Arguments
Positives and negatives of global warming
What the science says...
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Negative impacts of global warming on agriculture, health & environment far outweigh any positives. |
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Climate Myth...
It's not bad
"By the way, if you’re going to vote for something, vote for warming. Less deaths due to cold, regions more habitable, larger crops, longer growing season. That’s good. Warming helps the poor." (John MacArthur)
At a glance
“It's not going to be too bad”, some people optimistically say. Too right. It's going to be worse than that. There are various forms this argument takes. For example, some like to point out that carbon dioxide (CO2) is plant-food – as if nobody else knew that. It is, but it's just one of a number of essential nutrients such as water and minerals. To be healthy, plants require them all.
We know how climate change disrupts agriculture through more intense droughts, raging floods or soil degradation – we've either experienced these phenomena ourselves or seen them on TV news reports. Where droughts intensify and/or become more prolonged, the very viability of agriculture becomes compromised. You can have all the CO2 in the world but without their water and minerals, the plants will die just the same.
At the same time, increased warming is adversely affecting countries where conditions are already close to the limit beyond which yields reduce or crops entirely fail. Parts of sub-Saharan Africa fall into this category. Elsewhere, many millions of people – about one-sixth of the world’s population - rely on fresh water supplied yearly by mountain glaciers through their natural melt and regrowth cycles. Those water supplies are at risk of failure as the glaciers retreat. Everywhere you look, climate change loads the dice with problems, both now and in the future.
Please use this form to provide feedback about this new "At a glance" section. Read a more technical version below or dig deeper via the tabs above!
Further details
Most climate change impacts will confer few or no benefits, but may do great harm at considerable costs. We'll look at the picture, sector by sector below figure 1.
Figure 1: Simplified presentation of the five Reasons for Concern burning ember diagrams as assessed in IPCC AR6 Working Group 2 Chapter 16 (adapted from Figure 16.15, Figure FAQ 16.5.1).
Agriculture
While CO2 is essential for plant growth, that gas is just one thing they need in order to stay healthy. All agriculture also depends on steady water supplies and climate change is likely to disrupt those in places, both through soil-eroding floods and droughts.
It has been suggested that higher latitudes – Siberia, for example – may become productive due to global warming, but in reality it takes a considerable amount of time (centuries plus) for healthy soils to develop naturally. The soil in Arctic Siberia and nearby territories is generally very poor – peat underlain by permafrost in many places, on top of which sunlight is limited at such high latitudes. Or, as a veg-growing market gardening friend told us, “This whole idea of "we'll be growing grains on the tundra" is just spouted by idiots who haven't grown as much as a carrot in their life and therefore simply don't have a clue that we need intact ecosystems to produce our food.” So there are other reasons why widespread cultivation up there is going to be a tall order.
Agriculture can also be disrupted by wildfires and changes in the timing of the seasons, both of which are already taking place. Changes to grasslands and water supplies can impact grazing and welfare of domestic livestock. Increased warming may also have a greater effect on countries whose climate is already near or at a temperature limit over which yields reduce or crops fail – in parts of the Middle East and sub-Saharan Africa, for example.
Health
Warmer winters would mean fewer deaths, particularly among vulnerable groups like the elderly. However, the very same groups are also highly vulnerable to heatwaves. On a warmer planet, excess deaths caused by heatwaves are expected to be approximately five times higher than winter deaths prevented.
In addition, it is widely understood that as warmer conditions spread polewards, that will also encourage the migration of disease-bearing insects like mosquitoes, ticks and so on. So long as they have habitat and agreeable temperatures to suit their requirements, they'll make themselves at home. Just as one example out of many, malaria is already appearing in places it hasn’t been seen before.
Polar Melting
While the opening of a year-round ice-free Arctic passage between the Atlantic and Pacific oceans would have some commercial benefits, these are considerably outweighed by the negatives. Detrimental effects include increased iceberg hazards to shipping and loss of ice albedo (the reflection of sunshine) due to melting sea-ice allowing the ocean to absorb more incoming solar radiation. The latter is a good example of a positive climate feedback. Ice melts away, waters absorb more energy and warming waters increase glacier melt around the coastlines of adjacent lands.
Warmer ocean water also raises the temperature of submerged Arctic permafrost, which then releases methane, a very potent greenhouse gas. The latter process has been observed occurring in the waters of the East Siberian Arctic Shelf and is poorly understood. At the other end of the planet, melting and break-up of the Antarctic ice shelves will speed up the land-glaciers they hold back, thereby adding significantly to sea-level rise.
