A Report Not to Be Buried: Part II of the Fourth National Climate Assessment

November 26, 2018, 7:13 PM EST

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Above: Symbolic of the obstacles the U.S. economy faces from a failure to mitigate and adapt to climate change, this image shows a large boulder washed into Bella Vista Drive in Montecito, Calif., following heavy rains and mudslides on Jan. 9, 2018, when 21 people were killed by flash floods that ripped down the slopes of hills denuded by the Thomas Fire of 2017. That fire was California's largest fire on record, until the Mendocino Complex Fire of 2018. Image credit: AP Photo/Michael Owen Baker.

Fridays are traditionally known as the time to “bury” bad news by releasing it when most news organizations are ramping down for the weekend. Perhaps because of the inconvenience of its message, a major climate report was issued by the U.S. government weeks ahead of schedule, on arguably the quietest journalistic Friday of the year: the day after Thanksgiving. It’s not clear who is responsible for the odd timing, but in spite of that, Part II of the Fourth National Climate Assessment (NCA) is much too powerful a document to stay buried.

Cover of 4th National Climate Assessment, Part II

Friday’s report builds on the foundational science of Part I, which was released in late 2017 as the Climate Science Special Report. Part II takes this research and uses it to paint a portrait, in unprecedentedly vivid detail, of what we can expect human-produced climate change to deliver to the United States and its territories over the next few decades—as well as how we might avoid (mitigate) some of this change and how we might adapt to the rest. Both strategies are crucial, the report argues, if the nation is to avoid the worst possible consequences.

Previous NCAs were released in 2000, 2009, and 2013. By law, these assessments are supposed to be produced by the U.S. Global Change Research Program at least every four years, but the Bush administration downplayed the first NCA and obstructed the second one, resulting in a nine-year gap between the two.

Much like the major assessments of the Intergovernmental Panel on Climate Change (IPCC), the NCAs are not policy-prescriptive: they’re not designed to tell us what to do. Instead, they inform us—based on the latest science—what will most likely happen, depending on how we choose to respond or not to respond.

For a pithy summary, it's hard to beat the one the authors put in italics:

“[This report] concludes that the evidence of human-caused climate change is overwhelming and continues to strengthen, that the impacts of climate change are intensifying across the country, and that climate-related threats to Americans’ physical, social, and economic well-being are rising.

The follow-up is just as important:

“These impacts are projected to intensify—but how much they intensify will depend on actions taken to reduce global greenhouse gas emissions and to adapt to the risks from climate change now and in the coming decades.”

Heat and water (and the lack of it)

Some of the new conclusions are startling in their specificity. Estimates of the likely climate-related hit to the nation’s economy by the end of this century under the highest-emission IPCC scenario (RCP8.5), in 2015 dollars, include:

—$203 to $507 billion of cumulative loss in real estate expected to be below sea level by 2100

—$140 billion in cumulative tourism loss as a result of damage to U.S. coral reefs

—$32 to $87 billion per year in energy expenses, as greater demand for air conditioning overwhelms more modest savings in heating costs

—Up to $20 billion per year in temperature- and moisture-related damage to paved roads

The impacts to human health explored in the report are compelling in their own right. For example, shifts toward extreme heat and away from extreme cold may lead to a net increase of some 9000 premature deaths per year in 49 large U.S. cities (plus additional deaths outside those largest urban centers) by the end of the century under the IPCC’s highest-emission scenario. Even without adaptation, roughly half of these deaths could be avoided if the globe were to follow a lower-emissions path (RCP4.5). Similarly, atmospheric ozone is projected to cause an extra 500 premature U.S. deaths per year on a high- vs. low-emissions pathway.

Many of the messages in this report strike chords that are familiar from earlier assessments. It’s no surprise that the heaviest precipitation events are expected to get even heavier, since that’s already been the case for decades across much of the nation. Likewise, rising temperatures and shifts in wet-season timing are already exacerbating the impact of drought in California, even though long-term average precipitation hasn’t changed. Several of the most extreme U.S. disasters since the last NCA in 2013 have been closely tied to rainfall or the lack of it, including:

Hurricane Harvey, 2017:  89 killed, $125 billion in damage

Hurricane Florence, 2018:  53 killed, more than $10 billion in damage

—the “no-name” Louisiana flood, 2016: 13 killed, $15 billion in damage

—the pairing of extreme heat and a prolonged dry season that paved the way for California’s multiple fire catastrophes of 2017 (47 killed, $18 billion in damage) and 2018 (at least 103 killed, and more than $8 billion in damage).

