Quantifying the benefits of cutting carbon

BRACE project examines extreme heat, health, agriculture, and more

September 18, 2017 | Perhaps tens of thousands of studies have detailed the possible impacts of a changing climate: the amount of sea level rise we can expect by the end of the century, for example, or the regions of the world that will likely experience more severe droughts.

But rarely do these studies outline the actual benefits that society would gain from emitting fewer greenhouse gases instead of more. Instead, they often compare what is likely to happen in a particular scenario — such as emissions continuing unabated — to what the climate was like in the past.

"Most studies don't give us a good sense of what we have to gain by mitigating. What are the benefits of capping warming at 2 degrees Celsius instead of 3, for example?" said NCAR scientist Brian O'Neill. "That's important information for policy makers because mitigation has a price, and you need to understand the benefits to weigh them against the cost."

O'Neill and dozens of colleagues from NCAR and universities across the country have recently wrapped up a major effort to wrestle with this problem. The results from the project — the Benefits of Reduced Anthropogenic Climate changE (BRACE) — are published in a special issue of the journal Climatic Change.

For BRACE, scientists explored what the world would look like through 2100 in two different scenarios: a business-as-usual emissions scenario with a temperature increase of about 3.7 degrees Celsius over preindustrial levels and a moderate mitigation scenario where warming is reduced to about 2.5 degrees Celsius. By comparing the two worlds, they were able to calculate the benefits of moderate mitigation for a range of areas, from agriculture to health to extreme events.

Additionally, the researchers looked at how different pathways of societal development — including population growth, economic growth, and technological development — might play a role or even outweigh the climate effects.

"It's important to analyze the effects of societal change alongside the effects of climate change," O'Neill said. "Societal change can matter as much as climate change. And in some cases, it matters more."

The results of the studies vary significantly across sectors. When it comes to extreme heat, for example, the benefits of reducing emissions are clear and significant. In other areas, the findings are more nuanced. In agriculture, for example, the potential fertilizing effects of higher levels of carbon dioxide in the atmosphere and the ability of some farmers to adapt — by adding nitrogen fertilizer in regions where it isn't currently used, for example — could in some cases mute, or even cancel out, the benefits of mitigation.

A collage of areas studied by BRACE
The BRACE project has studied the benefits of reducing greenhouse gas emissions in a number of areas, including extreme heat, mosquito-borne diseases, droughts, tropical cyclones, and agriculture. (Photo collage, UCAR.)


Natural variability or climate change?

For the studies, the scientists relied on two sets of simulations from the NCAR-based Community Earth System Model. For each set — one representing a future with no mitigation and the other a future with moderate mitigation — the scientists drew from more than a dozen simulations. Having an "ensemble" of model runs allowed them to better estimate extreme events, which occur rarely by definition and are therefore difficult to capture with just one or two simulations. It also allowed them to distinguish the effects of human-caused climate change from natural climate variability.

In many cases, the ensembles allowed the scientists to demonstrate a substantial benefit of mitigating greenhouse gas emissions. One study found that extreme heat waves that today have just a one in 20 chance of occurring in any given year could become six to 20 times more common if greenhouse gas emissions continue unabated. In the moderate mitigation scenario, such heat waves would likely become just 2 to 4 times more common.

Other studies of extreme heat showed similar benefits. For example, scientists found the likelihood that a single summer between 2060 and 2080 would be hotter than any in the historical record to be 80 percent if greenhouse gas emissions continue unabated. But that risk is cut in half, to 41 percent, with moderate mitigation.

In some areas of study, however, natural variability still overwhelms the impact of climate change, making it difficult to measure the benefits of mitigation. This was true for several BRACE studies that looked at how tropical cyclones — and the damage they can cause — would differ between the two scenarios. The studies found that tropical cyclone damage (and number of storms) might actually increase if greenhouse gas emissions are reduced, though the intensity of storms might also be lessened.

However, the storm findings were not statistically robust, meaning the data were too noisy to detect a clear signal.

"Tropical cyclones are highly variable, so to show a real difference between the present and the future, we would need to run the model many more times to accurately describe how present and future storms are different," said NCAR scientist Andrew Gettelman, who led one of the studies.

Still, there are several plausible explanations for why hurricane activity might decrease if greenhouse gas emissions continue on their current trajectory. For example, uneven warming of the ocean basins could generate high-level winds, which make it more difficult for storms to form.

However, storms might still be more intense due to increases in ocean temperature, and indeed Gettelman and colleagues found increases in frequency for the most intense landfalling storms, despite decreases in overall storm numbers.

The BRACE researchers' work on this topic is made more difficult by the relatively short historical record that exists for hurricanes. Though observations of landfalling hurricanes stretch back as long as people have lived near coasts, a comprehensive record of how many cyclones form across the globe's oceans only began at the dawn of the satellite era in 1979. With only a few decades of data to create a baseline, it's difficult to detect future trends, Gettelman said.

