How environmental professionals acknowledge overpopulation – and then ignore it

We can have an environmentalism that ignores the fundamental causes of environmental problems, including lucrative careers treating overshoot’s many symptoms. Just not a successful environmentalism.

by Leon Kolankiewicz

Veteran population campaigners like me have long lamented the fact that at both the national and international scales, the environmental establishment (Big Green) and climate activists alike have for decades either avoided or disparaged the population issue out of some combination of cowardice, calculation, apathy, ignorance, inconvenience, ideology, political expedience, or hypocrisy.

Unfortunately this domain of deniers is not alone. I hate to say it, but most of my fellow environmental professionals – those who have the formal education and technical training to make a career out of managing or protecting the environment, in the public or the private sector – are pretty much in the same denialist or apathetic camp as the activists.

I have built a career as an environmental professional that now spans several decades, working both for government agencies charged with conservation and management of the environment and natural resources, and as a private contractor to those same agencies, analyzing the potential effects of their proposed projects on the environment. As a consultant to a dozen or more federal agencies, I have helped prepare scores of Environmental Impact Statements (EISs) and Environmental Assessments (EAs) under the National Environmental Policy Act (NEPA) in the United States. NEPA is sometimes referred to as the “Magna Carta” of American environmental laws. I like to call it the “Look Before You Leap” Act.

Many if not most of the projects (“proposed actions” in NEPA parlance) whose impacts I have helped analyze, predict, and disclose to the public and decision-makers are a direct consequence of human population size or growth. They run the gamut of notoriously environmentally-damaging actions: new water supply dams and reservoirs, flood control projects, coal-fired power plants, electricity transmission lines, wind farms, oil and gas drilling on public lands, new roads or road expansions, gas and water pipelines, even a uranium mine.

a) Bois d’Arc Creek, a tributary of the Red River, part of the border between east Texas and Oklahoma. Riparian hardwood forest is a biologically important, disappearing habitat in this region. Photo by Leon Kolankiewicz, as a consultant on the environmental impact statement for the project. (b) Bois d’Arc Lake, a 70-square kilometer reservoir authorized by the U.S. Army Corps of Engineers, which began to fill in 2021. The project was justified by expected future water demands based on projected enormous population growth in the Dallas — Ft Worth metroplex, one of the fastest-growing areas in the US. Photo by North Texas Municipal Water District.

The NEPA analyses I have prepared or managed, and many NEPA documents I have reviewed on behalf of agencies with jurisdiction over the decision in question, almost always clearly divulge the population connection. At the beginning of an EIS, in a mandatory section called the “Purpose and Need” of the proposed action, it’s virtually always made quite explicit that the increasing population of, say, water or electricity consumers (i.e., new residents, businesses, or industries) is driving the demand for a new environmentally damaging power plant, or dam and reservoir. Proposed state-of-the-art water or energy conservation and efficiency measures are simply not enough to offset the predicted increase in demand of electricity or water supply as a result of projected population growth.

And yet from what I have observed over the years, the environmental professionals themselves, those who draft these documents, have little or no interest in addressing the “root causes” of increasing human populations – whether from excessive fertility or immigration or both – which imposes an additional burden or “load” on environmental resources. Perhaps it’s the cynic in me, but I suspect that many in my profession acknowledge that it’s such perpetual growth that keeps them gainfully employed. After all, if there weren’t always new projects in the pipeline – to meet the ever-rising demands of more and more consumers – they (we) might be out of a job or even a career.

Some years back, I was an active member in a state-wide professional organization called the California Association of Environmental Professionals (AEP), whose aim was professional development and improving the quality and accuracy of environmental assessments. We were an eclectic, California-based organization of a thousand or more practicing wildlife and fisheries biologists, foresters, civil and environmental engineers, soil scientists, climatologists, archeologists and paleontologists, cultural resources specialists, environmental planners, geographers, geologists, cartographers and geographic information specialists, experts in such fields as air quality and noise modeling, sociologists and economists, and others of this ilk.

