Climate Change Mechanisms

We might as well start this discussion by highlighting the degree of polarisation between those on opposite sides of the climate change debate. This might be illustrated using the two graphs below, which compare the record of computer climate model forecasts with historical observations of global temperature change. On the left, we see a graph produced by John Christy, a climate scientist, and on the right, a graph produced by Gavin Schmidt , a climate scientist, which suggest two completely different pictures of the state of climate change.

Note: At this stage, it is unclear which of these two perspectives will be proved right in the coming decades based on actual change rather than predicted change based on today’s climate models. However, it needs to be recognised that all models are, by definition, a simplification of real-world complexity, such that any predictions have still to be questioned and not just accepted as proof. Equally, as an aside to the science, there may be reasonable grounds to raise legitimate concerns against some of the political and financial interests that now appear to surround the IPCC.

Given the scope of the two opposing views outlined above, this discussion will only attempt to outline some other factors, beyond just man-made CO2, which may have influenced the many climate change cycles that predate man-made CO2 emissions. Of course, we might also have to consider the idea that such mechanisms might also influence climate change in the future without ignoring the potential impact of man-made climate change. While this discussion is not directly intended to support the ‘sceptical’ viewpoint, it is felt that the IPCC perspective is both well-documented, well-funded and essentially pervasive, such that some wider discussion of other factors may not be so inappropriate as the 95% consensus might suggest. Therefore, we will start by simply listing some possible ‘other factors’ without really knowing the potential significance of each in terms of global climate change, i.e. past, present and future.

Milankovitch Cycles ,   Solar Cycles & Sunspots ,   Plate Tectonics Volcanism
Earth’s Oceans Earth’s Atmosphere Earth’s Albedo Space Weather
Cosmic Rays Geomagnetic Reversal Anthropogenic CO2

Clearly, this is quite a list of complex issues, which may all have some influence on climate change, in addition to the usual focus on CO2 concentration, irrespective of whether natural or man-made. However, it is not within the scope of this discussion to do more than provide an introduction of the totality of this complexity.

Milankovitch Cycles

As outlined in the previous discussion, the Milankovitch cycles involving eccentricity, axial tilt and precession operate over tens of thousands of years, such that the influence on climate change over just the next 100-year may be assumed to be minimal.

Solar Cycles & Sunspots

Note: While there is considerable empirical evidence that historic climate change has been triggered by variations in the solar cycles, there is still much debate about the actual processes at work within the Sun. In 1961, Horace Babcock proposed one of the first substantive models, now known as the Solar Dynamo Theory . A wider discussion of the Solar Cycles is provided by David Hathaway, while a more radical theory might be considered in terms of the Electric Sun Hypothesis .

 As also outlined in the previous discussion, there is still much debate about the causal mechanisms driving the 11-year solar cycle, such that we might have to question the accuracy of any prediction of the next grand solar minimum or maximum . However, there does appear to be enough empirical evidence to suggest that the Sun’s energy output does change in a cyclic manner over hundreds, not just tens of thousands of years. Based on this cyclic pattern, it would appear that we may be heading into a cooling period; whether this cooling period amounts to a grand solar minimum on the scale of the Maunder Minimum is still a matter of debate. While it is not easy to always find reliable data to quantify the underlying causal mechanisms, it might not be unreasonable to assume that the solar cycles in conjunction with changes in the number of sunspots might have a significant effect on climate change over the next 100 years. However, there are several proposed mechanisms through which the 11-year solar cycle could influence the Earth’s climate, as reflected in the diagram below which include:

 

  • A direct variability in the received solar energy on Earth’s climate subsystems might be quantified in terms of a variation in the total solar irradiance (TSI), although figures suggest that this variance would be less than 1%.

  • An indirect variability in the absorption of ultra-violet (UV) radiation in the upper stratosphere associated with the presence of ozone and other dynamics in the atmosphere.

