. And that is true of the Earth too. As the temperature of the Earth is quite low (compared to the Sun!), it will radiate most of its blackbody radiation in what we call the “thermal infrared” (long wavelengths).

We can work out the total power of the blackbody by working out the area underneath that curve. There’s a simple calculation there. The total power of a blackbody in a square metre of its surface emitted into space is sigma times Temperature to the power 4. (σT4. Sigma (σ) is the Stefan-Boltzmann constant and is 5.670 367 × 10-8 W m-2 K-4 .

If the Earth were perfectly black at both short wavelengths (visible, near infrared – the wavelengths the Sun emits) and at long wavelengths (thermal infrared – wavelengths the Earth emits), then we could write:

Incoming power in a square metre = outgoing power in a square metre

340 = σ × T4

So the temperature = 278 K = 5 ºC.

(To do this calculation yourself, remember that the Stefan-Boltzmann constant can be written with the decimal place moved 8 places, so 0.000 000 056 704 and to get from Temperature to the power 4 is something to Temperature is something you can press the square root button twice)

If, as is more realistic, the Earth has an average reflectance in the visible of 30% (so it reflects about 30% of the light from the sun straight back to space and absorbs 70%) but it is still perfectly black in the thermal infrared (not unreasonable), then

Incoming power in a square metre = outgoing power in a square metre

70% × 340 = σ × T4

So Temperature = 254 K = -18 ºC

Now, we have made A LOT of approximations here. The actual average reflectance of the Earth might not be quite 30%, and it’s not quite perfectly black in the thermal infrared – but the basic picture holds. If the Earth had no atmosphere at all, the average temperature across the whole world would be something close to -15 ºC to -18 ºC.

And just in case you think I’ve pulled the wool over your eyes, I thought I’d find out the average temperature of the moon – after all, it’s about the same distance from the sun as us and it’s about the same sort of reflectance. So I searched the internet for “average temperature of the moon” and found an answer here: https://socratic.org/questions/what-is-the-average-surface-temperature-of-the-earth-s-moon

I was most amused that it said:

“You could take an average of the mean maxima and minima to get a mean surface temperature of -23 °C, but it wouldn’t be very meaningful.”

I beg to differ – that is very meaningful – as it’s very close to what I calculated with my overly simplistic thought experiment. (The moon reflectance is a bit different and it’s fraction pointing towards the sun is perhaps a bit different and the average temperature is not necessarily the average of the minimum and maximum temperatures)

Of course, the reason that the average temperature of the Earth is not that cold – and is nearly 35 ºC hotter – is that we do have an atmosphere. But you might be surprised to know that if our atmosphere was 100% oxygen, nitrogen and argon (and not 99.9 % oxygen, nitrogen and argon) it would still be -18 ºC. I’ll explain why in the next lesson.

 

Lesson 3b: More on the solar spectrum lines

Source: N.A.Sharp, NOAO/NSO/Kitt Peak FTS/AURA/NSF
Published: November 30, 2017

390_highresolutionsolarspectrum1200w

I was asked:

‘In the sun’s spectrum there are black lines at special wavelengths’
This threw me a bit, and so in the continuing paragraph I also got confused. The ‘spectrum’ is the range of light yes? I’m not sure why there are black lines in there. And so then are these black lines acting as blackbody? Are the black lines matter? or more electromagnetic waves?

So let me explore that a little more.

The spectrum is the rainbow – but extending beyond the visible. It’s the light spread out in different wavelengths. Above is the photo NASA took of the spectrum from the top of a mountain in Arizona, USA. Note that really it’s one line going from red to blue, (it’s not 2D – they just made it that way to fit on a page!). The colour you see is the spectrum from about 800 nm (top left) to about 400 nm (bottom right) – the visible part of the solar spectrum. (Note that the solar spectrum actually goes from about 200 nm in the UV to about 3000 nm in the infrared – but we can only see this little bit of it).

