Forget modest cuts. Climate report says we need 'negative emissions'

Wind turbines stand in a field, Monday, June 2, 2014, in Adair, Iowa.
Image: Charlie Neibergall/Associated Press

The U.N. Intergovernmental Panel on Climate Change released the final installment of its mammoth Fifth Assessment Report on global climate change Sunday. The report amounts to a distillation of the most important climate science and policy findings.

It also lays out clear choices for policymakers to ponder during the next year of climate negotiations — which are supposed to culminate with a new global climate agreement in Paris next year.

The main message the report delivers is this: There is still time, and there are still options available, for the world to limit global warming to at or below 3.6 degrees Fahrenheit (2 degrees Celsius) above preindustrial levels. But it's going to take an unprecedented, near-Herculean effort.

Instead of talking about modest cuts to emissions of global warming gases, such as carbon dioxide and methane, as the world is now doing, countries need to get on a trajectory toward negative emissions by 2100. Otherwise we'll blow past the climate target, and increase the likelihood of more severe impacts.

Nations around the world, including the U.S., agreed to that temperature goal at the Copenhagen Climate Summit in 2009; many scientists and governments consider the 3.6 degree Fahrenheit target (often referred to as the 2 degree Celsius target) to be the threshold of "dangerous" manmade climate change, as spelled out in a binding climate treaty the world agreed to in 1992.

But that agreement never defined the meaning of the word "dangerous." According to scientists involved in the drafting of the final report, government representatives could not agree on wording that would help guide them in making a decision on what level of warming is truly dangerous, for fear it would commit their nations to emissions reduction targets that they were not yet prepared to embrace.

We are are maxing out our emissions credit card

The report finds that the world has already used up two-thirds of the global warming emissions that would take us to 3.6 degrees of warming by the end of the century, with just one-third left. The IPCC's carbon budget allows for spending (i.e. emitting) 2,900 gigatonnes of carbon dioxide from 1870 through 2100 to limit global warming to the agreed target.

IPCC Temperature Projections


Image: IPCC Synthesis Report Fig. 2.3

But we already emitted about 1,900 gigatonnes by 2011. More has gone into the atmosphere since. Once we blow past our allotted greenhouse gas spending allocation, scientists say, there's no turning back — given that a single molecule of carbon dioxide can remain in the atmosphere for centuries to millennia.

About half of all manmade carbon dioxide emissions since 1750 have taken place in just the last 40 years, the report found. Total manmade greenhouse gas emissions, which are causing global warming, have continued to climb from 1970 through 2010 "with larger absolute increases between 2000 and 2010."

The report details various greenhouse gas concentration "pathways" for the amount of greenhouse gases in the atmosphere through the year 2100, and unambiguously shows that we're currently following the highest and riskiest pathway. We're on the way to between 4.7 to 8.64 degrees Fahrenheit of warming by 2100, and possibly as high as 14 degrees Fahrenheit, relative to average temperatures during the 1986 to 2005 period.

That's not where we want to be, according to the IPCC, if we want to avoid costly extreme weather events, sharply higher sea levels, the likely destabilization of the entire Greenland ice sheet, and other nasty climate surprises. In fact, the IPCC found that such severe amounts of climate change could make routine tasks, such as working outdoors, difficult in parts of the globe.

"Rising rates and magnitudes of warming and other changes in the climate system, accompanied by ocean acidification, increase the risk of severe, pervasive, and in some cases irreversible detrimental impacts," the report says [emphasis added].

Pathways to "zero emissions or below"

According to the IPCC, in order to have a "likely" chance of limiting warming to at or below 3.6 degrees Fahrenheit by 2100, greenhouse gas concentrations in the atmosphere should be limited to 450 parts per million (ppm) or below.

That's going to be incredibly difficult, considering we hit 400 ppm this year — the highest level in human history — and are on course for about 1,000 ppm by 2100.

A target of 450 ppm would require cuts of 41% to about 71% below 2010 levels by 2050, and "emissions levels near zero or below in 2100."

Are you positive about going negative?


The IPCC, based on the peer reviewed studies on this topic, concluded that meeting the temperature target would require getting to zero emissions or below by 2100. This would mean that a combination of technological innovations that don't currently exist at a large scale, if at all.

Such negative emissions technologies include synthetic trees that suck the carbon out of the atmosphere, and natural carbon sinks that would remove more greenhouse gases from the air than humankind would be emitting. That's not a likely prospect, given the relatively paltry sums we are currently investing in energy research and development.

PPM warming

Key characteristics of the scenarios collected and assessed for WGIII AR5. For all parameters, the 10th to 90th percentile of the scenarios is shown. Focus on the RCP 2.6 scenario, this is the one with the lowest emissions and lowest chance of breaching the temperature target.

Image: IPCC Fig. SPM1

Because greenhouse gases pile up in the atmosphere like credit card debt, the IPCC says that the longer we wait to cut emissions, the steeper the emissions reductions will have to be. Steeper cuts will be more difficult and expensive to make, the IPCC says.

If we wait to start cutting emissions until 2030, and instead keep emissions at or above current levels, the IPCC says it will be difficult to impossible to stay below the 3.6 degree Fahrenheit limit.