The lead attorney for U.S. manufacturers and oil and gas companies on a climate change lawsuit didn't know the answer to a measurement fact when asked in court two weeks ago, court papers show.

At a Feb. 7 hearing of Juliana, et al v. United States of America, et al — a case a group of kids, young adults and environmentalists brought in 2015 against the U.S. government — Frank Volpe said he didn't know whether carbon dioxide levels had reached 400 parts per million, a measurement of atmospheric concentration.

Asked by Judge Thomas Coffin whether the groups he represents “acknowledge that the CO2 levels in the atmosphere are currently at 400 ppm,” Volpe did not answer.

“Do you deny that, or do you not know?” Coffin asked, according to a transcript.

“I would say that as we said in our answer, we don't know,” Volpe said.

“You don't know,” Coffin replied. Volpe said that determination would be up to an expert witness. Coffin tried again.

“So as we sit here today, do you have an expert witness that the intervenors intend to call that you can identify that will opine that the CO2 levels are not 400 ppm, but are something other than that and, if so, what?” he asked.

“I don't know, your honor,” Volpe responded.

Volpe is representing the American Petroleum Institute, American Fuel & Petrochemical Manufacturers and National Association of Manufacturers in the case. The groups intervened in the case to block it, and they have used similar language challenging widely accepted facts of climate science in their court filings.

Neither Volpe nor C. Marie Eckert, another attorney for the trade groups, responded to requests for comment about their clients' views on carbon dioxide concentrations.

The plaintiffs are arguing that the U.S. government has failed to protect them from the public health hazards of climate change, despite decades-old research on and knowledge of the phenomenon.

Judge Coffin says the nature, facts and drivers of climate change will be central to the case — including whether there is a threshold at which the concentration of CO2 in the atmosphere reaches a tipping point locking in irreversible planetary damage.

While scientists disagree on the specific dangers of CO2 levels — say, 350 versus 400 ppm — researchers track their concentration with laserlike focus.

The National Oceanic and Atmospheric Administration tracks CO2 levels measured at Mauna Loa Observatory, Hawaii. The monthly averages for January this year and last were both greater than 400 ppm, according to the agency.

Reached by phone and asked of the possibility that CO2 levels aren't at 400, a spokesman for the agency laughed.

Russell Schnell, deputy director of the Global Monitoring Division of NOAA's Earth System Research Laboratory, said in an email the agency compares its work against international research.

“Each CO2 data point is calibrated against high quality standards that are cross referenced with UN sanctioned standards,” Schnell said.

There are no peer-reviewed studies that show atmospheric levels of CO2 lower than the 400-ppm mark, he said.

“Multiple analyses from many institutions around the Earth confirm that average CO2 is above 400 ppm,” he said, adding that that fact remains true “even at the South Pole,” thousands of miles from significant carbon emission sources.

NOAA also organizes a network of research sites for greenhouse gas observation. There are 167 locations in 46 countries.

“CO2 levels are indeed currently demonstrably higher than 400 ppm,” Ralph Keeling, director of the Scripps CO2 Program and greenhouse gas expert, said in an email.

The Keeling Curve, a measurement of carbon dioxide levels taken from Mauna Loa since 1958, is named after his father, Charles David Keeling.

The government's attorneys said in court papers last month that the concentration of 350 ppm of CO2 isn't necessarily dangerous but acknowledged the country bears significant responsibility for fueling global warming (Climatewire, Jan. 18).

More than 25 percent of CO2 released globally between 1850 and 2012 came from the United States, Department of Justice lawyers said.

The case appears on track for trial in the fall.

Reprinted from Climatewire with permission from E&E News. E&E provides daily coverage of essential energy and environmental news at