CLIMATE CHANGE RETHINKING
Scientists Ditched a Scary Climate Scenario. What Now? - The New York Times
While global warming is still a threat, the decision to back away from a worst-case outlook raises questions about whether some risks have been overstated.
an international team of researchers published a major revision of the emissions scenarios used to study global warming.
When scientists try to model how hot Earth could get this century, they typically look at a range of possibilities for how much planet-warming pollution humans might pump into the atmosphere. These scenarios get updated every seven years or so.
In this latest update, the researchers abandoned a dire — and often criticized — high-emissions scenario known as RCP8.5 that has been prominently cited in thousands of climate studies over the past decade. The authors said the scenario was now “implausible” given recent energy trends.
For years, critics of the high-emissions scenario had argued that it was always unrealistic, in part because it envisioned that countries would burn coal at absurdly high rates. They argued that any studies or news reports relying on that scenario exaggerated the risks of climate change. Why, those critics now asked, did the course correction take so long?
How much hotter the world will get depends on two big factors: how many greenhouse gases humans will emit by burning fossil fuels and cutting down forests, and how the Earth’s climate will physically respond.
Predicting emissions over the next century is extremely difficult, since so much depends on future economic growth and technological changes. So scientists try to consider a wide range of possibilities.
In the early 2010s, climate scientists created a suite of standardized low-, medium- and high-emissions pathways. The goal, they said, was to capture every plausible outcome, even unlikely ones. The highest one, RCP8.5, envisioned annual carbon dioxide emissions roughly tripling this century.
The high-emissions pathway wasn’t meant to be a prediction, but more of a “worst case,” said Detlef van Vuuren, a climate scientist at Utrecht University and a leading figure in scenario development. “It’s simply exploring what happens if we end up with much higher emissions than we expect.”
“A lot of climate research is about understanding how Earth’s climate system behaves under certain conditions rather than a statement about what’s most likely to happen in the future,” said Robert Kopp, a climate scientist at Rutgers University. “They’re both important, but those are two different questions.”
Many scientific studies have wrongly referred to RCP8.5 as a “business-as-usual” scenario, suggesting that this was the pathway humanity is currently headed for. News stories about climate research often emphasized results based on RCP8.5 as a picture of what the world can expect unless countries slash their emissions, which isn’t right, either.
Costing out the effects of climate change
Episodes of severe weather in the United States, such as the present abundance of rainfall in California, are brandished as tangible evidence of the future costs of current climate trends. Hsiang et al. collected national data documenting the responses in six economic sectors to short-term weather fluctuations. These data were integrated with probabilistic distributions from a set of global climate models and used to estimate future costs during the remainder of this century across a range of scenarios (see the Perspective by Pizer). In terms of overall effects on gross domestic product, the authors predict negative impacts in the southern United States and positive impacts in some parts of the Pacific Northwest and New England.
Abstract
Estimates of climate change damage are central to the design of climate policies. Here, we develop a flexible architecture for computing damages that integrates climate science, econometric analyses, and process models. We use this approach to construct spatially explicit, probabilistic, and empirically derived estimates of economic damage in the United States from climate change. The combined value of market and nonmarket damage across analyzed sectors—agriculture, crime, coastal storms, energy, human mortality, and labor—increases quadratically in global mean temperature, costing roughly 1.2% of gross domestic product per +1°C on average. Importantly, risk is distributed unequally across locations, generating a large transfer of value northward and westward that increases economic inequality. By the late 21st century, the poorest third of counties are projected to experience damages between 2 and 20% of county income (90% chance) under business-as-usual emissions (Representative Concentration Pathway 8.5).
Abstract
Questions of environmental regulation typically involve trade-offs between economic activity and environmental protection. A tally of these trade-offs, put into common monetary terms—that is, a cost-benefit analysis (CBA)—has been required for significant regulations (e.g., those having an annual effect on the economy of $100 million or more) by the U.S. government for more than four decades (1–3). Ethical debate over the role of CBA is at least as old as the requirement itself (4), but the practical reality is that it pervades government policy-making. If estimates of environmental impacts and valuation are absent or questionable, the case for environmental protection is weakened. This is why the estimates of climate change damages reported by Hsiang et al. on page 1362 of this issue (5) are particularly important.
In 2017, a study by Justin Ritchie and Hadi Dowlatabadi pointed out that RCP8.5 envisioned a fivefold expansion in global coal use, including fueling cars with coal-based liquids, which they explained was “exceptionally unlikely.”
Central banks used it to test whether financial institutions are prepared for the effects of climate change. Governments often use it for adaptation planning, and insurance companies use it to set rates.
If the past 15 years of climate messaging had been based on a more realistic scenario instead, he added, “it wouldn’t make climate change go away, but it would certainly make climate change look far less apocalyptic, it would look more like a significant risk with serious consequences.”