Climate Change for Dummies Series

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Apr 262016


Maybe I shouldn’t have called this series ‘for dummies’, I don’t mean to imply anything by that but it’s done now!

I’m posting part 3 of the series today which took longer than I thought. The new article which is the previous blog post to this one is called “How we know that the Earth is warming up

If you haven’t read the first two blogs, please read them first. I designed the series to build up the evidence in an incremental fashion. The articles are just below in my blog, or you can use the links below:

Part 1: How do we know that recent increases in CO2 levels are man-made?

Part 2: How do we know that CO2 causes the planet to warm up?

Part 3: How we know that the Earth is warming up

Part 4: How we know for sure that we are responsible for warming the earth

I hope you find them informative and clearly written – that’s the goal anyway.

 Posted by at 3:21 pm

Climate Change for Dummies #3: How we know that the Earth is warming up

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Apr 102016


Chart from

You’d think that this was a simple question to answer, and it is really, but there are people who dispute this fact – despite the mountains of evidence we have. In this article, I’m not going to address why the Earth is warming, but just establish the fact that it is, and also that it is doing so at an unusually fast rate.

Firstly, let’s set the time-frame that is concerning us here. We know that the climate of the Earth has changed in the past, well before we humans were capable of influencing it. Indeed, the scientific study of palaeoclimatology has revealed that climate changes that have occurred in the past have helped us understand that the ‘recent’ warming is unusual. So, what does ‘recent’ mean? In this article ‘recent’ means since about 1850.

Now that I can use the word ‘recent’ so that we all know what it means, to be able to state that the level of warming that we measure is ‘unusual’ I need to define what ‘unusual’ means. I can hear the groans of dismay from here! However, bear with me because so much is made about how the recent changes can be dismissed as simply ‘natural’, I have to set a baseline of what is natural so that we can agree that the recent warming is ‘unusual’.

Past Global Temperatures

Because we have only been using instruments to physically measure the temperature around the world for  a hundred years or so, we need to use other ‘proxy’ methods. These include ice cores, tree rings, coral reefs and lake and ocean sediments amongst others. The data extracted using these proxies can be used to reconstruct various aspects of the climate, including global temperatures before instrumental records became available.

The chart in Figure 1 below is a recent version of one of the most talked about graphs in the history of science. It has been called the ‘Hockey Stick’ because the recent rapid upturn in global temperature looks like the upturned blade of a hockey stick.


Figure 1: The 1998 original Hockey Stick chart (blue), shown against a 2007 reconstruction by Wahl & Amman (red). More recent data from instruments are in black.

Michael Mann’s original 1998 version has been the subject of intense scrutiny over the years, and the denier community has done a very good job in attacking it to the point where many people think that the hockey stick is ‘broken’. However this could not be further from the truth. Several more recent reconstructions carried out independently using a variety of techniques and proxies have all verified the original findings. The denier community do this all the time – they just keep repeating a particular message again and again until it becomes fixed in the minds of the press and the public, even though it is not true. However, the scientific method keeps grinding on, and the climate scientists have concluded that the main ‘take-away’ is as follows:

The last few decades are the hottest in the last 500 to 2000 years (depending on how far back the reconstruction goes).

This answers the definition of what we mean by ‘unusual’ warming.

By the way, get used to looking at ‘anomalies’ when viewing data and charts. Just about every chart or table we look at in the field of climate science show anomalies of something, be it temperature or amount of ice loss. The anomaly values are always the difference compared to a baseline figure which will be plotted as the zero. The baseline will be a calculated average over a number of years. For example in the hockey stick chart shown above, the left axis describes the baseline as ‘Ref. 1902-1980’, and this averaged figure will appear at 0.0 on the axis. Temperatures warmer than the baseline appear above the 0.0 baseline, and cooler below.

Multiple Lines of Evidence

There are many lines of evidence that we can look at to find out if the recent warming is unusual (notice how I can use those two words now and we all know what I mean).

Direct measurements

We have global records of land and sea surface air temperatures as well as air temperatures over the oceans. Some of these temperature records go back to about 1850. You will hear a lot of nonsense from the denier community who regularly bleat on about badly placed measuring stations and urban heat island effects; but none of that detracts from the fact that all these calibrated measurements show the recent warming very clearly.  I might write a separate article about the badly-placed weather stations and why it is that we know they don’t make a jot of difference.

