2 Climate Change: The Science of Climate Change

2 Science of Climate Change

Climate change is controversial, complex and involves a lot of uncertainty. Climate change has a long history dating back to 1896 when the Swedish scientist Svante Arrhenius published a new idea. By burning fossil fuels such as coal, thus adding CO2 to Earth's atmosphere, humanity would raise the planet's average temperature. This "greenhouse effect," as it later came to be called, was only one of many speculations about climate change at the time.

In 1988 when scientists had first begun to call for restrictions on greenhouse gases, the world's governments created a panel to give advice on the issue. Although managed under the auspices of the United Nations, this Intergovernmental Panel on Climate Change (IPCC) was comprised of representatives appointed independently by each government. The IPCC has issued five reports, the last issued in October 2018. The IPCC reports are massive in part because there is a lot of uncertainty. If there were certainty, you could just state your results in a few sentences. There’s so much uncertainty, in fact, that the IPCC devised a nomenclature so that whenever they said something, you’d know exactly how certain they are of what they are saying.

The following summary terms are used to describe the available evidence: limited, medium, or robust; and for the degree of agreement: low, medium, or high. A level of confidence is expressed using five qualifiers very low, low, medium, high, and very high, and typeset in italics, e.g., medium confidence. Box TS.1, Figure 1 depicts summary statements for evidence and agreement and their relationship to confidence. There is flexibility in this relationship; for a given evidence and agreement statement, different confidence levels can be assigned, but increasing levels of evidence and degrees of agreement correlate with increasing confidence.

Box TS.1, Figure 1 | A depiction of evidence and agreement statements and their relationship to confidence. Confidence increases toward the top right corner as

suggested by the increasing strength of shading. Generally, evidence is most robust when there are multiple, consistent independent lines of high quality.

The following terms have been used to indicate the assessed likelihood:

Term - Likelihood of the outcome

Virtually certain 99–100% probability

Very likely 90–100% probability

Likely 66–100% probability

About as likely as not 33–66% probability

Unlikely 0–33% probability

Very unlikely 0–10% probability

Exceptionally unlikely 0–1% probability

* Additional terms (extremely likely: 95–100% probability, more likely than not: >50–100% probability, and extremely unlikely: 0–5% probability) may also be used when appropriate.

As far as I know, this level of carefulness is unprecedented in the history of panel reports, and is just one more indication of the controversy, complexity and uncertainty associated with global warming.

2.1 Causes of Global Warming/Climate Change

2.1.1 Physics of the earth’s atmosphere

See (https://www.sandia.gov/~jytsao/physics_global_warming_2008_04_notes.pdf)

The earth exists in the Goldilocks zone. It is neither too close to the sun to evaporate all the planet’s water nor to far from the sun such that all water is in the form of ice. A delicate balance is held in place by the earth’s atmosphere such that the global average temperature is 14C (57F), just about perfect for carbon and water-based life like us. Fourier was the first person to study the Earth’s temperature from a mathematical perspective. He calculated the earth’s temperature and determined the earth would be much colder than it is if the incoming radiation from the sun were the only warming effect.

Simple calculation of the earth’s temperature

Without an atmosphere the global temperature would be 6C (43F). The earth absorbs and reflects radiation from the sun during the day and emits radiation back into space principally at night.

30% of the sun’s radiation that is intercepted by the earth is reflected, and much of that reflection is from the clouds, and from ice and snow. And 90% of the radiation from the earth is absorbed in the atmosphere, and much of that absorption is by clouds and water vapor. Water is the most important molecule modifying the radiation balance of the earth. After water, there are many other molecules that also alter the heat balance of the earth, particularly through absorption of radiation from the earth, and collectively these are called greenhouse gases.

This figure shows the most important of these greenhouse gases (GHG), along with their absorption spectra. The bottom axis of the figure is the wavelength of the electromagnetic radiation in micrometers, or microns. Visible light, light that the human eye can see, has wavelengths between 0.4 and 0.7 microns, exactly the wavelength range in which the sun’s radiation is concentrated. That’s no accident, of course -- our eyes evolved to be sensitive to the sun’s radiation.

