I recently read World War Z, and one question nagged me throughout it- why did the Earth get colder during the Zombie apocalypse? The story suggested that most places were burning, which I thought would lead to a warmer planet, in many ways.
TL;DR: Fires left to burn unchecked around the world, nuclear weapons detonating in Iran, Pakistan, and China, and oil fields set ablaze in Saudi Arabia, all combine to fill the upper atmosphere with enormous amounts of ash and other particulate matter. This veil of pollution reflects a significant amount of sunlight - i.e., thermal energy, aka heat - back into space before it can warm the earth. As a result, global temperatures decline significantly until the skies clear and things return to normal. Brooks isn't ignoring the science of climate change - he's applying lessons learned from actual observation, and sophisticated computer modeling.
In the book:
Mundane fires and their impact on the climate:
Fire obviously didn't cease to exist when the zombies showed up. Early in the crisis, pitched battles, car crashes, people fleeing homes and businesses in panic, etc, would have led to a far higher number of fires than normal; shortly after everything went to hell, the lack of fire fighters, water pressure to sprinkler systems, etc, would allow these fires to spread unchecked.
Throughout the rest of the war, people were forced to fall back on open flames for cooking and heating (and they were still shooting, burning, and blowing up zombies and lots of other stuff), so houses and other shelters were still going up in flames and taking other structures along with them. Many people - like the family that fled to northern Canada - tried to survive in the wilderness, far from population centers; this made accidental wildfires a problem as well. Altogether, the number of fires that grew out of control during the rest of the war would have been a bit lower than in the first stage, but still higher than usual.
All that soot and ash goes up into the air, and before it comes back down, some of it reaches the upper atmosphere, where it lingers. While it is up there, it is reflecting sunlight - the planet's primary source of thermal energy (i.e., heat) - back into space before it can warm the surface. The ultimate result, of course, is that the skies grow darker, the temperature drops, winters grow longer and colder, and summers become shorter and cooler.
Here's every mention I could find related to changes in the climate, and it's connection to the fires:
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.
So abandoned cities, towns, and presumably some undeveloped areas were consumed by fire in the initial chaos, and as infrastructure was left to decay, other conflagrations broke out as well. The immediate result was large amounts of ash thrown into the atmosphere.
Nuclear weapons and their impact on the climate:
As noted in comments, and in ArgumentBargument's excellent answer, there was also a nuclear exchange between Iran and Pakistan, which would have added to the ash and smoke produced by the more mundane fires. On top of this, nuclear weapons were also used in China's civil war. Even more ash and particulate matter was thrust into the skies.
Iran and Pakistan:
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.
China's civil war:
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
The suggestion is that the sudden increase in airborne particulate matter from the initial fires, and the nuclear detonations in Pakistan, Iran, and China, reflected so much sunlight back out into space that temperatures dropped temporarily around the world.
All sources of airborne pollution taken as a whole:
The cumulative effect of the particulate output of mundane fires and nuclear explosions is most clearly laid out in the following passage, from the account provided by an astronaut who spent much of the war stranded aboard the 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.
8 To this day, no one knows why the Saudi royal family ordered the ignition of their kingdom's oil fields.
All of the quotes above are from World War Z: An Oral History of the Zombie Wars, by Max Brooks.
In real life:
This isn't such a strange idea. It has actually happened before - often as a result of massive volcanic activity, and occasionally as a consequence of meteors, asteroids, etc striking the planet. And we have ample evidence that a nuclear war - even a relatively limited, regional conflict - would have similar effects.
The eruption of Mount Tambora in 1815 caused such a significant drop in global temperatures that 1816 is now known as "The Year Without a Summer".
Another example is the eruption of 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.
It is also thought that large meteor impacts can cause similar - but much more dramatic - global cooling.
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
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
Computer models suggest that even a relatively small-scale nuclear exchange (involving 100 bombs like the one dropped on Hiroshima) would indeed cause significant, if temporary, global cooling:
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
In life as in World War Z, if you suddenly dump enormous amounts of particulate matter into the upper atmosphere, it is likely to cause short-term global cooling, because the dust and ash in the skies reflects heat and light back into space, which prevents the sun from warming the planet. In short, the planet cooled because it was receiving less thermal energy from the sun.