Ocean Acidification
Acidity is measured by the pH scale (0 = highly acidic, 7 = neutral, 14 = highly alkaline). The lowering of ocean pH is a cause for considerable concern without any counter-benefits at all. This process is caused by additional CO2 being absorbed in the water. Why that's a problem is because critters that build their shells out of calcium carbonate, such as bivalves, snails and many others, may find that carbonate dissolving faster than they can make it. The impact that would have on the marine food-chain should be self-evident.
Melting Glaciers
The effects of glaciers melting are largely detrimental and some have already been mentioned. But a major impact would be that many millions of people (one-sixth of the world’s population) depend on fresh water supplied each year by the seasonal melt and regrowth cycles of glaciers. Melt them and those water supplies, vital not just for drinking but for agriculture, will fail.
Sea Level Rise
Many parts of the world are low-lying and will be severely affected even by modest sea level rises. Rice paddies are already becoming inundated with salt water, destroying the crops. Seawater is contaminating rivers as it mixes with fresh water further upstream, and aquifers are becoming saline. The viability of some coastal communities is already under discussion, since raised sea levels in combination with seasonal storms will lead to worse flooding as waves overtop more sea defences.
Environmental
Positive effects of climate change may include greener rainforests and enhanced plant growth in the Amazon, increased vegetation in northern latitudes and possible increases in plankton biomass in some parts of the ocean.
Negative responses may include some or all of the following: further expansion of oxygen-poor ocean “dead zones”, contamination or exhaustion of fresh water supplies, increased incidence of natural fires and extensive vegetation die-off due to droughts. Increased risk of coral extinction, changes in migration patterns of birds and animals, changes in seasonal timing and disruption to food chains: all of these processes point towards widespread species loss.
Economic
Economic impacts of climate change are highly likely to be catastrophic, while there have been very few benefits projected at all. As long ago as 2006, the Stern Report made clear the overall pattern of economic distress and that prevention was far cheaper than adaptation.
Scenarios projected in IPCC reports have repeatedly warned of massive future migrations due to unprecedented disruptions to global agriculture, trade, transport, energy supplies, labour markets, banking and finance, investment and insurance. Such disturbances would wreak havoc on the stability of both developed and developing nations and they substantially increase the risk of future conflicts. Furthermore, it is widely accepted that the detrimental effects of climate change will be visited mostly on those countries least equipped to cope with it, socially or economically.
These and other areas of concern are covered in far more detail in the 36-page Summary for Policymakers from the IPCC AR6 Synthesis Report, released in March 2023. The report spells out in no uncertain terms the increasingly serious issues Mankind faces; the longer that meaningful action on climate is neglected, the greater the severity of impacts. The report is available for download here.
Last updated on 21 April 2023 by John Mason. View Archives
Banbrotam @375 , you seem to be applying selective vision to the situation.
First, you should clarify to readers (and to yourself) how much precisely of present-day rapid global warming is caused by human activity. If you hold that rather less than 50% of warming is anthropogenic, then you might well have a point that it would be a struggle to halt or reverse the global warming process. However, the facts are against you there — in actuality, the human causation is very close to 100% (as you will discover if you educate yourself about the issue). And therefore your denial of reversibility carries no weight.
Secondly, what are the "positives" of climate change (i.e. global warming) that you would wish to mention? Sure, you can point to some small areas, such as southern Patagonia and northern Russia, which would (from a human perspective) benefit from a few degrees of global warming. But — taken as a whole, the planet would be 95+% worse off. Especially for the human race in total, and also for most marine life [re temperature for the coral reef systems, and re acidification for much of the rest of the marine biosystem].
If you stop and think it through, Banbrotam, it will occur to you that the present-day plants and animals have evolved to suit the world temperatures (typified by the climate of approx 100 years ago) of the Holocene period. And so you would expect major disruption from very rapid rise in global surface temperature — and so you would be hard-put to find any definite "positives" arising from AGW. And so you would not be surprised that such "positives" [should they exist] are rarely mentioned in discussions.
Remember too, that the present large size of world human population is already pushing the limits of sustainabilty. Any small advantages to AGW (e.g. in northern Russia) are enormously outweighed by more general disadvantages — and particularly so in the Tropics.
