Global Warming. The arguments in favour.

Let's start with some basic information about the rock on which we live.

Our planet, our amazing Earth, is a ball of mostly molten (to at least some degree) rock, floating in the vastness of space.  We live on the solid crust, the lithospere, which is approximately 50 miles thick and comprises less than half a percent of the Earth's depth.  The core of our planet is 'solid' but still extremely hot.  It's solid state is not due to it being cool then, but is due to the enormous pressure preventing free movement of its constituent atoms.

The atmosphere that surrounds us is about 600 miles thick in total, but 90% of the air that makes up that atmosphere is contained in a layer just 7 miles thick.

The heat in the Earth - some 6,700 degrees Celsius at the core - has a minimal warming effect at the surface because rock is a poor conductor of heat.   On average the warming of the crust by heat flow from within the Earth is about 0.087 Watts of energy per square metre.  The source of that energy is primarily radioactive decay (80%).  The remaining energy comes from that left over from the collisions that created the Earth.

How does that heat from within the Earth compare with that received from the sun? It is infinitesimally small in comparison.  On average the amount of energy from our closest star is 164 Watts per square metre of the Earth's surface.  However, that varies because the output from the sun is not constant.  The sun goes through regular cycles, waxing and waning in power.  These cycles occur at regular 11 year intervals and can be observed by measuring the solar wind and numbers of sun spots. As the cycle reaches what is called solar minimum, and the number of sunspots decrease, so the amount of energy hitting the Earth falls.  How do we know this?  Well, sunspots have been recorded for thousands of years (Earliest known mention was 364 BC by the Chinese astronomer Gan De) and their absence is correlated with lower temperatures.  Correlation does not show cause and effect but modern astronomy has directly measured the energy of the sun as the number of sunspots wax and wane.  Since the laws of physics are not known to change, it is reasonable to conclude that the amount of energy reaching the Earth was low during low sunspot periods of the past.  Take the period of roughly 1645 to 1715, known as the Maunder Minimum. During that period there were virtually no sunspots and the temperatures plummeted.  That the sun's output was low during such periods of cold is borne out, not only by sunspot numbers but also by the relative amounts of such substances as Carbon 14 and Beryllium-10. The amounts of these substances increase as the amount of solar radiation decreases since they are formed by cosmic ray interaction with our atmosphere.  Cosmic ray levels increase as the solar wind decreases.  This solar wind makes Earth habitable by screening us from these rays.

So, the primary source of heat energy on Earth is the Sun.  Further evidence of this comes from our direct experience (toward the poles) of reduced solar radiation per square meter of the Earth during each Winter of the year. As the inclination of the Earth toward this primary heat source changes, so does the temperature.  Nothing else steps in to warm the Earth in the absence of the sun.  But why do we experience the temperatures we do? Because of the atmosphere.

The moon lacks an atmosphere but is, to all intents and purposes, at the same distance from the sun as Earth.  It orbits our planet at an average distance of about 240,000 miles. A little under a quarter of a million miles, then.  Given that Earth is an average 93 million miles from the sun, the fact that the moon moves toward and from the sun by a quarter million is irrelevant.  So what is the temperature like on the moon, with no atmosphere?  The day time temperature of the moon is 107 degrees Celsius - more than hot enough to boil water. At night the temperature drops to -150 degrees Celsius.  If the Earth did not have an atmosphere then similar figures would apply to our planet.  They would not be exactly the same due to the different rotational periods.  Earth rotates more quickly than the moon and so there would be less time to reach extremes.  Less time for any point to be exposed to the sun or to have no sun exposure.  However, that only makes slight differences. With no atmosphere, the average temperature of the Earth would be -18 Celsius.