One year, I was president of the Orange County Chapter of AEP and helped organize our annual statewide conference in Newport Beach, in the very heart of the toney seaside community made famous or infamous by “Real Housewives of Orange County.” As keynote speaker, I lined up the distinguished MIT professor Henry W. Kendall, a co-founder and board chair of the Union of Concerned Scientists (UCS), as well as a Nobel Laureate in Physics.

On behalf of UCS, Professor Kendall had recently spearheaded the “World Scientists Warning to Humanity,” a hard-hitting manifesto on the plight of humanity and the environment, signed by some 1,700 of the leading scientists in the world, including the majority of the recipients of the Nobel Prize in the sciences (physics, chemistry, physiology or medicine).

The Warning, published in 1992, started with a declaration that I have since seen repeated many times: “Human beings and the natural world are on a collision course.”

It had a major section focused on population, which started out with this unflinching message:

The earth is finite. Its ability to absorb wastes and destructive effluent is finite. Its ability to provide food and energy is finite. Its ability to provide for growing numbers of people is finite. And we are fast approaching many of the earth’s limits. Current economic practices which damage the environment, in both developed and underdeveloped nations, cannot be continued without the risk that vital global systems will be damaged beyond repair.

Pressures resulting from unrestrained population growth put demands on the natural world that can overwhelm any efforts to achieve a sustainable future. If we are to halt the destruction of our environment, we must accept limits to that growth.

Henry’s keynote address to the AEP conference that year (1994) elaborated on many of these themes, and it was relatively well received. However, I also put together a couple of panel discussions, one of which was focused on population growth as an environmental stressor. I thought it was a timely topic, given California’s status as by far the most populous (and overpopulated) state in the U.S., one in which the number of residents had grown by seven million in the previous decade alone (from 20 to 27 million, a whopping 35% increase), and one in which the environmental impacts of such rampaging growth were blatant.

In 2000, California was home to approximately 35 million people, a 15 million increase over the past 30 years. It took about 100 years to reach 10 million, but since California has been adding 10 million people every 20 years
California’s population growth, 1850-2000. Source: California Legislative Analyst’s Office. 2000. Cal Facts: California’s Population Has Increased Dramatically Over Time.

The two panelists I brought to the table were recognized experts and activists on population growth, with decades of combined experience at the state, national, and international levels. I was anticipating a lively, well-attended session. Instead I got three attendees, out of several hundred in attendance at the conference. What a dud.

It was a slap in the face. But more importantly, it was a vivid illustration of the indifference of my fellow environmental professionals concerning the population issue and what could be done about it. The most charitable explanation is that environmental professionals, who make their living from predicting and then mitigating the impacts of unsustainable human population growth, believe that it is an exogenous, natural phenomenon wholly beyond their control, like the seasons or the tides. Or perhaps they see it an issue freighted by too many controversies, unpleasantness, and career riskiness.

I won’t elaborate on the uncharitable explanations.

Over my long career, this vivid experience stands out, but it is not a one-off. Unfortunately, it’s the norm. Yet as I stated at the outset, environmental professionals are not outliers when it comes to eschewing overpopulation, but all too typical. In fact, the environmental activist establishment is still worse.

Take the Union of Concerned Scientists (UCS) for example. Today, if you search online for their original (1992) World Scientists Warning to Humanity, you can still find it at the UCS website. But before you ever read the message, you are treated to this woke disclaimer:

UCS is maintaining this page as part of our history. However, we understand that elements of this letter are deeply problematic. Specifically, centering population—with only a cursory nod to the consumption of wealthy nations and the wealthiest people—is a narrative rooted in colonialism and racism, and current-day unjust and inequitable socioeconomic systems.

I used to give UCS-approved talks on climate change, back in the 1990s. Politically correct posturing like this is why I dissociated myself from UCS a quarter-century ago.

I can’t dissociate myself from my chosen career as an environmental professional. But I can lament its refusal to be part of the solution when it comes to overpopulation, rather than merely a passive observer and data collector. More broadly, this unwillingness or apathy when it comes to facing facts and actively engaging with their implications, is symptomatic of humanity’s collective state of denial when it comes to recognizing and responding to limits to growth.