Banbrotam, the only real debate that remains, is how to expeditiously tackle climate change. A scientist (for instance: yourself) will of course realise that denial of reality is not "debate" but is simply slogan shouting [which here on SkS is named sloganeering].
Additional reference:
http://biodiversityandclimate.abmi.ca/
This is a mixed group of scientists trying to figure out the impact of climate change on Alberta. As part of this, they have constructured sets of maps corresponding to cool, medium and hot scenarios and have plotted changes in rainfall (not much different), temperature (scary difference, and ecozone shifts (really scarry difference) for the 2050's and 2080's. Note that each of these maps you can choose between the cool, medium and hot scenarios. The third link is with the hot scenario shows southern Alberta's desert regions (much like central Montana or Wyoming) will in places be up to the northern border.
I live an hour from Edmonton, pretty much in the center of the province. We're looking at a 6 C temperature rise in the next 70 years.
Mean Annual Precipitation
Mean Annual Temperature
Natural ecological zones
Hi everyone, I agree with commenter no 27: I too am puzzled there's no mention of Daniel Bailey's article about ice ages. His argument is that global warming has protected us from any risk of a new ice age. I would have thought that his point should be the first item on your list of pros and cons as an ice age would be cataclysmic and it makes the cons you mention seem insignificant. Surely averting an ice age is such a great achievement that any pros and cons list should come out heavily in favour of global warming?
David A
David @378 , your argument carries zero weight — for the simple reason that there is no ice age imminent.
The timing of the next glacial (the one that would have occurred, without human presence) has been discussed elsewhere on SkepticalScience. The coming of the next glacial age, may be considerably more than the few thousand years away which casual observation [of history] would suggest. An unusually low amount of orbital ellipticity (i.e. a more circular orbit) over the coming dozens of millennia . . . points to a greatly delayed "next" glaciation — delayed by tens of thousands of years, perhaps.
David, balanced against all the multiple major problems occurring now and increasingly during the next century or three, from AGW . . . an ice age that might come in 2,000+ years (or more likely 20,000+ years) is a non-event in current considerations.
And if such prediction of the zero threat of "imminent next ice age" should happen to prove wrong . . . then the history of the 20th Century demonstrates that we could easily scotch the ice, by 30 years of intensive coal-burning. Easy fix !!
David, in the current situation, there is zero benefit to warming, as a "preventative".
Thanks for the response Ec! I'm no expert, so I'm just going by what it says in other articles on this site that conclude there is a chance that global warming has prevented an imminent ice age. I would align myself more to the views made in the articles on this site, I think anyone emphasising a 'zero' scientific probability is not thinking with proper scientific scepticism. I agree with you that the correct preventative measure is man-made global warming though.
[JH] You are merely cherry-pickking to support your pre-determined conclusion. This tactic impresses virtually no one. In addition, you are skating on the thin ice of excessive repetition which is prohibited by the SkS Comments Policy.
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.
Agreed, David — should any "ultra-low probability" harmful global cooling happen to rear its head in the year 4000 AD or later . . . then we can easily use Plan B , which is the digging up & burning of 50 Gigatons of coal (we know it's easy, because we've already done that very thing several times over, during the past 100 years ! Easy-peasy !
However, right now and for the next couple of centuries, the only intelligent thing we can do — is use Plan A , which is the quick-as-we-can elimination of fossil fuels. Not quite such an easy achievement, but well within our capabilities over the next 5 decades.
"there's no mention of Daniel Bailey's article about ice ages"
Out of curiosity, what is this supposed article that I wrote? While I've written a variety of articles published in this and other venues, I don't recall writing one on that topic.
Link citations are best.
Thank you for posting!!
Normally, everything has two sides, positive and negative ways. For global warming, seem the negative ways are more than the positive one. The positive is affect only some area but the negative affect all, direct and indirect ways to human's health. In my opinion, CO2 didn't make sense much on agriculture in a positive way. More CO2 didn't say that more O2 plants could produce. While we have global warming, that means many trees had been cutting down. No helper to absorb CO2 as much as the past. So in agriculture, few trees cannot take all CO2 to change to O2. CO2 also causes many glaciers melted that made the fresh water mixed with an ocean. Decrease the water supply that is the factor for human's life. In conclusion, I think global warming didn't make our world be good.
