Arguments
Stop emissions, stop warming: A climate reality check
Posted on 18 December 2024 by Guest Author
This is a re-post from The Climate Brink by Andrew Dessler
One of the most important concepts in climate science is the idea of committed warming — how much future warming is coming from carbon dioxide that we’ve already emitted.
Understanding the extent of committed warming is vital because it informs our current climate situation. If there is a significant amount of committed warming already “locked in,” then we have much less ability to avoid the levels of warming that policymakers judge as dangerous.
In a previous post about what made me optimistic about the climate problem, I wrote:
When humans stop emitting greenhouse gases into the atmosphere, the climate will stop warming.
I received emails and comments from people who found that difficult to believe, so I thought I’d write a post about why this is true and shed light on the reasons behind the controversy surrounding it.
the 2000s
To understand why people are so confused about this, let’s step back to the 2000s. In the IPCC’s fourth assessment report (published in 2007), committed warming was defined to be:
If the concentrations of greenhouse gases and aerosols were held fixed after a period of change, the climate system would continue to respond due to the thermal inertia of the oceans and ice sheets and their long time scales for adjustment. ‘Committed warming’ is defined here as the further change in global mean temperature after atmospheric composition, and hence radiative forcing, is held constant. (from box TS.9)
Consider this simple example: humans emit CO2 until the year 2010, when the atmospheric concentration of CO2 reaches 400 ppm. After that point, the concentration of CO2 is held fixed at 400 ppm in perpetuity, as are all other components of the atmosphere (methane, aerosols, etc.).
In this scenario, maintaining a fixed atmospheric composition is analogous to setting a thermostat at a constant set point for the Earth's climate system. This is the resulting trajectory:
As you can see, the climate continues to warm well after concentrations are fixed (the vertical dashed line). The reason is the immense thermal inertia of the ocean. In much the same way that it takes a very long time for a hot tub filled with cold water to warm after you set the heater, the oceans will take a very very long time to fully warm to reach equilibrium with the fixed atmospheric composition.
a better understanding of cessation of emissions
In the late 2000s, scientists recognized that this was not the right way to think about this problem. The abstract of this 2010 paper says:
The perception that future climate warming is inevitable stands at the centre of current climate-policy discussions. We argue that the notion of unavoidable warming owing to inertia in the climate system is based on an incorrect interpretation of climate science. Stable atmospheric concentrations of greenhouse gases would lead to continued warming, but if carbon dioxide emissions could be eliminated entirely, temperatures would quickly stabilize or even decrease over time.
These two emissions scenarios are demonstrated in this figure:
The blue line shows the emissions time series for constant concentrations (e.g., holding CO2 fixed at 400 ppm). The red line shows emissions going entirely to zero. This can also be achieved by reducing emissions of CO2 as much as possible and then balancing any remaining emissions with CO2 removal (e.g., direct air capture), which is referred to as “net zero”.
Under a zero emissions scenario, temperatures stop rising after emissions cease, which is quite different from the scenario with fixed atmospheric concentration:
The reason is that, when emissions stop, atmospheric CO2 will begin to decline as it is absorbed by the ocean and land biosphere. This in turn reduces heating of the climate system — e.g., turns down the heater on the climate hot tub.
The figure below shows what happens when emissions stop in year zero of a set of global climate models. The left panel shows atmospheric abundance of CO2 starts to decline as soon as we stop emitting CO2. After 100 years, CO2 has dropped around 100 ppm.
The right panel shows temperature after emissions stop. Some models show a few tenths of a degree of warming and others a few tenths of a degree of cooling. However, the central estimate is that the global average temperature does not change much once emissions stop.
About 10 years ago, this was formalized in a nice way by Ricke and Caldeira, who showed that the maximum heating from a slug of CO2 emitted to the atmosphere occurred about a decade after you emit it. After that, enough of the slug has been removed from the atmosphere that the heating from the slug starts to decline (but the fall off is slow and it continues to heat the climate for a very long time).
