Hothouse Earth: Here’s What the Science Actually Does – and Doesn’t – Say
By Richard Betts
A new scientific paper proposing a scenario of unstoppable climate change has gone viral, thanks to its evocative description of a "Hothouse Earth."
Much of the media coverage suggests that we face an imminent and unavoidable extreme climate catastrophe. But as a climate scientist who has carried out similar research myself, I am aware that this latest work is a lot more nuanced than the headlines imply. So what does the hothouse paper actually say, and how did the authors draw their conclusions?
First, it's important to note that the paper is a "perspective" piece—an essay based on knowledge of the scientific literature, rather than new modeling or data analysis. Leading Earth System scientist Will Steffen and his 15 co-authors draw on a diverse set of literature to paint a picture of how a chain of self-reinforcing changes might potentially be initiated, eventually leading to very large climate warming and sea level rise.
One example would be the thawing of Arctic permafrost, which releases methane into the atmosphere. As methane is a greenhouse gas, this means the Earth retains more heat, causing more permafrost to thaw, and so on. Other possible self-reinforcing processes include the large-scale die-back of forests, the melting of sea ice, or the loss of ice sheets on land.
Global map of potential tipping cascades, with arrows showing potential interactions. Steffen et al / PNAS
Hothouse or Stabilized?
Steffen and colleagues introduce the term "Hothouse Earth" to emphasize that these extreme conditions would be outside those that have occurred over the past few hundred thousand years, which have been cycles of ice ages with milder periods in between. They also present an alternative scenario of a "Stabilized Earth" where these changes are not triggered, and the climate remains similar to now.
The authors make the case that there is a level of global warming which is a critical threshold between these two scenarios. Beyond this point, the Earth System might conceivably become set on a pathway that makes the extreme "hothouse" conditions inevitable in the long term. They argue—or perhaps speculate—that the process of irreversible self-reinforcing changes could in theory start at levels of global warming as low as 2°C above pre-industrial levels, which could be reached around the middle of this century (we are already at around 1°C). They also acknowledge large uncertainty in this estimate, and say that it represents a "risk averse approach".
A key point is that, even if the self-perpetuating changes do begin within a few decades, the process would take a long time to fully kick in—centuries or millennia.
Steffen and colleagues support their suggestion of a threshold at 2°C through reference to previously-published scientific work. These include other review papers which themselves drew on wider literature, and an "expert elicitation" study in which scientists were asked to estimate the levels of global warming at which "tipping points" for these key climate processes might be passed (I was one of those consulted).
The authors argue that 2°C can still be avoided if humanity takes concerted action to reduce its warming effect on the climate. In a similar way that the "Hothouse Earth" scenario involves huge changes in the climate system with multiple effects of one process leading to another, the concerted global action to avoid 2°C would, they suggest, also involve huge changes in the human system, again with several fundamental steps leading from one change to another.
Don't Ignore the Caveats
Personally, I found this an interesting and important think piece that was well worth reading. But since this is not actually new research, why is it getting so much coverage? I suspect that one reason is the use of the vivid "Hothouse Earth" term at a time when everyone's talking about heatwaves. Another is that it's clearly a dramatic narrative, and not surprisingly this has led to some sensationalist articles.
With some exceptions, much of the highest-profile coverage of the essay presents the scenario as definite and imminent. The impression is given that 2°C is a definite "point of no return", and that beyond that the "hothouse" scenario will rapidly arrive. Many articles ignore the caveats that the 2°C threshold is extremely uncertain, and that even if it were correct, the extreme conditions would not occur for centuries or millennia.
Some articles do however emphasise the more tentative nature of the work, and some push back against this overselling of the doomsday scenario, arguing that provoking fear or despair is counterproductive.
One thing that strikes me about the scientific literature on "tipping points" is that there are a lot of review papers like this that end up citing the same studies and each other – indeed, my colleagues and I wrote one a while ago. There is a great deal of interesting, insightful research going on using theoretical methods and calculations with large approximations. However, we have yet to see an equivalent level of research in the highly-complex Earth System Models which generate the kind of detailed climate projections used for addressing policy-relevant questions by the Intergovernmental Panel on Climate Change (IPCC).
Steffen and colleagues have made a good start at addressing such questions, going as far as they can on the basis of the existing literature, but their essay should motivate new research to help narrow down the huge uncertainties. This will help us see better whether "Hothouse Earth" is our destiny, or mere speculation. In the meantime, awareness of the risks—however tentative—can still help us decide how to manage our impact on the global climate.