So how does the atmosphere regulate the average temperature of the Earth?  The atmosphere acts like a blanket.  Just as a blanket on a bed slows the heating of an individual in a warm room and slows the cooling of the same individual in a cool room, so the atmosphere does the same for the Earth.  It reflects some of the incoming radiation back into space (about a third) and also, once that radiation is converted to infra-red radiation on contact with the surface, delays that radiation going back out into space.  The secondary effect of the atmosphere is to allow the spread of heat around the planet.  Streams of air carry the heat from day to night sides of the planet and from Summer to Winter hemispheres.  In other words, the atmosphere softens the extremes of global temperatures.

Does the temperature of the Earth rise and rise indefinitely? No.  All the energy of the sun is either re-radiated back into space (in one form or another) or, to a very small extent, converted to more permanent forms such as organic compounds. Living things use the energy of the sun to build their structures and then those structures are converted to oil, coal and gas.  That energy is released when the fossil fuels are burnt and converted back into heat that is again radiated back into space.

How does the atmosphere hold onto heat?  The atmosphere largely allows light from the sun to enter freely.  What happens to it then depends upon what it hits once it has finished it's journey through that atmosphere.  The simple physics of matter mean that things we humans perceive as light in color reflect the suns light back out into space.  Progressively darker surfaces reflect progressively smaller amounts of the energy they receive back out.  A black object absorbs virtually all of the light hitting it.  That's why it appears black to our eyes.  There is no light radiation for us to detect.  The energy of that light is not lost - a physical impossibility - it is changed.  What it becomes is infra-red radiation, otherwise known as heat.  We humans can't see it but our technology, and other creatures can.  Warm things radiate their energy in the infra-red spectrum.  That energy is radiated into space too.  However, some substances capture that infra-red radiation as it heads back up from the surface of the Earth.  These are what we call the Greenhouse Gases because, although they use a different method, their effect is the same as the glass in a greenhouse - they delay the passage of heat.

Not all of the gases in the atmosphere have this effect of capturing heat and are therefore not Greenhouse gases.  Only those which consist of two or more different atoms fit the category.  So, Nitrogen (N2), Argon(AR) and Oxygen (O2) are not greenhouse gases, Carbon Dioxide (CO2), Water vapour (H2O) and Methane (CH4) are.  The bonds between those atoms absorb the infra-red radiation and re-emit it in all directions.  Some will continue its journey out into space but some is sent back toward the Earth's surface.  How does it do this? Heat is simply the vibration of atoms.  This is basic physics.  As a substance cools, it's molecules vibrate less until at absolute zero they are motionless.  Photons of infra-red radiation strike the atomic bonds of molecules in the atmosphere, converting light energy into kinetic energy.  The increased vibration of these molecules is experienced as heat.  As the molecule returns to its pre-collision state and slows, its kinetic energy is converted back into infra-red radiation.

The more of these gases there are in the atmosphere, the more heat energy is retained and the higher the temperature we experience is.  We know this is the case because, as we saw above, simple physics which was developed long before any controversy over whether or not we are affecting the climate, states that the Earth should be some 33 degrees Celsius cooler than it actually is.   We also know the physics, described above, which shows why the Earth is warmer than it should be.  Greenhouse gases temporarily trap heat within the atmosphere.  This is what allows us to live on planet Earth in the first place.  This physical process also explains the temperatures of other bodies in the solar system. Venus is far hotter than it's size and distance from the sun would predict.  At 462 degrees Celsius the surface temperature is hotter than that of Mercury at 420 degrees Celsius.  This despite the fact that Venus, at a distance from the Sun of 76 million miles, is twice that of Mercury at an average distance of 35 million miles.  What is distinctive about Venus? It's massive atmosphere is 96% Carbon Dioxide!

