TL; DR: Incêndios deixados para queimar sem controle em todo o mundo, armas nucleares detonando no Irã, Paquistão e China e campos de petróleo incendiados na Arábia Saudita, todos se combinam para encher a atmosfera superior com enormes quantidades de cinzas e outras partículas. Esse véu de poluição reflete uma quantidade significativa de luz solar - energia térmica, também conhecida como calor - de volta ao espaço antes que possa aquecer a terra. Como resultado, as temperaturas globais diminuem significativamente até o céu clarear e as coisas voltarem ao normal. Brooks não está ignorando a ciência das mudanças climáticas - ele está aplicando lições aprendidas com a observação real e modelagem computacional sofisticada.
No livro:
Incêndios mundanos e seu impacto no clima:
Obviamente, o fogo não deixou de existir quando os zumbis apareceram. No início da crise, batalhas intensas, acidentes de carro, pessoas fugindo de casas e empresas em pânico, etc, levariam a um número muito maior de incêndios do que o normal; logo depois de tudo ir para o inferno, a falta de bombeiros, a pressão da água nos sistemas de aspersão, etc., permitiria que esses incêndios se espalhassem sem controle.
Durante o resto da guerra, as pessoas foram forçadas a cair em chamas para cozinhar e aquecer (e ainda estavam atirando, queimando e explodindo zumbis e muitas outras coisas), de modo que casas e outros abrigos ainda estavam subindo chamas e outras estruturas junto com elas. Muitas pessoas - como a família que fugiu para o norte do Canadá - tentaram sobreviver no deserto, longe dos centros populacionais; isso também fez dos incêndios acidentais um problema. No total, o número de incêndios que cresceram fora de controle durante o resto da guerra teria sido um pouco menor do que no primeiro estágio, mas ainda maior do que o habitual.
Toda essa fuligem e cinza sobe para o ar e, antes que desça, parte atinge a atmosfera superior, onde permanece. Enquanto está lá em cima, está refletindo a luz solar - a principal fonte de energia térmica do planeta (isto é, calor) - de volta ao espaço antes que possa aquecer a superfície. O resultado final, é claro, é que o céu fica mais escuro, a temperatura cai, o inverno fica mais longo e mais frio e o verão fica mais curto e mais frio.
Aqui estão todas as menções que pude encontrar relacionadas a mudanças no clima e sua conexão com os incêndios:
It was not easy Living, however. These people, no matter what their pre-war occupation
or status, were initially put to work as field hands, twelve to fourteen hours a day,
growing vegetables in what had once been our state-run sugar plantations. At least the
climate was on their side. The temperature was dropping, the skies were darkening.
Mother Nature was kind to them. The guards, however, were not. "Be glad you're alive,"
they'd
shout after each slap or kick.
...
As with so many other conflicts, our greatest ally was General Winter. The biting cold,
lengthened and strengthened by the planet's darkened skies, gave us the time we needed
to prepare our homeland for liberation.
...
Now he was a chimney sweep.
Given that most homes in Seattle had lost their central heat and the winters were now
longer and colder, he was seldom idle. "I help keep my neighbors warm," he said proudly.
I know it sounds a little too Norman Rockwell, but I hear stories like that all the time.
...
What do they say the average temperature's dropped, ten degrees, fifteen in some areas?6 Yeah, we had it real easy, up to our ass in gray snow, knowing that for every five
Zacksicles you cracked there'd be at least as many up and at 'em at first thaw.
6.Figures on wartime weather patterns have yet to be officially determined.
...
They say eleven million people died that winter, and that's just in North America. That
doesn't count the other places: Greenland, Iceland, Scandinavia. I don't want to think
about Siberia, all those refugees from southern China, the ones from Japan who'd never
been outside a city, and all
those poor people from India. That was the first Gray Winter, when the filth in the sky
started changing the weather.They say that a part of that filth, I don't know how much,
was ash from human remains.
...
Even in winter, it's not like everything was safe and snuggly. I was in Army Group North.