  • An indirect variability associated with energetic particles entering the thermosphere, mesosphere and upper stratosphere, especially at higher geomagnetic latitudes

  • An indirect variability in the generation of ions by galactic cosmic rays (GCR) penetrating into the troposphere, which may influence the formation of clouds causing more solar energy to be reflected.

It is not clear whether such factors are well represented in most IPCC climate models, which often appear somewhat preoccupied with just man-made mechanism, e.g. CO2 emissions. However, as indicated, there does appear to be substantive empirical evidence to support the idea that aspects of the solar cycles have had an effect on the Earth’s climate over the course of centuries, not millennium. Again, while in contradiction to the importance of anthropogenic CO2 in the PCC’s modelling, it does not seem unreasonable to highlight that naturally generated CO2 emissions may have contributed more than 95% to the overall CO2 total, while the rate of CO2 generation and that CO2 absorption may actually be driven by global temperature variations rather than causing it.

Plate Tectonics

From the perspective of geological time, plate tectonics have caused profound changes to the surface of the Earth. This process has also triggered huge earthquakes and volcanic eruptions throughout Earth’s history, which has led to changes in the composition of the atmosphere and oceans. While the scale of these changes undoubtedly led to shifts in global climate, the complexity of all the interactions is beyond the scope of this discussion to try to quantify. However, we might reasonably assume, barring a massive super-volcano eruption , that such effects might not be significant over the next 100 years.

Volcanism

A Wikipedia article lists some of the largest volcanic eruptions in Earth’s history. The largest listed, Guarapuava, occurred some 132 million years ago and ejected 8,600 cubic kilometres of rocks and ash plus greenhouse gases into the atmosphere. While there is still much debate about the scope of volcanism in the cycle of climate change, there is growing evidence that some of the earlier eruptions in Earth’s history led to mass extinctions due to changes in the biosphere. There is also a complex timeline between the onset of the eruption, the eruption itself and any subsequent change in temperature, possibly up to 10°C, triggered by a potentially huge outpouring of dust and greenhouse gases into the atmosphere, which then changed the acidity and CO2 concentrations in the oceans. In more recent times, the Krakatoa eruption (1883) and the Pinatubo eruption (1991) are known to have had an impact on the global climate for several years. Current theory suggests that this was due to changes in the Earth’s atmosphere caused by volcanic ‘aerosols’ that blocked short-wave solar radiation, which then contributed to a global cooling of the Earth’s surface, i.e. land and oceans. Today, the IPCC climate models assume that CO2 emissions from volcanoes constitute only 1% in comparison to man-made emissions. Therefore, we might not initially worry too much about this mechanism.

Earth’s Oceans

Earth’s oceans cover over 70% of its surface, i.e. 510 million square kilometres with a volume of 1332 million cubic kilometres. As such, the oceans are a huge source and sink of CO2 that is dependent on the global temperature, while ocean currents act as conveyer belts that transport warm water from the tropics towards the poles and cold water from the poles back to the tropics. This amounts to a huge movement of heat energy, presumably solar in origin, which can change global temperatures and the overall climate. The ocean currents also operate throughout the depths of the ocean and help circulate water from the surface to its depths and back again. The transport of heat energy is often described in terms of the thermohaline circulation , where reference might also be given to the Pacific Decadal Oscillations and Atlantic Multi-Decadal Oscillations . These mechanisms are known to be an important factor in ocean circulations, which if disrupted could cause a corresponding change in Earth’s climate in a relatively short period of time. During the last glacial period associated with Pleistocene epoch, temperatures in the northern hemisphere were subject to cycles every 1,500 years or so. While the causal mechanisms are not fully understood, there is some evidence emerging that suggests the ocean currents slowed during every one of these temperature changes, but more research is required to determine whether this is a cause or effect of climate change.

Earth’s Atmosphere

If we again make reference to the following diagram, we might begin to see the role of the Earth’s atmosphere in a number of energy exchanges, i.e. cloud reflections, atmospheric absorption and greenhouse effects. However, it also has to be highlighted that the Earth has to broadly remain within thermal equilibrium, otherwise the Earth would simply continue to heat up until all life dies, as might have been the case with Venus .