You also see black lines in the spectrum. These come from gases either in the outer part of the sun or in our own atmosphere that absorb light with particular wavelengths because that light has exactly the right energy (E = h nu) to make an electron jump from one orbit to another. Later the atom might release that energy going back down again – but, this is the crucial bit, it won’t do so in the same direction that the light was going in in the first place (and sometimes that will then cause another atom’s electron to jump up). So less light gets to us at those wavelengths than should – and we see black lines in our spectrum.

Lesson 3: My favourite equation

E=hnu picture

When I was 17 and doing A’level chemistry I learnt the equation E = h nu. This was the most exciting science lesson of my life. Seriously. I was only disappointed that chemistry rather than physics gave me that gift 😂

You see, I’ve always been fascinated by colour – the mixture of a prism for my tenth birthday and my dad being colourblind (a condition one of my two sons has inherited from the x-chromosome I got from my dad: the other son got the x-chromosome I got from my mum) meant that I really wanted to understand how “colour worked”. Now there are three bits to that – understanding our eyes (a detour I won’t go down now), understanding blackbodies – which explains white light – where lots of wavelengths are present, which we covered before, and understanding E = h nu which explains lights being coloured in and of themselves.

What this equation shows is how atoms interact with light. You’ll remember the simplified model of an atom with a central core and electrons in rings around it? Well, when an atom is “excited” the electrons can go up to a higher orbital. And when they fall back down to the ground state (the state where all the electrons are as close to the nucleus as possible while obeying the rules of how many electrons can be in each orbital) they release energy (based on the size of the jump) as light and the frequency of the light (nu) is proportional to the energy jump. So the bigger the energy of the jump, the higher the frequency of the light (more blue, less red).

You can see this yourself. Drop salt into a flame and the heat of the flame will excite the sodium atoms in salt to a higher energy state. As they fall back down they release yellow light. You might recognise that yellow light if you remember sodium street lights. In those electricity excited the sodium atoms. (Go on, try it!)

In the sun’s spectrum there are black lines at special wavelengths (called Fraunhofer lines). That is this process in reverse. The outer part of the sun is cooler than the inner part and absorbs light to make electrons jump up to higher orbitals. So the blackbody light from inside the sun (all wavelengths) loses light at the special E = h nu wavelengths. Of course those do re-emit those wavelengths as they drop back down, but here’s the crux: they re-emit in all directions including back towards the middle, so the amount of light coming towards us is lower.

This is how helium was discovered – they could match most Fraunhofer lines to lines they could get by putting elements into flames – but there were a set of lines they couldn’t match, so they proposed a new element – helium. (From Helios). Later they found helium on Earth. This is also how neon lights work – different jumps in neon give the different colours of neon light.

Now atomic lines are high energy so they tend to be in visible spectral bands. Later we’ll see that molecules have absorption/emission lines too. These are not from electrons jumping but from the molecule wobbling. Because whole atoms have to move the energy is much lower (heavier things don’t move as easily). So these lines are in the infrared.

But we’ll come back to that.

Lesson 2: Blackbody radiation

blackbody radiation

One of the most important bits of physics to understand, before we get to climate, is what blackbody radiation is all about. So this builds on lesson 1 about the electromagnetic spectrum. Here I’ve zoomed in on the middle bit and turned the scale around (so now IR – long wavelengths – are on the right and UV – short wavelengths – are on the left).

When something is perfectly black, it will absorb all energy falling on it (nothing is reflected) but if that were the whole story, things would get hotter and hotter for ever. Of course, that does not happen – because black bodies also radiate energy away as “light”. The blackbody curve (see picture) shows how black objects radiate “light” – and what wavelengths of electromagnetic radiation are radiated. That depends on how hot the object is.  Max Planck wrote down the theory of the blackbody curve back in 1900 and, almost by accident, invented quantum mechanics in the process (but let’s not get side tracked down that interesting alleyway).

As a blackbody gets hotter it emits more light and the spectrum shifts “up and left” (so more light, and the peak moves to shorter wavelengths).

Now that’s all a bit physicy and esoteric so let me link it to things you already know. You already know the idea of “red hot” – as you start heating an electric stove and it gets hotter there comes a point where it starts to glow red. What has happened is that the curve has shifted up and left enough that there’s enough red for you to see it. That’s somewhere around 600 degC (scientists call this ~900 kelvin as we like to start temperatures at absolute zero).

