We are well aware that our oceans are undergoing vast changes as a result of human activity and given their role as climate regulators and their huge contribution to primary productivity, understanding these changes is extremely important.
Looking back at the paleoecological record, many marine responses to climate change can be observed and these past insights are incredibly valuable in predicting the response of marine communities to human-induced climate change in the future.
Changes in temperature, sea level, circulation patterns and acidification are the major drivers of marine response to climate change with communities responding with range changes, changes in composition and changes in body size.
Temperature Change
The most significant effect of temperature change in the oceans is on species' distributions and records dating back several million years reveal range changes in response to temperature. For example, warmer temperatures in the Miocene (15-17 million years ago) led to range extensions of mollusks and plankton, with many species that are currently found in the tropics and subtropics extending much further to the north - some as far as Alaska. As the climate cooled again after the mid-Miocene global temperature peak, these species' distributions altered, retreating southwards once more.
Changes in species' ranges can lead to changes in community composition, some of which are transitory while others are longer-lasting. Global cooling that occurred around 3.5 million years ago led to the loss of larger predatory species such as crabs, sharks and many fish from Antarctic communities and as a result, these communities today are dominated by invertebrates such as star fish even at higher trophic levels.
Fossil data for the Pleistocene (last 2 million years) are abundant and indicate clear responses to temperature change such as latitudinal shifts whereby species' distributions shift polewards with warming and towards the equator with cooling: species that existed between Santa Barbara and Ensenada (both California) during a warm interglacial 125,000 years ago, exist only in (sub)tropical waters today (Roy et al. 1996).
Sea level change
Global warming fuels sea level change in two main ways: from thermal expansion of the oceans and from the melting of land ice. If the ice in Antarctica and greenland alone was to melt, global sea level would rise by approximately 75m (Hannah, 2011). It is important to note that melting of sea ice in comparison while having profound impacts on food webs, would not be a major contributor to global sea level rise (this is because sea ice already displaces water).
Past warming and cooling has raised and lowered sea levels continuously with Pleistocene glacial-interglacial cycles being one of the main drivers for change. Range shifts of marine organisms in response to sea level change exist as far back as 55 million years ago. For instance, bivalves in tropical Pacific islands changed distribution repeatedly, affecting community composition. Similar effects have also been observed in bivalves, gastropods and other species in places such as Fiji and Kenya.
Ocean Circulation
Ocean circulation plays an important role in all of the major functions of the ocean (heat and carbon dioxide absorption, the transportation of these and their mixing into deep waters). Evaluation of paleoecological data reveals that changes in two major types of ocean circulation have had exceptional influence on both climate and biodiversity in the past- these are thermohaline changes and changes in teleconnections.
Thermohaline circulation helps to maintain nutrient levels in the deep ocean which is linked to high productivity in phytoplankton and thus high biodiversity and abundance in other trophic levels o the food chain. When the breakdown of this circulation system occurs (often as a result of a large input in freshwater in the North Atlantic), deep waters become oxygen deprived which can lead to the death of many organisms. Oxygen deprivation in deep waters is believed to be linked to many of the major extinction events in the past.
Teleconnections include El Nino as well as the North Atlantic Oscillation (NAO). These result in sea surface temperature changes (see previous blog post) and El Nino episodes are associated with decreased upwelling of deep nutrient-rich waters and changed air circulation patterns over the Pacific. Marine systems respond rapidly to these changes, changes in fish stocks of species such as Atlantic cod, sardines and Pacific salmon, are particularly widespread.
Ocean Chemistry
Major changes in ocean chemistry (primarily carbon dioxide and pH levels) are recorded in the fossil record, especially in the shells and skeletons of marine organisms. Both ocean pH and carbon dioxide affect the ability of organisms to form hard calcium carbonate shells and remains of these shells can provide a useful indication of past sea condition. Corals and foraminifera are the leading indicators of past climate- analysis of the isotopic composition of their shells allow reconstruction of past ocean temperatures and conditions.
In my next post, more on the distribution of coral reefs through history and the fate of our reefs in the future.
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