Of course, as the diagram suggests, the process by which equilibrium is achieved can be quite complex and not necessarily completely stable. Within this model, 22.5% of the sun’s energy reaching the Earth is reflected back to space due to clouds, i.e. water vapour, and other particulates in the upper atmosphere and a further 8.7% reflected by the Earth’s surface in the form of snow, ice and deserts. While this reduces the energy in the climate model by 31.2%, major volcanic eruptions can introduce a higher concentration of particulates into the high atmosphere, which can cause a drop in Earth’s mean global surface temperature of about 0.5oC that can last for months or even years. Of the energy not reflected in this manner, 19.5% is initially absorbed by the atmosphere and 49% by the Earth’s surface in respect to the averaged figure of 342 W/m2 figure. However, as pointed out, the Earth has to broadly maintain thermal equilibrium, such that other mechanisms must radiate most of the 235 W/m2 absorbed back into space, which takes the form of longwave radiation. Before proceeding with the basic description of the energy model under discussion, we possibly need to add a little more detail about the composition of the atmosphere, where the following table lists the 11 most abundant gases found in the Earth's lower atmosphere by volume.

Name Formula % Volume
Nitrogen N2 78.08%
Oxygen O2 20.95%
Water H2O 0.4%
Argon Ar 0.93%
Carbon Dioxide CO2 0.0360%
Neon Ne 0.0018%
Helium He 0.0005%
Methane CH4 0.00017%
Hydrogen H2 0.00005%
Nitrous Oxide N2O 0.00003%
Ozone O3 0.000004%

Of the gases listed, nitrogen, oxygen, water vapor, carbon dioxide, methane, nitrous oxide, and ozone are the most important within Earth's biosphere. As most of us learn in school, nitrogen and oxygen are the main components of the atmosphere, making up ~99% of a dry atmosphere. Both of these gases are essential to life, where nitrogen is removed from the atmosphere and deposited at the Earth's surface mainly by nitrogen fixing bacteria plus lightning and rain. The addition of nitrogen to the soil provides nutrition for plant growth and is returned to the atmosphere when biomass is burnt or decays. In contrast, oxygen is used in both photosynthesis and respiration of plant and animal life. Photosynthesis also produces oxygen when carbon dioxide and water are chemically converted into glucose using sunlight and, in a somewhat reciprocal manner, respiration uses oxygen and releases both water and carbon dioxide. Given the physical appearance of the sky, we might realise that water vapor in the form of clouds is also one of the most abundant gases in the atmosphere, although subject to large variations in concentration. The highest concentrations of water vapor are found near the equator over the oceans and tropical rain forests, while the concentration over polar regions and subtropical deserts can approach zero.

Note: Water vapor has several very important functions. First, it redistributes heat energy via latent heat energy exchanges. Second, condensation of water vapor creates precipitation, which provides the necessary fresh water for plants and animals. Third, it helps warm the atmosphere as a greenhouse gas.

The fifth most abundant gas in the atmosphere is carbon dioxide, which has increased by 40% in the last 250 years, linked the burning of fossil fuels, deforestation and other forms of land-use change. Carbon dioxide is also a greenhouse gas. Finally, we also need to mention methane as it is a very strong greenhouse gas and estimated to have increased by more than 150%, since 1750, due to increased rice cultivation, domestic grazing animals, landfills and fossil fuel extraction.

Note: Despite all the political and media focus on CO2, it may only cause between 9-26% of the overall greenhouse effect. At this point, we might simply mention a cocktail of other man-made chemicals that also contribute to the greenhouse effect: sulfuryl-fluoride, trichlorofluoromethane, sulphur-hexafluoride, hexafluoroethane, trifluoromethane, ozone, nitrous-oxide. However, the top 3 greenhouse gases in reverse order are methane produced by swamps and termites plus man-made landfills and dairy-cows followed by carbon dioxide, as cited above and finally water vapour. So, within an overall greenhouse effect, water vapour, both in terms of humidity and clouds, may account for between 36-70% of the greenhouse gases. However, there are other factors which affect the ‘persistence’ of certain greenhouse gases being retained in the atmosphere.