This gives rise to one of the questions that opponents of global warming have to explain.  If the principles of physics tell us what the average temperatures of Earth and Venus should be, why are they not at those temperatures? How do we explain the observed differences from expectation in the two planets - consider the difference between Venus and Mercury especially the inverse relationship between temperature and distance.  One oft used argument is that water vapour is a potent greenhouse gas and it is that which is raising the temperatures of the planets and not CO2.  However: what causes water vapour levels to rise? The temperature!  Water vapour IS increasing and will add to the Greenhouse effect on Earth.  But increased water vapour is a symptom of increased warming not the initial cause!  Venus has virtually no water anyway.  But I get ahead of myself here.  Back to basics.

Ok, so the Earth is warmer than the physics of black-body radiation would suggest and it appears that this is because of its atmosphere and the presence of greenhouse gases, particularly carbon related gases such as Carbon Dioxide and Methane (the levels of which are also rising alarmingly) Methane has 25 times the greenhouse effect as Carbon Dioxide but each molecule only lasts about 8 years in the atmosphere, compered to over a hundred years for CO2.

Given the effects of physics on basic physical objects and the fact that an atmosphere blankets such a body, increasing it's surface temperature, what else affects the temperature of the Earth?  Let's go back to the absorption of light again.  Technically the absorption of light energy is said to depend upon the albedo of an object.  Simply put - how light in colour it is, from the Latin for 'Whiteness'.  The whiter an object is, the more light it reflects unchanged back into space.  We can see then that the more snow there is on the surface of the planet, the cooler it is because it absorbs less of the energy of the sunlight which hits it.  As we know, the continents slowly drift across the face of the planet.  This is driven by the heat of the inner Earth forcing apart the continental plates at the mid-oceanic ridges and the further edge of the plate sinking into the subduction zones. As these plates move they wander closer and further from the poles.  Extensive land masses toward the poles give a surface on which snow can accumulate and therefore reflect sunlight, cooling the planet.  Without a land mass upon which to accumulate there would be no white surface.  The north polar cap of the Earth , while consisting mainly of floating ice, is anchored by the surrounding land and the sea is not very deep, not deep enough to be an ocean.  At an average 3,400 ft deep it is actually a sea, part of a flooded continent plate, rather than ocean (average 14,000 ft deep).  Antarctica consists of land, upon which ice and snow have accumulated, plus the surrounding ice anchored to it.  At other times in Earth's history the continents have come together at the equator and there have been no polar ice caps.  200 million years ago the land masses were much closer toward the equator.  Subsequently Antarctica has drifted south while Europe and the America's further north.  Not only do continents directly affect the temperature by providing a surface for reflective ice and snow to accumulate on, they do it indirectly too.  The presence or absence of a continental mass allows or blocks the flow of ocean currents which distribute heat around the planet.  This is important because although that distribution does not affect the total amount of heat within our atmosphere, it does affect where it reaches.  If an area is cut off from the heat transported by the ocean it cools.  As a result it is again more likely to allow the accumulation of ice and snow and thereby reduce the absorption of heat by increasing the albedo.

Volcanism affects the the temperature of the Earth's surface but not in the way that is often cited by global warming deniers.  Volcanoes do, indeed, release large amounts of CO2.  However, they also produce large amounts of particulates and sulphur.  The net effect of volcanoes is to LOWER the temperature in the short term and to have a relatively insignificant effect (compared to us) in the long term.  The amount of CO2 released by volcanoes each year is (maximum estimate 300 million tons) How much is released by our burning fossil fuels? 29 BILLION tons.

So, is the Earth actually warming?  All the evidence would suggest that the Earth warmed dramatically during the latter half of the 20th century and that while the rate of heating has decreased in the early part of this century, it has continued.  There are various lines of evidence.  The actual measurements of temperature show that the temperature continues to rise.  According to NASA, 2012 was the hottest year on record.  Far from warming having stopped in 1997 (as some would have us believe) the ten warmest years recorded have all occurred since 1998.  The September sea ice mass for the Arctic has reached its lowest point ever recorded in 2012, a trend continued for at least the last thirty years.  The global ice mass measurement is down 500 billion tons on 2002 levels.  A constant downward trend for at least the last decade.  The Greenland ice mass loss is particularly worrying.  In 2012 the mass loss was 1,000 billion tons, again a continuous downward trend for the last decade.  Sea level continues to rise.  In 1998 the average sea level was 5mm higher than that in 1993.  With a slight dip in 2011 the rise has been steady and average sea level is 65 mm higher than 1993. Each year the records for high temperature are broken, glaciers and ice sheets shrink.

