Alsup asks for answers

Alsup asks for answers

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:

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.

Alsup’s Questions:

  1. 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?
  2. What is the molecular difference by which CO2 absorbs infrared radiation but oxygen and nitrogen do not?
  3. What is the mechanism by which infrared radiation trapped by CO2 in the atmosphere is turned into heat and finds its way back to sea level?
  4. Does CO2 in the atmosphere reflect any sunlight back into space such that the reflected sunlight never penetrates the atmosphere in the first place?
  5. Apart from CO2, 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?
  6. In grade school, many of us were taught that humans exhale CO2 but plants absorb CO2 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 CO2 back into oxygen? Given the increase in human population on Earth (four billion), is human respiration a contributing factor to the buildup of CO2?
  7. What are the main sources of CO2 that account for the incremental buildup of CO2 in the atmosphere?
  8. 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 11, 2018

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

    Ice age cycles from Snyder (2016)

    They vary in amplitude and phasing (becoming larger in the last 800,000 years), and moving from a dominant 40,000 yr periodicity in the first half to a 100,000 yr periodicity in the later period. It was discovered in the 1970’s that the pacing of the cycles seen in benthic foraminiferal oxygen isotopes was highly correlated to the Milankovitch cycles of orbital variability (Hays, Imbrie and Shackleton, 1976). More recent work has shown that the growth and collapse of the ice sheets is strongly tied to the insolation (Roe, 2006):

    The magnitude of the cycles is strongly modified by various feedbacks, including ice-albedo, dust, vegetation and, of course, the carbon cycle. Estimates of the drivers of global temperature change in the ice ages show that the changes in greenhouse gases (CO2, methane and nitrous oxide) made up about a third of the effect, amplifying the ice sheet changes by about 50% (Köhler et al, 2010).

  2. 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. Diatomic molecules (like N2 or O2) 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. Vibrational modes in triatomic molecules (H2O, CO2, O3, N2O) or in more complex modecules (CH4, CFCs, HFCs…) are easier to excite and so will absorb and emit lower energy photons (corresponding to the infrared bands, that just happen to be how the Earth loses heat to space).
  3. 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) adds to the warming at the surface.

    The figure shows the easiest description of the greenhouse effect.

  4. Not enough to matter. The latest update to the estimates of radiative forcing of CO2 (Etminan et al., 2016) shows a shortwave effect (i.e. a change in the absorption of downward solar radiation) is about -0.14 W/m2 for CO2 going from 389 to 700 ppm (compared to 3.43W/m2 in longwave forcing) – contributing to about a 4% decrease in the net forcing.
  5. 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.
  6. All animals (including humans) breathe in oxygen and exhale CO2. The carbon in the exhaled CO2 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 CO2 to the atmosphere that came from the atmosphere very recently). Note that any net change in biomass (whether trees, or cows or even humans) does affect atmospheric CO2, but the direct impact of human population growth is tiny even though the indirect effects are 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

    compared to current fossil fuel and deforestation emissions of ~10 GtC/yr (4 orders of magnitude bigger).

  7. Main sources of human CO2 emissions are fossil fuel burning and (net) deforestation. This figure is from the Global Carbon Project in 2017.

  8. 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.

    For instance, 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.

    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.


    1. C.W. Snyder, "Evolution of global temperature over the past two million years", Nature, vol. 538, pp. 226-228, 2016.

    2. J.D. Hays, J. Imbrie, and N.J. Shackleton, "Variations in the Earth’s Orbit: Pacemaker of the Ice Ages", Science, vol. 194, pp. 1121-1132, 1976.

    3. G. Roe, "In defense of Milankovitch", Geophysical Research Letters, vol. 33, 2006.

    4. P. Köhler, R. Bintanja, H. Fischer, F. Joos, R. Knutti, G. Lohmann, and V. Masson-Delmotte, "What caused Earth’s temperature variations during the last 800,000 years? Data-based evidence on radiative forcing and constraints on climate sensitivity", Quaternary Science Reviews, vol. 29, pp. 129-145, 2010.

    5. M. Etminan, G. Myhre, E.J. Highwood, and K.P. Shine, "Radiative forcing of carbon dioxide, methane, and nitrous oxide: A significant revision of the methane radiative forcing", Geophysical Research Letters, vol. 43, pp. 12,614-12,623, 2016.

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