Seran G. Gibbard

Climate Effects of Global Land Cover Change

There are two competing effects of global land cover change on climate: an albedo effect which leads to heating when changing from grass/croplands to forest, and an evapotranspiration effect which tends to produce cooling. It is not clear which effect would dominate in a global land cover change scenario. We have performed coupled land/ocean/atmosphere simulations of global land cover change using the NCAR CAM3 atmospheric general circulation model. We find that replacement of current vegetation by trees on a global basis would lead to a global annual mean warming of 1.6 C, nearly 75% of the warming produced under a doubled CO{sub 2} concentration, while global replacement by grasslands would result in a cooling of 0.4 C. These results suggest that more research is necessary before forest carbon storage should be deployed as a mitigation strategy for global warming. In particular, high latitude forests probably have a net warming effect on the Earths climate.

Titan's Atmosphere in Late Southern Spring Observed with Adaptive Optics on the W. M. Keck II 10-Meter Telescope

Abstract Using adaptive optics on the W. M. Keck II telescope, we imaged Titan several times during 1999 to 2001 in narrowband near-infrared filters selected to probe Titans stratosphere and upper troposphere. We observed a bright feature around the south pole, possibly a collar of haze or clouds. Further, we find that solar phase angle explains most of the observed east–west brightness asymmetry of Titans atmosphere, although the data do not preclude the presence of a “morning fog” effect at small solar phase angle.

Titan imagery with Keck adaptive optics during and after probe entry

[1] We present adaptive optics data from the Keck telescope, taken while the Huygens probe descended through Titan’s atmosphere and on the days following touchdown. No probe entry signal was detected. Our observations span a solar phase angle range from 0.05� up to 0.8� , with the Sun in the west. Contrary to expectations, the east side of Titan’s stratosphere was usually brightest. Compiling images obtained with Keck and Gemini over the past few years reveals that the east-west asymmetry can be explained by a combination of the solar phase angle effect and an enhancement in the haze density on Titan’s morning hemisphere. While stratospheric haze was prominent over the northern hemisphere, tropospheric haze dominated the south, from the south pole up to latitudes of � 45� S. At 2.1 mm this haze forms a polar cap, while at 1.22 mm it appears in the form of a collar at 60� S. A few small clouds were usually present near the south pole, at altitudes of 30–40 km. Our narrowband J,H,K images of Titan’s surface compare extremely well with that obtained by Cassini ISS, down to the small-scale features. The surface contrast between dark and bright areas may be larger at 2 mm than at 1.6 and 1.3 mm, which would imply that the dark areas may be covered by a coarser-grained frost than the bright regions and/or that there is additional 2 mm absorption there.

Speckle imaging of volcanic hotspots on Io with the Keck telescope

Abstract Using speckle imaging techniques on the 10-m W.M. Keck I telescope, we observed near-infrared emission at 2.2 μm from volcanic hotspots on Io in July–August 1998. Using several hundreds of short-exposure images we reconstructed diffraction-limited images of Io on each of three nights. We measured the positions of individual hotspots to ±0.004″ or better, corresponding to a relative positional error of ∼20 km on Ios surface. The sensitivity of normal ground-based images of Io is limited by confusion between overlapping sources; by resolving these multiple points we detected up to 17 distinct hotspots, the largest number ever seen in a single image. During the month-long span of our 1998 observations, several events occurred. Loki was at the end of a long brightening, and we observed it to fade in flux by a factor of 2.8 over the course of one month. At the 3-sigma level we see evidence that Lokis position shifts by ∼100 km. This suggests that the brightening may not have been located at the “primary” Loki emission center but at a different source within the Loki caldera. We also see a bright transient source near Loki. Among many other sources we detect a dim source on the limb of Io at the latitude of Pele; this source is consistent with 2.7% of the thermal emission from the Pele volcano complex being scattered by the Pele plume, which would be the first detection of a plume through scattered infrared hotspot emission.

Cloud Structures on Neptune Observed with Keck Telescope Adaptive Optics

We report on observations obtained with the adaptive optics system at the 10 m Keck II Telescope during engineering validation and early science observing time for the adaptive optics system. We observed Neptune at near-infrared wavelengths. Angular resolution was 005–006, corresponding to a spatial scale of approximately 1000 km at Neptune. We discuss the latitudinal structure of circumferential cloud bands and of compact infrared-bright features seen in the southern hemisphere, as well as their variation with wavelength. We determine the values of I/F (proportional to the ratio of reflected intensity to incident solar flux) in the J and H infrared-wavelength bands, including narrowband filters where there is strong methane absorption. We use the I/F values inside and outside of methane bands to estimate the altitude of clouds responsible for the brightest compact features in the infrared. Our data show that, on two of our four observing dates, the brightest region on Neptune contained highly reflective haze layers located below the tropopause but not deeper than a few bars.

Near-Infrared Observations of Neptune’s Tropospheric Cloud Layer with the Lick Observatory Adaptive Optics System

We provide one of the first constraints on the combined infrared single-scattering albedo and opacity of Neptunes upper tropospheric cloud layer. For the observations, we used the adaptive optics system on the Lick Observatorys 3 m Shane Telescope (Mount Hamilton, California). The cloud layer is thought to be composed of H2S and extend up to 3.5–4.5 bars. Previously, the single-scattering albedo was measured in the range 0.2–0.94 μm and found to be extremely high (>0.8), but decreasing with increasing wavelength. Assuming an optically thick cloud, we find the best-fit single-scattering albedo of a 3.5 bar layer to be 0.23 at 1.27 μm and 0.18 at 1.56 μm. Uncertainties in the column density of haze above the cloud layer, and from deconvolution to remove contaminating light scattered by the point-spread function from infrared-bright features, indicate that the cloud could be even darker, but it is unlikely to be brighter than we report. The cloud particles could be brighter than we report if the total near-infrared opacity of the cloud is very low or the clouds scattering phase function is significantly more forward-scattering at 1.2–1.6 μm than at 0.75 μm.