At first I thought we were golden, you know. Six months out of the year, I wouldn't have
to see a live G, eight months actually, given what wartime weather was like. I thought,
hey, once the temp drops, we're little more than garbage men: find 'em, Lobo 'em, mark
'em for burial once the ground begins to thaw, no problem.
...
It's already starting, slowly but surely. Every day we get a few more registered accounts
with American banks, a few more private businesses opening up, a few more points on
the Dow. Kind of like the weather. [Now that the war is over] Every year the summer's a little longer, the skies a
little bluer. Its getting better. Just wait and see.
Assim, cidades, vilas abandonadas e, presumivelmente, algumas áreas não desenvolvidas foram consumidas pelo fogo no caos inicial e, à medida que a infraestrutura foi deixada em decomposição, outras conflagrações também ocorreram. O resultado imediato foram grandes quantidades de cinzas jogadas na atmosfera.
Armas nucleares e seu impacto no clima:
Como observado nos comentários, e na excelente resposta da ArgumentBargument, houve também um intercâmbio nuclear entre o Irã e o Paquistão, o que aumentaria a cinza e a fumaça produzidas pelos incêndios mais mundanos. Além disso, armas nucleares também foram usadas na guerra civil da China. Ainda mais cinzas e partículas foram lançadas nos céus.
Irã e Paquistão:
We created a beast, a nuclear monster that neither side could tame... Tehran, Islamabad,
Qom, Lahore, Bandar Abbas, Onnara, Emam Khomeyni, Faisalabad. No one knows how
many died in the blasts or would die when the radiation clouds began to spread over our
countries, over India, Southeast Asia, the Pacific, over America.
...
We heard after reports of the "limited nuclear exchange" between Iran and
Pakistan, and we marveled, morbidly, at how we had been so sure that either you, or the
Russians, would be the ones to turn the key. There were no reports from China, no illegal
or even official government broadcasts.
Guerra civil da China:
And you decided to end the fighting.
We were the only ones who could. Our land-based silos were overrun, our air force was
grounded, our two other missile boats had been caught still tied to the piers, watting for
orders like good sailors as the dead swarmed through their hatches. Commander Chen
informed us that we were the only nuclear asset left in the rebellion's arsenal. Every
second we delayed wasted a hundred more lives, a hundred more bullets that could be
thrown against the undead.
So you fired on your homeland, in order to save it.
One last burden to shoulder. The captain must have noticed me shaking the moment
before we launched. "My order," he declared, "my responsibility." The missile carried a
single, massive, multi-megaton warhead. It was a prototype warhead, designed to
penetrate the hardened surface of your NORAD facility in Cheyenne Mountain,
Colorado. Ironically, the Politburo's bunker had been designed to emulate Cheyenne
Mountain in almost every detail. As we prepared to get under way, Commander Chen
informed us that Xilinhot had taken a direct hit. As we slid beneath the surface, we heard
that the loyalist forces had surrendered and reunified
with the rebels to fight the real enemy
A sugestão é que o aumento repentino de material particulado no ar dos incêndios iniciais e as detonações nucleares no Paquistão, Irã e China refletissem tanta luz solar de volta ao espaço que as temperaturas caíram temporariamente em todo o mundo.
Todas as fontes de poluição atmosférica, tomadas em conjunto:
O efeito cumulativo da produção particulada de incêndios mundanos e explosões nucleares é mais claramente exposto na passagem seguinte, a partir do relato fornecido por um astronauta que passou grande parte da guerra a bordo da ISS:
There were so many fires, and I don't just mean
the buildings, or the forests, or even the oil rigs blazing out of control -bleeding Saudis
actually went ahead and did it8 - I mean the campfires as well, what had to be at least a
billion of them, tiny orange specks covering the Earth where electric lights had once
been. Every day, every night, it seemed like the whole planet was burning. We couldn't
even begin to calculate the ash count but we guesstimated it was equivalent to a low-grade
nuclear exchange between the United States and former Soviet Union, and that's
not including the actual nuclear exchange between Iran and Pakistan. We watched and
recorded those as well, the flashes and fires that gave me eye spots for days. Nuclear
autumn was already beginning to set in, the graybrown
shroud thickening each day.