Reposted with permission from our media associate The Conversation.
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By Bob Jacobs
Hanako, a female Asian elephant, lived in a tiny concrete enclosure at Japan's Inokashira Park Zoo for more than 60 years, often in chains, with no stimulation. In the wild, elephants live in herds, with close family ties. Hanako was solitary for the last decade of her life.
Hanako, an Asian elephant kept at Japan's Inokashira Park Zoo; and Kiska, an orca that lives at Marineland Canada. One image depicts Kiska's damaged teeth. Elephants in Japan (left image), Ontario Captive Animal Watch (right image), CC BY-ND
Affecting Health and Altering Behavior<p>It is easy to observe the overall health and psychological consequences of life in captivity for these animals. Many captive elephants suffer from arthritis, obesity or skin problems. Both <a href="https://doi.org/10.11609/JoTT.o2620.1826-36" target="_blank">elephants</a> and orcas often have severe dental problems. Captive orcas are plagued by <a href="https://doi.org/10.1016/j.jveb.2019.05.005" target="_blank">pneumonia, kidney disease, gastrointestinal illnesses and infections</a>.</p><p>Many animals <a href="https://doi.org/10.1016/j.neubiorev.2017.09.010" target="_blank">try to cope</a> with captivity by adopting abnormal behaviors. Some develop "<a href="https://doi.org/10.1016/j.applanim.2017.05.003" target="_blank" rel="noopener noreferrer">stereotypies</a>," which are repetitive, purposeless habits such as constantly bobbing their heads, swaying incessantly or chewing on the bars of their cages. Others, especially big cats, pace their enclosures. Elephants rub or break their tusks.</p>
Changing Brain Structure<p>Neuroscientific research indicates that living in an impoverished, stressful captive environment <a href="https://doi.org/10.1016/j.jveb.2019.05.005" target="_blank" rel="noopener noreferrer">physically damages the brain</a>. These changes have been documented in many <a href="https://doi.org/10.1002/cne.903270108" target="_blank" rel="noopener noreferrer">species</a>, including rodents, rabbits, cats and <a href="https://doi.org/10.1006/nimg.2001.0917" target="_blank" rel="noopener noreferrer">humans</a>.</p><p>Although researchers have directly studied some animal brains, most of what we know comes from observing animal behavior, analyzing stress hormone levels in the blood and applying knowledge gained from a half-century of neuroscience research. Laboratory research also suggests that mammals in a zoo or aquarium have compromised brain function.</p>
This illustration shows differences in the brain's cerebral cortex in animals held in impoverished (captive) and enriched (natural) environments. Impoverishment results in thinning of the cortex, a decreased blood supply, less support for neurons and decreased connectivity among neurons. Arnold B. Scheibel, CC BY-ND<p>Subsisting in confined, barren quarters that lack intellectual stimulation or appropriate social contact seems to <a href="https://doi.org/10.1590/S0001-37652001000200006" target="_blank" rel="noopener noreferrer">thin the cerebral cortex</a> – the part of the brain involved in voluntary movement and higher cognitive function, including memory, planning and decision-making.</p><p>There are other consequences. Capillaries shrink, depriving the brain of the oxygen-rich blood it needs to survive. Neurons become smaller, and their dendrites – the branches that form connections with other neurons – become less complex, impairing communication within the brain. As a result, the cortical neurons in captive animals <a href="https://doi.org/10.1002/cne.901230110" target="_blank">process information less efficiently</a> than those living in <a href="https://doi.org/10.1002/dev.420020208" target="_blank">enriched, more natural environments</a>.</p>
An actual cortical neuron in a wild African elephant living in its natural habitat compared with a hypothesized cortical neuron from a captive elephant. Bob Jacobs, CC BY-ND<p>Brain health is also affected by living in small quarters that <a href="https://doi.org/10.3233/BPL-160040" target="_blank">don't allow for needed exercise</a>. Physical activity increases the flow of blood to the brain, which requires large amounts of oxygen. Exercise increases the production of new connections and <a href="http://dx.doi.org/10.1126/science.aaw2622" target="_blank">enhances cognitive abilities</a>.