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Monday 11 November 2019

Dying By Degrees

So, apparently, Donald Trump doesn't believe a report released by his own administration. That's his prerogative, of course, but he has responsibilities (I know; scary, right?)
The report in question is The National Climate Assessment, a comprehensive assessment of the impact of climate change on weather, food production and economy generally. I've only scanned the highlights, but it makes for some pretty hairy reading.

Here, I want to look at something that few treatements I've encountered on climate change have addressed properly, specifically the detailed physics of climate and energy budgets.

Global temperature variation is driven by many factors, including variations in the eccentricity of Earth's orbit (how elliptical the orbit is), variations in obliquity (axial tilt), axial precession, orbital precession and several others. These particular factors are known collectively as 'orbital forcing', and they're effectively cyclical, going through what are known as 'Milankovitch' cycles, after the Serbian geophysicist who proposed them. They had been proposed earlier but, without strong observational evidence to support them, they didn't gain much traction.

There are other factors, such as the evolution of the rotation rate of Earth (slowing due to tidal braking), variations in insolation (the amount of solar radiation received) and albedo (reflectiveness).

One often hears from those who rail against the scientific consensus that 'the climate changes; always has, always will', and this is, in fact, correct. The above factors are the things that, under normal circumstances, drive the long-term evolution of the climate over thousands of years. If that were the end of the story, then the naysayers would be right, and there'd be little to worry about. All else aside, most of these processes are cyclical, which means that they fluctuate around a long-term average. There are two clear exceptions to this, and both are important on long enough time scales, but only one of them is dynamic on the sort of timescales we need to be concerned with in terms of human habitability of the planet, because it's dynamic on the scale of decades; atmospheric composition.

Most of the atmosphere is composed of just two gases, nitrogen (N2)* at about 78% and oxygen (O2) at about 21% with the remainder being small amounts of argon (Ar) at just under 1% and carbon dioxide (CO2) at just 0.04% currently, along with bits of methane and other trace gases.

There's a single process that drives every other process in the universe, encapsulated in the Second Law of Thermodynamics, and known as entropy. Although popularly thought of as the tendency toward increased disorder, this doesn't really reflect what entropy is. What entropy really entails is simply the tendency of energy gradients downwards. It's a measure of how much energy in a system is not available to perform work. The simplest way to think of it is in terms of dissipation of heat from a hot body to its cooler surroundings, such as a hot cup of coffee going cold in a room. We can now think of the evolution of the climate in terms only of this energy cascade to see the problem in its simplest possible light.

There's a set of relationships that describe how the frequency of a 'particle' is a manifestation of its energy. These are known as the 'de Broglie' relations, and they tell us that, the higher the frequency, the higher the energy.

So, radiation in the form of photons comes in from the sun. Some is reflected straight back to space as light, some is absorbed and the rest is radiated back into space as light and heat. 

Some molecules, because of the way that their electrical charge distribution varies when vibrating, absorb some of this outgoing radiation, and this is where the problem occurs. This outgoing radiation is largely composed of lower frequency photons, photons that have shed energy due to their interactions with the planet, which means that they're lower energy and can be more easily absorbed. 

Diatomic gases (gases made up of molecules of two atoms of the same element), such as nitrogen and oxygen, don't undergo changes in the distribution of their electrical charge when they vibrate, which means that they don't interact strongly with photons in the infrared part of the spectrum, so they don't absorb much outgoing radiation. Monatomic elements like argon don't have vibrational modes, so they also don't strongly interact with outgoing radiation. Thus if the atmosphere were composed entirely of these three elements, the input/output ratio would be close to 1:1, and there would be no net change in the global average temperature given a constant insolation. With no greenhouse gases, the models tell us that the average global temperature would be somewhere around -18°C, some 33° cooler than at present.

Now let's look at greenhouse gases, beginning with the big one: CO2

COis a tiny fraction of the atmosphere. As we saw above, it constitutes only 0.04% of the atmosphere at present, which is minuscule and, one might think, therefore insignificant, but such a conclusion would be premature, and that's because of a very specific effect.

It's long been known, since the early 19th century, that colour spectra from stars are not uniform. William Hyde Wollaston conducted an experiment in 1802 using a spectrometer - a refined version of Newton's prism experiment - in which a lens was used to focus the sun's light on a screen. What he realised was that the light put out wasn't consistent across the spectrum, as he'd expected, but had dark bands where no light was received spread across the spectrum. This was further refined by Joseph Fraunhofer in 1815, who counted around 600 dark bands in the solar spectrum. We now call them absorption lines or Fraunhofer lines, and modern observations have revealed thousands of such lines in the solar spectrum.