Leon Kolankiewicz is a population activist and veteran environmental scientist and planner with professional experience in the government, non-profit, and consulting sectors. Leon helped write a comprehensive Environmental Impact Statement on US Immigration Policy published in 2016.

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24 thoughts on “How environmental professionals acknowledge overpopulation – and then ignore it

  1. RE the UCS disclaimer: Ok, consumption by rich countries is an issue too. As Phil Cafaro (and earlier Ehrlich and Holdren) pointed out, you multiply population times per capita consumption to calculate impact (or total consumption). Maybe the warning to humanity should have said more about that. But, given that most people want a decent standard of living, the numbers of poor people striving to get richer mining and clearing forests does matter. Rich countries could cut consumption quickly by, say, raising interest rates, raising bank reserve requirements, and a global carbon tax. Good luck with those. Family planning, on the other hand, has been, by comparison, an easier sell. Over half of world countries are below replacement fertility rates. Growth fell from 80 million to 67 million (covid played a small role). Family planning help from USAID and others played a big role in reducing family size under the leadership of governments of countries that saw overpopulation or population growth as problems. The “demographic dividend” is well established empirically and Africans from high fertility countries see it as a way to improve incomes. But the racist accusation is false, propaganda, serves a right wing, pro-growth agenda. Claiming policies to cut fertility rates are in support of “current-day unjust and inequitable socioeconomic systems” is false. What is racist is ignoring the plight of poor African women whose many children are stunted by malnutrition. Population growth makes labor cheap which increases profits. Paul Ehrlich, often accused of being a racist, organized sit-ins to integrate lunch counters–clearly an anti-racist. I’m proud to have spent a summer as a volunteer with SCLC during which I helped elect the first black candidates since Reconstruction in a rural Georgia county. I am an anti-racist. It really gets up my nose to see this lie that fertility transitions are racist repeated so often and so uncritically and so foolishly.

    1. Max,

      very important point that contra the usual story, it is easier to cut fertility rates than consumption rates. Most countries around the world have lower fertility rates and higher per capita consumption rates than they did 30 years ago.

    2. Max,

      Thanks for your thoughtful comments and observations, with which I couldn’t agree more. Among other things, you wrote: “What is racist is ignoring the plight of poor African women whose many children are stunted by malnutrition.” I was a Peace Corps Volunteer in Honduras in the late eighties, at a time when the total fertility rate in that impoverished country was about 5 and its population was increasing by more than 3% annually, on course to double every 20-25 years. The Catholic Church hierarchy in Honduras – from its archbishop on down – was outspoken and influential in its opposition to family planning and contraception. Yet it was self-evident that having large numbers of children helped keep families mired in desperate poverty.

      Even though Peace Corps brass would not touch population with a ten-foot pole, because of what a hot-button topic it was, I could see that such rapid growth coupled with poverty drove phenomena such as tropical rainforest deforestation and habitat destruction, water quality degradation, and the assault on the nation’s wildlife heritage from poaching, uncontrolled hunting, overharvest, overfishing, and habitat loss. Thus, I spoke out forcefully and often about the threat of overpopulation – to Honduras and Hondurans themselves – to hundreds if not thousands of Hondurans during my three years in the country. Never once was I smeared with the “racist” charge. That only happened after I returned to the United States.


      1. This is quite typical: there are a lot of things that are true about specific groups of people, that they know and will not disagree with, that you are officially not allowed to say simply because you are not one of them.

    3. These accusations of “racism” and “colonialism” (and sexism, etc) are so exhausting. The burden of proof should be on those making them; no one should be forced to prove they are not racist.

      (The irony, of course, is that when we talk about “population” we are just talking about people in general, under the assumption that numbers and consumption are what matters and not color/religion/culture etc. It doesn’t get any less racist than that. They are the ones putting race back into the discourse, as it so often happens lately with all these “anti racism” people)

      1. A good point! Our critics often. bring race into the discussion, then accuse us of doing so.

        In the academic world, one often sees pleas to pay more attention to race, to think more rigorously about it, etc. I think we would benefit more from a 25 or 30-year moratorium on all discussion of racial issues. Let the fog clear.