There are several datasets of global air temperatures, and a good example is the data from the Goddard Institute of Space Studies (GISS). The data in their GISSTEMP analysis comes from weather measurement stations, and you can download the raw data yourself. The chart below comes from the GISSTEMP data analyses:


Figure 2: Line plot of global mean land-ocean temperature index, 1880 to present, with the base period 1951-1980. The dotted black line is the annual mean and the solid red line is the five-year mean. The green bars show uncertainty estimates.

Satellite Measurements

I could put this under the ‘Direct Measurements’ section above, because satellites orbiting the Earth are equipped with sophisticated instruments that are making direct measurements of something; but they cannot directly measure temperature with a thermometer in the same way as a weather station does, or a ship which measures the temperature of the water at the surface of the ocean (often done by hauling in a bucket of water!). Satellite measurements can lead to an indirect measurement of the air temperature at various altitudes in the troposphere, but there are a lot of manipulations of the raw measurements required to do this. This is what a scientist who works with the Remote Sensing Systems (RSS) satellite dataset said recently:

They [satellites] are not thermometers in space. The satellite [temperature] data … were obtained from so-called Microwave Sounding Units (MSUs), which measure the microwave emissions of oxygen molecules from broad atmospheric layers. Converting this information to estimates of temperature trends has substantial uncertainties.

However, climate change deniers, like Senator Ted Cruz (R-Tx) have been holding up the RSS satellite dataset as “the best data we have”. They like to say this because the RSS data has, until recently, shown the least amount of warming in recent years. They like to say that the satellite data is more reliable than the ground based measurements which we discussed in the ‘Direct Measurements’ section above. However, there have been recent adjustments made to the RSS data which the deniers don’t like. Have a look at this video:

So, in summary, satellite measurements are valuable if correctly converted and calibrated, and they also show a warming, as shown in the chart below which is the latest RSS data which has been properly adjusted for diurnal variation (as described in the above video clip, and more information here).


Figure 3 – The global Middle Troposphere (TMT) anomalies from 1980 to now. The black line shows the old version, the light blue line the new. Note that the overall trend in the new version is 60% bigger than in the old version. The green line at the top shows the effect of the improved diurnal correction.

Just before we leave this section, it is interesting to note a denier trick to do with cherry picking the data. Take another look at the RSS data chart above, and notice the big peak around 1998 – that was due to a very big El Nino year. What people like Ted Cruz like to do is show a small section of this data where the left-hand side of the chart starts at that 1998 peak. They then draw a line through the data which appears to show cooling because the trend appears to go down, not up. This is also known as deliberately lying and abusing one’s position of authority – something that Ted Cruz seems to do on a regular basis.

Rise in Sea Level

There are two reasons why a rise in sea levels provides an indication that the world is warming. The first reason is due to the fact that water expands as it warms, and the second is the addition of extra water from melting ice caps and glaciers. But before going into any details we must again establish a baseline so that we can show that the recent rise in sea levels is ‘unusual’.

The basic summary of past sea levels goes like this: At the end of the last ice age (about 21,000 years ago), global sea levels rose 120 metres over several millennia and stabilised between 2,000 and 3,000 years ago. There is strong evidence to show that sea levels have changed very little from around AD 0 to about AD 1900. Since then there has been a marked, measured increase in global sea levels. So, the story for sea levels is very similar to the hockey stick described above for global temperatures, and it is clear to see that the recent measured rise is certainly ‘unusual’. This really is not surprising because the curve just has to follow the shape of the temperature anomaly curve because basic physics says that it should!

warm-03Figure 4 – Sea level evolution in North Carolina from proxy data (blue curve with uncertainty range). Local land subsidence is already removed. For more information see here

How do we measure the rise in sea levels? There are two main methods, and a bit like the case for temperatures there are direct measurements using tide gauges and also satellite measurements. However, for the case of satellites the sea level measurements are indeed direct measurements rather than indirect as in the case of temperature. This article explains the techniques used for both surface-based and satellite.