A counter argument put forward by deniers is that ice is actually increasing in Antarctica so that in some way counters the idea of global warming.  It does not.  As the above figures show, global ice levels continue to fall. The fact that ice accumulates in Antarctica is exactly what would be expected in a warming world.  Global water vapour levels have increased. In any area in which the temperature remains low enough to accumulate ice rather than lose it, that increased water vapour will lead to increased accumulation. 

So, what is happening that might explain the changes we are observing?  Is the sun increasing its output, is the earth capturing more heat?  Let's see.

Prior to the industrial revolution ice cores show that the CO2 concentration of the atmosphere was around 270ppm.  Today it is far higher and the levels have consistently risen over time.  In 1959 the CO2 level was 315ppm.  Today it is 393 ppm.  Every year the level has increased.  Given our knowledge of the physics of CO2 as a Greenhouse gas, the question for deniers is: Why, in your world view, has the increased CO2 NOT warmed the planet?  Basic physics, unconnected with the funding of global warming research, tells us that CO2 is a greenhouse gas that warms the Earth.  But massively increasing its levels in the atmosphere is supposed to have done nothing?  Where is that extra CO2 coming from? The burning of fossil fuels.  We release 29 billion tons of CO2, that nature had locked away, out of the carbon cycle, every single year.

We saw earlier that the sun is the primary source of heat for the Earth's surface.  What has it been doing?  Can it explain the current warming? For much of geological time, yes, rises and falls of temperature tie in very well with solar activity.  But not recently. The sun has been very quiet.  In the last 35 years, solar output and Earth's temperature have been diverging.  While the trend was increasing solar output from the start of the twentieth century to about 1960, subsequently the sun's output has been in decline.  In 2009 the total solar output was the lowest recorded in over 100 years.  The sun, far from heating our planet, may actually lead to a reduction in the consequences of our increasing the CO2 levels of the atmosphere.  There is even talk in the astronomical community that the next solar cycle might not even occur.

How can we tell what the sun was doing thousands of years ago?  That takes us back to Carbon 14 and Beryllium-10.  These are stored in the carbon taken up by living things (giving us Carbon dating) in the case of Carbon 14 and locked in ice cores and sediments, in the case of Beryllium 10.  10B has a half life of one and a half million years before decaying into Boron 10.  This means that measuring the ratio of 10B to 10Be means we can see what the sun was doing millions of years into the past.  This is because the formation of 10Be is the result of cosmic rays causing a nuclear reaction between Nitrogen and Oxygen in the air. Cosmic ray levels depend upon the levels of the solar wind.  This means we can get a good idea of what the solar wind was like over long periods and hence the output of the sun.

There is one other piece of evidence that indicates that it is a change in the atmosphere that is causing warming rather than any other influence and that is night time temperatures. The difference between day and night time temperatures is falling.   Since there is no extra heat being put into Earth's climate system just at night, the logical conclusion is that the atmosphere is simply not releasing the heat accumulated during the day as swiftly as it once did.

In conclusion then, it is entirely reasonable to conclude, upon the basics of the laws of physics, that human increases in the levels of CO2 are causing the temperature to continue to rise.  The Earth has indeed had natural cycles of warming that have exceeded those seen today.  But those cycles do not just happen out of no where.  They are not unexplained.  There are reasons.  These reasons do not apply now.  While science can not tell us all the details, all the evidence points to increasing CO2 levels being the primary cause of recent and continued warming.  There is no alternative hypothesis which accounts for all the evidence.