Summers in the Scandinavian Arctic are the warmest they've been in nearly 2,000 years, and this warming of the oceans worldwide could be leading to smaller fish, which has major implications for global fisheries, according to two new studies.  
 

Warmer oceans

William D’Andrea studies how Earth’s climate changes over time. In an article just published in the journal Geology, the associate professor at Columbia University’s Lamont-Doherty Earth Observatory charts 1,800 years of Arctic climate history, based on his analysis of sediment from a lake in the Norwegian archipelago of Svalbard.  
 
“This location hasn’t been as warm in the last 1,800 years as it has been in the last two decades,” he says.

D’Andrea and his colleagues reconstructed that climate history by examining traces of algae in the organic material and minerals that settled to the Arctic lake bottom over the millennia.

Scientists know that algae living in cooler water produce lots of unsaturated fats. In warmer water, they produce less. By measuring the fat content in algae retrieved in lake-bottom core samples, D’Andrea was able to track the Earth’s temperature over thousands of years.
 
“So what we have are little thermometers," he says. "These algae are producing thermometers and dropping them into the sediment and leaving them behind.”
 
D’Andrea says local air temperature records for the past 100 years match what he found in the lake-bottom sediment for the same period. He says the unique algae signatures can help scientists look into the past to see how Earth’s climate system behaves.

“And once we understand that, we get a better handle on how it does behave and why it responds in certain ways to different types of forcing, whether those forcing mechanisms are based on the sun’s output or volcanic eruptions or the level of greenhouse gases in the atmosphere,” D’Andrea says.

He adds that a clear picture of our climate history is essential to making accurate projections of our climate future.

Smaller fish
 
A second new study, published in the journal Nature Climate Change, looks at how fish are responding to rising ocean temperatures.  Lead author William Cheung and colleagues at the University of British Columbia’s Fisheries Centre, used computer models to project climate-induced physical changes in more than 600 species of fish.
 
“What we find is that across different ocean basins, meaning Pacific, Indian or Atlantic oceans, we are seeing on average around 14 to 24 percent reductions in maximum body size of the fish species that we investigated by 2050 relative to now,” Cheung says.
 
As the atmosphere is warmed by heat-trapping emissions from fossil-fueled power plants, buildings and automobiles, so too is the ocean.  And in a warmer ocean, there is less dissolved oxygen available to fish, who need it for normal growth.      

“So at some point the fish will stop growing because they just cannot get sufficient oxygen to support growth in addition to maintaining their normal body function,” Cheung says.  
 
The study is the first to predict that a warmer ocean could mean smaller fish in the decades ahead.  It also suggests that global warming may exacerbate the damage to fish populations already being done through overfishing, pollution and habitat loss.
 
“If you look at the fish population that is already depleted, that their critical habitat are deteriorated, they have a smaller capacity to respond to climate change compared to fish populations that are still abundant or that they are well managed and in good condition," Cheung says. "So we need to manage our marine ecosystems effectively as well.”
 
Cheung says the study concludes that failure to curb climate-changing greenhouse-gas emissions could risk further damage to marine ecosystems, global fisheries and an essential source of the world’s food.