  2. An all too familiar story.

    But, sadly, it’s a whole other world worse than the environmental orgs refusing to face up to the fundamentals of growthism:
    All those carefully researched reports that the environmental professionals delivered, were only read and argued over by other professionals in their fields. EIPs to developers and planners, economists and politicians are just impediments, they have to hire ‘antienvironmentalists’, to knock down, and, even if they can’t produce an environmental argument against them, the politicians, who won’t have read any of it, and couldn’t understand it if they did, will just keep changing the rules to allow them to go on and do whatever they wanted all along, ‘for the sake of The Economy’, which is the only mantra they know, and forcing ‘Growth’ to continue exponentially for ever, is what they see as the only purpose of governments at all levels. The ignorant politicians are bought and paid for by the developers and industrialists and financier speculators, and in their parties, they are ruled by the ‘whips’, so the vast majority of them will have seen no point in reading any of the literature about any impending decision at all, because their lives just consist of getting elected and then doing what the whips tell them. There is no way for the public to force the parties to change their minds, and most of the parties are right wing and see environmentalists as enemies to be destroyed, just to show them whio’s the boss. :/ I switched from campaigning on environment, and writing ‘proofs of evidence’ nobody cared to read, to campaigning on population, to produce evidence that neither politicians nor environmentalists would read, to campaigning to try and change UK politics, till I found there was no way to make them follow even their own codes of conduct, as they are judge and jury in their own courts. There seemed to be more hope for campaigners in the US, who only had two parties to deal with, and had a written Constitution to fall back on, but by this time, the whole system had been captured by well thought out planning by the industrialists, using their inexhaustible wealth, and their immortal corporate being status, to carry out propaganda and infiltration projects over multiple human working lifespans. We are now approaching ‘check mate’ for complete state capture: if the media succeeds in gaslighting Trump back into power, the game is over, and Humanity (in its civilised form at least) is finished.

    Apologies for this all being in one block of text: the panel is not accepting any formatting commands from this phone. :/

    1. Yes indeed. We are contending with a many-headed juggernaut. I myself oscillate between guarded optimism that Homo sapiens will be forced to recognize if not embrace limits in time, and despair or resignation that we won’t before our civilization and much of the biosphere comes crashing down with us.

      As the late systems scientist and pioneering computer simulation modeler Donella Meadows (lead author of The Limits to Growth) once wrote about the late ecological economist Herman Daly: “…as Herman Daly says, we are about to be hit by the hammer of necessity, but we are cradled on the anvil of desirability. We have no choice but to conform.”

  3. I have also been heavily engaged in this issue some 30 years. I started with the Seattle Chapter of ZPG (Zero Population Growth). I was asked to join the board which I did. We were a very active group but around 2000 we saw some changes coming. A new director said we need money to promote our message (more people means more money need to get out messages). At first it was the wealthy (Ted Turner for one) who were asked to join and give. Then things changed and it was announced we were no longer going to talk about immigration but focus on this country, the country with the world’s biggest footprint. Most of the board were already at burn out stage and left but a few stayed. We were pilloried and branded as racists. A PBS production “To the contrary” with Bonnie Erbe had a discussion and it was said that an anonymous source (we knew it was a group of corporations) offered $100M to both ZPG and the Sierra Club to remove immigration from the population growth equation. It was done. ZPG claimed the issue was their negative sounding name and changed it to the Population Connection. They mostly advocated family planning. Unfortunately, I see the same issue with many groups as the Nature Conservancy group. Too much money is needed to just fill the basic needs and since overpopulation issue is too difficult to control without stepping on toes (or being sued) it is dropped from most conversations. On the plus side, this issue is becoming harder and harder to ignore and I’m finding, more and more reports mention population growth. Even the National Geographic, now owned by Disney, has made population growth (but never overpopulation) comments. Sometimes changing words can change messages. I find when talking about this subject and using the term, population demographics, more people are willing to listen.