Normally, everything has two sides, positive and negative ways. For global warming, seem the negative ways are more than the positive one. The positive is affect only some area but the negative affect all, direct and indirect ways to human's health. In my opinion, CO2 didn't make sense much on agriculture in a positive way. More CO2 didn't say that more O2 plants could produce. While we have global warming, that means many trees had been cutting down. No helper to absorb CO2 as much as the past. So in agriculture, few trees cannot take all CO2 to change to O2. CO2 also causes many glaciers melted that made the fresh water mixed with an ocean. Decrease the water supply that is the factor in human's life. In conclusion, I think global warming didn't make our world be good.
I have a question. What would the world be in 100 years if we still release CO2?
First, my interest in agriculture is that, Increased warming may also have a greater effect on countries whose climate is already near or at a temperature limit over which yields reduce or crops fail. Second, Health, warmer climes will encourage migration of disease-bearing insects like mosquitoes. Third, Polar melting, the warming waters increase glacier and Greenland ice cap melt, as well as raising the temperature of Arctic tundra, which then releases methane, a very potent greenhouse gas (methane is also released from the sea-bed, where it is trapped in ice-crystals called clathrates). Next, positive effects of climate change may include greener rainforests and enhanced plant growth in the Amazon, increased vegetation in northern latitudes and possible increases in plankton biomass in some parts of the ocean. Negative responses may include further growth of oxygen poor ocean zones, contamination or exhaustion of fresh water, increased incidence of natural fires, extensive vegetation die-off due to droughts, increased risk of coral extinction, decline in global phytoplankton, changes in migration patterns of birds and animals, changes in seasonal periodicity, disruption to food chains and species loss.
Hello, not sure if a comment is the correct way to inform you about broken links, but I did not find another way to contact an author.
Please feel free to delete this comment, once the link was restored.
So which link is broken?
I was just reading the intermediate version of "positives and negatives of global warming" I think this is the direct link:
https://skepticalscience.com/global-warming-positives-negatives-intermediate.htm
Under "Sea Level Rise" there is a link to "Dasgupta 2009" which leads to a "server not found" page, this is the LINK.
I think either of these links might be better:
- http://hdl.handle.net/10986/4095
- https://openknowledge.worldbank.org/handle/10986/4095
It seems that one negative of the Artic melt is missing.
The amount of fresh water released by the Artic melt is diluting the Gulf Stream and has started to impede its normal circulation
This will bring intense cold to Europe as warm water will not reach the continent in winter
Almost any scientist looking at a new idea views it with deep skepticism and doubts it, and that skepticism is only overcome by a consistent preponderance of the evidence that keeps supporting the idea that that might be important - that global climate change driven by humans might actually be occurring. As that evidence has been accumulated, skeptic after skeptic among the scientists has decided, "Well, I'd better pay more attention to this." The physics of this is much more well understood. The models that incorporate all of our known aspects of physics and atmospheric chemistry and climatology and so on, all predict that what we're doing is going to lead to climate change. All these bits of evidence keep falling into place. They all keep saying, "Gee, we'd better pay more attention to this global climate change idea," because when we look at some data that maybe would have rejected it, it doesn't. It supports that idea. I guess what I would say is that the idea is so real now. There have been so many attempts to test it, so many attempts to reject the idea that we might be causing climate change which has not been successful, which keep supporting that hypothesis. I think it is now incumbent upon us to take it seriously and to do things to help slow the rate of climate change and hopefully stop it. If we find out in the long-term that climate change is not going to happen, we won't have done much to harm ourselves. But if we don't act now, we could have a runaway climate change that could basically greatly decrease the livability of the earth. The science is now solid enough that any reasonable person examining the scientific evidence would decide, "We have to pay attention to it. It's time to have some action."
You could add a link to this one -
https://www.tandfonline.com/doi/full/10.1080/14747731.2020.1807856
An economist's takedown of an economist's view of climate. Highly recommend.
[BL] Link activated.
The web software here does not automatically create links. You can do this when posting a comment by selecting the "insert" tab, selecting the text you want to use for the link, and clicking on the icon that looks like a chain link. Add the URL in the dialog box.
Hi Skeptical Science Team,
I am not trying to come across as a pessimist, in fact I am very jolly happy optimist. (it’s my nature).
However, looking around at all the attitudes that lack an understanding of how urgent our climate situation is, and the fact that not much has been done to ween the globe from fossil fuel burning, I can’t help but to think we are past the point of being able to curb the future disasters that are coming our way due to climate change. Those disasters are here and now already.