Flooding from Harvey in Houston, 8/27/17
Figure 1. People walk through the flooded waters of Telephone Road in Houston on August 27, 2017, as the Houston area grappled with catastrophic flooding from then-Tropical Storm Harvey. Image credit: Thomas B. Shea/AFP/Getty Images.

In the Midwest, the report’s projected long-term consequences of extreme heat are noteworthy in part because much of this region, especially the Corn Belt, has seen no major summer warming over recent decades (though other seasons have warmed). The Midwest’s summer atmosphere has become more moist on average, apparently the result of a mix of factors that includes heavier spring rains, expanded irrigation in the western Corn Belt, and/or higher planting densities since the mid-20th century. In turn, that sultry air has tended to push up average nighttime lows while tamping down average daytime highs. In recent years, there’s also been the much-discussed tendency toward hot upper-level ridging in the western U.S. and upper-level troughing in the central U.S., which has tended to focus extreme heat toward the west.

“The last three summers in the far west have been the three warmest on record for daytime high temperatures,” said Ken Kunkel (North Carolina State University), one of the authors of the NCA’s Midwest chapter, in an email. “It is not clear whether that is a signature of global warming or a natural climate fluctuation.”

Nevertheless, the NCA is confident that long-term warming will prevail across the Midwest by mid-century, with increasingly dire effects: “Warm-season temperatures are projected to increase more in the Midwest than any other region of the United States….Increases in temperatures during the growing season in the Midwest are projected to be the largest contributing factor to declines in the productivity of U.S. agriculture.” Barring major technical advances, the report finds that agricultural efficiency in the Midwest is projected to drop to 1980s levels as soon as 2050.

“The rate of change will be a challenge, especially if it is abrupt,” said Jim Angel (University of Illinois at Urbana-Champaign), lead author of the Midwest chapter, in an email. Dry springs will be especially prone to feed into very hot summers.

Map with projected increases in annual average U.S. temperatures in 21st century
Figure 2. Annual average temperatures across the United States are projected to increase over this century, with greater changes at higher latitudes as compared to lower latitudes, and greater changes under a higher scenario (RCP8.5; right) than under a lower one (RCP4.5; left). This figure shows projected differences in annual average temperatures for mid-century (2036–2065; top) and end of century (2071–2100; bottom) relative to the near present (1986–2015). Image credit: Fourth NCA, Part II, Figure 2.4, Ch. 2 (adapted from Vose et al. 2017).

New approaches in the Fourth NCA

More detail on state and regional outcomes. There are now separate analyses for the Northern and Southern Great Plains, a sensible switch from the previous lumping of states that stretch from Texas to North Dakota. I was especially impressed with a supplemental set of State Climate Summaries that provides succinct background on what to expect in each of the 50 states as well as the Western Pacific Islands and Puerto Rico/U.S. Virgin Islands. Each of these state reports is compellingly presented and easy to digest, even if the messages are often bleak.

—A new emphasis on overlapping and interactive risks arising from factors both related and unrelated to climate change. For example, the increasing threat posed by wildfire in California is related to an increase in “hot” droughts, which research shows is consistent with human-produced climate change. Also in the mix are decades of fire suppression and the flocking of Californians to housing within or near forested lands (the wildland-urban interface). In turn, California’s recent spate of catastrophic fires has led to intense air pollution episodes, infrastructure damage, and flash flood threats that have themselves proven deadly (see photo at top).

Extreme rainfall, heat, and sea level rise will pose a diabolic mix of threats to aging infrastructure, from highways to plumbing systems, which in turn will jeopardize economies and health in multiple ways.

“The full extent of climate change risks to interconnected systems, many of which span regional and national boundaries, is often greater than the sum of risks to individual sectors,” notes the report overview. An entire chapter of the new report is devoted to Sector Interactions, Multiple Stressors, and Complex Systems.

—New attention to how climate change will affect marginalized populations. A wide array of Americans are especially vulnerable to the myriad impacts of climate change, including the very old and very young, people on limited incomes, and those living in areas with multiple environmental risks (often communities of color). There’s a corollary: actions taken to reduce and deal with climate change can intensify or lessen societal inequities, depending on how they are crafted and executed. “Prioritizing adaptation actions for the most vulnerable populations would contribute to a more equitable future within and across communities,” the report notes.

One or the other, or one and the other?