World-wide, the vast majority of the world’s 170,000(+) glaciers are shrinking, a small number are actually growing, but this is due to global warming too! To understand why please see this superb video – it says it all much better than I could possibly describe it.


Frequency of Cold and Warm Nights and Days

This is an interesting one. From time to time, weather stations will record a record high or low temperature. If there were no global warming going on, we would expect that record lows and highs would average out over time. But this is definitely not what we see. What we observe is that record highs are outpacing record lows, and as time goes on, the ratio of record highs to lows is increasing. The following widget shows the current ratio of record highs and lows across the USA. It is updated on a daily basis. It is not possible to have such a large ratio in favour of the record highs unless there has been recent global warming. Please be sure to click on the “Learn More” link on the widget.

Ice Melt

I’ve already covered the melting of Glaciers world-wide and mentioned that melting ice is one contributing factor in the measured rise in sea levels, but there are other indications of a warming planet that can be seen and measured at both poles of the Earth.

Firstly, the Arctic and the Antarctic differ enormously in their makeup. The arctic is a frozen sea surrounded by land, whereas the Antarctic is a frozen continent surrounded by sea. But, there is also a similarity between them that is worth mentioning, and that is the phenomenon called Polar Amplification. In basic terms this means that any change in the net radiation balance (for example greenhouse gas intensification) tends to produce a larger change in temperature near the poles than the planetary average. So, changes such as the amount of ice cover are extremely important, as these effects have positive (bad) feedbacks associated with them. The obvious positive feedback effect here is that ice is bright and reflective which helps to bounce radiation from the sun back into space. However, warming melts the ice, exposing the sea. The sea is darker and absorbs more heat which melts more ice which exposes more sea which warms more and melts more ice – and so it goes on.

The Arctic

Let’s focus on the Arctic first. The extent of the arctic sea ice can easily be photographed and measured by satellites these days. The extent is a two dimensional indication to how far the sea ice covers the sea each year. Obviously there is a seasonal variation with an annual peak in the sea ice extent in the northern winter and a minimum extent at the end of the summer.

The chart below shows the trend in arctic sea ice extent from 1979 to 2015 (averaged to get rid of seasonal variations). There are clearly large troughs in extent – for example 2007 and 2012 when record lows in sea ice extent were measured. It seems that the sea ice has recovered from these record lows. However, the long-term trend is clearly downwards. It is very likely that a new record low, beating that of 2012 will be recorded by the end of this year – 2016. Record lows will continue to be beaten as the trend goes downwards.


Figure 5 – Shows the recent downward trend in Arctic sea ice extent.

However, sea ice extent alone does not reveal the whole picture because even the thinnest ice will be recorded in this metric. What is more revealing is the measurement of the volume of the sea ice. This is much harder to do and there are two main satellite measurement techniques that are used. Firstly, there is altimetry where satellites measure the height of the ice. Incredibly they can detect changes of just a few millimetres from an altitude of around 1000km! Secondly, exquisitely sensitive changes in the gravitational field of the Earth are measured using satellites, notably NASA’s GRACE satellites. This reveals the change in volume of the ice as mass is lost to the oceans. The chart below shows data from GRACE for the case of Greenland and the downward trend in volume is clearly seen.


Figure 6 – The solid blue line is the best-fitting linear trend. T Harig C, Simons FJ. Mapping Greenland’s mass loss in space and time. Proceedings of the National Academy of Sciences of the United States of America. 2012;109(49):19934-19937. doi:10.1073/pnas.1206785109.

The bottom line is that the arctic sea ice volume is reducing and at an increasingly fast rate. This is true even during the years where the sea ice extent grows.

The Antarctic

Climate change deniers like to talk about the Antarctic because the extent of the sea ice surrounding the continent has actually increased in recent years! Surely that can only happen if the Antarctic is cooling, not warming!

As always, the truth is slightly more complicated and, in fact, the increase in the sea ice extent is explained by warming, not cooling! Firstly, the chart below shows that the surface air temperatures over the ice-covered areas of the Antarctic are warming. Also, oceanographic studies reveal that the surface waters of the southern oceans are warming – and at a faster rate that the global average.


