His climate research must not be very effective in combatting climate change if 100 days of his time at work doesn’t pay for the carbon cost of a flight.

1/3 of a year is about 1% of a person’s career. If he can afford to not be working for 1% of his career, that implies the total effect of his work is less than 100 seat-flights, or less than one full passenger flight.

If a climate researcher’s total effect on carbon emissions, over the course of his entire career, is less than a single passenger flight then it makes sense to fire him.