Thus, climate scientists no longer routinely talk about committed warming in the same way they did in the 2000s because it’s not as relevant a quantity as previously thought. This is very good news.
Hansen
So why do people argue so much about this? I think that Jim Hansen’s recent paper, Global Warming in the Pipeline, is responsible for a lot of the confusion1.
In the abstract, they write:
Equilibrium global warming for today’s GHG amount is 10°C, which is reduced to 8°C by today’s human-made aerosols.
In other words, if we maintain today’s atmospheric amounts of greenhouse gases until the system reaches equilibrium (many thousands of years), we would get an enormous amount of warming. Their estimate is based on a climate sensitivity that is higher than most other estimates (4.8C), which I think it unlikely, but it’s not an entirely unreasonable calculation.
But then things go south. They write that “Equilibrium warming is not ‘committed’ warming”. This is confusing since these are literally the same thing: the IPCC defined committed warming to be the equilibrium warming from the constant atmospheric composition.
But the next sentence clarifies what they mean: “rapid phaseout of GHG [greenhouse gas] emissions would prevent most equilibrium warming from occurring.” Thus, we are not actually committed to this 8-10C of warming. If we can reduce emissions, then we can avoid most of this warming.
I think this is where the confusion arises. Most people read Hansen’s paper as saying that we are already committed to 8-10C of warming and there’s nothing we can do about it, but that’s clearly a misreading of the paper.
The sooner we can get emissions to (net) zero, the sooner we stabilize our climate system. And, as I said in this post, we have the technology to largely do that today. Whether we do so or not is a political decision, not a technical or scientific decision.
Nerd time
[skip this section if you don’t want a more technical description]
In case you want a slightly nerdier description of why temperatures stop rising, you’re in luck. When we stop emitting greenhouse gases, the future behavior of Earth’s temperature depends on the relative speed of two critical processes:
-
The rate at which CO? is removed from the atmosphere (through absorption by oceans and land).
-
The rate at which heat is transferred from the surface ocean (a layer with small heat capacity) into the deep ocean (which has very large heat capacity).
To illustrate this, consider two extreme scenarios:
Scenario 1: CO2 removal is very slow, ocean heat transfer is very fast. Under this scenario, the Earth’s temperature would continue to rise long after emissions stop. This happens because the persistent CO2 in the atmosphere would keep trapping heat, while rapid heat mixing into the deep ocean would increase the effective heat capacity of the surface ocean. A higher surface heat capacity means it would take longer for the surface ocean to reach equilibrium for any given CO2 level. Thus, when emissions cease, the surface layer is much cooler than equilibrium with atmospheric CO2 and it must warm over the following centuries to reach equilibrium.
Scenario 2: CO2 removal is very fast, ocean heat transfer is very slow. Under this scenario, CO2 levels drop quickly after emissions stopped. The slow mixing limits the heat capacity of the surface ocean, causing the temperature of the surface layer to remain very close to equilibrium with atmospheric CO2 levels. As CO2 concentrations decrease, the surface ocean will therefore cool to stay near equilibrium.
In our actual climate system, these two processes happen at roughly comparable rates. The cooling effect of declining CO2 levels tends to offset the warming caused by heat transfer into the deep ocean. This balance means that, after emissions stop, global temperatures are expected to remain relatively stable instead of significantly rising or falling.
Other stuff
Twitter/X is dead for climate science. If you were someone who went to Twitter to read about climate, head over to Bluesky — most of the climate community I engage with are and, as an added bonus, it’s not a Nazi bar. Read more about Bluesky on Andrew Rumbach’s recent substack post. I’m almost exclusively on bluesky; you can find me here.