    1. Thanks for those observations and memories, Jack. I too used to be a member of ZPG years ago (back in the early seventies, initially). But I left it behind in the nineties when it became too PC, coincidentally, about the same time that it changed its name from Zero Population Growth to Population Connection, whose initials are PC, which somehow seems fitting.

      Yes, words and terms matter on a controversial, loaded topic like population, and while we should avoid triggering folks (especially potential allies) unnecessarily, we should also avoid euphemisms whenever possible.

      I hope you’re right that the “issue is becoming harder and harder to ignore.”

      Best, Leon

  4. The corporates saw the 1960s environmental movement as a big threat to endless growth, and they have effectively neutered it. This is well documented by Kerryn Higgs and others.

    On the other hand, they’ve read the 21st century climate movement and its “net zero” as a wonderful camouflage for endless growth, and embraced it with gusto.

    1. I think you’re spot-on, Stable. And this was also revealed in the angry reaction on the part of many climate and renewable energy activists to the Michael Moore / Jeff Gibbs film “Planet of the Humans” a couple of years ago, with its critical take on the “renewable energy transition” to “save the climate.” The neolib mainstream — including corps, pols, foundations, and big enviro inc. — utterly rejects the ideas of ecological overshoot and limits to growth, because their political, economic, and cultural implications are too radical and scary.

      1. Agreed. This near-exclusive focus by environmental groups on climate change is not doing us any favors. It covers up the underlying problem : growth of an already excessive human presence on Earth

  5. We should have a job requirement for all politicians to attend regular meetings of a spiritual recovery program, to learn to be more honest & less selfish. It works for alcoholics & addicts,
    so it’ll work for anyone!
    Years of Graft Doomed 2 Dams in Libya, Leaving Thousands in Muddy Graves
    By Vivian Yee, Sept. 27, 2023, New York Times
    [ . . . ]
    Analysts say the prime minister has also paid to fend off the threat from Khalifa Hifter, the military strongman who controls eastern Libya and has tried repeatedly to seize the west, by giving him access to a share of Libya’s oil revenue. After years of east-west conflict, the two men have settled into an arrangement in which their power and profits are guaranteed, but basic services are lacking, analysts and activists say. “This kleptocratic boom, in east and west Libya alike, has led to the decay of infrastructure all across the country, including the Derna dams,” says a forthcoming report on corruption in Libya from The Sentry, a Washington-based nonprofit.

    Some infrastructure has been built. But the investments both administrations make tend to be in visible symbols of progress like roads or bridges that may not be needed but give them something to show Libyans — and help line the authorities’ pockets, said Walid Madi, an engineer who worked in Libyan infrastructure for years, now studying in Glasgow. “They’re not erecting or designing or deciding on new projects based on the country’s needs,” Mr. Madi said, “but based on how they can profit from these projects.”

    Many Libyans rely on their own generators and wells to compensate for regular power cuts and lack of running water. Even normal amounts of rain can flood Tripoli, the western capital, and other cities. Libya’s meteorological service, which issued warnings about the storm that hit Derna, but not about the dangers the dams posed, is chronically underfunded, leaving it short-staffed and unable to perform basic maintenance on equipment, Petteri Taalas, the secretary general of the World Meteorological Organization, said in an interview. In Derna, state auditors in Tripoli said in a 2021 report, more than $2.3 million allocated for maintaining the two dams was never used.
    A previous fund dedicated to rebuilding Derna after it was partly destroyed in post-2011 conflict ultimately resulted in few tangible improvements, analysts say, leading Libyans to accuse the officials who oversaw the fund of stealing it.

    Analysts and experts say the divided and dysfunctional governments also contributed to the chaotic early response to the floods. In the absence of a coordinated official response, Libyans from across the country rushed to Derna to help. “At a time when all aspects of Libyan society are working together,” said Mr. Dorda of Libya Desk, “Libya’s politicians are in their own reality, figuring out how they can divide the cake between themselves.”