It was like looking down on an alien planet, or on Earth during the last great mass
extinction. Eventually conventional optics became useless in the shroud, leaving us with
only thermal or radar sensors. Earth's natural face vanished behind a caricature of primary
colors.
8To this day, no one knows why the Saudi royal family ordered the ignition of their
kingdom's oil fields.
Todas as citações acima são de World War Z: Uma História Oral das Guerras dos Zumbis, por Max Brooks.
Na vida real:
Esta não é uma ideia tão estranha. Na verdade, isso já aconteceu antes - geralmente como resultado de atividade vulcânica maciça e, ocasionalmente, como consequência de meteoros, asteróides, etc. atingindo o planeta. E temos amplas evidências de que uma guerra nuclear - mesmo um conflito regional relativamente limitado - teria efeitos semelhantes.
Vulcões:
A erupção do Monte Tambora no 1815 causou uma queda tão significativa nas temperaturas globais que o 1816 agora é conhecido como "O ano sem verão".
Outro exemplo é a erupção de Krakatoa:
In the year following the 1883 Krakatoa eruption, average Northern Hemisphere summer temperatures fell by as much as 1.2 °C (2.2 °F). Weather patterns continued to be chaotic for years, and temperatures did not return to normal until 1888. The record rainfall that hit Southern California during the “water year” from July 1883 to June 1884 – Los Angeles received 38.18 inches (969.8 mm) and San Diego 25.97 inches (659.6 mm) – has been attributed to the Krakatoa eruption.
The Krakatoa eruption injected an unusually large amount of sulfur dioxide (SO2) gas high into the stratosphere, which was subsequently transported by high-level winds all over the planet. This led to a global increase in sulfuric acid (H2SO4) concentration in high-level cirrus clouds. The resulting increase in cloud reflectivity (or albedo) would reflect more incoming light from the sun than usual, and cool the entire planet until the suspended sulfur fell to the ground as acid precipitation.
- Wikipedia
Impactos do meteoro:
Também se pensa que grandes impactos de meteoros podem causar um resfriamento global semelhante - mas muito mais dramático.
A huge meteor collided with Earth 2.5 million years ago, causing a gigantic tsunami that swallowed everything in its path. But according to a new study, published in the Journal of Quaternary Science, the meteor impact may have had a much larger effect on the planet -- it may have ushered in the ice age.
"The tsunami alone would have been devastating enough in the short term, but all that material shot so high into the atmosphere could have been enough to dim the sun and dramatically reduce surface temperatures," Goff said. "Earth was already in a gradual cooling phase, so this might have been enough to rapidly accelerate and accentuate the process and kick start the Ice Ages."
- IFLS: How a Giant Meteor Caused the Ice Age
E:
The impact of a medium-sized asteroid would create a 9-mile-wide crater, clogging the atmosphere with large amounts of dust. If the asteroid struck somewhere outside an area like a desert with small amounts of vegetation, the impact would ignite multiple wildfires, sending soot into the protective ozone, says the Apex Tribune.
The dust and soot that would fill the atmosphere would reduce the amount of sunlight to reach the surface by 20 percent for the first two years and lead to a drop in precipitation of about 50 percent.
"This is due to the lost heating and the lost temperature, so we lose convection; we don't have as many [weather] fronts," Bardeen said.
- Weather.com: Earth Could Experience a Mini Ice Age If Hit By a Medium-Size Asteroid, New Study Claims
Guerra nuclear:
Modelos de computador sugerem que mesmo uma troca nuclear de escala relativamente pequena (envolvendo bombas 100 como a que foi lançada em Hiroshima) realmente causaria um resfriamento global significativo, embora temporário:
Even a regional nuclear war could spark "unprecedented" global cooling and reduce rainfall for years, according to U.S. government computer models.
