Raghu MurtuguddeMar 23, 2019 16:04:25 IST
An old adage in marine ecology is that nature makes the rules but biology finds the loopholes.
One such loophole is how pockets of cold water persist near the South American coast even during the El Niño. Anchovies survive El Niño due to this loophole. Through such mechanisms, El Niño has served as a driver of the rich biodiversity in the Galapagos archipelago.
Another loophole has also been identified in the armpit of Isla Isabela in the Galapagos, where warming-driven wind changes have produced a cold patch of water and an increase in the Galápagos penguins as a result. Galápagos penguins are listed as an endangered species.
It is said that climate is what you expect and weather is what you get. This is true of the ocean too. While global mean temperature and global mean sea level continue to rise, all global warming is local. Local weather changes matter most for daily living. This also applied to the ocean. Yes, the ocean, too, has weather and the ocean weather, too, is getting more extreme.
It is critical to understand how marine life responds to ocean weather for future projections to be reliable in a warming world and sustainable navigation of marine resources is to be effective. Temperature is known to be the most important limiting factor for the distribution of microbiome and phytoplankton in oceans.
Many studies have reported on how global warming causes large-scale changes in primary productivity in oceans – something the entire food web depends on for its survival. And yet, analysis of high-resolution sea surface temperatures and chlorophyll show many regions where warming actually leads to more chlorophyll, more phytoplankton, more underwater fauna.
The narrative has long been that oceans are slow as molasses compared to the atmosphere – providing a fairly uniform and slowly-changing environment for organisms that call it home. Such an environment is also considered responsible for the much lower number of species in the ocean compared to that over land. The ocean, however, is full of eddies (analogs of tornadoes), filaments (cold and warm fronts) and fronts (lines of thunderstorms in the atmosphere) which constantly generate ocean weather and numerous environmental niches for life to evolve.
This animation of the global ocean depicts this beautifully.
We need to remember that we are pretty much still in the dark about how many species exist in the middle (mesopelagic), lower (bathypelagic) and deep (abyssopelagic) layers of the oceans. The ocean floor from the coasts to the deep dark ocean appears to be just as rich in biodiversity as land.
An extensive study of global marine biodiversity change indicates that there is no systematic local biodiversity loss because of diversity changes over time, i.e., the community composition changes over time without a loss in total biodiversity. This essentially means that biodiversity might need to be evaluated with holistic measures like the health of an ecosystem – its diversity, its resilience to shocks, and its total productivity.
Ocean temperatures can differ by more than 10ºC over short distances – in minutes, days or weeks – and some tides, storms, and eddies can reach from the surface to the seabed several kilometers below. A Comment authored by a team of 17 marine scientists in Nature August 2018 warned that ignoring ocean weather can jeopardize both the conceptualization and conservation of marine biodiversity.
The same also seems to be true in terrestrial ecosystems. While seasonal temperature ranges strongly control species range in midlatitudes, tropics have a stronger rainfall control. The rest of the world appears to be controlled by daily temperature ranges. This again points to how weather timescales control life on earth. How global warming affects life will thus depend on how weather variability will change.
Ocean weather includes not only temperature but also nutrients, acidity or pH, and salinity. Large scale averaged quantities are not the best indicators of marine ecosystem responses. For example, even as there was large scale bleaching of the Great Barrier Reef, some fish suffered while others responded well. Only high-resolution data can tell us what loopholes may have been generated in the complex symbiotic system supported by the reef that allowed some species to survive.
Corals themselves are not responding uniformly to warming; some corals in the Florida and Caribbean regions are adapting to warming. Maybe new genetic techniques are needed to understand the intra-species genetic differences but it is clear that local ocean weather and biological loopholes will need to be understood completely as well.
Two recent studies have reported that marine heat waves, or days when the ocean temperatures warm above the 90th percentile of the expected values, are increasing in number, intensity and duration. They are projected to continue to increase with global warming. Ocean heat uptake is also reported to be accelerating.
Large scale weather patterns such as the anomalously warm Pacific blob in the Gulf of Alaska which occurred from 2013 through 2016, the Atlantic cold blob in the Greenland-Iceland-Norwegian Seas which has persisted for much longer, and the warmest ever recorded sea surface temperatures off of West Australia detected during February of 2011, are all ocean weather manifestations of global warming.
The good news is that both in situ and remotely sensed data continue to grow exponentially; especially due to the revolution in autonomous platforms that can reach places in the ocean that was not possible before. Matching the data to the scales which life cares about is crucial for eliciting the processes that generate biological loopholes.
Oceans take up more than 90 percent of the excess energy being trapped by anthropogenic greenhouse gas emissions and about 25 percent of the carbon emissions. Ocean weather is responding to this generosity with increasing extremes which clearly cannot be good news. But we cannot know just how bad the news is until we map in detail all the loopholes biology is finding even as nature’s rules continue to warm, raise and acidify the oceans.
The author is a Professor of Atmospheric & Oceanic Science and Earth System Science at the University of Maryland, currently a Visiting Professor at IIT Bombay.
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