    1. Well, can we ignore the SIERRA CLUB EXAMPLE? I served on their national population-issues subcommittee (working directly out of the director’s office) back when then director Carl Pope ordered us to print a bumper sticker, “The United States: The World’s Most Overpopulated Nation.”

      Then, a year or two out, the self-same Pope had put a gag order on local chapters, was calling esteemed members of the Club “racist” for their concerns about immigration and population, and all that in return, as revealed by THE LOS ANGELES TIMES, for a multimillion-dollar contribution linked to the Club remaining SILENT ON IMMIGRATION. You know? A bribe!

      Meanwhile, I remain shocked that so many people allegedly concerned about population fail to understand all the “racist” rhetoric on immigration stems out of CORPORATE-OWNED media and environmental groups eyeing the bottom line, and we’re talking MILLIONS!

  6. For what it’s worth, I’ve lost a few friends specifically for things I had said about mass migration. They stopped talking to me; one attacked me viciously in public.
    They live much more lavishly than me, but virtue-signalling, as it’s called now, makes them feel better about themselves and better than people such as myself. Perhaps most disheartening, of course beside the lack of concern for nature and willingness to cut friendships over this, is the disregard for logic. It should be a no-brainer that it’s impossible to let in all the people who want to come, in Europe especially but even in the US or Australia. We are talking about hundreds of millions, and more, if you consider the baked-in-the-cake demographic and climate projections from places such as Nigeria, Bangladesh, Egypt or Pakistan. If this is true, and there’s no way to argue it isn’t, then we should sit down and decide what to do about it. But we do not. I am very worried.

    1. Sorry to hear that, Gaia, but I can relate to it, because I too have experienced the same thing, as I suspect, have many who read this. I recall one former Chicana (Mexican-American) friend of mine in California: our congenial outings and conversations were replaced with scowls and silence as we passed each other in the hallway of the government office building of the environmental agency in which we both worked. I could tell that she had lost all respect for me. It was as if I now had a prominent red “R” stamped on my forehead that she had somehow missed before. The fact that I was actually married to and had a child with an immigrant Latina (whom I met as a Peace Corps Volunteer in Honduras) only seemed to make me more contemptible, perhaps a hypocrite, in her eyes.

      What is an acceptable, appropriate level of immigration for any given nation state — environmentally, ethically, economically, culturally, demographically — is one of the key international issues of our time — with enormous “downstream” or future consequences for both sending and receiving countries. But because the issue is so fraught and divisive, it seems all but impossible to address openly, dispassionately, and respectfully. I used to jokingly refer to the “I-word;” a veritable third rail.

      And it may well be that “we ain’t seen nuthin’ yet.”

      Certain projections by reputable climate scientists in eminent scientific journals contemplate not just potentially hundreds of millions but a billion or more climate refugees as the century unfolds and climate disruption proceeds. If those higher-end projections do come to pass, then all bets are off concerning any coordinated, effective response of the highp-emission, developed nations in the “Global North” to the challenges of reducing greenhouse gas emissions, climate adaptation, and sustainable, affordable energy alternatives.

      These nations will be too consumed with trying to maintain social cohesion and coping with the unprecedented influx of newcomers from a host of divergent, sometimes disparate, backgrounds. And BTW, these folks and their descendants, even if they do succeed economically and do attain the higher average standard of living of the countries in which they settle, will raise aggregate country GHG emissions, of course. (Barring the realization of “net zero” or “decoupling” fantasies.)

      I too am very worried.

      1. Interestingly, I’ve seen that some immigrants at least here in Italy are hostile to the new waves of migration, partly because they make them “look bad” (all migrants are lumped together in the eyes of natives, according to this view), partly because they feel they had it harder and had to make an effort to integrate, whereas people now are just sweeping in.
        I’m not saying it’s all of them, but I’ve heard it sometime.

        Europe is even less equipped to deal with this mass migration than the US and Canada: we are very easily accessible from both Africa and Asia and already severely overpopulated. We are also not used to this: the US is a country of immigrants (after having decimated the native population), but Europe is not, and does not seem to be prepared, for a number of reason, to deal with this huge challenge.