Widespread famine and disease would likely follow, experts speculate.
To see what climate effects... a regional nuclear conflict might have, scientists from NASA and other institutions modeled a war involving a hundred Hiroshima-level bombs, each packing the equivalent of 15,000 tons of TNT—just 0.03 percent of the world's current nuclear arsenal.
The researchers predicted the resulting fires would kick up roughly five million metric tons of black carbon into the upper part of the troposphere, the lowest layer of the Earth's atmosphere.
In NASA climate models, this carbon then absorbed solar heat and, like a hot-air balloon, quickly lofted even higher, where the soot would take much longer to clear from the sky.
The global cooling caused by these high carbon clouds wouldn't be as catastrophic as a superpower-versus-superpower nuclear winter, but "the effects would still be regarded as leading to unprecedented climate change," research physical scientist Luke Oman said during a press briefing Friday at a meeting of the American Association for the Advancement of Science in Washington, D.C.
Earth is currently in a long-term warming trend. After a regional nuclear war, though, average global temperatures would drop by 2.25 degrees F (1.25 degrees C) for two to three years afterward, the models suggest.
At the extreme, the tropics, Europe, Asia, and Alaska would cool by 5.4 to 7.2 degrees F (3 to 4 degrees C), according to the models. Parts of the Arctic and Antarctic would actually warm a bit, due to shifted wind and ocean-circulation patterns, the researchers said.
After ten years, average global temperatures would still be 0.9 degree F (0.5 degree C) lower than before the nuclear war, the models predict.
- National Geographic: Small Nuclear War Could Reverse Global Warming for Years
Conclusão:
Na vida como na World War Z, se você repentinamente despejar enormes quantidades de material particulado na atmosfera superior, é provável que cause um resfriamento global de curto prazo, porque a poeira e as cinzas nos céus refletem o calor e a luz de volta ao espaço, o que impede o sol de aquecer o planeta . Em resumo, o planeta esfriou porque estava recebendo menos energia térmica do sol.
Esse é um mecanismo diferente do aquecimento global via gases do efeito estufa, porque os gases do efeito estufa retêm o calor, enquanto os céus escuros dos aumentos repentinos e maciços de material particulado atmosférico impedem que o calor entre na atmosfera.
A longo prazo, depois que o céu clarear e o clima se recuperar do frio temporário, imagino que as temperaturas retomariam os aumentos que vemos em nosso próprio mundo, devido às emissões de gases de efeito estufa antes da guerra, mas o aquecimento global atingia o pico e começaria a reverter muito mais rapidamente do que sugerem nossas projeções atuais. Isso ocorre porque a população global foi drasticamente reduzida - não sabemos o número exato de mortes, mas é certamente o bilhão de pessoas. O mundo do pós-guerra parece confiar menos em combustíveis fósseis e no comércio internacional; a indústria e o consumo ainda estão se recuperando anos 12 após o fim da guerra; os recursos são mais escassos do que antes da crise. Simplificando, com menos pessoas restantes no planeta, as pessoas não estão causando tanto dano quanto costumavam.
A vida é muito mais simples e mais sustentável no mundo do pós-guerra. Há menos demanda por produção de energia e bens de consumo. Danos à infraestrutura industrial significam menos produção. A crise econômica causada pela guerra significa que as pessoas não estão consumindo tantas coisas que causam emissões de gases de efeito estufa. Populações mais centralizadas e menos foco nas atividades de conveniência e lazer reduzem ainda mais o consumo. Como resultado, menos fábricas estão lançando fumaça; menos carros estão nas estradas; menos aviões estão nos céus; menos usinas estão queimando carvão, gás e petróleo; menos prédios estão sendo aquecidos com combustíveis fósseis; menos gado e outros animais estão sendo criados e consumidos; menos florestas estão sendo cortadas; e assim por diante. Nossa pegada coletiva de carbono é uma fração do que era antes da guerra.