The climate-change zeitgeist in recent weeks has centered on mitigation—in particular, how to make the drastic cuts in greenhouse-gas emissions needed by 2030 for a reasonable chance of keeping global temperature rise to 1.5°C and avoiding the most catastrophic change, as laid out in a high-profile IPCC report.

It’s long been easy to view adaptation as a backstop, a flag of surrender rather than a full-throated response to an existential threat. However, it’s obvious throughout this new NCA that the authors see adaptation as being every bit as crucial as mitigation in reducing national harm from climate change. The new report has full chapters on both approaches, and nearly every sentence that refers to mitigation in the Summary Findings also refers to adaptation.

In an ImaGeo post for Discover on Sunday night, Tom Yulsman, who directs the Center for Environmental Journalism at the University of Colorado Boulder, dinged a number of leading media outlets for minimizing or omitting the adaptation angle. As Yulsman put it, “The report couldn’t have been any clearer: How we choose to adapt to change is another key variable in determining how millions of Americans—not to mention billions of people worldwide—will feel the impacts of climate change.”

Some types of adaptation and mitigation can pair well. For example, certain techniques for storing more carbon in farmland can also tamp down on the runoff and erosion expected to increase with more extreme precipitation events. Other pairings are more counterproductive: ramping up the use of air conditioning or desalinated water, for instance, could push emissions upward.

Extreme heat, one of the most serious threats outlined in the new report, is one place where adaptation could make a real difference. Over the last 10 to 15 years, many U.S. cities have taken steps to reduce the risk of catastrophic heat-wave impacts, including neighborhood cooling centers, outreach efforts, and design features such as white roofs and tree-lined streets. Perhaps in part due to these advances, the U.S. has yet to see another heat wave as deadly as the Chicago disaster of 1995, when at least 739 people were killed.

Phoenix extreme heat projections and locations of cooling centers
Figure 3. Left: average annual number of days above 100°F in Phoenix, Arizona, for 1976–2005, and projections of the average number of days per year above 100°F by the late 21st century (2070–2099) under lower (RCP4.5) and higher (RCP8.5) emission scenarios. Dashed lines represent the 5th–95th percentile range of annual observed values. Solid lines represent the 5th–95th percentile range of projected model values. Right: hydration stations and cooling refuges in Phoenix in August 2017. Such measures are expected to be needed at greater scales if the adverse health effects of more frequent and severe heat waves are to be minimized. Image credit: Fourth NCA, Fig. 1.16, Ch. 1. Sources: (left) NOAA NCEI, CICS-NC, and LMI; (right) adapted from Southwest Cities Heat Refuges (a project by Arizona State University’s Resilient Infrastructure Lab), available here. Data provided by Andrew Fraser and Mikhail Chester, Arizona State University.

The new NCA Part II includes a U.S. Climate Resilience Toolkit, designed to help local stakeholders determine what assets they value most and figure out how to investigate and prioritize options for reducing risks to these assets posed by climate change. Of course, whether these and other adaptations will manage to keep up with the projected rise in impacts as our climate evolves through this century is a huge question that remains to be answered.

Mitigation won’t be a cakewalk either. The new report notes that U.S. emissions in 2016 were at their lowest levels since 1994, and are projected to drop to as much as 20% below 2005 levels by 2025. Yet that’s still well short of the 26-28% drop pledged by the U.S. just before the Paris Agreement (and since abandoned by the Trump administration). And even if the U.S. and other nations meet their original Paris-related pledges, the world will fail to achieve the emission cuts needed to avoid 1.5°C of global warming.

Clearly, there’s work to be done on multiple fronts. I found this to be the single most haunting, and most motivating, sentence of the Fourth NCA:

“While Americans are responding in ways that can bolster resilience and improve livelihoods, neither global efforts to mitigate the causes of climate change nor regional efforts to adapt to the impacts currently approach the scales needed to avoid substantial damages to the U.S. economy, environment, and human health and well-being over the coming decades.”

Postscript: On Monday afternoon, President Donald Trump weighed in with his opinion of the NCA Part II's findings on potentially devastating economic losses: "Yeah, I don't believe it." A weather.com article has more reactions to the report from Trump and others.

Dr. Jeff Masters contributed to this post.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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Bob Henson

Bob Henson is a meteorologist and writer at weather.com, where he co-produces the Category 6 news site at Weather Underground. He spent many years at the National Center for Atmospheric Research and is the author of “The Thinking Person’s Guide to Climate Change” and “Weather on the Air: A History of Broadcast Meteorology.”


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