At the end of November, climate negotiators from around the world will descend on Paris for a United Nations conference on global warming. They'll try to lock in each country's pledge to cut greenhouse gas emissions. One key question will be whether the talks incorporate the notion of "cumulative carbon."
Throughout the industrial age, burning fossil fuel has created waste.
"At one point we had a transportation system that had horses," said Penn State University geologist Richard Alley. "And the horses made horse ploppies. If our CO2 came out of the car as horse ploppies, it would sort of be a pound per mile. And if you put that on the roads of America, in a year it would average an inch deep. And as I like to say, we would have no more joggers in America. We'll all be cross-country skiers."
One relevant question is how much carbon waste counts as too much. Which brings another analogy: imagine our atmosphere as a landfill.
"Right now we're using the atmosphere as a free and unregulated waste dump for carbon dioxide," said University of Oxford physicist Raymond Pierrehumbert. "And we have been doing that for a long time."
Pierrehumbert explained it's important to focus not just on the trucks dumping waste into the landfill – i.e. the rate of emissions – but on the big trash heap itself. Total cumulative carbon stays in the atmosphere for hundreds, perhaps thousands, of years, trapping heat.
"Really, the one knob you have to pay attention to that actually determines how warm it's going to get is cumulative carbon emissions," he said.
Imagine the landfill holds a trillion tons of carbon. Scientists generally agree that amount is likely enough to raise global temperatures 2 degrees Celsius.
"That's 3.6 Fahrenheit," Alley said. "And the land warms more than the ocean. So that's 4 or 5 or a bit more Fahrenheit where most of us live."
At that point, scientists say the risk of extreme weather events, sea level rise, fires, water shortages and crops yield may go beyond a level humans have ever experienced. The bigger the rise, the bigger the risk.
How humanity arrived at this point is a story of industrial and energy history.
Eighteenth-century steam engines enabled the 19th century of machines and railroads. Early steam came from burning a powerful source of densely packed energy: coal. Thus the first truckloads of carbon waste dumped in the planetary landfill.
Not long after that, scientists began writing about the idea of carbon waste and the greenhouse effect. Swedish chemist Svante Arrhenius published a paper on what he called the "greenhouse law" in 1896.
"He's the first scientist who points out that when you burn fossil fuels, it emits carbon dioxide to the atmosphere," said Harvard science historian Naomi Oreskes. "And this has the potential to warm the climate. The basic physics and chemistry of this is very old, well established. It's more than a century old."
Arrhenius didn't get it all right. He assumed warming might take 500 years. Back then, there was barely any trash in the earth's carbon landfill.
Even the advent of the "horseless carriage" – the automobile – didn't speed things up much. A full decade after the Model T, the landfill was 9 percent full.
"Not much actually happened in that first hundred years," Oreskes said. "It's actually surprising how little total carbon dioxide was released in that first hundred years compared to what happens later."
Later, as in post-World War II. Many scientists call this period the Great Acceleration, a measurable boom in all kinds of things: population, GDP, urbanization, energy use.
"It's mobility," Oreskes said. "It's electricity use. It's the size of our houses."
Fossil fuels lighted and heated those homes, and powered ships and trucks delivering food long distances around the world. Along with this progress came the carbon waste.
"Rather than picking out one thing and saying, 'Well if we just hadn't invented the car,' we'd have had something else," Alley said. "Because we're clever and we love the good we get from the energy we use.
In the 1950s and 60s, scientists were just starting to measure emissions in the atmosphere, and detect an increase. One science paper landed on the desk of President Lyndon Johnson, who went on to deliver a special message to Congress on the topic.
One section read: "This generation has altered the composition of the atmosphere on a global scale through radioactive materials and a steady increase in carbon dioxide from the burning of fossil fuels."
It was 1965. By then the carbon landfill was a quarter full.
"Now, no one in 1965 thought that climate change was already under way," historian Oreskes said.
She said by then, carbon emissions were rising, but there was not yet consensus the planet had begun to warm. It took until 1995, she said, for an intergovernmental panel of scientists to announce temperature rise and a "discernible human influence."
More trucks were dumping more waste in the earth's carbon landfill than ever before. But even then, most emissions came from rich countries. Oreskes said emerging economies didn't really crash the CO2 trash party until after the turn of the millennium.
And did they ever. Which brings today's reality.
"We are roughly half full," Pierrehumbert said. "But because the usage of fossil fuels has grown so much since then, it won't take another 200 years or so to fill it up the rest of the way. At the current rate, we will fill up that trillion-ton limit somewhere around 2030."
That is the incredible fill rate that climate negotiators are racing against in the lead-up to the next round of talks in Paris.
From a science perspective, Pierrehumbert said, the key point for global leaders is not just to slow dumping of carbon, but eventually bring it to zero.