Dr. James Hansen

Dr. James E. Hansen visited Denison at the end of September for the Denison High School Class of 1959 reunion and sat down with the Bulletin and Review to discuss climate policy in the United States.

This is the second part of the discussion.

Hansen has a BA in physics and mathematics, an MS in astronomy and a Ph.D. in physics, all from the University of Iowa. He is the director of the Climate Science, Awareness and Solutions Program of the Columbia University Earth Institute.

Since the 1980s, Hansen has been an advocate of action to avoid the dangers of climate change.

Greenhouse gases, of which carbon dioxide (CO2) is the most prevalent, increase the atmosphere’s opacity in the infrared regions, he explained.

“When we add CO2 and methane and nitrous oxide to the atmosphere, it’s like putting the planet in a blanket, so it radiates less energy to space,” Hansen said.

The planet’s energy imbalance, caused by global warming, can be determined by measuring the earth’s reservoirs, of which the largest by far is the ocean.

“We now have more than 3,000 Argo floats, which are distributed around the world’s oceans,” Hansen said. “They dive down to two-kilometer depths, make measurements and come back to the surface and radio the data to satellites.”

Measurements by the Argo floats show that the earth is out of balance by three-quarters of a watt per square meter, Hansen said.

“Three-quarters of a watt doesn’t sound like much energy, and it’s not, but globally it’s equal to 500,000 Hiroshima atomic bombs per day, every day of the year,” he said. “It is quite a lot of energy that is going into the ocean, where it warms the ocean and melts ice.”

We’ve already passed the point at which we could have made changes that would have allowed for an easy, soft landing, according to Hansen.

“We’re concluding that we’re already past the safe level of CO2,” he said. “If you left the atmospheric CO2 at 450 ppm (parts per million) for centuries, you’d guarantee that we’d lose all of our coastal cities.”

Determining just how quickly the earth’s ice sheets will disintegrate is not possible, but sea level rise will be the single most devastating of the climate impacts of high CO2, he said.

CO2 is currently about 410 ppm and continues to rise unabated.

“We’ve already reached a point where we’re going to have to get CO2 to decrease and that’s actually not so implausible,” Hansen said. “Presently, we’re injecting five parts per million of CO2 into the atmosphere each year by burning fossil fuels, but if you measure the amount of CO2 in the atmosphere, it’s only increasing 2.5 parts per million. The other 2.5 parts per million is going mostly into the ocean, into the soil, into the biosphere.”

The system is capable of soaking up a lot of CO2.

“If we reduce emissions from five to 2.5 percent, then CO2 would stabilize, temporarily, but you really need to get it to decrease,” Hansen said.

He and other scientists have concluded that CO2 should be reduced to 350 parts per million.

“And probably somewhat less than that, if you want to stabilize shorelines at approximately where they’ve been in the last several thousand years,” he said.

Two degrees of warming would make the atmosphere as warm as it was in the last interglacial period about 120,000 years ago, when the sea level reached six to nine meters higher than it is today.

“If you just go up one to two degrees, it’s not clear how long it will take the ice sheets to come to equilibrium with that higher temperature,” he said.

If “business as usual” continues, sea level will rise several meters in 50 to 150 years, he said.

“That means today’s young people would face the consequences of losing coastal cities, combined with the fact that low latitudes, tropics and sub tropics, are becoming uncomfortable,” he said.

“It’s difficult to work outdoors when it gets too hot and more than half the jobs (in those regions) are outdoors. It’s going to increase the immigration pressure from those countries, combined with immigration from losing coastal cities.”

The planet could become ungovernable.

Climate change is increasing the extremes at both ends of the hydrologic cycle.

“Dry places get drier and wet places get wetter,” he said. “We also get more extreme rainfall events, because as the planet gets warmer the atmosphere holds more water vapor.”

“The Mediterranean, the Middle East and the southwest United States are just getting hotter and drier,” Hansen said. “That’s what the models show and that’s what the data shows.”

Hansen noted that of the 10 highest crests of the Missouri River at Omaha, Nebraska, six have occurred in the last 10 years; three of those were in 2019.

“It’s been a pretty extreme change, which is noticeable,” he said. “Now, suddenly, people are realizing we have a problem.”

People continue to deny the problem, but their arguments don’t make any sense, he said.

“Ask any national academy of science of any country,” Hansen said. “Globally, the problem is understood.”

The problem won’t be solved quickly.

“It requires national and global policies and it’s going to require timescales of decades,” he said. “We are going to have to get emissions to peak and start going down to a point that the amount of CO2 in the atmosphere is 350 parts per million and maybe a little less. If we do that, we not only have a good chance of preserving the ice sheets and sea level but also (stopping) the ocean acidification, which is damaging, if not destroying coral reefs, and causing other problems in the oceans.”

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