Variable Effects of Global Warming on Vegetation of the Pennsylvanian ‘Coal-Age’ Tropics The Pennsylvanian Period (299-318 million years ago) was, like our modern world, a time of glaciation and low CO2. However, it also includes intervals of significant global warming and deglaciation. One such interval, about 306 million years ago, was accompanied by threshold-like changes of great magnitude in the composition and structure of the vegetation of the lowland, wetland biome – the “coal swamps”. However, recent studies have revealed an alternation of wetland and seasonally dry floras in the lowlands, the latter exhibiting great persistence and little evidence of change in response to warming. This seasonally dry biome may have been the temporally dominant vegetation of the lowlands and appears to have persisted for long periods in the western, coastal parts of the equatorial belt and perhaps in mountainous regions. (Javascript is required to view Mediasite content)