This is a different mechanism than global warming via greenhouse gases, because greenhouse gases hold on to heat, whereas the darkened skies from sudden, massive increases in atmospheric particulate matter prevent that heat from entering the atmosphere in the first place.
Long term, after the skies clear and the climate recovers from the temporary chill, I imagine that temperatures would resume the increases we see in our own world, due to pre-war greenhouse gas emissions, but global warming would peak and begin to reverse far more quickly than our current projections suggest. This is because the global population has been drastically reduced - we don't know the exact death toll, but it is certainly in the billions of people. The post-war world appears to rely less on fossil fuels and international trade; industry and consumption are still recovering 12 years after the war ended; resources are more scarce than before the crisis. Simply put, with fewer people left on the planet, people aren't causing as much damage as they used to.
Life is much simpler and more sustainable in the post-war world. There is less demand for energy production and consumer goods. Damage to industrial infrastructure means less production is possible. The economic downturn caused by the war means people aren't consuming as much of the things that cause greenhouse gas emissions. More centralized populations and less focus on convenience and leisure activities further reduce consumption. As a result, fewer factories are spewing smoke; fewer cars are on the roads; fewer airplanes are in the skies; fewer power plants are burning coal, gas, and oil; fewer buildings are being heated with fossil fuels; fewer cattle and other livestock are being raised and eaten; fewer forests are being logged; and so on. Our collective carbon footprint is a fraction of what it was before the war.
Pakistan and Iran had engaged in what seems to be all-out nuclear war. I don't have my copy of the book to get supporting quotes, but I would assume that this resulted in a nuclear winter. While I'd recommend reading the Wikipedia article, the synopsis is that when there are a large number of fires (not even necessarily fires caused by nuclear bombs, and there were clearly a lot of non-nuclear fires), the smoke particles produced remain in the atmosphere long enough to reflect a significant amount of heat from the sun, causing hemisphere-wide if not global temperature drops.
When I read the book, I didn't get the sense that the climate changed that much. Just that the breakdown of society made dealing with a "slightly colder then normal" winter much more difficult.
With no fuel delivery, mining, and infrastructure, it's not like you could get your normal delivery of propane.
In real life most of the world is uninhabitable without some form of organized civilization to help offset the current climate and weather in winter. Even in history, villages had prepared sources of fuel for files. Tribes either were nomadic or reserves of one type of fuel or another. Take all that away and, plop down your standard suburban family, with no supplies in the middle of a mild winter, and...
Now what the book does mention is the "Gray" winter, or the pollution in the sky "changing" the weather. But I always filter that through two main facts.
First the book is an account of other peoples accounts. How many times have you heard someone describe a winter as "The coldest winter ever" or "the single coldest day in their entire lives" etc. etc. It's common, specially when people are telling stores that the cold is actually described as colder then it actually is. Specially when they are going through hard times.
The account of Gray skies from city wide fires or such. Winter, when your really right out in it, is pretty "gray" normally. It can feel bleak and desolate all on it's own. Even today. Add a few clouds and some heavy smoke and it can be down right nasty. Most people see winter (even when they know it's not true) as this perfect white shiny time of the year. But if you stop plowing roads, stop driving down them, and still live in an area. Man it's gets nasty, gray, and funky in just a couple of days.
So basically what I see is that people are being people. They take the negative and nasty and spread it around. They smoke from fires, and other things "may" have effected the weather, but mostly people, just being people, remember those dark, cold nights as the "bad times" and what came before as the "golden age" where the winters weren't as cold, the summers weren't as hot, and the food actually tastes better. The winters may have been "a little" colder, and a little "grayer", but essentially they were the same as now, and just the people changed.
Read some of the accounts of winter time war actions, and you will get the same result. Soldiers trapped in fox holes, or entrenchment lines and the like reporting that the winter was the coldest ever in existence, etc. It may not be true on paper, but it certainly felt that way to them.
TL;DR While it might have been a little colder, most of the reported effect is because your "listening" to stories told by people that collectively went though a bleak and "dark" time. Not necessarily because the actual climate changed as much as it is perceived to have changed.