Pollution, ocean temperature and 'natural' disasters

18 July 2012

Met Office research suggests links between industrial pollution, Atlantic sea-surface temperatures and disasters such as drought, flooding and storm damage.

Nature cover Published in the journal Nature, the study is the first to identify mechanisms that may link human emissions to temperature variations in the Atlantic Ocean. Previously it was thought that the changes were down to natural variability, in a large part because no physically based models have reproduced Atlantic changes, consistent with observations, in response to historical emissions.

There are well established links between this historical Atlantic variability and past climate changes and 'natural' disasters. Now, using a state-of-the-art climate model, Met Office scientists have demonstrated a link between man-made pollution and changes in the Atlantic.

Temperature fluctuations

Changes in the temperature of the Atlantic affect rainfall in parts of Africa, South America and India, as well as hurricane activity in the North Atlantic. In some cases, extreme weather has caused drought, flooding and storm damage.

Dr Ben Booth, a Met Office Climate Scientist and lead author of the research, explains: "Until now, no one has been able to demonstrate a physical link to what is causing these observed Atlantic Ocean fluctuations, so it was assumed they must be caused by natural variability.

"Our research implies that far from being natural, these changes could have been largely driven by dirty pollution and volcanoes. If so, this means a number of natural disasters linked to these ocean fluctuations, such as persistent African drought during the 1970s and 80s, may not be so natural after all."

Findings suggest industrial air pollution may have driven large-scale, long-term changes in Atlantic Ocean temperatures which may have had widespread impacts on climate. This means that human activity may have, and may continue to have, the potential to drive large-scale impacts on regional climate.

Aerosols and the Atlantic

This research explores the complex interactions between aerosols and clouds. Aerosols are tiny particles suspended in the air which have a direct impact on sunlight, scattering or absorbing part of it before it reaches the surface of the Earth, leading to cooler surface conditions.

Increased numbers of aerosols make clouds brighter and longer-lasting so they reflect more energy from the Sun, further reducing surface temperatures. Over the past 10 years, scientists have started to understand and quantify the impact of aerosol-cloud interactions and how they respond to other factors like temperature, relative humidity and air currents. It is only now that we are starting to see widespread inclusions of these processes in current state of the art climate models.

Sulphates, soot and black carbon were included in the study, but the sulphates are by far the largest group of aerosols. There are both natural and human sources of sulphates. Marine biology and active volcanoes represent a consistent natural source of emission that determines the background level of aerosols in the atmosphere.

Sulphur dioxide, the precursor to sulphates, is an industrial pollutant. Most of the sulphate variability seen in the past comes from this source. Volcanoes can have an important impact on the atmosphere by emitting with enough force to put material in the upper atmosphere. With volcanoes, sulphates make up an important component but, unlike lower-level emissions, the high level nature of these aerosols from big volcanic events put them above the level of the clouds.

Knock-on impacts

The research revealed that sulphate particles attract water vapour creating lots of tiny droplets within clouds, making clouds brighter. Bright clouds reduce the amount of sunlight that reaches the sea surface which has a large effect on North Atlantic sea surface temperatures. The new work shows that inclusion of these processes highlights a much more enhanced role of aerosols on the climate, compared to previous assessments.

Highs and lows in industrial air pollution have exerted a large influence over the Atlantic, either cooling or warming the surface of the ocean. These changes have led to significant regional climate changes and associated human and ecological impacts.

Further understanding of the relationship between pollution and ocean temperatures could help to predict and potentially avoid future changes in the Atlantic region. It could also help to reduce the impact of the knock-on impacts such as drought, flooding and storm damage.

The work opens up the potential that man's emissions played a large role in past regional climate changes. There will now be an emphasis on how other next-generation climate models reproduce this result, and thoughts will turn to novel ways of using satellite data to confirm these processes.

  • Find out more about the lead author of the research, Met Office Climate Scientist, Dr Ben Booth.

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