Monitoring global vegetation dynamics with coarse and moderate resolution satellite data

Abstract

Earth's annual average temperature has increased by about 0.6°C during the past three decades. This warming pulse has brought many changes in the climatic system. For example, the Amazon forests of South America experienced frequent droughts possibly from altered air-sea interaction patterns in the Pacific and Atlantic oceans. The response of vegetation to this unprecedented rate of warming is the subject of this dissertation. Vegetation greenness levels, a surrogate of vegetation photosynthetic activity, recorded by satellite-borne sensors offer repetitive synoptic views of the Earth's vegetation. This period of extraordinary warming coincided with the availability of multiple data sets of vegetation greenness levels from different satellites, thus providing an unique opportunity to assess the impact of warming on vegetation. The Amazon region has suffered two severe droughts during this decade - the so-called "once-in-a-century" drought in 2005 and an even stronger drought in 2010. Vegetation browning during the 2010 drought was four times greater than that in 2005 (2.4 million km^2). Notably, 51% of all drought-stricken forests showed browning in 2010 (1.68 million km^2) compared to only 14% in 2005 (0.32 million km^2). This large-scale decline in vegetation greenness denotes significant loss of photosynthetic capacity of Amazonian vegetation and thus a major perturbation to the global carbon cycle. In the northern latitudes (>50°N), vegetation seasonality (SV) is tightly coupled to temperature seasonality (ST). As ST diminished, so did SV. The observed declines of ST and SV are equivalent to 4 and 7° (5 and 6°) latitudinal shifts equatorward during the past 30 years in the Arctic (Boreal) region. Analysis of simulations from 17 state-of-the-art climate models indicates an additional ST diminishment equivalent to a 20° equatorward shift this century. How SV will change in response to such large projected ST declines is not well understood. Hence there is a need for continued monitoring of northern lands as their seasonal temperature profiles evolve to resemble those further south. The results presented in this dissertation provide a better understanding of the impact of recent warming on three pristine ecosystems - the Amazonian forests, and the Arctic and Boreal ecosystems.