We now know that the greenhouse gases act as a ‘blanket’ to all of the sources of longwave radiation coming from the Earth’s surface, where the top 3 are water vapour, carbon dioxide and methane. Of course, there is a complexity in the greenhouse effect of water vapour because while clouds do have a heating effect as a greenhouse gas, they also have a cooling effect by first reflecting incoming solar energy as it arrives. The complexity of the Earth’s climate model is also compounded by a number of other sub-system mechanisms. For example, the Earth is a sphere, such that more solar energy arrives per metre2 at the tropics than at higher latitudes. This leads to energy thermals (7%) from the equatorial regions to higher latitudes by both air and sea. In the process, energy is also evaporated from the sea and land surfaces, triggering latent heat (22.8%) to be released that is also a primary cause of atmospheric circulation and ocean currents. Other mechanisms are linked to the direction and magnitude of Earth’s rotational velocity, which drives many of the global weather patterns. These weather patterns are triggered by moving low-high pressure systems and associated with cold-warm fronts, compounded by temperature differences over the oceans and geographical features, such as mountain ranges and ice sheets. Of course, many of these mechanisms are also subject to change over geological time for a variety of reasons that cannot really be detailed in this overview. However, what might be reasonably concluded is that even the Earth’s internal climate mechanisms have a lot of complexity that are not really fully understood.

Earth’s Albedo

 The term albedo, which is the Latin for white, is commonly applied to the overall average reflection coefficient of an object, e.g. the Earth. The albedo in the visible spectrum falls within a range of about 0.9 for fresh snow to about 0.04 for charcoal. However, we might consider the idea of the Earth’s albedo in terms of the reflection of incident solar energy at different altitudes as shown in the diagram below.

When seen from a distance, the ocean surface has a low albedo, as do most forests, whereas desert areas have some of the highest albedos among landforms. Most land areas are in an albedo range of 0.1 to 0.4, while the overall average albedo of Earth is about 0.3, which is higher than the ocean’s that cover about 70% of the Earth’s surface due to the reflection of clouds. Clearly, if the global temperature increases, for whatever reason, it might lead to more clouds, which might then act a feedback mechanism that blocks more of the Sun’s incident energy at the top of the atmosphere, but which in-turn creates a greater greenhouse effect that might offset the loss of incident solar energy. In part, this last feedback mechanism is one of the key issues that climate models are still trying to resolve. For in order to predict the climate several decades into the future, the model needs to accurately understand many of the interacting mechanisms within the overall climate system, where one critically important aspect appears to be the role of clouds. As suggested, changes in the global temperature may affect cloud formation, where water evaporates in tropical regions and condenses in colder regions and altitudes.

  • Clouds often form as water vapour condensing on aerosol particles.
  • Clouds come in many shapes and size and can exist at many different altitudes.
  • Clouds cool Earth's surface by reflecting incoming sunlight.
  • Clouds warm Earth's surface by absorbing heat emitted from the surface
  • Cloud re-radiate energy back down toward the surface.
  • Clouds supply water to the surface by forming precipitation.

We might first assume that Earth's climate should warm due to the greenhouse effect. However, a consequence of this change would be the weather patterns and associated cloud formation would also change. This secondary change might then cause a mechanism of either negative or positive feedback in the climate system, resulting in a warming or cooling along with either increased or decreased precipitation in different regions.

Space Weather

We might initially define ‘space weather’ as changes that surround planet Earth’s upper atmosphere, but which can affect Earth bound weather systems. Using the vocabulary of weather, w e might also link the source of space weather to ‘storms’ that take place within the Sun’s corona, which are reflective of sunspot and magnetic activity that trigger solar flares and fuel the solar wind.

