Fengyun 3 08: a weather eye on climate change

In the early hours of 27 September, a Long March 4C rocket lifted off from the Jiuquan launch centre and carried the Fengyun 3 08 satellite into orbit. The Fengyun series is one of the oldest and most dependable strands of the Chinese space programme, the family of meteorological satellites that has fed weather forecasts, day and night, for decades, and the 08 satellite is the latest instalment of the polar-orbiting branch, the Fengyun 3 line. The mission’s description, however, carried an addition that reflects a shift in the series’ mandate: the satellite will continue the operational weather work, imaging clouds, sounding the atmosphere, and it will also place a particular emphasis on monitoring climate change.

The weather and the climate are different jobs that share instruments. A weather satellite measures the state of the atmosphere now, the clouds, temperatures and moisture that go into tomorrow’s forecast; a climate record is built from the same measurements accumulated over decades, the slow changes in those same quantities that the forecast never notices. The Fengyun 3 08 satellite does both at once, and its addition to the fleet strengthens a record that is valuable in proportion to its length: the polar-orbiting observations, continuous, calibrated and comparable across the years, are the raw material of the climate story.

The launch is a small event in a long history. The Fengyun programme dates to the 1980s, and its continuity is itself a form of infrastructure: forecasters plan around it, climate scientists build on it, and the series’ reliability is as much a product as any instrument. The polar-orbiting branch has been especially important for climate work, because its global coverage, every point on Earth passed over at regular intervals, produces the kind of complete record that regional or geostationary views cannot match.

The 08 satellite’s particular emphasis is worth noting precisely because it is a departure. Meteorological programmes historically had one customer, the weather service; the addition of a climate-monitoring role is a recognition that the same data have a second, longer-term customer, the society that needs to track the planet’s changing state. The distinction is reflected in the instruments and in the demands on them: climate monitoring requires a level of calibration and stability that weather forecasting can tolerate without, because a climate signal is small and must be measured against decades of noise. A drift that would never trouble a forecast can corrupt a climate record, and the demands of the longer job are, in that sense, the stricter ones.

The satellite will enter service quietly, its data flowing into the operational system and, over the years, into the climate record, and its contribution will be legible only in retrospect. That is the nature of the work: a weather satellite is judged tomorrow, a climate satellite over decades, and the 08 satellite will be both. A rocket rose in the dark over Jiuquan, and the continuous, patient record of the planet’s weather and its slow changes grew, as it does, by one more unit.