Spectroscopy is among the most fruitful areas of study in the modern age, too. It's central to whole swathes of discoveries in physics, for example. The first discovery of helium was done using spectroscopy. Helium had never been found on Earth at the time; the first observation of helium was conducted during a solar eclipse in 1868 by Jules Janssen, who named it after Helios, the Greek god of the sun. Spectroscopy underpins much of the development of quantum theory, including the blackbody spectrum identified by Max Planck, the photoelectric effect and Bohr's model of atomic structure.

We now know that those absorption lines are unique to each element, and this is one of the means by which we measure the expansion of the universe via redshift. As the universe expands, the light coming to us from distant sources is shifted toward the red end of the spectrum (their wavelengths are stretched), which means that the absorption lines are also shifted toward longer wavelengths, but they maintain their relationships to each other, meaning that we can still identify the elements contributing to the light source.

So what does all this tell us about CO2?

Well, it isn't only elements that have identifiable spectra, molecules do too and, as noted above, this is because of the way the distribution of their electric charge varies when they vibrate. It's this knowledge that underpins much forensic science, such as drug testing, etc. These vibrations create 'resonances' at very specific frequencies, which are a reflection of the frequencies of radiation that they absorb. This becomes important when we look at the energy budget of the planet.

Here's a nice little group of graphs that shows what the problem is.

Image result for atmospheric absorption spectrumAt the top is a graph showing the outgoing radiation from the planet's surface as it is now with all the contributors to absorption included. As we can readily see, the largest contribution to absorption is water vapour, but it's far from the only one. In fact, comparing the water vapour contribution to the COcontribution, it's fairly easy to see that, while CO2's contribution largely overlaps (meaning that it increases absorption of some of the frequencies absorbed by water vapour), it also contributes to absorption in the area where the maximum absorption of water vapour tails off, meaning that even more frequencies of infrared radiation are absorbed. There's an important point to be made here, which is that all these processes are statistical in nature. Where there's an absorption bump at some frequency, it isn't the case that all outgoing photons will be absorbed. It requires that a photon with energy in those frequencies (remember that a photon is a quantum entity, so that it doesn't possess a single wavelength, but a superposition of wavelengths) meets an appropriate molecule to absorb some of its energy. Additionally, we have to consider saturation. The more appropriate molecules there are, the more probable it is that an individual photon will encounter one on its outward journey. This becomes important when we begin to look at the effect of humans on the atmosphere.

There's also a notably large absorption bump around 5 μm, which corresponds to a gap in the water vapour absorption spectrum, as well as a few bumps elsewhere. These are where the problems really manifest themselves. That bump in particular, because it corresponds to frequencies not routinely absorbed by water vapour, is an increase in the final range of frequencies absorbed and not allowed to escape into space. More importantly, because those correspond to higher frequencies, we know via the de Broglie relationships that those regions are higher in energy. There are several natural sources of carbon dioxide in the atmosphere - volcanoes, naturally-caused forest fires, among others - and they've long had an influence on the planet's energy budget, as we can readily see from ice-cores, which are firm representations of the amount of atmospheric COat any given time. 

It's also important to note once again that these absorptions are statistical. It would be easy to look at those graphs and come away with the conclusion that, where a bump in absorption by CO2 corresponds to a frequency range where there is also absorption by water vapour, that the CO2 absorption doesn't contribute, but this would be a mistake, because what it really represents is an area in which more outgoing photons are absorbed than would otherwise be absorbed, so they still contribute positively to absorption, and therefore average temperature.

Those sources are an important contribution, but they aren't the only ones, and they aren't the sources that are of major concern in terms of the evolution of the climate. What is of major concern is how human activities are contributing to its evolution.

Firstly, atmospheric CO2 has increased by 5% in the last few decades alone as a direct result of industrial activity, among other anthropogenic sources. Once again, this looks like a small number, but it isn't in terms of impact. Every percentage point represents an increase in average temperature.

Secondly, there are other factors to consider, and one of the most important of them is methane. This requires a little more explanation, and it's critical, because the graphs make it look like the absorption by methane is tiny, and it is, in terms of the range of frequencies absorbed but, once again, the frequencies aren't the only important point in absorption. As we've seen, absorption is statistical in nature, and the graphs don't show the individual absorption coefficients of the gases involved. The higher the absorption coefficient of a gas, the more efficiently it will absorb outgoing radiation. The absorption coefficient of methane is approximately 30 times that of CO2, meaning that it's 30 times more effective at absorbing outgoing radiation. This is a huge problem, and here's why:

As the average temperature of the planet increases, permafrost lines retreat Northward. Among other things, there are massive amounts of methane trapped in the world's permafrost. Thus, as the frostlines retreat, huge volumes of methane are being realeased into the atmosphere. This in turn increases the overall absorption coefficient of the atmosphere as a whole, which in turn accelerates the increase in temperature, which makes the permafrost retreat further, which releases more methane, and so on, in a positive feedback effect, which eventually becomes runaway warming. In fact, there are those who suggest that it may already be beyond recovery, notably James Lovelock, who suggested this well over a decade ago.