As such, the Sun is not only a source of the electromagnetic energy being received by the Earth, but also the source of a host of charged particles in the form of the solar wind , which in-turn interact with other highly energetic particles in the form of cosmic rays . More specifically, the composition of the solar wind is a mixture of ionized hydrogen, i.e. protons and electron with an 8% component of helium-alpha particles and trace amounts of heavier ions, which can have a velocity anywhere in the range: 3-400km/s. When these charged particles reach the Earth, they interact with the Earth's magnetosphere and are accelerated down magnetic field lines where they collide with other particles within the Earth’s atmosphere, particularly at poles where the magnetic field lines are focused.

Note: While it appears obvious that the Sun is the main source of electromagnetic energy that drives the Earth’s climate, there are other factors that may impact other climate  mechanisms, as previously outlined. As such, it may be possible that variable space weather also affects the composition and intensity of solar radiation underpinning so many of Earth’s climate sub-systems.

As early as 1911, Charles Wilson forwarded the idea that ionizing radiation could affect atmospheric cloud formation in contradictory ways. For increased cloud cover in the upper troposphere can reduce long-wave radiation, which may result cause warming, but where increased cloud cover in the lower troposphere may reduce incoming radiation and cause cooling. There was also a suggestion that variations in the solar wind and the Earth’s magnetic field might also change the amount of cosmic ray interaction in the upper atmosphere, such that there is now research being conducted into how changes in cosmic rays may play a role in climate change.

Note: One hypothesis currently being considered is that the process of ionisation in the upper atmosphere helps to form ‘aerosols’ that may then facilitate cloud formation via cloud condensation nuclei (CCN). This process allows water vapour in the atmosphere to condense and create lower altitude clouds, which may exert a cooling effect, such that the level of cosmic rays may also influence climate change.

As has been outlined, while the Milankovitch cycles may well have had a significant effect of global climate change over thousands of years, the long cycle times do not appear to provide an explanation of shorter term climate cycles. Likewise, the effect of the 11-year solar cycle on the energy received by the Earth, i.e. 1372 W/m2, appears be too small to be significant to any perceived changes in global climate, such that anthropogenic CO2 is often assumed to be the only major factor in the climate change debate. However, there is a suggestion that changes in the strength of the solar wind might be correlated to sunspot activity, which then changes cosmic ray interaction with Earth’s atmosphere, which we might see in the following chart.

So while the approximate 11-year solar cycle may not significantly change the energy received by the Earth, the variability of its strength, which we might describe in terms of the grand solar maximum and minimum may act as an amplifier of solar activity and its effect on climate change. As such, we might consider the idea that a change solar activity, linked to the solar cycle, may also cause a change in the strength of the solar wind, which then affects the level of cosmic rays reaching Earth. Likewise, if cosmic rays do affect the level of ionisation in the troposphere, increased solar activity will translate into a reduced level of ionization, which may then reduce cloud cover at lower altitudes, which is known to have a net cooling effect.

Note: Cloud cover has been observed from space since the 1980's and, by the 1990's, enough cloud data had been accumulated to provide empirical evidence for some form of solar/cloud mechanism. Using the satellite data, Henrik Svensmark has shown that cloud cover varies in with the variable level of cosmic rays, which appears synchronised to the 11-yr solar cycle. This idea is also being pursued by Nir Shaviv .

The work of Svensmark, who is director of the Centre for Sun-Climate Research at the Danish Space Research Institute, appears to downplay the significance of the effects of man-made increases in atmospheric CO2 on recent and historical global warming. He argues that while the role of greenhouse gases in climate change is considerable, the solar variations is larger. The following chart reflects Svensmark’s research and appears to show a correlation between variations in cosmic rays in red and a change in sea temperature in black.

Svensmark and his colleagues have also shown a correlation in cloud cover and modulation of cosmic ray within the solar cycle, as illustrated below. However, while these charts are suggestive of a link between cosmic rays and climate change, further proof of a causal link is still required.

However, there does appear to be mounting evidence that support a causal link between cosmic rays and climate change beyond the observed cloud cover variations as variations appear to have left a paleoclimatic imprint in the geological records. While full details are beyond the scope of this outline, Nir Shaviv has provided further evidence that cosmic rays are also subject to the position of the solar system within the spiral-arm of the Milky Way and although subject to geological time, changes can be correlated with geological sedimentation records, when Earth’s climate was subject to cycles between hothouse and icehouse conditions.