We are now living on a planet that is in the beginning stages of driving humans, as well as other animal species towards extinction. We’ve already caused extensive death and destruction. Over 50 percent of the world's coral reefs have died in the last 30 years and up to 90 percent may die within the next century. The rate of normal background extinction is hundreds, or even thousands of times higher than the natural baseline rate.
Also, since CO2 lingers around in the atmosphere for 1000’s of years and we are continuing to pump ~50 Billion tonnes of carbon dioxide equivalents a year into the atmosphere, how on earth are we going to prevent the creation of a runaway greenhouse effect?
Am I alone in thinking we’ve reached a point of no return?
TVC15 @390 :-
Permit me some general waffle : my comment is that you should be half'n'half ~ half optimist, half pessimist. The global situation is going to get bad but not catastrophic. Yes, we are going to blow straight past the 1.5 degree mark in global surface temperature rise since pre-industrial. The rise already (over 170 years) is about 1.1 degrees, and this makes a mockery of any contrarian who opines that the CO2-doubling Climate Sensitivity is less than roughly 2.0 degrees (and seems most likely to be in the 2.5 to 3.0 degree range at equilibrium ~ which also fits with non-historic data e.g. the paleo data).
With extraordinarily good management, we might conceivably halt the rise by 2.0 degrees . . . but our political track record so far is poor.
It is not just the politics, but technological advances which are still required. Sure : cheaper solar & wind technologies are coming, but we really should have started seriously developing these at least 10 years before we actually did. (But the past cannot be changed.)
The 2050 date for "carbon neutral" will require more than the present-day solar & wind, even at half of today's prices. Energy storage is absolutely necessary ~ and I am looking to bulk storage of electrolytic hydrogen. Hydrogen to provide electricity via fuel cells (at small scale) or steam-driven turbines at large scale (possibly combined-cycle?).
The second leg to stand on, is a hugely-increased supply of liquid hydrocarbon [octane / kerosene / diesel types] produced from non-fossil feedstocks, by means of catalytic / enzymatic / fermentational technology. In brief, we need to produce these hydrocarbons at a scale little short of present-day fossil fuel consumption. For a great amount of our energy usage, these hydrocarbons are necessary ~ and I suspect it will take many decades after 2050 , before we could replace such hydrocarbon fuels.
Extinction of a very large slice of animal/plant species . . . is arguable. Extinction of the human race ~ certainly impossible. The casualty rate may be high in the future ~ but extinction, no way. Mass migration of "climate refugees" will increase as sea level rise and heat waves occur, and there will be major social disruption. "Interesting Times" , as the old Chinese saying goes.
I'm sort of where Eclectic is, too.
There is an old saying: an optimist believes that we live in the best of all possible worlds; a pessimist fears that this is true.
Even if we can't stop the car before it hits the wall, slowing it down helps. And in the case of climate change, trying to stop at 1.5 or 2C but failing will still help us avoid 3 or 4 later on. And even if we fail to stop the bad consequences, knowing what they are going to be helps us prepare for them.
Anyone that thinks "it can't possibly get worse" is fooling themselves - there is always some creative genius out there who can find a way to make it worse.
While I wish stronger actions were being taken now, it is way better than it was ten years ago. I remember thnking "how can we get people to build out wind and solar when they are 5 times more expensive than fossil fuels?" Now renewable energy is the cheapest so they are building out renewables in Texas!! No greenies behind those wind turbines and solar farms, they are building them because it is cheapest.
Carbon Brief reports many coal plants planned 5-10 years ago that were not built yet are being cancelled. That is not because they are worried about climate change, it is because renewables are cheaper.
I am worried about the future, but it looks way better to me than it did 10 years ago.
@393
Michael,
That's a very good point. I recently heard a denialist argument talking about how the models were wrong. The interesting thing about his argument was he used the "worst case do nothing" projection and then tried to claim, "See? The models were exagerated!"
Ironically the reason we are no longer on that old worst case scenario projection is because actions were taken! When the proper scenanio was selected and projected, the models proved to be almost uncannily accurate!
"it's not that bad." only because we are making the changes needed. Maybe not enough. But we are moving in that direction!
Potholer put out a great video about that 3 years ago! Projections still look bleak. We can do much more. But it is not as bad as it could have been.
Red Baron @394 ,
There's no hope for direct persuasion of the sort of denialist who comes out with the kind of "Models are All Wrong" argument that you mention. The deniosphere is full of that deliberate mental blindness.
