I began this blog with the belief that understanding the history of this planet is necessary in helping us to predict our future, particularly in an era where the future of climate and biodiversity remains uncertain.
Each week I explored a past instance of global change and noticed big parallels with modern issues. For example, deforestation experienced in the Carboniferous and in the present have strikingly similar effects on biodiversity e.g. declines in amphibian populations, despite differences in atmospheric composition and climate. There are also parallels between modern and archaic events of eutrophication and species invasion, which we tend to consider modern concerns.
I was also surprised (and concerned) at the number of features shared between modern climate change and extinction rates to that observed in the Permian, which is arguably the closest that complex life has come to complete annihilation. If the Permian is anything to go by, we could be looking at a future with extreme seasonality, widespread aridity and major biodiversity crashes, perhaps pushing life to the brink (or beyond) once more. Speaking of biodiversity crashes, it was worrying to see that we could end up observing biodiversity losses comparable to that seen in the KT extinction and be heading into a sixth mass extinction.
It was interesting to see that the PETM is one of the best comparisons for modern global warming, in terms of rapidity and temperature increase, but that it did not result in very high extinction levels. Could this suggest that present extinction levels are high because of other factors such as habitat loss and ocean acidification, rather than temperature increase itself?
It has been really enjoyable discovering how these drastic changes in global systems in the past altered the course of evolution, such as the CRC allowing for the evolution of the reptiles and the Permian Extinction promoting the diversification of the diapsids, or how the KT extinction was crucial for the rise of mammals and subsequently. Without these events, we as a species may not be here at all, the world may still have been dominated by amphibians or dinosaurs. It also promotes the question of how our actions now are affecting the course of evolution amongst ourselves and other species.
It has also been interesting exploring the idea of “threshold” levels of change e.g. sea level rise, temperature rise and release of methane clathrates, atmospheric composition and cooling/warming, ice volume, and nitrogen/phosphorus composition in freshwater. Yet if the theory of planetary boundaries holds up to scrutiny, we should be vigilant of going beyond the threshold or else we could find ourselves in some sort of runaway situation.
One thing I have taken from this blog is that we tend to think of modern climate change as an increase in temperature and melting ice caps, but a rise in temperature actually has severe implications on a whole host of issues e.g. oceanic and atmospheric circulation and some of which one would never think to associate with global warming such as eutrophication, species invasion and range shifts.
Despite similarities between modern and past events of global change, we are in a situation never experienced before. Never has there been a time in history where a single species has been as abundant and widespread as us, nor has there ever been a species that has had such a profound impact on planetary systems through activities such as agriculture and fuel production. Yet we are also in a situation that humans have never experienced before; humans originated and evolved in a time of regular glacial-interglacial cycles, never in our history has there not been ice in the poles and with current melting rates, we could be heading for a future without ice.
To conclude, I hold the belief even more strongly that understanding the past is key to predicting the future. Without studying these past events it would be impossible to know how the world and its organisms respond to dramatic global change. Rockstrom et al. 2009 defined planetary boundaries as a “safe operating space for humanity”, but it is important to realise that we are not the only inhabitants of this earth. Whilst we may be in the “safe zone” for our existence in many boundaries, we are well outside of the “safe zone” for other species. Perhaps a less human-orientated definition of planetary boundaries be more appropriate in assessing the state of our planet.
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