However, while there is some evidence in support of an ever-growing link between climate change on Earth and the wider context of space weather, caution is still required. As such, it remains as only one of possibly many climate change mechanisms that might be worthy of further consideration.

Geomagnetic Reversal

A geomagnetic reversal is a change in Earth’s magnetic field such that the positions of magnetic north and magnetic south are reversed. The timespan of these reversals appears somewhat random in that they can occur between 100,000 and 1,000,000 years, with an average of 450,000 years, where the reversal itself may take between 1,000 and 10,000 years. The latest one, the Brunhes–Matuyama reversal, occurred 780,000 years ago, while there is now growing evidence that the next one may have already started and cause climate effects within a human lifetime.

How would a reversal of the Earth’s magnetic field affect climate change?

Obviously, we might see some correlation with the mechanisms previously outlined, where cosmic rays are affected by the solar wind and the Earth’s magnetic field. However, analysis of the movement of the Earth’s magnetic poles over the last 105 years appears to suggest a strong correlation between the position of the north magnetic and geomagnetic poles with global temperatures, but especially in the northern hemisphere. While many may still question this correlation, statistical analysis suggests that there is less than a 1% chance it being random, causal mechanisms are still unclear. Current proposals range between the idea that the Earth’s magnetic field affects the energy transfer rates from the solar wind to the Earth’s atmosphere, which in turn affects the North Atlantic Oscillation and that the movement of the poles changes the geographic distribution of galactic and solar cosmic rays on regionally sensitive zones in the climate system. However, the movement of the magnetic poles may also change the distribution of ultraviolet radiation on the oceans, which then changes the death rate of carbon sinking in the form oceanic plant life, such as phytoplankton.

Anthropogenic CO2

 So, finally we return to the concern that man-made global climate change is being triggered by increases in CO2 emissions. However, along the way, it has been highlighted that CO2 is only one of a number of greenhouse gases in the atmosphere, which represents about 0.3% of the total by volume. It has also been pointed out that the man-made contribution assumed to be entirely responsible for the increase from 280ppm to 390pmm since 1750, i.e. 40%, would only represent a 0.11% difference in the composition of the atmosphere. It has also been highlighted that CO2 concentration have been much higher in Earth’s history and that there is some evidence that CO2 levels lag temperature rather than necessarily driving it, i.e. it is an effect not necessarily the primary cause. Finally, it is unclear how the various climate models account for all the other potential causal mechanisms outlined in this discussion.

 

So why is there a suggestion that 95% of climate scientists are primarily focused on CO2?

In part, this question might be too simplistic, such that it misrepresents the current state-of-play. Therefore, we might attempt to clarify the wider scope by citing an extract from the 2014 IPCC Summary for Policymakers, which states that warming of the climate system is 'unequivocal' with ‘unprecedented’ changes over decades to millennia, including the warming of the atmosphere and oceans, loss of snow and ice, and sea level rise.

Note: Given the level of the scientific debate, it is unclear that anything is ‘unequivocal’ at this stage or that the actual changes to-date are ‘unprecedented’ given the long history of climate change outlined.

The summary report does appear to recognise that greenhouse gas emissions have been driven largely by economic and population growth, which may not be easily reversed. However, the report does continue to assert that greenhouse gas concentrations along with other anthropogenic drivers are the most likely cause of the observed global warming since the mid-20th century.

“Continued emission of greenhouse gases will cause further warming and long-lasting changes in all components of the climate system, increasing the likelihood of severe, pervasive and irreversible impacts for people and ecosystems. Limiting climate change would require substantial and sustained reductions in greenhouse gas emissions which, together with adaptation, can limit climate change risks.”