</p><p>In their native habits these animals must move to survive, covering great distances to forage or find a mate. Elephants typically travel anywhere from <a href="https://www.elephantsforafrica.org/elephant-facts/#:%7E:text=How%20far%20do%20elephants%20walk,km%20on%20a%20daily%20basis." target="_blank">15 to 120 miles per day</a>. In a zoo, they average <a href="https://doi.org/10.1371/journal.pone.0150331" target="_blank" rel="noopener noreferrer">three miles daily</a>, often walking back and forth in small enclosures. One free orca studied in Canada swam <a href="https://doi.org/10.1007/s00300-010-0958-x" target="_blank" rel="noopener noreferrer">up to 156 miles a day</a>; meanwhile, an average orca tank is about 10,000 times smaller than its <a href="https://www.cascadiaresearch.org/projects/killer-whales/using-dtags-study-acoustics-and-behavior-southern" target="_blank" rel="noopener noreferrer">natural home range</a>.</p>
Disrupting Brain Chemistry and Killing Cells<p>Living in enclosures that restrict or prevent normal behavior creates chronic frustration and boredom. In the wild, an animal's stress-response system helps it escape from danger. But captivity traps animals with <a href="https://doi.org/10.1073/pnas.1215502109" target="_blank">almost no control</a> over their environment.</p><p>These situations foster <a href="https://doi.org/10.1037/rev0000033" target="_blank">learned helplessness</a>, negatively impacting the <a href="https://doi.org/10.1155/2016/6391686" target="_blank" rel="noopener noreferrer">hippocampus</a>, which handles memory functions, and the <a href="https://doi.org/10.1016/j.neuropharm.2011.02.024" target="_blank" rel="noopener noreferrer">amygdala</a>, which processes emotions. Prolonged stress <a href="https://doi.org/10.3109/10253899609001092" target="_blank" rel="noopener noreferrer">elevates stress hormones</a> and <a href="https://doi.org/10.1523/JNEUROSCI.10-09-02897.1990" target="_blank" rel="noopener noreferrer">damages or even kills neurons</a> in both brain regions. It also disrupts the <a href="https://doi.org/10.1016/j.neubiorev.2005.03.021" target="_blank" rel="noopener noreferrer">delicate balance of serotonin</a>, a neurotransmitter that stabilizes mood, among other functions.</p><p>In humans, <a href="https://doi.org/10.1006/nimg.2001.0917" target="_blank" rel="noopener noreferrer">deprivation</a> can trigger <a href="https://doi.org/10.3389/fnins.2018.00367" target="_blank" rel="noopener noreferrer">psychiatric issues</a>, including depression, anxiety, <a href="https://doi.org/10.3389/fnins.2018.00367" target="_blank" rel="noopener noreferrer">mood disorders</a> or <a href="https://doi.org/10.1177/1073858409333072" target="_blank" rel="noopener noreferrer">post-traumatic stress disorder</a>. <a href="https://doi.org/10.1007/s00429-010-0288-3" target="_blank" rel="noopener noreferrer">Elephants</a>, <a href="https://doi.org/10.1371/journal.pbio.0050139" target="_blank" rel="noopener noreferrer">orcas</a> and other animals with large brains are likely to react in similar ways to life in a severely stressful environment.</p>
Damaged Wiring<p>Captivity can damage the brain's complex circuitry, including the basal ganglia. This group of neurons communicates with the cerebral cortex along two networks: a direct pathway that enhances movement and behavior, and an indirect pathway that inhibits them.</p><p>The repetitive, <a href="http://dx.doi.org/10.1016/j.bbr.2014.05.057" target="_blank">stereotypic behaviors</a> that many animals adopt in captivity are caused by an imbalance of two neurotransmitters, dopamine and <a href="https://doi.org/10.1016/j.neubiorev.2010.02.004" target="_blank" rel="noopener noreferrer">serotonin</a>. This impairs the indirect pathway's ability to modulate movement, a condition documented in species from chickens, cows, sheep and horses to primates and big cats.</p>
The cerebral cortex, hippocampus and amygdala are physically altered by captivity, along with brain circuitry that involves the basal ganglia. Bob Jacobs, CC BY-ND<p>Evolution has constructed animal brains to be exquisitely responsive to their environment. Those reactions can affect neural function by <a href="https://www.penguinrandomhouse.com/books/311787/behave-by-robert-m-sapolsky/" target="_blank">turning different genes on or off</a>. Living in inappropriate or abusive circumstance alters biochemical processes: It disrupts the synthesis of proteins that build connections between brain cells and the neurotransmitters that facilitate communication among them.