Figure 7 – Annual mean surface air temperature averaged over the ice-covered areas of the Southern Ocean. Straight line is trend line (Zhang 2007).

So, how can the increase in Antarctic sea ice extent be explained? I can’t beat this very concise explanation by John Cook from the brilliant website

There are several contributing factors. One is the drop in ozone levels over Antarctica. The hole in the ozone layer above the South Pole has caused cooling in the stratosphere (Gillet 2003). A side-effect is a strengthening of the cyclonic winds that circle the Antarctic continent (Thompson 2002). The wind pushes sea ice around, creating areas of open water known as polynyas. More polynyas leads to increased sea ice production (Turner 2009).

Another contributor is changes in ocean circulation. The Southern Ocean consists of a layer of cold water near the surface and a layer of warmer water below. Water from the warmer layer rises up to the surface, melting sea ice. However, as air temperatures warm, the amount of rain and snowfall also increases. This freshens the surface waters, leading to a surface layer less dense than the saltier, warmer water below. The layers become more stratified and mix less. Less heat is transported upwards from the deeper, warmer layer. Hence less sea ice is melted (Zhang 2007).

Antarctic sea ice is complex and counter-intuitive. Despite warming waters, complicated factors unique to the Antarctic region have combined to increase sea ice production. The simplistic interpretation that it’s caused by cooling is false.

There are other indicators that point to the fact that the world has warmed over and above what is expected due to natural variations in recent years. I think I have laid out enough evidence in this article to close the case. However, as an exercise for the reader you might like to look into these topics:

  • Spring arriving earlier
  • Increase in ocean heat content
  • Rise in specific humidity

There are others…

 Posted by at 10:07 am

Climate Change for Dummies #2: How do we know that CO2 causes the planet to warm up?

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Apr 042016

In part 1 of this series, I put forward the evidence on how we are sure that CO2 levels are rising and how we know that human activities are the cause. In this article I show how it is that we know that CO2 causes the planet to warm up. I’ll start with a small rant…

Small Rant

If the Earth, like our satellite the Moon, had no (or very, very little) atmosphere, the temperature measured at its surface would be very much colder than the balmy conditions we enjoy as a species of animal that has evolved on this watery blue jewel of a planet that we call home. And let’s be sure of one thing – this mote of dust that we inhabit, with its fragile and thin atmosphere, is the only place we are ever likely to survive. Why? Because the Universe is a big place, and we will never manage to abandon this oasis of our birth to populate another rock which we may royally fuck-up in a similar manner as we have here. Rant over!

Warmer than theory

If we considered the Earth to be an ideal theoretical ‘blackbody’ then we could calculate that the radiation from the Sun would warm its surface to about 5°C. If you then allow for the fact that the Earth is quite reflective due mainly to its bright oceans, clouds and polar caps, this temperature would drop to a theoretical value of about -18°C. Something here does not add up! The actual, measured average temperature of the Earth is about +14°C, so what is it that causes the Earth to be over 30°C warmer than simple theory predicts?

The answer lies with the atmosphere – the layer of air that extends from the surface of the Earth to the edge of space. It is often said that if the Earth was the size of a bowling ball, then the atmosphere would be only as thick as a coat of varnish, and that is certainly about right, but all our weather happens in this tenuous layer of gases, and this article will explain how certain gases in the air are responsible for raising the temperature.

Isn’t this old news?

You might think that our understanding of the climate is a very recent thing, perhaps in the last few decades, but in fact we have known about the basic mechanisms that drive changes in the climate for well over a century, and the first clue came from the great French mathematician and physicist Joseph Fourier around 1824. He calculated from the size of the Earth and its distance from the Sun what the temperature should be if the only factor was the heat it receives from the Sun. As mentioned above the answer to this turns out to be rather chillier than it is, but he then went on to suggest that the air might absorb heat rising from the Earth’s surface, preventing it from escaping into space, thus warming the planet. This idea was inspired by earlier experiments carried out by Horace Bénédicte de Saussure, and so Fourier was the first person to describe “the greenhouse effect” (although he didn’t use that term).