While there is much discussion surrounding the scientific debate, despite the assertion of a 95% consensus, actual policy decisions appear to be subject to wider considerations, which we might present in terms of the following statement by the US National Research Council:

The question of whether there exists a ‘safe’ level of concentration of greenhouse gases cannot be answered directly because it would require a value judgment of what constitutes an acceptable risk to human welfare and ecosystems in various parts of the world, as well as a more quantitative assessment of the risks and costs associated with the various impacts of global warming. In general, however, risk increases with increases in both the rate and the magnitude of climate change.

This statement possibly represents the view of a wider spectrum of scientists, inclusive of social scientists, medical experts, engineers and ‘even’ philosophers on the implications of climate change. As such, climate change policy is discussed in terms of climate change mitigation, climate change adaptation, climate engineering, politics of global warming, climate ethics and the economics of global warming. However, if we put such issues aside for the moment, we might return to the central question.

What is the evidence that humans alone are responsible for global climate change?

It has been suggested that the proof that man-made CO2 is the cause of climate change follows a chain of evidence. This evidence then leads to the basic conclusion that CO2 keeps the Earth warmer than it would have been without the anthropogenic CO2 being added by humanity. The sub-text of this position is often that this additional CO2 is mainly produced by burning fossil fuels, which clean renewable energy sources, such as solar and wind, might reverse. It is also asserted that there is empirical evidence that the rising temperatures are being caused by the increased CO2, which acts as a greenhouse gas that wraps the Earth in an invisible blanket. However, while this blanket is essential to maintaining most life on Earth, it can also lead to a run-away effect as in the case of Venus . In this respect, we might recognise that the moon’s temperature varies from +100°C during its day-time, while plunging to -173°C during its night-time. In comparison, the coldest temperature on Earth was recorded in Antarctica: −89.2°C, while the hottest was 56.7°C.

How is the effect of CO2 being measured?

One of the ways of measuring the effect of CO2 is by using satellites to compare how much energy is arriving from the sun, and how much is leaving the Earth. From this, it is believed that there has been a gradual decrease in the amount of energy being re-radiated back into space over the last few decades. Of course, this approach is based on the assumption that the amount of energy arriving from the sun has not changed and no other mechanisms are at work.

What keeps the excess energy from escaping back into space?

Broadly, it is assumed that the only answer is the greenhouse effect and the composition of greenhouse gases. For certain gases ‘capture’ energy, where the primary gases of interest to the climate models are carbon dioxide (CO2), methane (CH4), water vapour, nitrous oxide and ozone, which collectively make up about 1% of the air – see earlier table for more details. While only a small percentage of the atmosphere, i.e. its greenhouse gases, they are assumed to have a significant effect within the climate model by keeping the planet 33°C warmer than it would otherwise be without them. However, as indicated, much of the focus appears to be orientated towards CO2 levels, as defined by the following chart.

While atmospheric CO2 levels are probably best understood as a percentage of the total atmosphere, the percentage of CO2 is so small, that it is also expressed as parts per million (ppm). However, the previous chart has displayed the amount of CO2 in the atmosphere in gigatons, possibly because such a large number might appear more worrying. So, over the last 1000 years, we see that the percentage of man-made CO2 emission with respect to the total CO2 emission increasing from approximate zero, then rising steeply throughout the 20th century until today, where man-made CO2 is said to amount to about 40% of the total. Based on this accepted fact, it is concluded that CO2 is the primarily cause of climate change, as CO2 is known to trap energy at very specific wavelengths, while other greenhouse gases trap different wavelengths. We might attempt to quantify this effect using the next chart.

This chart shows the spectrum of the greenhouse radiation as measured at the surface of Earth, but where the significance of the greenhouse effect from water vapor has been filtered out. Among the spikes, we can see energy being radiated back to Earth by ozone (O3), methane (CH4), and nitrous oxide (N20), where the spike for CO2 on the left dwarfs all the other greenhouse gases. However, this chart does not explain the other gases are even smaller as a percentage of the total than CO2 or provides any relative perspective with respect to water vapour, which other sources have suggested might account for 36-70% of the greenhouse effect. As such, we will now turn our attention towards the climate models on which so many predictions are based.