The biggest sustainability problem we have is not “climate change”, it’s the cause of that change which is the emission of greenhouse gases that are warming the planet at a rate exceeding prediction. Releasing greenhouse gases into the atmosphere is a clear case of market externalities, like the example we gave of a factory dumping toxic chemicals into a river.

It is entirely understandable that we didn’t realize there was a cost to releasing greenhouse gases. Any kid looking up at the sky sees a vastness that extends seemingly forever. On a clear cold winter night though, that same kid might begin to understand how thin that insulating layer above us really is. The atmosphere is made up almost entirely of nitrogen, oxygen, and argon (yes argon at about 1%!). These gases constitute 99.5% of the atmosphere and do not contribute to the greenhouse effect. Carbon dioxide at .039% is a powerful greenhouse gas, and the amount of warming it causes is directly proportional to the number of molecules. Since 1850, humanity has added 1 trillion metric tons of carbon dioxide to the atmosphere, 33 billion tons in 2019 alone. The result has been a substantial increase in the number of molecules of CO2 in the atmosphere: 421 ppm as of May 2022 up from 280 ppm before the industrial revolution, an increase of 50% in the number of CO2 molecules heating the planet^[1].

There is a natural carbon cycle on earth in which as much carbon dioxide, released by various “sources” into the atmosphere, is absorbed back from the atmosphere by various “sinks” such as rocks and sediment, the oceans and plant and animal life. This cycle is “sustainable” since it maintains equilibrium over time^[2]. Perturbations to this cycle occur when there are unusual releases of carbon dioxide, as through periods of major volcanic activity. After these increased emissions stop it takes hundreds of years for the atmospheric CO2 levels, and global temperatures, to fall back to their pre-disturbance levels because the rate of absorption of CO2 into long term sinks, such as rocks, is fairly slow^[3]. This means that the climate change that has occurred to date will persist for at least the next few hundred years and can only get worse with more human emissions^[4]. And CO2 is only one of several human-released greenhouse gases, methane and nitrous oxide have even greater warming power.

Clearly continuing to release greenhouse gases into the atmosphere is not sustainable. Every additional billion tons of greenhouse gases will cause increased warming, and, at some point, it will become prohibitively expensive or impossible to deal with the consequences. There is now a lot of talk about “mitigation” and “adaptation” to deal with the effects of climate change, but the quicker we reduce greenhouse gas emissions, the lower the costs of damages, mitigation and adaptation. As the old saw says, “an ounce of prevention is worth a pound of cure”.

Climate Models   There are many excellent explanations of climate models online (see footnote) from which this is drawn[5]. Climate models are computer simulations of our earth that predict climate changes over time in the atmosphere by dividing the surface into squares of about 100 KM (60 miles) on a side and layers about 1 KM (.6 mile) high extending up into the atmosphere. Detailed historical data about the conditions at each point in this lattice are obtained from observations used for weather forecasts and other purposes. Calculations show how the conditions at each point in the lattice change over time according to physical principles. Models are tuned by running them on historic data to see how well they predict the past. In addition to the initial conditions, the models are given data on items that they can’t predict such as the intensity of the sun’s output, human greenhouse gas emissions, and aerosols from various sources. Recent models also include ocean circulation, ice sheets, and biological and geologic cycles into “coupled” global models. Over a million lines of code are required for some of these models, and they have to be run on the most advanced supercomputers.   The point of running such models is to get a picture of how the atmosphere, the oceans, the biosphere, and geochemistry co-evolve over time given various levels of the input greenhouse gas emissions and other factors both natural and human. The models can predict how changes in climate will be distributed geographically and show how the various components of climate interact. For example, as the climate warms there may be more clouds that will somewhat cut down on incoming solar energy. You can see the amazing detail of these climate predictions for any location in the US using Future Climate Projections – Graphs & Maps | NOAA Climate.gov   On the global scale, these models arrive at much the same short term temperature predictions as a simple calculation based on the historic relationship between CO2 emissions, CO2 atmospheric concentration, and temperature. Simply put, the relationship between CO2 concentration and temperature increase has been close to linear as shown in the graph below, and CO2 concentration grows linearly with emissions. Using this one can predict future global temperatures from current CO2 concentration and hypothetical future emissions[6].

Figure 72: Linear Relationship Between CO2 and Increasing Temperatures. Source: https://factsonclimate.org/infographics/concentration-warming-relationship , WW193

The fact that simple extrapolation of historic experience and the most sophisticated and detailed climate models pretty much agree makes it hard to deny the evidence. We know how much CO2 we are releasing, and how that directly relates to warming, both from physics and past observation. Many possible confounding factors such as level of solar emissions are considered in the models.

^[1] Heat radiating from the ground is absorbed and re-released by CO2 molecules anywhere in the atmosphere. When you stand on a hot asphalt street in summer, CO2 all around you is absorbing heat and then releasing it in all directions. There are many good summaries of the science such as https://gml.noaa.gov/outreach/carbon_toolkit/

^[2] It is important to realize that CO2 is constantly absorbed and released by the major short-term sinks such as the biosphere and oceans. So even though any individual CO2 molecule is absorbed by a “sink” within about 4 years, other CO2 molecules are released from that “sink”. Extra CO2 is only taken out of circulation when it passes into a long-term sink such as sediment or rock.

^[3] https://www.carbonbrief.org/explainer-will-global-warming-stop-as-soon-as-net-zero-emissions-are-reached/

^[4] I’m discounting CO2 sequestration post release.

^[5] See UCAR E&O – Randy Russell – Climate Model Resolution ; What Are Climate Models and How Accurate Are They? ; Q&A: How do climate models work? – Carbon Brief

^[6] There are roughly 7.82 Gigatons of CO2 per one part per million in the atmosphere. Using the recent historic relationship of .01 degree C of warming for every 1 parts per million increase of CO2, and noting that about 47% of added emissions stay in the atmosphere, one can compute how much warming there would be from various levels of CO2 emission. For example, continuing to emit 37GT of CO2 per year until 2100 results in 611 PPM of CO2 with a short-term temperature rise of about 3 degrees C. Climate models using a medium emissions scenario come up with a CO2 concentration of 670 PPM in 2100 and warming of 3-4 degrees C.