But things are hopeful in other directions. Denialists are in some alarm about the current European Union moves to establish (more extensively) what is in effect a Carbon Tax on imports. This must cast a shadow into the future ~ a shadow influencing the investment choices of financiers & large corporations, and tilting their future decisions away from coal etc.
And in the USA, the new Biden Administration - however temporary it may be - must be having a chilling effect on the commercial advocates of future fossil-fuel developments.
Red Baron, your own interest in carbon-fixing (in the soil) does also add the possibility of lowering CO2, by a small but ultimately significant amount. We shall see !
And for the persuadable Middle Americans who like to drive "gas-guzzling pick-up trucks" , it seems the Ford F150 Lightning (due 2022) will have both the traditional looks & the carrying/hauling capabilities they fancy. An all-battery truck, with adequate range and muscular performance ~ quite a breath-taking acceleration, in fact. Forget the wimpy Prius and nerdy Tesla cars, because this "truck" will soften the gasoline addiction of even the average red-blooded right-winger.
I have a friend who likes muscle cars and owns a high power Mustang. I saw an ad for an electric car that beat a bunch of muscle cars off the line in a drag race. I asked my friend and he said the electric car would beat his car for 1/8 of a mile (by a lot) but that after they got up to 100 mph his car had better acceleration and would win the 1/4 mile. I guess if you want to go 150 on the street (this friend does) you need a more powerfull electric car.
Michael Sweet ~ LOL, in a Mustang maybe : but I wouldn't like to drive the average country road at 150mph in a pick-up truck. The Ford F150 is heavy, but has 4WD electric motors of total 420 horsepower (in lower spec.!!) Not too shabby. The battery has a 9 Kw outlet for portable power tools, or for powering your house during a blackout. Due late 2022, so not far away.
My point is, that many Americans will find this type of electric vehicle attractive. And this will shed a halo glow onto other "electric stuff" . . . and, I hope . . . soften some of the pro-fossil attitudes, and this will spill over into the more political arena.
Eclectic:
We agree. At one time people complained that gas cars would never replace horses. Once electric cars are established and charging stations are built everywhere everyone will accept them. Then in the future people will wonder why we put up with the air pollution killing so many people for so long.
TVC15 @351,
To throw in my own ten pen'orth.
If anyone says “those disasters are here and now already,” it's a bit of an end to the discussion. I think your intended message is that the "disasters" are starting to happen. But as the analogy set out by Bob Loblaw @353 runs "if we can't stop the car before it hits the wall, slowing it down helps." So we are seeing disasters. We are in the process of hitting the wall. It is a very slow process. Things will get a lot worse before we come to a stop. But as we crash into the wall, it will help greatly if we take our foot off the accelerator and slam harder onto the brake.
As for where we will end up, as Eclectic @352 says, the idea that humanity could drive itself to extinction through AGW is not on the cards.
If we fail to wean ourselves off FF where will we be? As michael sweet @354 suggests we are seemingly weaning ourselves off coal. That is significant. The +4ºC-and-higher 'outcomes' turn to +3ºC 'outcomes' (and higher) if we stop burning coal. I add the 'and higher' as +3ºC 'outcomes' also greatly increase the uncertainty relative to +2ºC 'outcomes'. Yet even +2ºC 'outcomes' will have bad consequences such as the melt-down of Greenland if we stay at +2ºC for too long. So we shouldn't lose the limit of +1.5ºC if we don't want 7m SLR in the next millennia or so.
And if we fail to buck up our game, still buried within the +3ºC (and higher) is the potential for our nation states to start tussling over real estate and resources and 'devil take the hindmost', thus the breakdown of the civilised world order. How bad a breakdown? There are 193 nation states in the world. So pick a number between 193 and zero.
In terms of climate, I don't think we have reached a point where we cannot with tomorrow's technologies reverse the climate forcings that is causing AGW. But that doesn't mean we reverse all the consequences. If we reverse the climate forcings there will already be irreversable events, certainly within the biosphere. There are thus many 'points of no return' we are creating but the idea of a 'runaway GH-effect' that would turn us into a carbon-copy of Venus; that is not going to happen.
The Basic Edition of this rebuttal says "deaths attributable to heatwaves are expected to be approximately five times as great as winter deaths prevented" (and oddly, only the Basic version says this).
What is the source of this claim?