If you can’t get enough of my perspectives on climate and energy, I gave a “fireside chat” at the Texas A&M Innovation Forward conference a few weeks ago. You can watch it here:
Although I agree with the theoretical aspects of no warming after reaching net-0 emissions, the danger I see with the underlying message in this paper is that we are broadcasting the concept that the future is in our hands. This is not only an arrogant position, but may backfire. The average person is not reading SkS and is not grounded in legitimate climate science, but may be getting a fuzzy, positive feeling when they see the number of solar panels, wind turbines, and EVs on the road increasing. They see what looks like great progress deploying renewable energy and EVs, and therefore conclude wrongly that we're decreasing CO2 emissions, and so now can relax and rest assured that the future will be fine. As long as they continue to see the deployment of renewable energy projects and EVs, they are satisfied that we are doing what is needed. Now that we've got the climate back on track, let's go elect leaders to get the economy back on track.
This at a time when CO2 concentrations are increasing at a rate of 2.5 ppm/year and fossil-fuel use continues to increase year after year.
In my opinion it will never work to broadcast that the future is in our hands and that we just need to get to Net-0 emissions to stabilize the climate. The message is arrogant and really just a concept that we cannot possibly hope to effectively quantify. In my opinion, achieving it will require more than we've ever demonstrated we're capable of.
I hope my opinion is wrong!
Having said this, I'm still not sure what the best messaging is. I think what SkS is doing is critically important because it is helping people understand what is happening and why. So I offer my comments in an effort to put the message of this paper into context and to temper what I see as an overly optimistic message.
The message that I prefer to give people is this.
"The current CO2 concentration is 420 ppm. That concentration is sufficient to warm the planet to 1.7C if we don't bring it down. Every time we emit CO2 we are actively destroying Earth's life-support systems. We need to reduce our greenhouse gas emissions as fast as possible and to support local, national, and global initiatives that do that."
This is a message that is consistent with climate science and refers people back to the Keeling Curve to monitor how we're doing. If 420 ppm is enough to take us to 1.7C, then anything higher will take us to a higher temperature. The message is also that any level of emissions is bad, and that we need to do all that we can to reduce GHG emissions.
My understanding is that the committed / equilibrium warming is apparently zero, provided emissions drop abruptly to zero in some given year, as opposed to trailing off slowly (?). But it very unlikely that emissions will abruptly drop to zero, so for all practical purposes we have some committed warming!
However assuming purely for the sake of argument that the committed warming is Hansens 10 degrees, that will take many thousands of years to evolve and so won't affect humanity significantly for a very long time, so its not a reason for us to give up on reducing emissions.
I hear what Evan is saying. There is perhaps also a tendency for people to assume things like emissions trading schemes or carbon taxes are fixing the problem when they are not doing this adequately. However counter balancing this people must also be aware progess reducing emissions is going too slowly, given its been in the media often enough. So I'm not sure that too many people would assume the problem is being adequately solved.
Evan says "The message is also that any level of emissions is bad, and that we need to do all that we can to reduce GHG emissions."
Exactly.
Skeptical Science asks that you review the comments policy. Thank you.
[snip]
TODAY!......... Climate Shift. ..??.. DID the Climate Fanatics of the 1970’s CAUSE the Climate Crises of Today?
Funny as the ''Climate Craze'' back in the 1970's was the New Ice Age..... Yes ''they'' said that Pollution (partials) were being thrown up into the upper atmosphere and causing the suns light to be reflected back into space., This was causing a New Ice Age to destroy the earth.
(Scientists say sprinkling diamond dust into the sky could offset almost all of climate change so far — but it'll cost $175 trillion)
Story by Sascha Pare 12-19-2024
Today those same people (Rainmakers) are selling yet another climate ''Crises''.
Note; To STOP this New Ice Age, the USA went 'seriously' into protecting the 'Environment' way back in the 1970's with President Nixon signing the Environmental Protection Act (EPA) into law.
Today the USA only produces as much steel as it did in 1950, this is as an example of those EPA efforts.
High Gas Prices?.. EPA will Not allow a New Oil refinery to be built in America.
And this is also a major reason for the loss of Millions of very good paying jobs, I might add. 'clean', comes with a very steep 'price'.
