On that flight too, in transit between the Cook Islands and New Zealand, a friend called to warn him of clouds on his route. There were no nearby airports. "Sometimes you have to go through because you cannot go back," Lenarcic later recalled. The moist, tropical clouds tossed the Virus and frightened Lenarcic, but he collected the data, which Mocnik and colleagues at the Slovenian company Aerosol, which makes the instrument, are preparing for publication. However the itinerary lacked Arctic soot, so Lenarcic set his course north in April. This time, facing the prospect of icy clouds choking the engine’s air intake, Lenarcic pulled back the throttle on the cockpit floor and descended towards Eureka.
Two months before Lenarcic's polar flight, an international team of scientists announced that most climate models underestimate black carbon's impact on atmospheric warming by up to a factor of three. "We think [black carbon] is a really powerful warming agent on its own,” says Shindell, “But without really understanding what it does to clouds, it's hard to draw a more definitive conclusion." In warm temperatures, for example, black carbon particles can seed clouds, which reflect sunlight and may cool the atmosphere. But to understand how freezing clouds behave, researchers need fresh aerial data on soot and clouds.
Researchers studying Arctic climate change say that there is little manoeuvring room for mitigating human-provoked ice melting. But they say reducing black carbon emissions may be the best short-term bet. The Arctic is responding faster to climate change than are other regions on Earth. Black carbon acts over short timescales. It sinks quickly out of the atmosphere compared to greenhouse gases such as carbon dioxide, making it a good target for reducing atmospheric warming in the short term, according to a 2012 study by Shindell and colleagues.
International organisations are already taking note. In February, Arctic environment ministers agreed to cooperate on estimating and eventually reducing emissions of short-lived pollutants including soot. The UN Economic Commission for Europe issued its first reporting guidelines for black carbon emissions at a mid-May meeting in Istanbul. Arctic residents say regulation cannot happen fast enough. Last month the international treaty organisation Arctic Athabaskan Council petitioned the Inter-American Commission on Human Rights for help regulating Canadian black carbon emissions.
But reporting, says Mocnik, is not as good as measurement. Reporting tends to be a calculation based on official estimates of what an economy is burning. Since people do not report home woodstove burning to tax authorities or economists, for example, those estimates are often too low to be useful. Researchers will need real-world data such as those Nasa and Lenarcic are collecting. And the most cost-effective way to do that is still up in the air.
Having slowed his descent to Eureka, Lenarcic still had to land with his data intact. The plane's shadow slowed over the snow as Lenarcic circled the airfield and aligned the plane's nose with the runway. He drew back the throttle just enough to establish a gentle descent and waited until crossing the runway threshold before he reached up to the cockpit roof, pulled the air brake lever and committed to the landing. The Virus' wheels met the snow-drifted runway and rolled to a stop.
"This is not a very pleasant place to stay for a long time," he told me from the shelter of the eight-man weather station the next day. Before landing the temperature had been -18C (-0.4F). He fretted about starting the Virus' engine again. Such deep cold can sap the starter battery and the howling winds can press sharp ice crystals deep into the engine's tubes and wires.