I wrote an essay addressing the challenge of global warming. Joseph Romm subsequently raised four major objections in a response essay. I then reviewed why I believe that each of these four objections is misplaced. Mr. Romm has now responded by reiterating the first of these four objections. To review the bidding on this point, I had written in my original essay that:
The current IPCC consensus forecast is that, under fairly reasonable assumptions for world population and economic growth, global temperatures will rise by about 3°C by the year 2100.
Mr. Romm disputed this, saying that it was “not correct,” and instead that:
…the latest IPCC report finds that, absent a sharp reversal of BAU trends, we are headed toward atmospheric levels of carbon dioxide far exceeding 1,000 parts per million by 2100. IPCC’s “best estimate” for temperature increase is 5.5°C (10°F)…
I replied by, among other things, citing and explaining the specific document and table (WG1 SPM, Table SPM.3) that supports my assertion (the table seemingly straightforwardly labeled “Projected global average surface warming and sea level rise at the end of the 21st century”), and making the point that neither Mr. Romm’s assertion nor its material equivalent can be found in the document that he links to with the words “best estimate for temperature increase is 5.5°C (10°F).”
Mr. Romm has now replied to this by correcting his link, and repeating his assertion that I am in error on this point. I strongly suspect that this dispute is, increasingly, of interest only to the two of us. I am tempted to leave it to the reader to make a decision between these viewpoints upon the evidence presented, but I think this is too critical a point to just let drop. Therefore, I’ll focus on what appears to me to be the core of Mr. Romm’s latest reply.
In effect, Mr. Romm has presented his own forecast for warming and claimed that it is the IPCC’s best estimate for warming in a business-as-usual case.
Here is Mr. Romm’s restatement of his argument that he is presenting the viewpoint of the IPCC:
Buried on page 16 of the Working Group 1 Summary for Policymakers is perhaps the most alarming yet under-reported paragraph in the entire document, …
… if annual emissions average 11 GtC this century, we risk the real, terrifying prospect of seeing 1,000 ppm carbon dioxide in the atmosphere ….
Carbon emissions from the global consumption of fossil fuels are currently above 8 GtC per year and rising faster than the most pessimistic economic model considered by the IPCC. Yet even if the high price of energy from fossil fuels and power plants combines with regional climate initiatives to slow the current rate of growth somewhat, we will probably hit 11 gigatonnes of carbon emissions per year by 2020.
The first question to ask is — what is the IPCC’s ‘best estimate’ warming for 1000 ppm of CO2? Here I used the wrong link in my essay. This does not come from the WG1 Summary for Policymakers. The right link is the WG1 Technical Summary… . In any reasonable business-as-usual case where there is no greenhouse gas constraint, then you would expect steady rises in the levels of other greenhouse gas emissions.
The best estimate warming for 1000 ppm of CO2eq is 5.5°C and the best estimate for 1,200 ppm of CO2eq is 6.3°C. Given how much effort will be required to merely stabilize atmospheric concentrations of CO2 alone at 1,000 ppm (discussed below), I think it is very safe to say that total business-as-usual warming is at least 6.3°C and that 5.5°C is conservative.
How fast could this happen? Again, climate scientists don’t spend a lot of time studying this nightmare scenario because they can’t imagine humanity would be so stupid as to let it happen. But as I cited in my original essay, one very credible model suggests we could hit 1,000 ppm of CO2 in 2100 with a total warming from preindustrial levels in 2100 of about 5.5°C.
Let me see if I’ve got this straight.
Mr. Romm does not use any of the emissions scenarios that are the starting point for IPCC climate projections, but instead has developed a forecast for carbon emissions (which are “rising faster than the most pessimistic economic model considered by the IPCC”). Under Mr. Romm’s forecast, “we will probably hit 11 gigatonnes of carbon emissions per year by 2020.” He then further develops his emissions forecast with the assumption that “[i]n any reasonable business-as-usual case where there is no greenhouse gas constraint, then you would expect steady rises in the levels of other greenhouse gas emissions.” He asserts that this forecast is a “business as usual” baseline, which is a concept that the IPCC does not use.
Next, in order to convert his emissions forecast into a forecast for warming, he cites an exhibit (Table TS.5, page 66) in an IPCC technical summary that relates atmospheric carbon dioxide concentration to equilibrium temperature change versus pre-industrial temperature, without mentioning the crucial fact that this is an equilibrium projection. In this context, “equilibrium” means roughly where temperature will eventually settle down in the long run, which can be a very long time. (See WG1 Glossary, page 945, for the distinction between “Equilibrium and transient climate experiments.”) So when Mr. Romm says that “[t]he best estimate warming for 1,000 ppm of CO2eq is 5.5°C,” this means that (1) such a concentration should eventually lead to such a temperature change, and (2) this temperature change is versus pre-industrial temperature, not versus today’s temperature. This is very far from the same thing as projected temperature change between today and 2100. Presumably recognizing that the world scientific community uses global climate models of some complexity to translate emissions scenarios into projections for warming by specific dates, he refers to “one very credible model” that “suggests we could” hit total warming from pre-industrial levels of about 5.5°C by 2100.
In other words, Mr. Romm has used external sources to develop his own emissions business as usual baseline, and then used other external sources to create his own climate forecast based on this emissions forecast. I have great respect for Mr. Romm’s technical capabilities, and his forecast for warming may or may not turn out to be correct, but it is very far from being an IPCC forecast. If the information “buried” in the Summary for Policymakers in combination with the referenced table in the technical summary demonstrates Mr. Romm’s point, this appears to have escaped the authors and editors of the IPCC Assessment Report.
As a separate point, Mr. Romm says that:
Manzi is under the serious misimpression that the IPCC scenarios like B1 or A1F1 either separately or averaged together, represent a consensus or business-as-usual forecast, when in fact most of those scenarios assume the kind of aggressive action to deploy clean energy technologies that he does not support.
Here is what the IPCC says about this (WG1 SPM, page 18):
The SRES scenarios do not include additional climate initiatives, which means that no scenarios are included that explicitly assume implementation of the United Nations Framework Convention on Climate Change or the emissions targets of the Kyoto Protocol.
Or see WG1 Chapter 10, Table 10.26, page 803 which defines all marker scenarios as “non-mitigation” scenarios.
Or see the IPCC Special Report on Emissions Scenarios, Section 4.4.7 for detailed assumptions for technological change by scenario.
In my essay, I did not render an opinion on “aggressive action to deploy clean energy technologies” per se. Any reasonable scenario for global development over the next century is likely to project technological change, which as we have seen in the past century would likely include changes in energy creation and consumption technologies, as well as the propagation of the kind of normal environmental policies that we have seen in the developed world in the past century, such as control of sulfur pollution. The specific actions that I opposed in my essay were: (1) a cap-and-trade system for carbon emissions, (2) a carbon tax, and (3) $5 trillion of U.S. government spending between now and 2100 on technology designed to reduce greenhouse gas emissions. None of these are assumed in any IPCC marker scenario.