The Greenhouse Effect

A quick physics refresher here: All light, be it visible, infrared, ultraviolet, radio, microwave, x-ray or gamma is the same stuff – electromagnetic radiation. The different categories are to do with the wavelength of the light. Wavelength is sort of the inverse of Frequency. The higher the frequency, the higher the energy of the light (and the shorter the wavelength). Talk of wavelength and frequency suggest something is waving, and indeed it is, for light has both magnetic and electrical field components, which are the things that are waving about. When one wiggles it generates the other and vice versa. End of physics lesson.

The greenhouse effect goes like this: The electromagnetic radiation reaching the Earth from the Sun consists mainly of visible and ultraviolet wavelengths. Not all of it reaches the ground, some is reflected back into space before that, but the planet absorbs the energy that does reach the surface and then reradiates it as infrared radiation. You can’t see infrared light with your eyes, but you can feel it on your skin – it’s what we call heat. On the way back up, greenhouse gases absorb the infrared light and reemit it in all directions. Some of it eventually escapes back into space; some goes back down towards the Earth’s surface. The result is a warming in the lower part of the atmosphere – think of it like putting a blanket on at night to keep warm.

The picture below is taken from NASA’s website here


Being just a little bit of a pedant about these things, the term “greenhouse effect” is not a bad one, but a glass greenhouse does not keep the air inside it warm for quite the same reason as the so-called greenhouse gasses (GHGs) do, and that’s because the infrared radiation in a greenhouse is trapped by the fact that glass is opaque to infrared radiation (and won’t let it back out), rather than the absorption/re-emission process described above.

GHGs vs. Non GHGs

Before discussing the gases that do contribute to global warming, let’s first look at the other gases that are present in the atmosphere that don’t. This is quite instructive for a couple of reasons which will become apparent.

The air we breathe is mostly made up of Nitrogen (78%), Oxygen (21%) and Argon (0.93%). The rest of the components of (dry) air are present in trace amounts as we will see when we look at GHGs.

Why is it that some molecules are affected by infrared radiation yet others aren’t? Well, it’s complicated, but Nitrogen and Oxygen exist in the atmosphere as N2 and O2 which are known as diatomic molecules (two atoms in the molecule). Argon exists as a monatomic molecule. When these molecules vibrate, there is very little change in the distribution of their electrons and hence very little change in the electrical charge distribution around the molecule. This makes them almost totally unaffected by infrared (IR) radiation – in other words, IR radiation is just not in tune with them; not on their wavelength so to speak.

There are several gases in the air that do contribute to global warming. Before discussing them individually, let’s just explain how it is that these GHG molecules are affected by IR radiation. If you can, imagine a molecule of Carbon Dioxide (CO2) which has one atom of Carbon bound to two atoms of Oxygen. When this molecule is excited by an incoming infrared photon of light, it vibrates and absorbs the photon, and then reemits another photon of IR in a different direction. It just so happens that the frequency that the molecule ‘likes’ to vibrate at is ‘in tune’ with the frequency of infrared radiation; and this is why the IR photons get absorbed. Photons of visible or ultraviolet light, for example, do not cause the molecule to ‘ring’ and are ignored.

A diagram is worth a thousand words and the following animation of a CO2 molecule being excited by a photon of IR radiation is from the UCAR website here

Human-caused climate change deniers (henceforth referred go as ‘deniers’) like to point out how small the contribution of GHGs like CO2 is to the overall makeup of the atmosphere; we just learnt that they are only present in trace amounts (just over 400 ppmv or 0.04% in the case of CO2). They ask “how could such small traces make any difference to the temperature of the Earth?” There are several answers to this, and one is that the greenhouse effect cannot be diluted by the non-GHGs. In other words doubling or halving the percentage of Nitrogen or Oxygen will not have any effect of the ability of the GHGs to absorb and reemit IR radiation. Additionally just because the GHGs are present in trace amounts, and people find it hard to believe they can be dangerous is not a very scientific way of going about things as there are lots of examples of things that humans have trouble imagining that are nonetheless true. For example even 0.000001% of Arsenic in the water supply is considered a danger to Human health.