</p><p>There is strong evidence that <a href="https://doi.org/10.1523/JNEUROSCI.0577-11.2011" target="_blank">enrichment</a>, social contact and appropriate space in more natural habitats are <a href="https://doi.org/10.1111/j.1748-1090.2003.tb02071.x" target="_blank" rel="noopener noreferrer">necessary</a> for long-lived animals with large brains such as <a href="https://doi.org/10.1371/journal.pone.0152490" target="_blank" rel="noopener noreferrer">elephants</a> and <a href="https://doi.org/10.1080/13880292.2017.1309858" target="_blank" rel="noopener noreferrer">cetaceans</a>. Better conditions <a href="https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5543669/" target="_blank" rel="noopener noreferrer">reduce disturbing sterotypical behaviors</a>, improve connections in the brain, and <a href="https://doi.org/10.1038/cdd.2009.193" target="_blank" rel="noopener noreferrer">trigger neurochemical changes</a> that enhance learning and memory.</p>
The Captivity Question<p>Some people defend keeping animals in captivity, arguing that it helps conserve endangered species or offers educational benefits for <a href="http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.574.3479&rep=rep1&type=pdf" target="_blank" rel="noopener noreferrer">visitors to zoos and aquariums</a>. These justifications are questionable, particularly for <a href="https://animalstudiesrepository.org/acwp_zoae/8/" target="_blank" rel="noopener noreferrer">large mammals</a>. As my own research and work by many other scientists shows, caging large mammals and putting them on display is undeniably cruel from a neural perspective. It causes brain damage.</p><p>Public perceptions of captivity are slowly changing, as shown by the reaction to the documentary <a href="https://en.wikipedia.org/wiki/Blackfish_(film)" target="_blank" rel="noopener noreferrer">"Blackfish</a>." For animals that cannot be free, there are well-designed sanctuaries. Several already exist for elephants and other large mammals in <a href="https://www.elephants.com/" target="_blank" rel="noopener noreferrer">Tennessee</a>, <a href="https://globalelephants.org/overview/" target="_blank" rel="noopener noreferrer">Brazil</a> and Northern <a href="http://www.pawsweb.org/about_our_sanctuaries.html" target="_blank" rel="noopener noreferrer">California</a>. Others are being developed for large <a href="https://whalesanctuaryproject.org/" target="_blank" rel="noopener noreferrer">cetaceans</a>.</p><p>Perhaps it is not too late for Kiska.</p>
By Tara Lohan
Maybe we can blame COVID-19 for making it hard to hit the streets and gather signatures to get initiatives on state ballots. But this year there are markedly fewer environmental issues up for vote than in 2018.
While the number of initiatives may be down, there's no less at stake. Voters will still have to make decisions about wildlife, renewable energy, oil companies and future elections.
Here's the rundown of what's happening where.
Return of an Apex Predator<p>Wolves are on the ballot in Colorado. <a href="https://leg.colorado.gov/ballots/reintroduction-and-management-gray-wolves" target="_blank" rel="noopener noreferrer">Proposition 114</a> would require the state's Parks and Wildlife Commission to create a plan by 2023 for the reintroduction and management of gray wolves (<em>Canis lupus</em>) in areas west of the continental divide.</p><p>Gray wolves once roamed across the western United States but were mostly eradicated by the 1930s. Slowly efforts are being made to bring them back. The reintroduction of gray wolves to Yellowstone National Park in 1996 has been hailed as a <a href="https://www.theguardian.com/environment/2020/jan/25/yellowstone-wolf-project-25th-anniversary" target="_blank" rel="noopener noreferrer">rewilding success</a>.</p><p>"The argument is that by putting back in wolves — an apex predator that has evolved alongside their prey species — we're putting things back into ecological balance," University of Colorado Boulder ecology professor Joanna Lambert <a href="https://therevelator.org/wolf-reintroduction-colorado/" target="_blank">told <em>The Revelator</em></a> in a February interview about the science behind wolf reintroductions.</p><p>The Rocky Mountain Elk Foundation and the Colorado Farm Bureau are two of the top donors to the opposition groups.</p><p>The measure does include compensation for losses of livestock caused by gray wolves.</p><p>"What we're all hoping for is a landscape where we can coexist with the species that were originally here, but also acknowledging that humans need to make a living and that the costs of this initiative will be felt by some folks more than others," Lambert said.</p>
