Some of you might have read about the lawsuit by a number of municipalities (including San Francisco and Oakland) against the major oil companies for damages (related primarily to sea level rise) caused by anthropogenic climate change. The legal details on standing, jurisdiction, etc. are all very interesting (follow @ColumbiaClimate for those details), but somewhat uniquely, the judge (William Alsup) has asked for a tutorial on climate science (2 hours of evidence from the plaintiffs and the defendents). Furthermore, he has posted a list of eight questions that he’d like the teams to answer.

It’s an interesting list. They are quite straightforward (with one or two oddities), but really, pretty much textbook stuff. Andrew Dessler made a quick stab at answering them on Twitter:

Here are answers to questions posed by the Judge Alsup re: climate science (https://t.co/DLFDT70PdL). Turns out answers to those questions are actually pretty well known. 1/ — Andrew Dessler (@AndrewDessler) March 8, 2018

But I think we can do better. So what I propose is that we crowd-source the responses. They should be pithy, to the point, with references (not Wikipedia) and, preferentially, accompanied by a good graphic or two. If we can give a credible uncertainty to any numbers in the answer that’s a bonus. I’ve made a start on each, but further voices are needed. Put your response in the comments and I’ll elevate the best ones (giving credit of course) to the main post. If you have any other comments or edits to suggest, feel free to do so. The best of those will also be incorporated. [Update: I realise I can’t possibly incorporate all the good suggestions while still keeping this short. So be sure to read the comments too for additional material. Also, as I should have said to start with, the best responses to these kinds of questions (though not to these specifically) are to be found in the FAQ of the IPCC report, the Royal Society/National Academies report, and the US. National Climate Assessment science report.]

Alsup’s Questions:

What caused the various ice ages (including the “little ice age” and prolonged cool periods) and what caused the ice to melt? When they melted, by how much did sea level rise? What is the molecular difference by which CO 2 absorbs infrared radiation but oxygen and nitrogen do not? What is the mechanism by which infrared radiation trapped by CO 2 in the atmosphere is turned into heat and finds its way back to sea level? Does CO 2 in the atmosphere reflect any sunlight back into space such that the reflected sunlight never penetrates the atmosphere in the first place? Apart from CO 2 , what happens to the collective heat from tail pipe exhausts, engine radiators, and all other heat from combustion of fossil fuels? How, if at all, does this collective heat contribute to warming of the atmosphere? In grade school, many of us were taught that humans exhale CO 2 but plants absorb CO 2 and return oxygen to the air (keeping the carbon for fiber). Is this still valid? If so, why hasn’t plant life turned the higher levels of CO 2 back into oxygen? Given the increase in human population on Earth (four billion), is human respiration a contributing factor to the buildup of CO 2 ? What are the main sources of CO 2 that account for the incremental buildup of CO 2 in the atmosphere? What are the main sources of heat that account for the incremental rise in temperature on Earth?

Alsup’s Answers:

Note this is an updating text. Last edit: March 16, 2018

The “ice ages” are the dominant cycles of change over the last 2.5 million years (Snyder, 2016)







They vary in extent and duration. They generally were larger in the last 800,000 years, and the duration changed from about 40,000 years in the first half to about 100,000 years in the later period. It was discovered in the 1970s that the cycles were highly correlated to changes in the variability of the Earth’s orbit – the so-called Milankovich cycles (Hays, Imbrie and Shackleton, 1976) (Roe, 2006)







The magnitude of the cycles is strongly modified by various feedbacks, including ice-albedo, dust, vegetation and, of course, the carbon cycle which amplify the direct effects of the orbital changes. Estimates of the drivers of global temperature change in the ice ages show that the changes in greenhouse gases (CO 2 , methane and nitrous oxide) made up about a third of the effect, amplifying the ice sheet changes by about 50% (Köhler et al, 2010) The sea level changes over these cycles was large. The difference between the last glacial maximum (20,000 yrs ago) and today is about 120 meters (400 ft), but the high levels during some of the warmest interglacials were 6-9 meters (20 to 30 feet) higher than today. These changes are dominated by the amount of ice volume change. The so-called “Little Ice Age” was a cooling of the Northern Hemisphere climate (and possibly less markedly in the Southern Hemisphere) in the period of the fourteenth century to the the 1850’s, approximately. It came after a period of a relatively warm climate called the Medieval Warm Period. The cause of this relatively short lived cooling (it was not a true “ice age”) is likely due to an increase in volcanic eruptions and with some role for a slightly reduced solar activity. Over the Holocene (last 11,000 yrs) there is a small but persistent cooling trend due to the orbital cycles mentioned above. Greenhouse gases are those that are able to absorb and emit radiation in the infrared, but this is highly dependent on the gases molecular structure. Vibrational modes in molecules with three or more atoms (H 2 O, CO 2 , O 3 , N 2 O, CH 4 , CFCs, HFCs…) include bending motions that are easier to excite and so will absorb and emit low energy photons which coincide with the infrared radiation that the Earth emits. Thus it is these molecules that intercept the radiation that the Earth emits, delaying its escape to space. More detailed discussion including the importance of the gases dipole moment can be found here. Diatomic molecules (like N 2 or O 2 ) have stretching modes (with the distance between the two molecules expanding and contracting), but these require a lot of energy (so they absorb only at higher energies. Some absorption is possible in the infrared due to collisions but calculations suggest this is a very small part (~0.2%) of the overall greenhouse effect (around 0.3 W/m2, compared to a total effect of 155 W/m2) (Höpfner et al, 2012)



Figure showing the vibrational modes for CO 2 . Arrows indicate the directions of motion. Vibrations labeled A and B represent the stretching of the chemical bonds, one in a symmetric (A) fashion, in which both C=O bonds lengthen and contract together (in-phase), and the other in an asymmetric (B) fashion, in which one bond shortens while the other lengthens. The asymmetric stretch (B) is infrared active (allowed by quantum mechanics) because there is a change in the molecular dipole moment during this vibration. Infrared radiation at 2349 (4.26 um) excites this particular vibration. The symmetric stretch is not infrared active, and so this vibration is not observed in the infrared spectrum of CO2. The two equal-energy bending vibrations in CO 2 (C and D) are identical except that one bending mode is in the plane of the paper, and one is out of the plane. Infrared radiation at 667 (15.00 um) excites these vibrations. (source)

The Earth’s surface emits infrared radiation. This is absorbed by greenhouse gases, which through collisions with other molecules cause the atmosphere to heat up. Emission from greenhouse gases (in all directions, including downwards) adds to the warming at the surface.