Even IF the ‘Clean’ is ONLY here and all that pollution was just Moved to China, along with all the Jobs.
Good thing we don’t use the same Air as the Chinese. Otherwise it would ALL have been a waste of time and Money.
TODAY!......... Climate Shift. ..??.. DID the Climate Fanatics of the 1070 CAUSE the Climate Crises of Today?
(Scientists say sprinkling diamond dust into the sky could offset almost all of climate change so far — but it'll cost $175 trillion)
Story by Sascha Pare 12-19-2024
Sprinkling diamond dust into the atmosphere could offset almost all the warming caused by humans since the industrial revolution and "buy us some time" with climate change, scientists say.
New research indicates that shooting 5.5 million tons (5 million metric tons) of diamond dust into the stratosphere every year could cool the planet by 1.8 degrees Fahrenheit (1 degree Celsius) thanks to the gems' reflective properties. This extent of cooling would go a long way to limiting global warming that began in the second half of the 19th century and now amounts to about 2.45 F (1.36 C), according to NASA.
The research contributes to a field of geoengineering that's looking for ways to fight climate change by reducing the amount of energy reaching Earth from the sun.
https://www.msn.com/en-us/weather/topstories/scientists-say-sprinkling-diamond-dust-into-the-sky-could-offset-almost-all-of-climate-change-so-far-but-it-ll-cost-175-trillion/ar-AA1w6MuP?ocid=msedgntp&pc=HCTS&cvid=2dfb5c2f1669448799854ec819ce98bf&ei=43
[BL] Apart from getting facts wrong, this is essentially yet another uninformed political rant.
Please note that posting comments here at SkS is a privilege, not a right. This privilege can be rescinded if the posting individual treats adherence to the Comments Policy as optional, rather than the mandatory condition of participating in this online forum.
Please take the time to review the policy and ensure future comments are in full compliance with it. Thanks for your understanding and compliance in this matter.
AdriantheHistorian said: "Today the USA only produces as much steel as it did in 1950, this is as an example of those EPA efforts."
Not necessarilly. This is from "History of the iron and steel industry in the United States" on Wikipedia: "US production of iron and steel peaked in 1973, when the US industry produced a combined total of 229 million metric tons of iron and steel. But US iron and steel production dropped drastically during the recession of the late 1970s and early 1980s. From a combined iron and steel production of 203 million tons in 1979, US output fell almost in half, to 107 million tons in 1982. Some steel companies declared bankruptcy, and many permanently closed steelmaking plants. By 1989, US combined iron and steel production recovered to 142 million tons, a much lower level than in the 1960s and 1970s. The causes of the sudden decline are disputed. Among the many causes alleged have been: dumping of foreign imports below cost, high labor costs, poor management, unfavorable tax policies, and costs of environmental controls."
It seems most likely that the EPA contributed to a relatively small part of the stagnation in steel production if anything. I think is a price worth paying to look after the environment and have clean air and water and so forth. It's a values issue.
AdriantheHistorian @4,
In addition to reviewing the comments policy, the SkS home page helpfully offers 3 big boxes near the top for people who are relatively unaware or lack a reasonable understanding of the issue:
A very helpful part of the Newcomers, start here page (linked here) is the section: Good starting points for newbies.
After becoming more familiar with the issue you should understand and appreciate the lack of legitimacy, lack of merit, lack of value, of the beliefs you shared in your comment @4.
Obvious questions about constantly pumping massive amounts of diamond dust, or other materials, into the atmosphere are:
Since my opinion is in agreement with Evan, I too hope we are both wrong.
"When humans stop emitting greenhouse gases into the atmosphere, the climate will stop warming." Not just keep co2 emissions from increasing but stop any GHG pollution entering our air.
Here is part of why I think "committed warming" is the real world norm.
Take for example the USA,www.wri.org/insights/interactive-chart-shows-changes-worlds-top-10-emitters while its per capita co2 emissions has peaked, historically this one country has put 25% of the co2 in our atmosphere! It is number 2 in the current yearly co2 emitters and to get some perspective-this is the equivalent to the mass of 6,300+ small cars x a million in this year alone!
