Climate Change & the Humboldt Current

Over the past 100 years, the sea surface temperatures have risen between 0.3°C and 0.6°C and is generally believed to be a general trend across the globe. There are deviations from this in isolated cases. 

Again, we’re going back to South America and looking at the strong upwelling in the Humboldt current. Here, observations in the Humboldt Current's large marine ecosystem have shown a cooling over the past 25 years. Although this is not a consistent cooling over 25 years, any long term warming episodes have been attributed to El Niño events where the entire oceanic circulation of the Pacific shifts. The cooling effect has been connected to increased strength of the upwelling occurring across the eastern Pacific, as a direct result of the increasing strength of the trade winds, associated with global warming. The trade winds run in an upwelling-favourable equatorward direction. The increasingly rapid deep water being pulled to the surface draws heat from the air and thus cools the environment.

We have already noted that this region of the world houses the world’s most productive fisheries. Unfortunately, these are in decline due to human impact of overfishing. The change in catch composition has shifted remarkably over the years, particularly in the occurrence of achoveta and sardines. Studies have shown a trophic level collapse, particularly since the 1950s where the sardine, a low-trophic level species, declined rapidly, with fishery explosion occurring simultaneously. It is estimated that today that 80% of stocks are overexploited or have collapsed, and 80% landings are from these collapsed stocks. It’s a shocking statistic.

Mean Trophic Level in the Humboldt Current (Sea Around Us, 2007)

Stock-Catch Status Plots for the Humboldt Current Large Marine Ecosystem, 

showing the proportion of developing (green), fully exploited (yellow), 

overexploited (orange) and collapsed (purple) fisheries by number of stocks (top) 

and by catch biomass (bottom) from 1950 to 2004 (Heinemann al., 2009).

Whilst upwelling could be seen as an opportunity for relief from this detrimental human activity, it is far from it. Upwelling provides nutrients to the surface waters which promote primary productivity. The graph below shows that there has been an increase in primary productivity in the sea, as you would expect. 

Humboldt Current Large Marine Ecosystem trends in chlorophyll a (left) and primary productivity (right) 

1998-2006, from satellite ocean colour imagery. (O’Reilly & Hyde, cited Heinemann et al. 2009)

But the benefits to this bloom (like below) are dwarfed by the relentless harvesting by fisheries, where annual catches may sum to over 18 million tonnes.

Planktonic bloom off the coast of New Zealand due to local upwelling