The John Holdren portion is long enough to separate out.
Business as usual could increase the 2004 emissions of 6.4 (metric) tonnes C (this counts just the carbon portion of the CO2 molecule) to 21 tonnes by 2100. Today’s figures also include 1.6 tonnes from deforestation and 0.2 tonnes from cement production. Deforestation is expected to decline by 2100, presumably because the Earth doesn’t have that many forests. Climatologists are worried that small changes in temperature can lead to changing the state of the climate – big changes in precipitation patterns, ocean upwelling, etc. Many of the pollutants we have added to the atmosphere reflect back sunlight, partially offsetting and partially masking climate change. But these pollutants kill people and their use will drop.
In order to reduce carbon emissions, we can reduce population, GDP/capita, energy/GDP, or carbon intensity/energy. The fastest and cheapest changes will be in the energy intensity of GDP. More slowly we can change the mix of energy sources to include less carbon, or carbon capture and storage. A population of 8 billion is easier to deal with than 10 billion, and fortunately some of the paths to lower population, such as educating women, are good in their own right.
Soot is an important contributor to climate change, from two-stroke and diesel engines [note; much, much less so from the ultra low sulfur diesel], biomass, agriculture, and fossil fuel use in general. With technology change there can be sharp reductions.
Many technology suggestions won’t work as hoped. Increasing building reflectivity (light roofs) is important for the urban heat effect, but has small global benefits. Scrubbing carbon from the atmosphere is 5 – 10 times as expensive as carbon capture and storage. Experiments with ocean fertilization [adding iron and other nutrients to increase photosynthesis to absorb carbon] indicate that it will be of limited benefit. Afforestation and better agricultural techniques was assumed by the 2001 IPCC reports to take up 100 Btonnes of carbon by 2050, but this estimate appears to be 20% optimistic.
How much mitigation? We signed and ratified the 1992 UN Framework, and it is the law of our land. We agreed to stabilization of greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system. The temperature increase has already been 0.8 C, will be 0.6 C more due to thermal lag in the oceans. Under business as usual by 2050, the increase will be 2 C, and 3 C by 2100 over pre-industrial temperatures. [Note: these are lowish values for BAU estimates.] A temperature increase of 1.5 C will drive the coral reefs and polar bear to extinction. A 2 C increase could lead to catastrophic melting of 3 – 4 m sea level rise/century. A 2.5 C increase would decrease crop yields worldwide.
Until a few years ago, a 3 C increase in temperature (about 550 ppmv) was seen as a compromise, as both the highest increase we could live with and the fastest mitigation possible. Now 2 C is seen as the maximum safe increase over pre-industrial times, with the atmospheric concentrations maximizing at 400 – 450 ppmv by 2100. [Note: ppmv is parts per million volume – out of every million pieces of air, 400 would be carbon dioxide.]
Temperature increases as a function of atmospheric carbon concentrations are obtained by using probabilistic modeling. The main uncertainty is about the size of the masking. Climate sensitivity is thought to be between 1.5 and 4.5 C [note: the increase in temperature after doubling atmospheric carbon], but it may be above 4.5 C, which means maximum safe levels of atmospheric carbon are even lower.
There is uncertainty about the movement of carbon into and out of the atmosphere. Feedbacks not accounted for may require even lower levels of stabilization.
The cost of delay is likely to be substantial. [Note: assume this is an understatement.] Further delay may make the 450 ppmv goal impossible.
Emissions cap and trade or a tax is necessary. There are many policies that are considered win-win. [Note: win-win polices satisfy other goals, such as lowering prices and decreasing pollution.] If the proposed solution is not equitable, it won’t be achievable. All people must have an equal right to put carbon into the atmosphere.
Again: Paul Baer focuses on ecoequity as first-third world squabbles could easily preclude any climate change solution.