Additionally, global climate models are typically run at a resolution that is too coarse for individual cyclones to form. While hurricanes do form in climate models when run at a high resolution, it is not yet clear that those storms are behaving in a way that matches what would happen in the real world.

The societal effect 

Even if climate models could accurately simulate tropical cyclones, it might still be difficult to determine the benefit, at least in terms of decreased damage, that might be realized if greenhouse gas emissions were cut. That's because societal choices also play an important role. For example, in the study led by Gettelman, the researchers found that any potential changes in storms and the damage they cause were swamped by the projected increase in societal vulnerability: more people and more buildings are projected to be near coastlines, in harm's way.

This societal effect also dominated the climate effect in other areas studied for the BRACE project, including heat-related mortality in Houston, and total population exposure to the Aedes aegypti mosquito, which carries diseases including Zika, dengue, and chikungunya.

"When it comes to human health outcomes, climate change usually becomes an exacerbating factor," said NCAR scientist Andrew Monaghan, who led the study on Aedes aegypti. "Other important factors are governance decisions, socioeconomic development, and population growth trajectories."

Monaghan and colleagues found that the amount of land that is climatically suitable for the mosquito to live would increase by 13 percent without any mitigation of greenhouse gases, compared to 8 percent in a moderate mitigation scenario. But the number of people who would be exposed to the mosquito would increase by as much as 130 percent as the population grows in regions where the mosquito can thrive.

The societal effect may also be dominant in certain aspects of agriculture, according to the BRACE studies. Fertilizing or irrigating additional land could increase crop yields, for example. And crops that are particularly vulnerable to climate change could be replaced with hardier varieties.

Agricultural uncertainty

The BRACE studies that tried to quantify the agricultural benefits of mitigation also had to contend with another area of uncertainty: How much of a fertilizing effect will increased carbon dioxide in the atmosphere have on crop yield? Plants need carbon dioxide to grow, and greater availability of carbon dioxide can bolster crop yields. But there are other limiting factors at play, including available water, soil nutrients, and extreme heat.

One BRACE study that included the fertilizing effects of increased carbon dioxide found that crop yields would grow by a global average of 11 percent if greenhouse gas emissions continue on their current trajectory. By comparison, the global average would increase just 6 percent with moderate mitigation.

"We've already observed a massive ramp-up in crop yields from the 1960s through the current day, due to changes in fertilizer, crop varieties, management, and carbon dioxide levels," said NCAR scientist Peter Lawrence, who worked on the agricultural aspects of the BRACE project. "The big question is what happens as we move forward. We know that our models can accurately represent the relationship between increased carbon dioxide and increased plant growth. But there are a lot of processes our models don't capture, including flooding and the effects of heat waves while plants are flowering."

Other agricultural studies in the BRACE project that looked at specific crops found net positive benefits from reducing greenhouse gases. For example, one study found that climate change would decrease potential maize yields in the future (relative to what they would be in the absence of climate change). Maize yields would decrease by a global mean of 27 percent if emissions continue unabated vs. 15 percent with moderate mitigation. Another study found that the exposure of crops to damaging heat above a critical threshold for growth would be reduced by about a third in the moderate mitigation scenario compared to the scenario with no mitigation.

The agricultural studies highlight the stubborn uncertainties that persist when trying to determine how crops will be affected in a warming world. And while this makes quantifying the benefits of mitigation more difficult, it also helps set the direction for future research.

O'Neill says he hopes all of the more than 20 studies published so far in the BRACE framework will inform the direction of future work, including what questions need to be asked and what kinds of model simulations would prove most useful for finding answers.

"Even though the BRACE project involved dozens of scientists to research a broad range of sectors, it is really just a start," said O'Neill.

About the article

Title: The Benefits of Reduced Anthropogenic Cliamte changE (BRACE): a synthesis

Authors: Brian C. O’Neill1, James M. Done, Andrew Gettelman, Peter Lawrence, Flavio Lehner, Jean-Francois Lamarque, Lei Lin, Andrew J. Monaghan, Keith Oleson, Xiaolin Ren, Benjamin M. Sanderson, Claudia Tebaldi, Matthias Weitzel, Yangyang Xu, Brooke Anderson, Miranda J. Fix, and Samuel Levis

Journal: Climatic Change DOI: 10.1007/s10584-017-2009-x

Collaborators

Argonne National Laboratory
Brigham Young University
City University of New York
Colorado School of Mines
Colorado State University
Johns Hopkins
Indian Space Research Organization, India
Lanzhou University, China
Lawrence Berkeley National Laboratory
National Oceanic and Atmospheric Administration
Stanford University
Stony Brook University
Sun Yat-sen University, China
Swiss Federal Institute of Technology
Texas A&M
The Climate Corporation
University of Arizona
University of Colorado Boulder
University of Illinois Urbana
University of Washington


Writer/contact:
Laura Snider, Senior Science Writer and Public Information Officer

 

 


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