The worst top three add 46% of climate change pollutants with the worst 10 making this amount to over 66%.
I'm tired of the hopium of scaled co2 "scrubbers", of a wake-up of humanity and forcing our leaders to think decades ahead and to get the transition moving more quickly.
The map for 2017 ourworldindata.org/contributed-most-global-co2 shows the large inequalities of contribution across the world that the first treemap visualization has shown. The USA has emitted the most to date: more than a quarter of all historical CO2 — twice that of China, which is the second largest contributor.
In contrast, most countries across Africa have been responsible for less than 0.01% of all emissions over the last 266 years.
What becomes clear when we look at emissions across the world today is that the countries with the highest emissions over history are not always the biggest emitters today. The UK, for example, was responsible for only 1% of global emissions in 2017. Reductions here will have a relatively small impact on emissions at the global level – or at least fall far short of the scale of change we need. This creates tension with the argument that the largest contributors in the past should be those doing the most to reduce emissions today. This is because a large fraction of CO2 remains in the atmosphere for hundreds of years once emitted.3
This inequality is one of the main reasons why it’s so challenging to find international agreement on who should take action.
I think future warming is inevitable because of our flawed human nature..
I read in my local newspaper, The Daily Breeze, serving South Bay Cities of Los Angeles County, the following quote, and was wondering if anyone has a response to it. It was titled "Progressive myths harm the honest discourse." Author was Michael Huemer.
"Virtually no serious scientists think that global warming is an existential threat. Mainstream researchers anticipate global warming contributing perhaps a quarter percent to the excess death rate by mid-century and costing us 2.5% of GDP by 2100, making global warming less serious than many other world problems that have received far less attention."
rkolph@8
I repeatedly hear from top climate scientists that the scope and severity of climate change is proceeding faster than climate scientists thought it would X years ago. One of the top climate researchers, Prof. Richard Alley of Penn State, is on record saying that sea level rise could be 15 ft of more by 2100. That statement alone is sufficient to counter the positive outlook presented in the Daily Breeze.
If you watch videos of Prof. Alley's talks, you will quickly learn that he is a very measured and disciplined scientist who carefully chooses his words. He is not an alarmist. For him to say that you can not rule out sea level rise of 15 to 20 ft is alarming.
Prof. Alley has also researched past climatic changes and notes that if we push the system hard enough it can switch states in a matter of years through Abrupt Climate Change. The previous link is to a paper that is behind a paywall, but if just read the abstract visible on the website, you get the idea. Abrupt climate changes are hard to predict, but have happened before, and could likely happen again, given just how hard we're pushing the system.
How hard are we pushing the system?
Typical ice-age cycles see the predominant greenhouse gas, CO2, change by about 100 ppm over 100,000 years, causing a fluctuation of sea level by 400 ft! We are currently increasing atmospheric CO2 concentrations by 2.5 ppm/yr, year after year after year. In just 40 years, we increase CO2 by the same amount that natural processes require 1000's of year to do.
The idea that the human effect on the climate will be mild and managable are wishful thinking. We are actively damaging our life-support systems, but making precise predictions about how this will play out is difficult.
My recommendation is that you google "Richard Alley Climate Change" and start watching vidoes of his talks. You will learn a lot with which to counter the myth that the effects of climate change will be mild and managable.
rkolph@8
You said: "Virtually no serious scientists think that global warming is an existential threat. "
Some well qualified scientists do think climate change is an existential threat to humanity:
phys.org/news/2023-10-life-earth-existential-threat-climate.html
www.forbes.com/sites/davidrvetter/2023/10/24/we-are-afraid-scientists-issue-new-warning-as-world-enters-uncharted-climate-territory/
You said "Mainstream researchers anticipate global warming contributing perhaps a quarter percent to the excess death rate by mid-century and costing us 2.5% of GDP by 2100, making global warming less serious than many other world problems that have received far less attention."
