Reconciling Paris Agreement goals for temperature, emissions

March 26, 2018

BOULDER, Colo. -- As society faces the challenge of limiting warming to no more than 2 degrees Celsius, new research finds an apparent contradiction: Achieving that goal doesn't necessarily require cutting greenhouse gas emissions to zero, as called for in the Paris Agreement. But under certain conditions, even zero emissions might not be enough.

The Paris Agreement, a global effort to respond to the threats of human-caused climate change, stipulates that warming be limited to between 1.5 degrees C (2.7 degrees Fahrenheit) and 2 degrees C (3.6 degrees F). It also stipulates that countries achieve net-zero greenhouse gas emissions in the second half of this century. But the relationship between the two -- is the emissions goal sufficient or even necessary to meet the temperature goal? -- has not been well understood.

In a new study published in the journal Nature Climate Change, scientists used a computer model to analyze a variety of possible future scenarios to better understand how emissions reductions and temperature targets are connected. The study, published March 26, was led by Katsumasa Tanaka at the National Institute for Environmental Studies in Japan and co-authored by Brian O'Neill at the U.S. National Center for Atmospheric Research.

"What we found is that the two goals do not always go hand in hand," Tanaka said. "If we meet temperature targets without first overshooting them, we don't have to reduce greenhouse gas emissions to zero. But if we do reduce emissions to zero, we still might not meet the temperature targets if we don't reduce emissions quickly enough."

The team also found that whether temperatures overshoot the target temporarily has a critical impact on the scale of emissions reductions needed.

"If we overshoot the temperature target, we do have to reduce emissions to zero. But that won't be enough," Tanaka said. "We'll have to go further and make emissions significantly negative to bring temperatures back down to the target by the end of the century."

The research was supported by the Environment Research and Technology Development Fund (2-1702) of the Environmental Restoration and Conservation Agency in Japan and by the U.S. National Science Foundation, NCAR's sponsor.

Drafted in 2015, the Paris Agreement has been ratified by more than 170 countries. President Donald Trump announced last year the intention to withdraw the United States from the agreement.

Modeling the problem from both sides

For the study, the researchers used a simplified integrated assessment model that takes into account the physical connections between greenhouse gases and global mean temperature in the climate as well as the economic costs of emissions reductions.

"We investigated the consistency between the Paris targets in two ways. First we asked, what happens if you just meet the temperature target in a least-cost way? What would emissions look like?" said O'Neill, an NCAR senior scientist. "Then we said, let's just meet the emissions goal and see what kind of temperatures you get."

The team generated 10 different scenarios. They found that Earth's warming could be stabilized at 1.5 or 2 degrees C -- without overshooting the goal -- by drastically cutting emissions in the short term. For example, total greenhouse gas emissions would need to be slashed by about 80 percent by 2033 to hit the 1.5-degree target or by about two-thirds by 2060 to meet the 2-degree target. In both these cases, emissions could then flatten out without ever falling to zero.

Due to the difficulty of making such steep cuts, the scientists also looked at scenarios in which the temperature was allowed to temporarily overshoot the targets, returning to 1.5 or 2 degrees by the end of the century. In the 1.5-degree overshoot scenario, emissions fall to zero by 2070 and then stay negative for the rest of the century. (Negative emissions require activities that draw down carbon dioxide from the atmosphere.) For the 2-degree temporary overshoot scenario, emissions fall to zero in 2085 and also become negative, but for a shorter period of time.

On the flip side, the scientists also looked at scenarios where they set the emissions levels instead of the temperature. In those cases, they analyzed what would happen if emissions were reduced to zero around mid-century (2060) or at the end of the century (2100). In the first case, the global temperature peaked around the 2-degree target and then declined. But in the second case, the temperature rose above 2 degrees around 2043 and stayed there for a century or more.

"The timing of when emissions are reduced really matters," O'Neill said. "We could meet the goal set out in the Paris Agreement of reducing emissions to zero in the second half of the century and still wildly miss the temperature targets in the same agreement if we wait to take action."