The Greenhouse Gases

  • Water vapour (H2O) is the most abundant greenhouse gas and is the biggest contributor to the ‘natural’ greenhouse effect. The amount present in the air varies depending on how ‘wet’ the air is. Water vapour provides us with an example of a bad ‘positive feedback’ in the climate meaning that more of it causes more warming which in turn causes more water vapour to form and so on. Human activity has little direct effect on the abundance of water vapour aside from the fact that warming caused by CO2 emissions causes warming, which causes more moisture, which causes more warming…
  • Carbon Dioxide (CO2) is probably the most important of the GHGs. It has a much longer lifetime in the atmosphere and this can be anything between 30 and just below 100 years (By comparison, water vapour molecules hang around for about 9 days on average). We know for certain that human activities are increasing the amount of CO2 in the atmosphere.
  • Methane (CH4) exists in quantities in the air about 200 times lower than CO2. The thing about methane is that it is a more ‘potent’ GHG than CO2 as it has a higher greenhouse ‘potential’. However, it has a shorter lifetime in the air than CO2 at about 11 years. Methane is currently responsible for about 20% of the non-natural greenhouse effect. It comes from sources such as cattle and drilling for natural gas. Recently there was a huge leak of methane in California which we really could have done without! A big worry with methane is another (bad) positive feedback effect where recent warming is melting the not-so-permafrost in places such as Eastern Siberia, this is venting methane which causes more warming which cause more venting…
  • The are other GHGs including Nitrous Oxide, Ozone and CFCs, but their impact is not so important as those described above.

The Evidence

To be honest, when I write this stuff, I get to the point where I think that anyone reading this would already be convinced by the story so far. But I’ve only really introduced the science and haven’t presented any evidence of warming or that it is caused by GHGs. Let’s start by thinking about some experiments we could carry out to prove our case.

The first experiment I can think of is some way of looking down at the Earth from space and measuring the amount of radiation that is coming back from the surface. If we could measure the amount at the different wavelengths of light then that would be great. Secondly, it seems sensible to propose that, if the warming is being caused by GHGs in the lower part of the atmosphere, then if we could measure changes in temperature vertically through the atmosphere that would also prove valuable evidence. It turns out, of course, that we can do both these things.

The first experiment has been carried out by satellites orbiting the Earth since 1970. They have been measuring the brightness of the radiation coming back up from the surface at various wavelengths. The chart below from a paper in published in Nature (Harries 2001) shows the change in the brightness over the infrared part of the electromagnetic spectrum from 1970 compared to 1997. This is a period over which the Earth has been measured to have warmed. The chart shows that the brightness decreases most at the IR wavelengths which are ‘tuned’ to the GHG molecules as described above. Here we can see the decrease due to Carbon Dioxide (CO2), Ozone (O3) and Methane (CH4), plus some other CFC molecules. In a nutshell, the Earth has got slightly dimmer over time in the IR wavelengths as seen from above, because more GHG molecules have held the IR ‘captive’.


The second experiment involves measuring the temperature changes over time at varying heights in the atmosphere. Some useful terminology here: The troposphere is the name for the lower region of the atmosphere covering from ground level up to roughly 18 km. The stratosphere takes over after that and goes on up to about 50 km in altitude (the mesosphere takes over higher than that).

We can easily measure that GHGs are warming the troposphere. We use ground based weather stations and satellites. If the GHGs are preventing some of the IR radiation from leaving the troposphere we would also expect the upper atmosphere to cool over time as the lower atmosphere warms over time. Interestingly, the situation would be reversed if it was the Sun causing the warming.

Fortunately, since 1979 The National Oceanic and Atmospheric Administration (NOAA) have been operating the Stratospheric Sounding Units (SSUs). These satellites have provided near global stratospheric temperature data above the lower stratosphere. These meaurements clearly show that the stratosphere has been cooling whilst the troposphere has been warming. The chart below is taken from a study by Ramaswamy et al., Reviews of Geophysics, Feb. 2001. The data covers the time perion of 1980 to 1995.


There really is no other convincing way of explaining a cooling in the upper atmosphere whilst the lower atmosphere and surface warms.

Just one more thing. Satellite measurements have detected that the stratosphere is shrinking, this is exactly what you would expect as contraction occurs with cooling.

Case closed.

 Posted by at 9:58 am