Note that no researchers are named. You invariably find its the economist William Nordhaus. Just look up his wikipedia entry and read the expert criticisms of his DICE eoconomic model of climate change, near the end of the article. His assumptions are often unrealistic and he leaves out entire aspects of climate change like sea level rise.
One thing. He assumes quite high levels of economic growth in the future will offset climate problems. However economic growth has slowed relentlessly in developed countries since the 1970s until presently, with every sign developing countries will follow that trend later this century, and we live in a world of finite resources, with many fast being depleted and we have many countries with aging demographics and market saturation. This suggests future global economic growth will be low.
And thats before you consider the negative impacts of climate change on economic growth. Some experts calculate it will be considerably more than Nordhaus assumes:
"The largest impact of climate change is that it could wipe off up to 18% of GDP off the worldwide economy by 2050 if global temperatures rise by 3.2°C, the Swiss Re Institute warns."
www.weforum.org/stories/2021/06/impact-climate-change-global-gdp/
18% is huge and would severely impact the world. And this is still based on middle range warming estimates, and assumes critical tipping points won't be crossed.
rkrolph @ 8, sorry I didn't mean to say "you said" those quotes, they were by Michael Heumer.
rkrolph @8,
In addition to the helpful comments by Evan @9 and nigelj @10, as a Professional Engineer with an MBA I would add that a proper evaluation of GDP has to exclude any economic activity that is a 'repair of or recovery from damage done by climate change'. Not excluding those activities that are required to address the harm done is like saying that the clean up of environmental damage done by an economic activity counts as a 'boost to the economy'.
Also, the likes of Nordhaus usually exclude any 'external negatives' like the displacement of, or harm done to, people who are not significant parts of the economic activity they are evaluating.
But the most misleading thing that the likes of Nordhaus do is 'heavily discount' future negatives. They apply high discount rates to create misunderstanding that make it appear that future negative impacts are 'justifiably significantly less meaningful to people today'.
rkrolph @ 8:
The devil is in the details on the quote you provided. What exactly do they mean by "existential threat"? That is the sort of subjective, emotive phrase that is very hard to pin down (and therefore hard to argue against or even have any sort of reasonable discussion about). Until such terms are clearly, unambiguously defined, trying to debate the statement is a fool's errand.
As for the "other world problems" part of that statement, it sound like they are channelling Bjorn Lomborg, who uses that shtick all the time. The shtick is so common that there is even an XKCD comic about it:
rkrolph @8,
The quote you provide comes from a 900 word essay entitled 'Progressive myths harm the honest discourse' by Michael Huemer, a professor of philosophy at the University of Colorado at Boulder. The essay is really no more than an advert for his book 'Progressive Myths' (Amazon preview here).
In both book and essay he rails against "political activists" saying that "Nearly every piece of information they disseminate is a distortion or outright lie," and also that their influence is pervasive. In the essay he cites three exemplar "lies" promulgated by such "political activists." The three exemplars given are:-
(1) Women earn just 82 cents for every dollar that men earn for the same work;
(2) Police shootings show a marked racial bias against Black Americans;
(3) Global warming is an existential threat to America and the world.
These are, of course 'progressive' lies as are the nine "myths" featured in his book (according to this book review) and with Huemer apparently a 'libertarian' (according to the reviewer of the book who does say but not convincingly Huemer "also addresses falsehoods from the far right"). With the subject of the book being titled "Progressive Myths", some significant bias should bring no surprises. The Amazon book review linked above shows the book's Part VI containts three chapters:-
19 The Global Warming Consensus.
20 Existential Climate Risk.
21 Mask Science, which presumably is about spread of the recent pandemic.
(I should point out that, as I am a more-progressive less-libertarian Brit sat on the other side of the pond, I would consider the egregious lies and denials spread by 'libertarians' in the US should be far more of an issue and a concern. Thus I see the book as the lesson of Matthew 7:3-to-5 at play here.)