Confusion Over Clean Energy<p>In Nevada voters will take a second swing at a constitutional amendment to require that electric utilities source 50% of their electricity from renewables by 2030. Voters passed the same measure, <a href="https://www.nvsos.gov/sos/home/showdocument?id=8826" target="_blank">Question 6</a>, in 2018, but state law requires that constitutional amendments be passed in two consecutive even-numbered election years.</p><p>More clean energy for the state may seem good. But there's concern that enshrining 50% renewables by 2030 in the state's constitution isn't that ambitious and it will make it harder to continue the push for 100% renewables in the future. To do that would be another constitutional amendment that would again take four years and two consecutive ballot wins to move the needle.</p><p>Also, the state is already on its way to the same renewable goal.</p><p>A legislative effort to achieve 50% renewables by 2030 — but with a slightly different timeline for the increments to get there — was signed into law in April 2019 by Nevada Gov. Steve Sisolak. Renewable advocates hope the state will do even better than that benchmark, but passing Question 6 would make it harder.</p>
Paying a Fair Share<p>If California's <a href="https://www.sos.ca.gov/elections/ballot-measures/qualified-ballot-measures/" target="_blank">Proposition 15</a> passes, commercial and industrial properties will need to start paying taxes based on their current market value, instead of paying based on the purchase price from decades prior (which stems from Proposition 13 passed back in 1978). The initiative would exempt agricultural land, small businesses, renters and homeowners.</p><p>Reassessing the worth of large commercial properties could bring in between $7.5 billion and $12 billion a year that would go toward supporting local governments, school districts and community colleges.</p><p>Most of the <a href="https://ballotpedia.org/California_Proposition_15,_Tax_on_Commercial_and_Industrial_Properties_for_Education_and_Local_Government_Funding_Initiative_(2020)" target="_blank">opposition</a> has come from big business and anti-taxation groups.</p><p>The California Teachers Association Issues PAC is the biggest supporter of the effort, but a number of <a href="https://www.yes15.org/endorsers-environment" target="_blank" rel="noopener noreferrer">environmental groups</a> have also endorsed the measure, which would likely see oil companies and other big industrial polluters having to kick in more money.</p><p>"The oil industry has used Prop. 13 loopholes to evade tens of millions of dollars in property taxes," <a href="https://www.nrdc.org/experts/victoria-rome/nrdc-announces-support-californias-proposition-15" target="_blank" rel="noopener noreferrer">wrote Victoria Lome</a>, California legislative director for the Natural Resources Defense Council. "Companies like Chevron, Exxon, Phillips 66, Shell and Tosco are paying taxes based on assessments taken prior to 2000. Prop. 15 would end this hidden subsidy to dirty energy."</p><p>Oil companies could stand to lose in Alaska, too. Voters there will weigh in on <a href="https://ballotpedia.org/Alaska_Ballot_Measure_1,_North_Slope_Oil_Production_Tax_Increase_Initiative_(2020)" target="_blank" rel="noopener noreferrer">Ballot Measure 1</a>, which would increase taxes on big oil producers (those that have produced more than 400 million barrels overall or 40,000 barrels a day in the past year) operating in three established oil fields in the North Slope.</p>
Taking the Wind Out of the Sails of the Electoral College<p>Colorado's <a href="https://leg.colorado.gov/ballots/adopt-agreement-elect-us-president-national-popular-vote" target="_blank">Proposition 113</a> isn't about environmental issues directly but could cause big shifts in how presidential elections are run and what states and issues are considered important.</p><p>The initiative would add Colorado to the <a href="https://www.nationalpopularvote.com/written-explanation" target="_blank">National Popular Vote Interstate Compact</a>. That effort is aimed at ensuring the presidential candidate who wins the popular vote wins the election. It doesn't eliminate the Electoral College, but it saps its power.</p><p>The compact needs states representing at least 270 Electoral College votes to go into effect. It's currently at 196.</p><p>If Colorado's proposition is passed, and if the National Popular Vote Interstate Compact eventually gets enough votes to go into effect, then Colorado's nine electoral votes would go to the presidential candidate who wins the popular vote, not to the one who gets the most votes in Colorado.</p>
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