The new study is part of a growing body of research that seeks to better understand and define what it will take to comply with the Paris Agreement. For example, another recent study -- led by Tom Wigley, a climate scientist at the University of Adelaide who holds an honorary appointment at NCAR -- also looks at the quantity and timing of emissions cuts needed to stabilize global temperature rise at 1.5 or 2 degrees above preindustrial levels. This work focuses in particular on implications for emissions of carbon dioxide, the main component of the broader greenhouse gas emissions category that makes up the Paris emissions target.

O'Neill and Tanaka believe their work might be useful as countries begin to report the progress they've made reducing their emissions and adjust their goals. These periods of reporting and readjusting, known as global stocktakes, are formalized as part of the Paris Agreement and occur every five years.

"Our study and others may help provide countries with a clearer understanding of what work needs to be done to meet the goals laid out in the agreement. We believe that the Paris Agreement needs this level of scientific interpretation," Tanaka said.
-end-
The University Corporation for Atmospheric Research manages the National Center for Atmospheric Research under sponsorship by the National Science Foundation. Any opinions, findings and conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

About the article:

Title: Paris Agreement zero emissions goal is not always consistent with 2°C and 1.5°C temperature targets
Authors: Katsumasa Tanaka, Brian C. O'Neill
AJournal: Nature Climate Change, DOI: 10.1038/s41558-018-0097-x

National Center for Atmospheric Research/University Corporation for Atmospheric Research

Related Greenhouse Gas Emissions Articles from Brightsurf:

Using materials efficiently can substantially cut greenhouse gas emissions
Emissions from the production of materials like metals, minerals, woods and plastics more than doubled in 1995 - 2015, accounting for almost one-quarter of all greenhouse gas (GHG) emissions worldwide.

Climate change: Ending greenhouse gas emissions may not stop global warming
Even if human-induced greenhouse gas emissions can be reduced to zero, global temperatures may continue to rise for centuries afterwards, according to a simulation of the global climate between 1850 and 2500 published in Scientific Reports.

Climate-friendly Cooling Could Cut Years of Greenhouse Gas Emissions and Save US$ Trillions: UN
Energy-efficient cooling with climate-friendly refrigerants could avoid up to 460 billion tonnes of greenhouse gas equivalent being added to the atmosphere through 2060 - roughly equal to eight years of global emissions at 2018 levels.

Forests can be risky climate investments to offset greenhouse gas emissions
Given the tremendous ability of forests to absorb carbon dioxide from the atmosphere, some governments are counting on planted forests as offsets for greenhouse gas emissions -- a sort of climate investment.

Switching from general to regional anaesthesia may cut greenhouse gas emissions
Switching from general to regional anaesthesia may help cut greenhouse emissions and ultimately help reduce global warming, indicates a real life example at one US hospital over the course of a year, and reported in the journal Regional Anesthesia & Pain Medicine.

Women generate lower travel-related greenhouse gas emissions, NZ study finds
Women use more diverse modes of travel and generate lower greenhouse gas emissions than men, despite men being more than twice as likely to travel by bike, a New Zealand study has found.

Great potential in regulating plant greenhouse gas emissions
New discoveries on the regulation of plant emissions of isoprenoids can help in fighting climate change - and can become key to the production of valuable green chemicals.

Cable bacteria can drastically reduce greenhouse gas emissions from rice cultivation
The rice fields account for five percent of global emissions of the greenhouse gas methane, which is 25 times stronger than CO2.

Sugar ants' preference for pee may reduce greenhouse gas emissions
An unlikely penchant for pee is putting a common sugar ant on the map, as new research from the University of South Australia shows their taste for urine could play a role in reducing greenhouse gases.

Seeking better guidelines for inventorying greenhouse gas emissions
Governments around the world are striving to hit reduction targets using Intergovernmental Panel on Climate Change (IPCC) guidelines to limit global warming.

Read More: Greenhouse Gas Emissions News and Greenhouse Gas Emissions Current Events
Brightsurf.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to Amazon.com.