      2. Many thanks for sharing your article and experiences, Leon.

        Do you have 1-2 ref.s to support your comment, “projections by reputable climate scientists of … a billion or more climate refugees as the century unfolds”?

        For others who might be interested, I did find this 2017 article in the journal
        Land Use Policy: Impediments to inland resettlement under conditions of accelerated sea level rise” by Charles Geisler and Ben Currens.

        What other studies were you thinking of?

    2. I’ve had the same experience, unfortunately: losing friends over my views on immigration. Then again, I’m not sure someone who will cut you over policy differences is a real friend

      1. Funny enough, the only people who’ve never cancelled me are on the right. I’ve had self-professed “fascists” tell me things like “well, you have your idea, I have mine, it’s ok”, and supposedly open-minded people on the left not letting me near their children for something I had said (usually along the lines of what’s being said here). Like you say, it makes you wonder what people base their relationships and respect on.
        We live in a society in which what you say is most of the time more important than what you do, and that explains why we are stuck with so many problems we can’t solve, and environmental and inequality inaction especially.

  7. Davykydd asks: ‘Do you have 1-2 ref.s to support your comment, “projections by reputable climate scientists of … a billion or more climate refugees as the century unfolds”?’

    Leon: I could’ve sworn that in just the last couple of months or so, I glimpsed a 2023 paper making that prediction or claim in a peer-reviewed journal such as Science Advances or Nature Climate Change. However, I’ve been unable to find it. And it’s not the only one I’ve seen.

    “Climate refugees” of course, doesn’t necessarily mean international migrants or refugees; many of them would probably be internal refugees (remaining within their countries of origin).

    In the meantime, here’s a 2020 paper in the Proceedings of the National Academy of Sciences (PNAS), “Future of the human climate niche”:

    “We show that for thousands of years, humans have concentrated in a surprisingly narrow subset of Earth’s available climates, characterized by mean annual temperatures around ∼13 °C. This distribution likely reflects a human temperature niche related to fundamental constraints. We demonstrate that depending on scenarios of population growth and warming, over the coming 50 y, 1 to 3 billion people are projected to be left outside the climate conditions that have served humanity well over the past 6,000 y. Absent climate mitigation or migration, a substantial part of humanity will be exposed to mean annual temperatures warmer than nearly anywhere today.”

    And whether in the hundreds of millions of climate refugees or more, the world is facing dire prospects, as described in a 2018 paper by noted climatologist James Hansen:

    “Low latitudes will become hotter and less hospitable, if fossil fuel emissions and global warming continue unabated. Evidence of this trend is beginning to emerge already in tropical regions and in the subtropics.

    “Recent scientific research confirms what common sense suggests: violence and conflicts increase in regions that become hotter and less livable. Interpersonal violence and conflicts between groups and nations are both found to rise significantly with excessive temperature.

    “Pressures to emigrate from low latitudes will increase, if global temperature continues to increase. Continued global warming will also cause sea level to rise more rapidly, which will be another source of pressure to emigrate.

    “The world already has enough difficulty caring for refugees. If these potentially huge numbers of climate refugees should become reality, young people will inherit a planet in chaos.”

    –James Hansen, “Climate Change in a Nutshell: The Gathering Storm,” 18 Dec 2018

    1. (Sorry to be commenting on everything here)
      I remember reading about that 13 degrees ideal, and then finding out that’s exactly the mean temperature in my home region. I was then thinking about this and what it means to live in an fertile area with a mild climate and, presently, peace; or to be “lucky” and have something others may want and are coming for. It’s both a blessing and a curse, because what to you is special and worth protecting and the only home you have, for others is just an opportunity as good as any other. I’ve wondered if this is how some Native Americans felt when Europeans started pouring in demanding that they shared what they had (which they initially were happy to).
      To the Natives, it was home, history, family, and a sacred place; to Europeans, it was just land, a plot as good as any other. Or money, or gold.
      This is also true for nature, in a slightly different way: the most fertile and hospitable places are run over by people first.

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