To billions of people who live in coastal cities, by 2040, sea-level rise has become obvious, dangerous and costly as governments ponder how and where to move people out of harm's way, according to the scenario. They have had some recent experience to draw upon because rising water from melting polar ice has already made many small island nations uninhabitable, forcing multiple rescue efforts and triggering mass migrations to nations that agreed to take the refugees.
And that was only one link of a rapidly forming chain of emergencies that finally convinced all major world powers to take more drastic steps. They have deployed technology to shade the Earth against the sun's increasingly heat and have tried using machines that remove carbon dioxide from the atmosphere around the clock.
"That was the idea of this paper," explained Simone Tilmes, an atmospheric chemist at the National Center for Atmospheric Research (NCAR). "What if we start late on this large-scale mitigation? What can we do then?"
Tilmes' first study, which emerged last fall, argued that rapid decarbonization of the atmosphere is necessary. It has gained more attention now that the Trump administration has rejected the Paris Agreement. The pact, agreed to by 195 nations, calls for keeping global temperatures from rising more than 2 degrees Celsius (3.6 degrees Fahrenheit) above the world's preindustrial levels.
The second study assumes that by 2040, with greenhouse gases still accumulating on a business-as-usual basis, the 2 C barrier settled upon at Paris has already been reached and many people are enduring extreme events that are near or beyond recorded human experience.
"What you're seeing right now as record events would be very common in the future," said Jadwiga Richter, an expert on the dynamics of the atmosphere who is working with Tilmes and others on the new study.
This one will explore the side effects of geoengineering, or man-made attempts to reduce the results of the planet's warming.
Impacts include sea-level rise, melting of polar ice and permafrost, massive forest fires, record-setting heat levels, extreme drought, and heavy precipitation that can happen when what scientists call the globe's hydrologic cycle gets out of kilter.
The ideas of injecting aerosols such as sulfate particles into the stratosphere to shade the Earth, or developing machines such as "man-made trees" that more efficiently ingest CO2 from the atmosphere, have been discussed for decades. The ongoing Tilmes-Richter study doesn't pick potential winners among the technologies, which remain undeveloped, but explores their likely side effects.
Their studies show that if geoengineering works, it will be imperfect. Some dangers, such as the increasing acidification of the oceans, which threatens seafood industries, and others, like rising global temperatures — which also threaten food supplies — may not be stopped until after 2100, when sufficient CO2 has been removed from the atmosphere.
Meanwhile, aerosols, or tiny particles that can reflect the rays of the sun, and other man-made attempts to drop temperatures will affect the weather — and they might be blamed for helping to cause extreme events or creating imbalances in nature.
'A huge risk'
One scholarly paper has already examined the risks involved. One example is the possibility of delaying or preventing the monsoon that annually waters Pakistan's crops. The result could be famine. Another impact could be nuclear war if Pakistan suspects that its traditional enemy, India, had some hand in manipulating the weather.
Tilmes, Richter, and their team are working on new ways that those effects could be potentially minimized to make geoengineering approaches safer.
Read more at Should We Change Earth to Halt Warming? Scientists Say Maybe
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