The figure shows the easiest mathematical description of the greenhouse effect. The downward radiation from greenhouse gases can be easily measured at the surface in nights under clear skies and no other heat sources in the atmosphere (e.g. Philipona and Dürr, 2004). Yes, but not enough to matter. The latest update to the estimates of radiative forcing of CO 2 (Etminan et al., 2016) 2 for CO 2 going from 389 to 700 ppm (compared to 3.43W/m2 in longwave forcing) – contributing to about a 4% decrease in the net forcing. Direct heat generated by the total use of fossil fuels and other forms of energy adds up to about 18TW [IEA,2017]. Spread over the planet that is 0.04W/m2. Compared to anthropogenic forcings since 1750 of about 2.29±1.1W/m2 [IPCC AR5, Figure SPM 5], it’s about 1/100th the size. Locally however (say in cities or urban environments), this can be more concentrated and have a bigger impact. The grade school calculation is still valid. All animals (including humans) breathe in oxygen and exhale CO 2 . The carbon in the exhaled CO 2 comes from the food that the animals have eaten, which comes (ultimately) from carbon that plants have taken from the atmosphere during photosynthesis. So respiration is basically carbon neutral (it releases CO 2 to the atmosphere that came from the atmosphere very recently). Plants do take up CO 2 as they are growing. With higher CO 2 concentrations (and higher temperature), plants in fact increase their CO 2 uptake somewhat but not as much as would be needed to absorb all the human-caused emissions. Of these emissions only about a quarter is absorbed by plants, while another 20% is absorbed by the oceans, but about half of the emissions stay in the atmosphere. Note that any net change in biomass (whether trees, or cows or even humans) does affect atmospheric CO 2 , but the direct impact of human population growth is tiny even though our indirect effects have been huge. For scale, the increase of 3 billion people over the last 40 years, is equivalent to: 0.185 (fraction of carbon by mass) * 80 kg (average mass of a human) * 3 billion (additional humans) * 10-3 (conversion to GtC) / 40 years = 0.001 GtC/yr which, compared to current fossil fuel and deforestation emissions of ~10 GtC/yr is 4 orders of magnitude too small to be relevant. Main sources of human CO 2 emissions are fossil fuel burning and (net) deforestation. This figure is from the Global Carbon Project in 2017.





Prior to ~1750, atmospheric CO 2 had been stable (within a few ppm) for millenia sustained by a balance between natural sources and sinks. This figure shows the changes seen in ice cores and the instrumental record.





This is the biggie. What is the attribution for the temperature trends in recent decades? The question doesn’t specify a time-scale, so let’s assume either the last 60 years or so (which corresponds to the period specifically addressed by the IPCC, or the whole difference between now and the ‘pre-industrial’ (say the decades around 1850) (differences as a function of baseline are minimal). For the period since 1950, all credible studies are in accord with the IPCC AR5 statement:

It is extremely likely that more than half of the observed increase in global average surface temperature from 1951 to 2010 was caused by the anthropogenic increase in greenhouse gas concentrations and other anthropogenic forcings together. The best estimate of the human-induced contribution to warming is similar to the observed warming over this period. The US National Climate Assessment attribution statement is a bit more specific than the one in IPCC: The likely range of the human contribution to the global mean temperature increase over the period 1951–2010 is 1.1° to 1.4°F (0.6° to 0.8°C), and the central estimate of the observed warming of 1.2°F (0.65°C) lies within this range (high confidence). This translates to a likely human contribution of 93%–123% of the observed 1951–2010 change. It is extremely likely that more than half of the global mean temperature increase since 1951 was caused by human influence on climate (high confidence). The likely contributions of natural forcing and internal variability to global temperature change over that period are minor (high confidence). This summary graphic is useful:





Basically, all of the warming trend in the last ~60yrs is anthropogenic (a combination of greenhouse gases, aerosols, land use change, ozone etc.). To get a sense of the breakdown of that per contribution for the global mean temperature, and over a longer time-period, the Bloomberg data visualization, using data from GISS simulations is very useful.



The difference in the bottom line for attribution for the last ~160 years is that while there is more uncertainty (since aerosol and solar forcings are increasingly shaky that far back), the big picture isn’t any different. The best estimate of the anthropogenic contribution is close to the entire warming. The potential for a solar contribution is slightly higher (perhaps up to 10% assuming maximum estimates for the forcing and impacts). In all cases, the forcing from anthropogenic greenhouse gases alone is greater than the observed warming.



Figure 10.5 from IPCC. Assessed likely ranges (whiskers) and their mid-points (bars) for attributable warming trends over the 1951–2010 period due to well-mixed greenhouse gases (GHG), other anthropogenic forcings (OA) (mainly aerosols), natural forcings (NAT), combined anthropogenic forcings (ANT), and internal variability.

The role of internal climate variability gets smaller as the time-scale increases, but needs to be accounted for in these assessments. Note too that this can go both ways, internal variability might have wanted to cool overall in one period, and warm in another.

References