Because the earth is much cooler than the sun, it emits infrared light, light that the human eye cannot see, at wavelengths between 8 and 20 microns. This is exactly the wavelength range which is absorbed by the various greenhouse gases. There’s a narrow window at a wavelength of 10 microns through which some infrared light is transmitted, but by and large the rest is absorbed. You can see that water vapor is the overwhelmingly most important greenhouse gas, mainly because it is such a large fraction, 1%, of the earth’s atmosphere. Carbon dioxide is also important, but less so, mainly because it is a much smaller fraction, 0.04%, of the earth’s atmosphere. Ozone, methane and nitrous oxide are also greenhouse gases, but there is much less of them still, and so they are correspondingly less important. Note, though, that methane, because it is so abundant in the earth, is also considered relatively important.

Interestingly, the two molecules that are most abundant in the atmosphere, nitrogen at 78%, and oxygen at 20.9%, don’t absorb in the infrared at all, and hence don’t contribute to the greenhouse effect. So, here’s a slight digression, but an interesting one: why don’t they absorb in the infrared?

Let’s start by asking why the other molecules do absorb in the infrared. Here I’ve sketched an infrared light wave –

basically a rapidly oscillating electric and magnetic field that is traveling in space. We know that electric charges interact with electric fields – positive charges move one way and negative charges move the other way. But atoms and molecules usually have the same number of protons and electrons, so they are neutral, and don’t interact as a whole with electric fields.

But what if a molecule is composed of two or more different kinds of atoms. And what if the nuclei of the different atoms have a different propensity to attract electrons. Then, when the atoms form chemical bonds with each other, the electrons that are in those bonds might tend to be a little closer to one atom or the other. One atom might end up slightly negative, and the other atom slightly positive. Then, in an electric field, the slightly positive atom moves one way, and the slightly negative atom moves the other way, and the molecule begins to vibrate. This vibration represents an energy gain by the molecule, and an energy loss from the light wave.

In fact, virtually all molecules composed of atoms that are different will have some internal charge imbalance and will absorb infrared light. And even molecules composed of atoms that are the same but that see different local environments – like ozone with one oxygen atom at the center but two off to the side – will have some internal charge imbalance and will absorb infrared light. But molecules that are composed of atoms that are the same and that have exactly the same local environments – like oxygen or nitrogen with two identical and symmetric atoms – have no internal charge imbalance and won’t absorb infrared light at all.

Good thing, too, because if either nitrogen or oxygen absorbed infrared light, there’s so much of them that the earth would undoubtedly end up being a furnace!

2.1.2 CO2 and Climate Change

Looking at the 1,000-year temperature history of the earth we see that, indeed, from 1000 to 1900 there has been a general cooling trend, consistent with the idea that on a 100,000 year time scale we’re heading into an ice age. But, suddenly, in the 1900’s, the temperature starts to shoot up. And, at the same time, the concentration of carbon dioxide in the atmosphere also starts to shoot up.

There is a solid consensus amongst climate change scientists that these increases are real. Not only that, it looks to be different than previous warmings, which were determined by changes in solar radiation hitting the earth, amplified by water and other greenhouse gases like carbon dioxide.

It is extremely likely (95%-100% probability) that human activities caused more than half of the observed increase in global average surface temperature from 1951 to 2010. More than half of the observed increase in global average surface temperature from 1951 to 2010 is very likely due to the observed anthropogenic increase in GHG concentrations.

2.1.3 Earth’s Sluggish Carbon Balance

The illustration shows all of the various sources and sinks of carbon on earth. The black numbers are pre-industrial values, and the red numbers are post-industrial additions. So, the atmosphere used to hold about 597 GtC, but now it holds an additional 165 GtC. The shallow ocean holds even more – pre-industrially about 900 GtC, and now an additional 18 GtC. Vegetation and soil hold yet more – pre-industrially about 2300 GtC, and now an additional 251 GtC. However, the biggest reservoir by far of carbon is the deep ocean – it holds more than 10 times all of the other reservoirs combined!

In fact, if you lump the atmosphere, vegetation, and the shallow ocean into something we might call the biosphere – the part of the earth that biological systems interact closely with – then we have a picture that looks like this. There is a tiny biosphere, with a certain concentration of carbon, and then there is this huge deep ocean reservoir, also with a certain concentration of carbon.

