5.2.2 Global Change from NISTAR

Our original hopes for using the NISTAR to measure global change were muted by the realities of the Triana halo orbit (ranging from 4 to 15 degrees away from the Earth-Sun line and not the same from one year to the next). This use of NISTAR would only be perfect if Triana were exactly at L-1. Especially in the shortwave, the halo orbit scrambles global and seasonal change in a way that would be nearly impossible to unscramble in a few-years mission.

However, because of:

1. surface and atmosphere thermal lag times,
2. the relatively slow change of Earth's effective temperature with latitude, and
3. the high longwave opacity of the atmosphere outside the 8-12 µm window,

the longwave radiance is much less affected by this scrambling than the shortwave. Thus, in spite of the halo orbit, we expect NISTAR to be able to "take the Earth's temperature" in an integral sense and obtain a result at least as meaningful as (although certainly not equivalent to) the observed globally averaged surface temperature.

Of course, unlike the ~4000 separate instruments in the surface network, the NISTAR is a single absolutely calibrated instrument with no data voids over the ocean. (Although there is one large data void over the Earth's night side where the Earth is 1-2 degrees cooler, perhaps this can be predicted using global models and known thermal lag times.) And the NISTAR automatically integrates over half of the diurnal cycle. Thus, in spite of the orbit drawbacks, we think that the NISTAR longwave result will be a valuable addition to the small arsenal of integrative measures of global change.