Saturday round-up: Dead zones reach skywards

University of Maryland's Louis Codispoti, who predicts increased nitrous oxide emissions from the sea could worsen climate change

The sea’s production of a greenhouse gas, caused in part due to human effluent, could increase as the world warms up. The nitrous oxide emitted due to changes in ocean make-up could worsen climate change, according to Louis Codispoti of the University of Maryland.

In the atmosphere, nitrous oxide strongly retains heat that would otherwise be lost into space. How much is released from the sea depends upon how much oxygen microbes living there have access to. If a given body of water were to fall from its maximum oxygen concentration to 1% of that, its nitrous oxide output would be 20 times greater.

Currently, human activity is thought to be reducing the oxygen content of many areas of the ocean, creating “dead zones”, also known as hypoxic waters. Sewage and fertiliser run-off promotes excessive plant growth in these regions. When these plants die, they are fed upon by micro-organisms that consume oxygen dissolved in the ocean, making it hard for fish and other creatures to live there.

“As the volume of hypoxic waters expand along our coasts, their ability to produce the greenhouse gas nitrous oxide increases,” explains Codispoti. “With low-oxygen waters currently producing about half of the ocean’s net nitrous oxide, we could see an additional significant atmospheric increase if these dead zones continue to expand.”

As well as human effluent, warming of the ocean will play a part in raising nitrous oxide emissions. “Oxygen solubility decreases with warmer water temperatures,” Codispoti writes in Science this week. As burning fossil fuels raises atmospheric CO₂ levels, CO₂ is in turn absorbed by the sea and makes it more acidic. This is expected to reduce how well small animals can produce carbonate mineral skeletons, which help them sink to the sea bottom. Instead they are likely to remain in the upper part of the ocean, already the most hypoxic, consuming even more oxygen.

Also this week, German researchers have published research showing that the world would warm regardless of the influence of the sun if CO₂emissions remained unchanged. Through history there has been a close relationship between solar activity seen as sunspots – dark regions on the sun – and the Earth’s temperature. From 1645 to 1715, for example, the Sun experienced an extended period of low activity which corresponded to a cold period referred to as the “Little Ice Age”.

The Solar & Heliospheric Observatory (SOHO) shows no visible sunspots. Credit: NASA and ESA

Sunspot activity usually changes between high and low activity over 11-year cycles, but the most recent minimum has been unusually long. This prompted Georg Feulner and Stefan Rahmstorf of the Potsdam Institute for Climate Impact Research to look at what another Little Ice Age would mean for climate change. They used two different scenarios previously suggested for what happened during the Little Ice Age, namely a 0.25% and a 0.08% reduction in the total energy falling from the sun onto the Earth. They input these figures into models based on “business as usual” Intergovernmental Panel on Climate Change IPCC scenarios of human-produced climate change.

Feulner and Rahmstorf's projections for the impact of lower solar activity on global warming are shown in dashed lines, with the solid lines representing normal 11-year cycles. Credit: Potsdam Institute for Climate Impact Research

“Global mean temperatures in the year 2100 would most likely be diminished by about 0.1°C and should not be larger than 0.3°C” Feulner and Rahmstorf write in Geophysical Research Letters. “Comparing this to the 3.7°C and 4.5°C temperature rise relative to 1961–1990 under the IPCC scenarios, a new solar activity minimum cannot offset the global warming caused by human greenhouse gas emissions.”