In reality, while our models are solid, and we know unambiguously that the impact of humans on the long-term evolution of the climate is significant, there's still an awful lot we don't know. I recall objections to the predictions of climate science being the source of much hilarity among climate deniers at a not unreasonable suggestion that the trend of warming might lead to an ice age (actually, we're still in an ice age, but we're enjoying a period of relative warmth, known as an interglacial). On the surface, this could look like a contradiction, but it isn't, because there are processes on the planet that regulate the temperature. One such is the North Atlantic conveyor system (AMOC; Atlantic Meridional Overturning Circulation), which is what keeps Northern Europe from suffering the cold experienced at similar latitudes in North America and elsewhere. This conveyor system brings warm water from the tropics up into the North Atlantic, which in turn warms the air.

What it carries, of course, is salt water. Where the warmer salt water meets the cold fresh water from melting ice in the Arctic, the salt water dives down and is carried back Southward, as a result of differential density. This process is fairly stable for the time being, but it may not always be so. The greater the saturation of fresh water in the North Atlantic, the further South this conveyor system is pushed and, more importantly, it's far from being beyond the realm of possibility that this system could be cut off entirely if the influx of fresh, cold water is sufficient. Once cut off, the Northern hemisphere will cool appreciably, and this could lead to an increase in the planet's albedo, which in turn could end the interglacial. Alternatively, dependent on timing and temperature of cutoff, we could see ice extending much further South with temperatures very hot at the equator. Certainly the effects on Northern Europe would be huge.

Update 05/08/21: We now have reports that a decline in AMOC has been measured, and may be approaching a critical threshold. Where the point is that it simply shuts off is not known.

Science Daily article.

The effects of climate change aren't merely off in the future, either. We can see their effects even now. Entomologists are reporting record lows in the accounting of insect numbers the world over (there's one disturbing story of a couple of entomologists who revisited a jungle in South America that used to be awash with the noise of insects going about their business, but is now almost deathly silent). It's thought that insects are experiencing extinction rates in the region of eight times that of other animals. Coral reefs in the South Pacific, notably the Great Barrier Reef, are suffering increased bleaching, effects known to occur with an increase of only 1ºC, an increase which has already been exceeded. Coral reefs aren't just living organisms in themselves, they're entire ecosystem, which means that their deaths also represent the deaths of many species contingent upon them.

While weather (short term local effects) does not represent a change in climate, we can still see the effects of climate change there. The increase in severity and frequency of tropical storms and hurricanes, for example, is a clear pointer to an increase in average global temperature. The increase in frequency and severity of forest fires all across the globe, can be tied at least in part to a warming trend, as ambient temperatures inch closer to natural combustion points. The severe droughts in Texas and elsewhere that led science-denier Ted Cruz to enjoin people to 'pray for rain' are again increased in frequency and severity by an increase in average temperature.

Here's a sobering animation produced by NASA that shows the data gathered over the last 140 years. Note that what's shown here is not absolute temperatures, but variations from the norm, so that variations from the norm upwards are shown in red, while the variations downward are shown in blue.

You'll note that there are pockets of average temperature decreasing throughout, even in recent times, but the average overall shows a crystal clear trend upward.

Here's the thing, and this should really be the take-home from this post. Even if you don't believe what the science unequivocally tells us, even if you think it's all a scam, the responsible thing to do is to treat it as if it's real, because this isn't some abstract we're talking about here, this is the future of our species, and the future of many, many other species. Far too many are treating this as a political issue, when it really isn't. Too many talking points are thrown around, such as that giving in to carbon emission targets will have negative impacts on economies. Nothing could be further from the truth. Necessity drives technology, and we've already seen that new technologies employed in reducing carbon emissions in manufacturing are in fact paying dividends economically, with massive benefits in overheads and manufacturing efficiency, driving down costs and increasing productivity.

I'm going to leave this topic here, except to give a single recommendation for anybody genuinely interested in the subject. It's one of the best, most informative sources for information about climate change I'm aware of. The channel is run by science journalist Peter Hadfield, a geologist by training, and comprehensively debunks all the myths regarding climate change, as well as offering much information about how industry can not only play its part in addressing this emergency, but also in how it can benefit economically from doing so. He's among the most ardent sceptics I've ever encountered, and really does know how to research scientific topics using the primary literature.


Here's the first video in his climate change series:

I'll shortly be doing a supplement to this post addressing some of the objections.

*The subscript here denotes the number of atoms present. Nitrogen gas is molecular, meaning that the gaseous state requires more than one atom - two in this case. The same is true for oxygen.

† Audio friends will recognise this name, as this is the name of a popular codec used to encode MP3 files.

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