The rates at which carbon dioxide is exchanged within the biosphere are all relatively fast, so, at least on a 10-20-year time scale, we can treat the biosphere as one unit. But the rate at which carbon dioxide is exchanged between the biosphere and the deep ocean is very very slow. So slow that it takes about 100 years for the concentrations of carbon dioxide in the two reservoirs to come into equilibrium with each other.

So, the sequence of events kind of looks like this.

Pre-1900’s, before we were dumping a lot of carbon dioxide into the atmosphere, there was plenty of time for the biosphere to come into equilibrium with the deep ocean. So, the concentration of carbon dioxide in the biosphere was pretty much the same as the concentration of carbon dioxide in the deep ocean.

In the 1900’s and 2000’s, we started dumping a significant amount of carbon dioxide into the biosphere. That means the concentration of carbon dioxide in the biosphere is now higher than the concentration of carbon dioxide in the deep oceans. Since all things want to flow from places of high concentration to places of low concentration, there is a net flow of carbon dioxide from the biosphere to the deep ocean. But the flow is tiny, because the exchange between the shallow and deep oceans is slow. So, the concentration of carbon dioxide in the biosphere continues to build up.

Post-2000’s, if we were to suddenly stop dumping carbon dioxide into the atmosphere, then the concentration of carbon dioxide in the biosphere would stop increasing. And in fact, it would start decreasing, as it tries to equilibrate with the deep ocean. But again, because the exchange of carbon dioxide between the shallow and deep oceans is so slow, it would decrease very very slowly.

It would only be post-2100’s that, finally, the concentration of carbon dioxide might become “normal” again. But by that time, the earth’s temperature would have gone through a significant cycle of warming.

So, this is one of the central problems of global warming, there is already too much carbon dioxide in the biosphere, and because of the inertia of that carbon dioxide, even if we started reducing carbon dioxide emissions now, it would take a hundred years before we would recover.

2.1.4 Experimental Evidence

2.1.4.1 Surface measurements of downward longwave radiation

The increase in atmospheric CO2 and other greenhouse gases has increased the amount of infrared radiation absorbed and re-emitted by these molecules in the atmosphere. The Earth receives energy from the Sun in the form of visible light and ultraviolet radiation, which is then re-radiated away from the surface as thermal radiation in infrared wavelengths. Some of this thermal radiation is then absorbed by greenhouse gases in the atmosphere and re-emitted in all directions, some back downwards, increasing the amount of energy bombarding the Earth's surface. This increase in downward infrared radiation has been observed through spectroscopy, which measures changes in the electromagnetic spectrum.

2.1.4.2 Satellite measurements of outgoing longwave radiation

In 1970, NASA launched the IRIS satellite that measured infrared spectra between 400 cm-1 to 1600 cm-1. In 1996, the Japanese Space Agency launched the IMG satellite which recorded similar observations. Both sets of data were compared to discern any changes in outgoing radiation over the 26 year period. The resultant change in outgoing radiation was as follows:

Change in spectrum from 1970 to 1996 due to trace gases. 'Brightness temperature' indicates equivalent blackbody temperature .

What they found was a drop in outgoing radiation at the wavelength bands that greenhouse gases such as carbon dioxide (CO2) and methane (CH4) absorb energy. The change in outgoing radiation is consistent with theoretical expectations. Thus the paper found "direct experimental evidence for a significant increase in the Earth's greenhouse effect".

This result has been confirmed by subsequent papers using more recent satellite data. The 1970 and 1997 spectra were compared with additional satellite data from the NASA AIRS satellite launched in 2003 . This analysis was extended to 2006 using data from the AURA satellite launched in 2004. Both papers found the observed differences in CO2 bands matching the expected changes from rising carbon dioxide levels. Thus, we have empirical evidence that increased CO2 is causing an enhanced greenhouse effect.

2.2 Consensus View

2.2.1 World

The consensus view is that the emission of greenhouse causes has led to the acceleration in the increase in global average temperature. Both the United Nations (https://www.un.org/en/sections/issues-depth/climate-change/) and the Intergovernmental Panel on Climate Change (IPCC) (https://www.ipcc.ch/reports/) have concluded

• The concentration of GHGs in the earth’s atmosphere is directly linked to the average global temperature on Earth;

• The concentration has been rising steadily, and mean global temperatures along with it, since the time of the Industrial Revolution;

• The most abundant GHG, accounting for about two-thirds of GHGs, carbon dioxide (CO2), is largely the product of burning fossil fuels.