With that preamble from me, is there any merit to the notion of "global warming is an existential threat to America and the world" being nothing but a "progressive myth," as Huemer says? Is it indeed a lie? And do "Virtually no serious scientists think that global warming is an existential threat"?
The first thing required to be clear is what is meant by "existential threat."
There are some lunatics who talk of an "existential threat" to humanity, apparently suggesting that the Homo Sapiens species could become extinct. But such a notion is not being considered by Huemer.
The future exisitence of "America (USA) and the world" is the issue at hand. In the Amazon book review linked above which was lilely written by Huemer, the question is put "Is global warming really going to destroy human civilization?" Put another way, could we be** stoking a collapse of the USA and/or enough of the sovereign states of the world to collapse the world economic order. Note that more will be in play that AGW itself. Without collapsing the entire world order, the remaining sovereign states will almost certainly be arguing over resources, with the environmental impacts of AGW thus precipitating political conflict and thus further chaos.
(** There is considerable uncertainty with the climate effects of AGW, even when a global level of warming is a given. There is thus a lot of uncertainty even before the level of AGW is converted into a measure of economic impacts.) The evident uncertainties within any assessment of the economic damage from AGW means assessment has to account for a less-than precise answer. The average of the potential results does not really provide a worthy assessment. It would be properly some assessment of the worst likely outcome.
And that leads to the work apparently setting-out what will be the financial impacts of AGW. In the most recent IPCC AR6 the conclusion is that no identifiable range of economic impacts globally is apparent due to the varying methodologies producing such a wide range of results. This range has increased since the limited analyses reviewed in AR5. Further complications include there being non-linear impacts with increasing AGW and there will be significant regional variation.
But to at least put some numbers to it, the range shown in AR6 for +4ºC of AGW is +3% to +33% with the CI ranging from negative to +66%. (Note the authors of these lower evaluations do come under fire and the likes of Richard Tol are well known for presenting a denialist stance.) This range compares to the "2.5% of GDP by 2100" stated by Huemer without any mention of the level of AGW assumed. It also compares with the range given in AR5 Box 3.1 "These incomplete estimates of global annual economic losses for temperature increases of ~2.5°C above pre-industrial levels are between 0.2 and 2.0% of income (medium evidence, medium agreement)."
There are many difficulties facing these researchers trying to set some sort of economic cost to AGW, mitigated or unmitigated, some examples being:-
☻ The 2100 time-frame usually chosen ignores some very serious issues, not least Sea Level Rise over multi-century timescales. Greenland melt down will become inevitable at some point below 2ºC AGW if it continues at that level. Thus it becomes a certainty for continuing AGW of +2ºC to resultant +7m SLR over a millennium or so. At +4ºC, there would be an additional +8m SLR from other land ice loss.
☻ There are many saying the undeveloped nations will see negative economic growth under unmitigated AGW. This may well not have such a big simplistic impact on global economic growth as the deveolped world accounts for the vast majority of the global economy. So if say Madagascar were to melt into the Indian Ocean and disappear, the global economy shrinks by just 0.1%. But also the 30 million inhabitants would thus be looking for some sort of future beyond their lost homeland. Some may see such migrations boosting economies elsewhere while others may see it as a more significant annual cost than the $500/head/y lost from their present day autochthonic productivity.
☻ The potential size of unmitigated AGW has been reduced in the minds of some researchers because the world has turned against using coal. This is argued because there are insufficient non-coal FFs to create much more than +3ºC AGW. Yet such an assumption remains to be fully argued out.
And do "Virtually no serious scientists think that global warming is an existential threat"? There is another philosopher who talks as though no scientist could seriously say it is not an exisitential threat. "In the worst-case scenarios in scientists’ climate models, human-caused climate change is a threat to the continued existence of many species and to human society as we know it."
To conclude, Huemer presents a predictably denialist (and he insists he is not an AGW denialist) with his outlandish pronouncements entirely out-of-kilter with him being a growed-up philosopher and all.