It is probably the most influential paper on climate science today. But few outside scientific circles even know it exists.
Though just six pages long, its dense, technical writing makes it largely incomprehensible to non-experts. And yet this paper is transforming the climate change debate—prompting the financial world to rethink the value of the world's fossil fuel reserves and giving environmental activists a moral argument for action.
That's because behind its complicated terminology is a simple question that affects every aspect of society and business: How much time do we have before the burning of fossil fuels pushes the climate system past tipping points? In a worst-case scenario, about 11 years at current rates of fossil fuel use, according to the paper.
"Once you hear the numbers, at least for me, there is no more room for wishful thinking, for speculation or for doubt," said Bill McKibben, founder of the activist group 350.org. Last year, McKibben plucked the science from popular obscurity and used it in a Rolling Stone article and speaking tour to stoke the moral case for carbon controls.
The paper, "Greenhouse-Gas Emission Targets for Limiting Global Warming to 2C," was published in April 2009 in Nature, the prestigious science journal. It was the work of researchers from Germany, the UK and Switzerland, led by Malte Meinshausen, a climatologist at Germany's Potsdam Institute for Climate Impact.
The study filled a factual void in a simmering debate over climate change. By 2006, the year the scientists began their research, many world governments had endorsed the scientific consensus that global temperature rise should be kept below 2 degrees Celsius in this century. But governments didn't know how far down the path of global warming they had already gone—and how much further they could safely go.
Meinshausen and his team, which included his brother Nicolai, a statistician at the University of Oxford, took up the puzzle. "It seemed the obvious thing to do with so many governments asking the question," Meinshausen said.
The scientists created what is called a global "carbon budget," which details how much carbon countries have emitted in the atmosphere from burning coal, oil and natural gas—and how much more they can "spend" before crossing 2 degrees. They didn't invent the concept—many others had crunched carbon budgets. But none were as rigorous.
The paper's methodology was groundbreaking. It was the first to incorporate hundreds of uncertainties in the climate system into a single climate model—factors that had never been modeled together or that hadn't been given proper weight in previous studies, such as radiative forcing or unknowns in the carbon cycle like how much carbon is stored in the deep ocean. In total, 400 environmental parameters were run under 1,000 different emissions scenarios.
What they found was stark: To have a 50-50 chance of keeping temperature rise below 2 degrees, humans would have to stick to a carbon budget that allowed the release of no more than 1,437 gigatons of carbon dioxide from 2000 to 2050.
To have an 80 percent chance of avoiding that threshold, they would have to follow a stricter budget and emit just 886 gigatons.
The paper found that by 2006, nations had already spent a quarter of that amount, or 234 gigatons. Meaning, the planet's carbon budget would be exhausted by 2024—11 years from now— if emissions levels stayed the same, or even earlier if they continue their upward trend.
From a scientific point of view, burning all of the world's proven fossil fuel reserves isn't an option, the paper suggested. The reserves "vastly exceed the allowable CO2 emission budget for staying below 2C" of warming, it said.
In 2009, the findings were used by the International Energy Agency (IEA), a policy group that advises 28 countries about their energy policies, to make the case for steep reductions in climate-changing gases. "We are currently eating into these CO2 budgets at a disproportionate rate," the authors wrote in its World Energy Outlook that year.
Less than four years later, the paper is one the most cited environmental science studies ever. To date, it has garnered 262 citations in scientific articles, according to Web of Science, an online citation index run by media conglomerate Thomson Reuters, putting it in the top 0.1 percent cited environmental papers in recent years, and just missing being among the top 0.01 percent. The paper has racked up more than 600 citations in Google Scholar, a more inclusive citation index that includes mentions in books, professional societies and on university websites.