If you keep heating something up it will go orange hot as you start getting orange and red wavelengths too. Your old fashioned tungsten lightbulb (I hope by now replaced by an LED!) had a tungsten filament at around 2500 K – 3000 K (subtract 273.15 to convert to degrees Celsius). That was a yellowy-white. The outside of the sun is about 5500 K and that is “white hot” – the peak of the blackbody curve is in the middle of the visible so all the wavelengths are there and they mix to look “white” (remember Newton splitting white light with a prism to show all the wavelengths are there). There are stars hotter than our sun that are “blue hot” as their peak is in the UV and their spectrum is already dropping in the visible – with blue much higher than red.

But you can see from the graph that the sun also emits plenty of what we call “short wave infrared” (incidentally that was the problem with tungsten lamps – almost all their radiation was actually infrared and thus invisible.).

Of course it doesn’t stop there. Almost everything has a blackbody curve. Blackbodies around room temperature (~300 K) have a peak at wavelengths around 10 micrometres. That’s a wavelength we call “the thermal infrared” and it’s what those thermal imaging cameras you see in science museums (the ones that give people red faces and blue noses and black glasses) measure. Even space itself has a microwave blackbody signal – that represents a temperature of around 3 K (just above absolute zero) – and is the temperature the Big Bang has cooled down to.

Now real objects aren’t perfect blackbodies – they reflect some light – but these basic ideas hold up. And the sun and earth are, to a first approximation, blackbodies at 6000 K and 300 K respectively. And this matters because it’s how the sun heats the Earth up and how the Earth cools back down again.

(Don’t worry, we’ll start getting onto the climate soon! This is the background stuff that makes the explanation meaningful. I know this stuff is hard, so please feel free to ask questions. Lesson three will involve my favourite equation and how atoms and molecules emit and absorb single wavelengths of light rather than blackbodies emitting and absorbing broad spectra … and then we’ll start talking about the temperature of the Earth!).

Lesson 1: Electromagnetic Radiation

EM Spectrum image

To understand climate change, we first need to understand light. (Personal aside: I got a prism for my tenth birthday and told everyone that one day I’d get a job splitting light into pretty colours – so of course I start here)

Light is electromagnetic waves that travel at the “speed of light”. The properties of light depend on the wavelength (how many times the electromagnetic field vibrates). Short wavelength light vibrates lots and the wavelength is small enough to get inside you and damage you – that’s “ionising radiation”: ultraviolet that damages your skin and x-rays and gamma rays that go inside.

Long wavelength light is radio and microwaves and the infrared. That can’t damage your molecules directly, but (and we’ll come back to this), some infrared and microwaves can make molecules vibrate which heats things up.

In the middle is visible light – the bit we can see. At 400 nm (nanometre – that means 0.000 000 400 m) wavelength we start to see blue light (if we don’t have cataracts) around 555 nm it looks quite green – and our eyes are most sensitive. At 800 nm we just about see a deep red (unless colour-blind and lacking red sensors).

Now it’s no coincidence that this is the bit of the electromagnetic spectrum that we see best. This is the peak of the sun’s spectrum – and all we see on Earth is visible electromagnetic radiation from the sun (or one of our artificial lights) that reflects from the Earth.

But the Earth itself does glow – just in wavelengths we don’t see. We call that the thermal infrared. In lesson two I’ll explain about blackbody radiation.

Climate Lessons Introduction

This is my personal blog. I’ve been using it for a few years to post my thoughts. At first it was a place where I explored my thoughts about topics that were controversial – particularly those that people argued about on Facebook. I also explored my faith and how I connected my science to my faith.

However, recently, I started writing “lessons on climate change” on Facebook. I did so because I wanted to help my friends understand some of the basic principles of climate science. I soon realised Facebook was not the best place for such lessons, so I moved them here. Because the blog still has some more personal posts, I’ve left this anonymous. I may change that later – but I’m guessing most of my readers know exactly who I am.

I’ve now posted quite a few climate blogs – so I’ve created a page to help you find them easily.