2.2.2 United States view(https://www.globalchange.gov/nca4)

Human activities continue to significantly affect Earth’s climate by altering factors that change its radiative balance. These factors, known as radiative forcings, include changes in greenhouse gases, small airborne particles (aerosols), and the reflectivity of the Earth’s surface. In the industrial era, human activities have been, and are increasingly, the dominant cause of climate warming. The increase in radiative forcing due to these activities has far exceeded the relatively small net increase due to natural factors, which include changes in energy from the sun and the cooling effect of volcanic eruptions. (Very high confidence)

2.2.3 Dissention from the consensus view

Wikipedia currently maintains a list of scientists who disagree with the scientific consensus on global warming. The site includes links to the dissenting published papers (https://en.wikipedia.org/wiki/List_of_scientists_who_disagree_with_the_scientific_consensus_on_global_warming)

The website NoTricksZone (https://notrickszone.com/) has claimed for several years to have hundreds of peer reviewed scientific papers that cast doubt on the position that anthropogenic CO2 emissions function as the climate’s fundamental control knob…or that otherwise question the efficacy of climate models or the related “consensus” positions commonly endorsed by policymakers and mainstream media. However, a quick review of many of these papers shows they are legitimate papers that discuss particular climate change issues but do not cast doubt on the validity the IPCC conclusions.

Learning from mistakes in climate research (https://link.springer.com/article/10.1007/s00704-015-1597-5)discusses papers that reject anthropogenic global warming. The abstract of that paper discusses flaws that were found in a number of the papers.

Among papers stating a position on anthropogenic global warming (AGW), 97 % endorse AGW. What is happening with the 2 % of papers that reject AGW? We examine a selection of papers rejecting AGW. An analytical tool has been developed to replicate and test the results and methods used in these studies; our replication reveals a number of methodological flaws, and a pattern of common mistakes emerges that is not visible when looking at single isolated cases. Thus, real-life scientific disputes in some cases can be resolved, and we can learn from mistakes. A common denominator seems to be missing contextual information or ignoring information that does not fit the conclusions, be it other relevant work or related geophysical data. In many cases, shortcomings are due to insufficient model evaluation, leading to results that are not universally valid but rather are an artifact of a particular experimental setup. Other typical weaknesses include false dichotomies, inappropriate statistical methods, or basing conclusions on misconceived or incomplete physics. We also argue that science is never settled and that both mainstream and contrarian papers must be subject to sustained scrutiny. The merit of replication is highlighted and we discuss how the quality of the scientific literature may benefit from replication.

There are individual dissenting views from the consensus but no alternate explanation that is supported by the observational data.

• Nir Shaviv claims that solar activity changes have contributed between half to two thirds of the warming over the 20th century,[20] and that climate sensitivity should be on the low side ΔTx2=1.3±0.4 °C compared with IPCC's range of ΔTx2=1.5 to 4.5 °C per CO2 doubling (https://en.wikipedia.org/wiki/Nir_Shaviv#Rejection_of_human-caused_climate_change)

• Roy Spencer claims most climate change is natural in origin, the result of long-term changes in the Earth's albedo and that anthropogenic greenhouse gas emissions have caused some warming, but that its warming influence is small compared to natural, internal, chaotic fluctuations in global average cloud cover. (https://en.wikipedia.org/wiki/Roy_Spencer_(scientist))

• John Cristy – Global warming prediction overestimate the temperature increase per decade by about a factor of 2. (https://www.mdpi.com/2072-4292/2/9/2148/htm) based on satellite data

Skeptical Science (https://skepticalscience.com/) examines and rebuts many skeptical claims.

Note: For an understandable and comprehensive discussion of the science of climate change see David Archer’s course on climate change (http://forecast.uchicago.edu/) which includes a link to the on-line version of this course (https://www.coursera.org/learn/global-warming) and videos of lectures (http://forecast.uchicago.edu/lectures.html). The accompanying book is available as an ebook (https://books.google.com/books/about/Global_Warming.html?id=DewbAAAAQBAJ&source=kp_book_description) and the first edition is available as a pdf (https://pdfs.semanticscholar.org/7de3/0f30e44a42899effa2e1dda947c2d22f79eb.pdf)