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There’s more bad news from the climate scientists. Not about global warming, where the news is never good, but about the impact nuclear weapons would have on the atmosphere, our climate, and food production if even a very small number were used in a limited, regional conflict.

The basic narrative—a nuclear famine bombing run, if you will—has not changed much since 2007, when Brian Toon, Alan Robock, and other experts in atmospheric science simulated the use of 100 Hiroshima-sized bombs over cities in India and Pakistan and found that enough soot and smoke would be injected into the upper atmosphere from the resulting firestorms to block sunlight from reaching substantial areas of the Earth’s surface for a decade or more. This, in turn, would cause sudden and dramatic global cooling, significant loss of rainfall, and shortened growing seasons in key agricultural regions.

Based upon those initial scientific findings, International Physicians for the Prevention of Nuclear War (IPPNW) first estimated that as many as a billion of the world’s most vulnerable people would face starvation as a result of global food shortages. New studies revealed that even this shocking conclusion was an underestimate, and last year IPPNW published a second edition of our Nuclear Famine report, warning that at least two billion people—more than a quarter of the world’s population—could starve to death in the years following the use of a tiny fraction (6 one-thousandths!) of the nuclear weapons that exist today. So how does it get worse than that?

Using even more comprehensive computer modeling—what they call an “Earth system model including atmospheric chemistry, ocean dynamics, and interactive sea ice and land composition components,” Michael Mills, Toon, Robock, and Julia Lee-Taylor have explained just how much worse it could get in an open-access research article they’ve published in the journal Earth’s Future.

For those of you who understand and relish the technical details, a full description of the model and the methodology is presented in the article. The conclusions are what matter to all of us. The soot (5 teragrams of black carbon, initially) gets lofted higher and persists longer—at least 15 years instead of the 10 previously calculated. Global average cooling is more extreme and persists longer—two decades or more—because of thermal inertia in the oceans after “more than a decade of prolonged cooling.” Drops in precipitation rates are not quite as steep during the first five years, compared with the previous studies, but in year five they continue to fall and are still down 4.5 percent at the end of a decade, when the earlier models were already showing a recovery.

Sea ice expands significantly in both the Arctic and Antarctic for a number of years, affecting the transfer of energy between the oceans and the atmosphere and further cooling the surface through reflection of sunlight. Ocean organisms, of course, are hit hard. (These ocean impacts are the previously unaccounted-for part, and they prolong the overall cooling for a good 25 years. Actually, come to think, that’s a pretty bad 25 years.)

Then we get to ozone loss, which Dr. Mills has already studied extensively. The black carbon heats the middle atmosphere severely, leading to “massive ozone loss” during the first five years after the nuclear war, with only modest recovery at the end of a decade. Increases in UV radiation resulting from the ozone loss confirmed in this new study are “dramatic.” Peak UV Index values for mid-latitudes in summer would be “off the charts” at 12-21, with 11 considered “extreme.”

The new study confirms that the impacts on agriculture—loss of rainfall, shortened growing seasons, genetic damage to plants from UV exposure—would be catastrophic. While the effects in the American Midwest and South America are “somewhat smaller,” the average growing seasons in Russia, North Africa, the Middle East, and the Himalayas are even shorter than calculated in previous studies.

Here’s the bottom line:

The ozone loss would persist for a decade at the same time that growing seasons would be reduced by killing frosts, and regional precipitation patterns would shift. The combination of years of killing frosts, reductions in needed precipitation, and prolonged enhancement of UV radiation, in addition to impacts on fisheries because of temperature and salinity changes, could exert significant pressures on food supplies across many regions of the globe….It is conceivable that the global pressures on food supplies from a regional nuclear conflict could, directly or via ensuing panic, significantly degrade global food security or even produce a global nuclear famine.

…These results illustrate some of the severe negative consequences of the use of only 100 of the smallest nuclear weapons in modern megacities. Yet the United States, Russia, the United Kingdom, China and France each have stockpiles of much larger nuclear weapons that dwarf the 100 examined here….Knowing the perils to human society and other forms of life on Earth of even small numbers of nuclear weapons, societies can better understand the urgent need to eliminate this danger worldwide.

Yes, things could get a lot worse. They could also get a lot better. The scientific and medical case for banning and eliminating nuclear weapons doesn’t get much clearer—or come with greater authority. As IPPNW science advisor Alan Robock wrote earlier this year on The Huffington Post, “The only way to be sure we do not annihilate the human population is to destroy the weapons.”

John Loretz is the Program Director of International Physicians for the Prevention of Nuclear War and syndicated by PeaceVoice.