With 71% of our planet covered in water, there is little surprise that we have affectionately named ourselves ‘the blue planet’. For something that takes up some much space (360,600,000 km2 to be precise) it has gone relatively unnoticed in the climate change story.
That all changed this week with the release of a new IUCN report, entitled Explaining ocean warming: causes, scale, effects and consequences. This report gives us a stark warning of the dangers we face as a result of our rapidly warming oceans.
More than 93% of the enhanced heat resulting from human activities, since the 1970s, has been absorbed by the ocean. To help put that in perspective, the report explains: if the same amount of heat that has gone into the top 2km of the ocean between 1995 and 2010 had instead gone into the lower 10km of the atmosphere, Earth would have seen a warming of 36°C.
The ocean plays a key role in climate regulation. Its low albedo means it absorbs much of the radiation from the sun and it takes heat up 4000 times as effectively as air. It also plays a key role in redistribution of heat from the tropics to the poles. Together with ocean acidification and sea level rise, excessive warming of the ocean will lead to a series of cascading effects, affecting not just the life within it, but humans too.
The impacts of ocean warming are wide ranging, affecting biological process within cells to changing ocean currents. The report shows that warming is already affecting ecosystems from the tropics to polar regions, and causing entire groups of species like plankton, jellyfish and seabirds up to 10 degrees of latitude towards the poles – at a rate five times faster than the shifts seen by species on land. Every creature, from minute microbes to the largest whales will be impacted in some way.
Microorganisms represent the vast majority of ocean biomass and inhabit all oceanic habitats, from deep sea floor to the surface layer. Microbes carry out an impressive away of metabolic process, influencing most geochemical processes which in turn drive the functioning of the entire ocean. For something so small, microbes have a big affect. Little is known at present about that the likely impacts of climate change will be on these organisms and the information is conflicting. There is widespread thought that sensitivity to climate change increases with trophic level, meaning these base organisms can carry on, business as usual. However, recent evidence suggests that microbes have higher sensitivity to changes in temperature. Potential effects include oxygen depletion and reduced primary productivity; even minor changes at the bottom of the food chain can be amplified exponentially up the food chain.
Despite covering less than 0.1% of the ocean floor, coral reefs are worth a staggering US$9.8 trillion in ecosystem services. If that wasn’t impressive enough, the reefs provide habitat for 25% of marine fish species. Coral reefs are a key indicator ecosystem, responding quickly to changes in environmental conditions. Maximising growth and reproduction close to their upper temperature limits, coral reefs are pretty resilient. However, once this threshold is reached, corals can experience bleaching. The stress of warmer temperatures results in the coral expelling the colourful algae that live within them. Some corals can recover but often the entire ecosystem, which once supported a huge array of diversity, disappears and the once rainbow reefs turn to ghostly white.
Marine fish are sensitive to temperature change because physiological performance is dependent on the ambient temperature. Tropical and polar fish are the most sensitive due to their narrow range of tolerance. In response to a warming ocean, many fish species are showing range shifts up to hundreds of kilometres; this can lead to species invasion, local extinctions and dominance of warm water species.
Seabirds are primarily affected via their dependence on oceans to provide food sources, but other changes associated with warming seas, such as retreating sea ice and increased storminess, is also likely to damage populations. Significant declines can already be demonstrated in several species. For example, emperor penguin populations have declined by 50% as a consequence of reduced sea ice. Rockerhopper penguins at Campbell Island have decreased by 96% as sea temperature has increased. Another problem for seabirds is that many species are faithful to nesting sites, choosing to return to the same site every year. In some cases, young seabirds will return to the colony were they fledged. Although birds have had previous success at these sites, the warming ocean may affect distribution of food, or rising sea levels could make the site at risk of flooding, again impacting on bird populations.
Turtles are being dealt a double blow when it comes to climate change, feeling the effects of changes both in the sea and on land at their nesting sites. Being ectotherms, turtles are reliant on external sources of heat to regulate their temperature; warming sea temperatures will obviously impact this, as well as potentially affecting distribution of vital food sources. Changes in temperature on nesting beaches can also have significant impacts on embryo development and, importantly, hatchling sex ratio. Turtles have a narrow temperature range where eggs can develop successfully, and within this range is a critical temperature which varies between species. Gene expression differs at this key temperature point; below, males develop and above, females are produced. As a result, warmer temperatures yield more females. This is a complex process and it is not yet certain how significantly warming temperatures will affect sex ratios of turtle populations. However, warming temperature is likely to reduce overall hatchling success. On the west coast of Costa Rica, leatherback hatchling success is predicted to decline from 42% to 18%.
Marine mammals are a relatively small taxonomic group but their huge biomass and position at, or near, the top of the food chain, makes them ecologically important. The main impact on this group is likely to be changing availability of prey. However, shifts in range, timing of migrations, reduced reproductive success can all be linked with increased ocean temperature. As many whale species are highly migratory, they also face challenges in multiple environments. The other potential impacts on marine mammals are perhaps less obvious. Human food security issues, as a result of fishery collapse, for example, may lead to increased human reliance on marine mammals as a food source. Conversely, collapse of fisheries could increase populations of some marine mammals due to the removed risk of bycatch. Warming of waters at higher latitudes might increase aquaculture leading to culling of marine mammals or eutrophication. Changes could also change the risk of chemical pollution or increase the presence of pathogens, providing another challenge.
That leaves us. The ocean provides a huge array of ecosystem services to the human population. It is difficult to put a value on the oceans; our interaction with it is so complex and far reaching. Under business as usual scenarios, fisheries and aquaculture are likely to be affected. The detrimental effects of ocean warming on key coastal habitats such as reefs, mangroves and oyster beds will reduce their ability to protect our coastlines from extreme weather events which are becoming more and more common (The number of hurricanes, for example, has increased at a rate of around 25-30 per cent for every 1 degree Celsius increase in warming, according to the report). We are also likely to see increased incidences of water-borne diseases; there is growing evidence of a cholera causing bacterial disease and algal bloom species. The ocean is sick, and it will make is sick too.
The report says that the some of the damage is already done, and we will see the impacts from historic warming for the foreseeable future. It is important that changes are made now to prevent even more damage. Options include mitigating CO2 emissions, protecting marine and coastal ecosystems, adapting to the projected changes and repairing damaged ecosystems.
The scariest part is that the absorbed heat and carbon dioxide is not fixed in the ocean, and could one day be released back into our atmosphere. The report makes reference to a warning given by the meteorologist Carl-Gustav Rossby in 1956 – “Nature can be vengeful. We should have a great deal of respect for the planet on which we live.”
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